WO1999021803A2

WO1999021803A2 - Dielectric glasses for low dielectric loss, low temperature cofired ceramics with medium dielectric constants

Info

Publication number: WO1999021803A2
Application number: PCT/CA1998/001001
Authority: WO
Inventors: Galina Kniajer
Original assignee: The Westaim Corporation
Priority date: 1997-10-29
Filing date: 1998-10-29
Publication date: 1999-05-06
Also published as: AU9731098A; WO1999021803A3

Abstract

Dielectric glass compositions which can be used to produce glass ceramics having medium dielectric constants (⊂r 15 to 100), and being formed from: (a) a major proportion of a glass composition comprising 14 to 30 % by weight SiO2, 5 to 11 % by weight Al2O3, 20 to 35 % by weight SrO, and 8 to 28 % by weight TiO2; and (b) a minor proportion of one or more additives selected from the group consisting of 0 to 7 % by weight MgO, 0 to 20 % by weight La2O3, 0 to 20 % by weight PbO, 0 to 20 % by weight Nb2O5, 0 to 8 % by weight CdO, and 0 to 10 % by weight B2O3. Also provided are mixtures of host glass powders with partially crystallized glass powders of substantially the same composition, the partially crystallized glass having crystalline and amorphous phases in a volume ratio between 5:95 and 95:5, and having been crystallized at a temperature between T1 and T2, wherein T1 is a temperature which is not more than 40 °C below a first exothermic peak on a differential thermal analysis curve for the glass, and T2 is a temperature at least 20 °C below a temperature which causes complete crystallization of the glass. The partially crystallized glass provides improved dimensional stability to sintered glass ceramics formed from such mixtures, along with the ability to tailor the properties of the glass ceramic to the required needs or application. Methods to produce the partially crystallized glass powder, and methods of forming dense glass ceramic bodies with or without the partially crystallized glass, are also provided.

Description

"DIELECTRIC GLASSES FOR LOW DIELECTRIC LOSS, LOW TEMPERATURE COFIRED CERAMICS WITH MEDIUM DIELECTRIC CONSTANTS" Field of the Invention The invention relates to dielectric compositions useful as low dielectric loss, low temperature cofired ceramics. More particularly, the invention relates to glass materials that are sintered at low temperatures to produce dense bodies useful as solid substrates, multilayered structures for integrated microcircuits, discrete electronic components, electromagnetic shielding and the like working at radio and microwave frequencies. Background of the Invention Low temperature cofired ceramics (LTCC) are known in the art. LTCC technology has been introduced as a method for fabricating multilayered circuits and allows the use of highly conductive metals such as gold, silver and their alloys. The vast majority of commercial LTCCs [1] have low relative dielectric constants (e_r J around 4 - 10 that enable high signal propagation speeds in a microcircuit, according to the known equation: t = (e_r)°-⁵l/c wherein t is signal propagation time, e.is the dielectric constant of the medium, 1 is the distance the signal must travel, and c is the speed of light. However, in certain applications, low dielectric loss LTCCs with medium range dielectric constants (15 to 100) can offer design and functional benefits for electronic packaging without speed deterioration. LTCCs with medium dielectric constants possess such attractive properties as low firing temperature, ability to cofire or be cofired with highly conductive metals such as gold and silver, and a thermal expansion coefficient (CTE) close to that of semiconductors. LTCCs with a medium range of dielectric constants offer size reduction because the guide length of metal conductors of a microcircuit shortens by the same factor (e_r )⁰⁵ and, accordingly, the integrated circuit (IC) size can also be decreased. This is very useful in applications in which the size of the IC is a major concern. The main advantage of multilayered LTCCs with medium e_r is that they allow for design of miniaturized three-dimensional ICs for a wide range of frequencies. This is in part because the ratio of the conductor line width to the substrate thickness decreases with an increase in e_r thus providing more opportunities to manipulate these parameters to create better multilayered structures with reduced parasitic coupling and optimized parameters. To provide microcircuits with the benefits described above, there is a need for LTCCs having the following properties: • Dielectric Constant in the range from 15 to 100 • Dissipation Factor (tanδ) as low as possible at working frequencies • Compatibility with high conductivity metals such as gold and silver • Firing temperatures below about 950 °C • Absence of porosity • Thermal Expansion Coefficient (CTE) close to that of the semiconductors Currently, this combination of properties does not exist in LTCCs. Exemplary of patents which disclose dielectric compositions with low dielectric constants are U.S. Patent 5,258,335, to Muralidhar et al., which teaches a dielectric composition for use in tape in the fabrication of multilayered circuits wherein a dielectric material has a dielectric constant less than 7 at 10 MHz; U.S. Patent 4,755,490, to Di Lazzaro which describes glass ceramic materials with a dielectric constant between 4.5 and 6.1; and U.S. Patent 4,672,152, to Shinohara et al., which describes a dielectric material with a dielectric constant of 5.1 to 6.0. Ceramic materials with medium dielectric constants and very low tanδ are also described in the art. Kyocera Corp. provides electronic microwave ceramic materials having dielectric constants from 21 to 110 and very low tanδ at microwave frequencies, as disclosed in its catalogue [2], and in such U.S. Patents as 5,223,462, 4,866,017 and 4,670,409. However, these materials have higher than desirable thermal expansion coefficients, generally about 8 to 10 ppm/K, that do not match the CTE of desirable semiconductors. Further, such materials cannot be used for co-fired multilayered structures with high conductivity metals due to their higher firing temperatures of about 1200°C. LTCCs with medium dielectric constants are also disclosed in other literature. There are two materials available from Intertec Southwest LLC [3] which have a) e_r 20 to 30, tanδ 3 X lσ⁴ at 1 MHz, CTE 5.5 ppm/°C, firing temperature 900°C, and b) e_r 40 to 45, tanδ 3 X 10^"4 at 1 MHz, and firing temperature 1000°C, CTE 6 ppm/°C. Electro-Science Laboratories described a LTCC which has e_r 85 - 1 10, tanδ 25 X 10^-2 at 1 MHz and firing temperature of 850 °C [4]. U.S. Patent 4,628,404 to Yokoe et al. and assigned to Kyocera Corp., describes a dielectric ceramic composition prepared from a composition comprising by weight percent: 18-27% BaTi0₃; 31.6-36.3% Nd₂0₅; 27.6-35.5% Ti0₂; 2.5-8.6% Bi,0₃; and 5.6-9.0% Pb₃0₄ with an addition of a small amount of additives B₂0₃ (0.1-1.3%), SrO ( 1-3%) and InO (0.5-3%). These additives are disclosed as forming an amorphous liquid phase which promotes the sintering of the ceramic. The dielectric ceramic is reported to have e_r in the range of 69-83 and quality factor 1900-2700 (i.e. tanδ 3.7 x 10 to 5 x 10^"4) at 1 MHz. The sintering temperature is reported to be 1000 - 1050°C. Japanese Patent No. 07- 182922 issued to Kyocera Corp. teaches a material which is made of a composite containing 35 - 65 wt% of the composition of the molecular formula - aNd₂0₃ - bAl₂0₃ - cCaO - dTi0₂ and 35 - 65 wt% of boron silicate glass. The material is produced at 850 - 950°C and has e_r in the range of 14 - 18 and tanδ 2 - 3 X 10³. Japanese Patent No. 08-295558 issued to Kyocera Corp. describes LTCCs with firing temperatures of 850 - 1000°C with e_r 10-14. The composition relates to the composite comprising glass of the Si0₂-Al₂0₃-MgO-ZnO-B₂0₃ system with the addition of ZrO_: fillers. Japanese Patent No. 07-330427 issued to Kyocera Corp. teaches materials with e_r 10 - 20 and tanδ from 2 X 10^'3 to 1.4 X 10^"3 and firing temperatures of 850 - 950°C. These materials consist of crystalline phases such as (Mg₀₅Cao₅)Ti0₃, Ba(Mg_1/ Ta_2/3)0 , BaTi₉O₂₀, CaTi0₃, BaTi0₃, or SrTi0₃ blended with boron silicate glass wt % Si0₂ 50; A1,0₃ 5; B₂0₃ 10; CaO 12. Japanese Patent No. 08-169760 issued to Kyocera Corp. describes materials with e_r 17 to 21, tanδ 2.3 X 10^"3 to 1 X 10^"4, and firing temperature at 950°C. The composition relates to the composite made of crystalline phase aBaO-bMgO-cW0₃-dTa0_5/2 and additives B₂0₃ and Li₂C0₃ which form a liquid phase reacting with the major crystalline phase. While some materials exist with dielectric constants in the range of 10 to 1 10, with relatively low tanδ, no materials, to the inventor's knowledge, combine all the relevant properties with low firing temperatures and suitable CTE. It can generally be seen from the above review of the art that the main approach in the past is to provide mixtures of crystalline compounds, having high dielectric constants and high CTE, with a low temperature glass, usually having a low dielectric constant and a low CTE, which in most cases are of boron silicate compositions. However, in many cases, the attempt to lower the sintering temperature of a dielectric ceramic by adding boron containing compounds has a conflicting nature with respect to improving the dielectric properties of the ceramic after firing. When the amount of the ceramic crystalline phase is too large, sintering does not occur. However, when the amount of glass is too large, the dielectric properties deteriorate and different types of shape distortion or dimensional stability (warpage or via hole misalignment) are observed. A dielectric material is needed which has high dimensional stability to prevent any warpage or shape distortion which may occur on sintering. This is especially useful for electronic packaging applications where a good alignment of via holes is of paramount importance. Shape distortion or warpage is typically a problem in glass ceramics produced by a powder technique. In glass ceramics which are made by bulk glass technology, nucleation and subsequent crystallization processes begin at relatively high viscosities. Such glasses form a supporting crystalline frame which compensates for a decrease in the viscosity of the remaining amorphous phase until most of the amorphous phase crystallizes. In contrast, in glass ceramics which are made by glass powder techniques, the densification is provided by glass viscous flow. In this case, viscosities should be quite low to provide sintering before the onset of crystallization. If it is overlapped by crystallization, the process of viscous sintering is subsequently slowed down and finally cut off. The final product in such cases is usually porous. Sintering may take place but only at noticeably higher temperatures and very fast ramp rates to a peak temperature that is not always acceptable [5]. If crystallization begins at a temperature much higher than the material's softening point, shape distortion and warpage can occur. Since the amorphous phase is considered to be responsible for warpage, there are different approaches in the prior art to provide a glass with a rigid supporting frame. Usually refractory materials are employed as fillers to develop a supporting frame. The filler role is very diverse. It can be used to provide a reaction with a glass matrix with the formation of new crystalline phases, or to accelerate the crystallization process. U.S. Patent 3,778,242 issued to Francel et al. teaches a sealing glass in which a precrystallized glass in an amount of 0.0001 to 0.003 wt.% in combination with a refractory material are added to an uncrystallized glass to provide the necessary flow properties and crystallization speed. In most cases a filler preferably should be inert to the surrounding glass matrix. One of the most important features of a material to be a filler is its wettability with glass allowing for a sufficient amount of the crystalline compounds to be introduced into the glass matrix. However, the wettability of crystalline fillers by glass is poor, and in many cases glass does not accept a ceramic material in a sufficient amount to provide needed properties and undergoes undesirable crystallization or uncontrolled reaction with fillers. Many efforts have been directed to improving the wettability of refractory materials. U.S. Patent 4,812,422 issued to Yuhaku et al. describes a dielectric paste for a ceramic multilayer wiring substrate which employs several ceramic oxides as fillers reacting with the glass matrix. To improve the wettability, a part of the glass powder is mixed with a ceramic powder, and the mixture is heated at a high temperature, about 1300°C, so that the ceramics and glass react with each other, and a very thin reaction product and glass is formed on the surface of the ceramic particles which improvs the wettability with the raw glass at firing. U.S. Patent 5,141,899 issued to Jean et al. teaches a LTCC material which was made of a mixture of a low temperature borosilicate glass and high temperature titanium silicate glass. The latter acts as a supporting filler. Due to the obvious fact that glasses in general are very compatible in terms of wettability, the amount of the high temperature glass was increased up to 50 wt %. However, glass fillers have disadvantages in that they cannot control the crystallization process and can react with the glass matrix very easily to produce undesirable effects such as increased sintering temperatures and changing of the end properties. U.S. Patent 3,947,279 issued to Hudecek discloses a thermal crystallizable glass of lead zinc borate composition which includes very small amounts of essentially fully crystallized glass particles of same composition. However, the amount of the fully crystallized material that can be added is so small (parts per million) that a costly and not very effective operation known as a "Master Blend" of the crystallized particles with the uncrystallized particles must be used. From the foregoing it can be seen that crystalline materials have poor wettability while amorphous materials possess good wettability but do not have the other properties which are necessary to be a filler for a dielectric glass ceramic. Hence, the development of fillers which combine good wettability with the ability to control sintering and crystallization is desired for glass ceramic materials produced by powder techniques. Summary of the Invention The present invention solves the problems of the prior art efforts by providing dielectric glass compositions which do not contain high boron-containing glasses, which can be sintered into dense bodies at temperatures of 950 °C and lower, and which have dielectric constants in the range of 15 to 100, low dielectric losses at radio and microwave frequencies, and thermal expansion coefficients in the range from 6.0 to 8.0 ppm/°C. The glass compositions of the invention also achieve dimensional stability on firing, making the materials highly desirable for electronic packaging applications needing via holes. The compositions have application to substrates or multilayered microcircuits, resonators, filters, capacitors etc. The invention provides a dielectric glass from which a dielectric glass ceramic may be formed having the above-mentioned properties. The composition of the dielectric glass comprises: (a) a major proportion of a glass composition comprising 14 to 30% by weight Si0₂, 5 to 11% by weight A1₂0₃, 20 to 35% by weight SrO, and 8 to 28% by weight Ti0₂; and (b) a minor proportion of one or more of the additives selected from the group consisting of 0 to 7% by weight MgO, 0 to 20% by weight La₂0₃, 0 to 20% by weight PbO, 0 to 20% by weight Nb₂0₅, 0 to 8% by weight CdO, and 0 to 10% by weight B₂0₃. In another aspect of the invention, the above composition of the dielectric glass is altered to include the above-listed ingredients with only 8 to 25% by weight Ti0₂ and only 0 to 5% by weight B₂0₃. Preferably the glass comprises 70-80% by weight of components (a) and 20-30% by weight of components (b). The glasses of the present invention may contain small amounts of other common glass additives which do not adversely affect the desired glass ceramic properties, but preferably the glasses contain no more than 2% by weight of other additives or impurities. The following additives or impurities are preferably included in only very small amounts so as not to significantly alter the properties of the composition: • alkali metal oxide < 0.5% • barium oxide < 1 % • calcium oxide < 0.5% • cerium oxide < 1 % • zirconia < 1 % • bismuth oxide < 0.3% Glass additives which may be tolerated in larger amounts, but are preferably less than 2% by weight, include Sb, Pr, Nd, Ta, Zn, Sn, and As, generally in oxide forms, or as oxide precursors, i.e. compounds which yield the oxides on heat treatment. The glasses of this invention are unique examples of glass compositions having high amounts of such components as Ti0₂, and sometimes of Nb₂0₅, and La₂0₃ but which surprisingly have a "surface" type of crystallization. Typically, glasses containing such a high concentration of Ti0₂ as up to 20% by weight, have a "volume" type of crystallization. When such glasses are used for making glass ceramics by bulk technology, the sintering and crystallization processes are controlled by homogeneous or heterogeneous nucleation. This results in dense, non-porous glass ceramic bodies. However, these glasses with "volume" type of crystallization are not always suitable for making glass ceramics by powder techniques because, when finely ground, they are very prone to rapid, uncontrolled crystallization at high viscosities (close to the softening point). In such cases, crystallization overlaps sintering and, as a result, a porous material is produced. Unlike those glasses which undergo "volume" crystallization, the glasses of this invention behave in the same manner as glasses with no, or only small amounts, of the components Ti0₂, Nb₂0₅, and La₂0₃, as for example borosilicate glasses which are used to make low dielectric constant glass ceramics. This surprising behavior makes the glasses of the present invention quite unique and allows them to be used for producing glass ceramic materials with medium dielectric constant, which requires the presence of a large amount of Ti. The invention also extends to dielectric glass ceramic compositions formed by sintering the above dielectric glasses. The crystalline structure formed on sintering consists of a predominantly perovskite phase SrTi0₃, along with two other crystal phases (unidentified silicate and titanosilicate phases), with the perovskite phase being significantly modified by various substitutions by the additives for the Sr and Ti sites. It was found that the substitution at the Sr sites provides the higher dielectric constant and tanδ variations, while the substitution for the Ti sites makes it possible to adjust the tanδ values without a significant alteration of the dielectric constant. The proportion of these phases controls the dielectric properties and CTE values. The higher the perovskite phase content, the higher is the e_r . In general, higher e_r values are accompanied by higher tanδ and CTE values. The invention also extends to methods of producing a dielectric glass comprising: (a) producing a glass frit or glass flakes from a dielectric glass of the composition set out above; and (b) grinding the glass frit or flakes to produce a finely divided glass powder. The invention further extends to producing dense dielectric glass ceramic bodies from the dielectric glasses so produced, by the additional steps of: (c) mixing the glass powder from step (b) with an organic system; (d) forming a green body from the mixture of step (c); and (e) sintering the green body at a temperature not greater than about 950 °C to produce a dense dielectric glass ceramic material. The addition of low thermal expansion coefficient ceramic fillers may be used to improve the dimensional stability of the sintered dielectric glass ceramics. Low thermal expansion coefficient ceramic fillers are known in the art. They typically have a CTE < 7ppm/°C and are included in an amount between 0 and 20% by weight, (or preferably between 0 and 10% by weight) in a particle size range of 1-10 μm. Exemplary are one or more of lead, strontium, magnesium or aluminum titanates, titanium dioxide, niobium pentaoxide, cordierite, and the like. By cordierite is meant low cordierite having an orthorhombic crystal structure and a low thermal expansion coefficient. In general, these low CTE ceramic fillers can be mixed in varying proportions, depending on the desired end properties of the dielectric glass ceramic. Strontium titanate can be used, but should not exceed about 1 - 2% wt. The invention further extends to glass compositions as set out above in a partially crystallized form, useful as fillers in the production of dielectric glass ceramics. Such partially crystallized glasses contain crystalline and amorphous phases in a volume ratio between 5:95 and 95:5. They are formed by crystallizing at a temperature between T, and T₂, wherein T, is a temperature which is not more than 40 °C, and preferably not more than 30 °C, below a first exothermic peak on a differential thermal analysis (DTA) curve for the glass, and T₂ is a temperature at least 20°C, and preferably 50°C, below a temperature which causes complete crystallization of the glass. As is known in the art, DTA curves are usually used to characterize the crystallization process of the glass. T, can be readily generated from a DTA curve generated from glass powder for the particular dielectric glass composition, in a well known manner. The temperature T₂ is determined after microscopic analysis of samples of the frit or flakes which have undergone crystallization at a number of high temperatures, for a reasonable, but set, crystallization time such as a time in the range of 0.5 to 5 hours. The interval of temperatures T, - T₂ characterizes a change in the amorphous/crystalline phase ratio from 95:5 to 5:95. The amorphous/crystalline phase ratio is evaluated by examining the partially crystallized frit or flakes by microscope using magnification of 10 to 50 times. Based on the desired amorphous/crystalline ratio in the partially crystallized glass product, a crystallization temperature within T, - T₂ is chosen. Surprisingly, it was found that the addition of this partially crystallized glass, in finely divided form, to uncrystallized glass powder significantly decreased shrinkage and increased the dimensional stability of glass ceramics resulting therefrom. This is especially beneficial for glass ceramics having complex shape and structure. The invention also extends to a method of producing partially crystallized glass, comprising: (a) producing a glass frit or glass flakes from a dielectric glass of the composition set out above; (b) heating the glass frit or glass flakes at a temperature between T, and T₂, wherein T, is a temperature which is not more than 40°C, and preferably not more than 30°C, below a first exothermic peak on a differential thermal analysis curve for the glass, and T₂ is a temperature at least 20 °C, and preferably 50 °C, below a temperature which causes complete crystallization of the glass frit or glass flakes, for a time sufficient to produce a partially crystallized glass frit or glass flakes having crystalline and amorphous phases in a volume ratio between 5:95 and 95:5; and (c) grinding the partially crystallized glass frit or glass flakes to produce a finely divided, partially crystallized glass powder. The temperature for crystallization in step (b) will vary with the particular glass composition, but will generally be between 850 and 1 100°C. For use in glass powder techniques, the partially crystallized glass frit or flakes are preferably ground to a particle size in the range of 2 - 4 μm. The invention also extends to mixtures of glass powders, comprising: (a) a host glass in a finely divided form; and (b) 2 to 50 % by weight, based on the host glass, of a partially crystallized glass in a finely divided form and having a composition which is substantially the same as that of the host glass, said partially crystallized glass having crystalline and amorphous phases in a volume ratio between 5:95 and 95:5, and having been crystallized at a temperature between T, and T₂, wherein T, is a temperature which is not more than 40°C, and preferably not more than 30 °C, below a first exothermic peak on a differential thermal analysis curve for the glass, and T₂ is a temperature at least 20°C, and preferably 50°C, below a temperature which causes complete crystallization of the glass. The host and partially crystallized glass will generally have a particle size in the range of 1 - lOμm, most preferably 2 - 4 m for forming glass ceramics by powder techniques. The invention also extends to methods of producing dense glass ceramic bodies from a host glass with a partially crystallized glass, comprising: (a) producing a glass frit or glass flakes of a host glass; (b) grinding the glass frit or glass flakes of the host glass to produce a finely divided host glass powder; (c) heating a glass frit or glass flakes having a composition which is substantially the same as the host glass at a temperature between T, and T₂, wherein T, is a temperature which is not more than 40°C, and preferably not more than 30°C, below a first exothermic peak on a differential thermal analysis curve for the glass, and T₂ is a temperature at least 20°C, and preferably 50°C, below a temperature which causes complete crystallization of the glass, for a time sufficient to produce a partially crystallized glass frit or glass flakes having crystalline and amorphous phases in a volume ratio between 5:95 and 95:5; (d) grinding the partially crystallized glass frit or glass flakes to produce a finely divided, partially crystallized glass powder; (e) mixing the host glass powder and the partially crystallized glass powder, the partially crystallized glass powder being included in an amount of 2 to 50% by weight, based on the host glass powder; (f) forming a green body from the mixture of step (e); and (g) sintering the green body to produce a dense glass ceramic body. Preferably, the host glass powder and the partially crystallized glass powder are of a composition that forms predominantly a titanium-based perovskite crystal phase on sintering, such as strontium titanate based glass ceramics. It will be understood that one or more host glass compositions may be mixed with one or more partially crystallized glass compositions. However, the compositions will be substantially the same. By this is meant that the proportions of the same ingredients may vary, but the major ingredients will generally be present with one or more of the minor ingredients being changed. Most preferably both the host glass powder and the partially crystallized glass powder have the compositions of the dielectric glasses set out above, with variations in the minor ingredients, and in the proportions of any ingredient being possible. The partially crystallized glass frit or flakes are preferably formed by heat treatment at temperatures between about 850 - 1 100°C, for 0.5 - 5 hours. The peak temperature and holding time at this temperature used for the crystallization are generally selected based on the desired ratio of crystallized to amorphous phase in the partially crystallized glass. The time and temperature for crystallization varies with the composition of the glass and will depend on the desired end properties for the glass ceramics made therefrom. The temperature T, may be routinely determined from DTA curves generated for each glass composition in a powder form in a manner well known in the art, while T₂ may be determined by microscopic analysis of the glass frit or flakes which have been crystallized at high temperatures. In general, the lower the degree of crystallization in the partially crystallized glass, the higher the amount of partially crystallized glass that can be introduced into the host glass. Use of excessive partially crystallized glass ceramic can lead to undesired porosity in the sintered part. The higher fractions of the partially crystallized glass can be used when the crystallization temperature used is the lowest (i.e. toward the 850°C end of the range), owing to the increased wettability of the partially crystallized material when it has less of a crystalline character and is therefore more similar to the host glass. The final properties of the sintered materials which include the partially crystallized glass ceramic are not negatively affected by the fraction of the partially crystallized glass or the conditions used for the crystallization. This is achieved by maintaining the chemical composition of the host glass and the partially crystallized glass substantially the same, that is most of the same major components and most of the minor additives are included, although not necessarily in the same proportions. Being of substantially the same composition, no adverse chemical reaction can occur between them. A major advantage of using the partially crystallized glass as a filler in glass ceramics is that dimensional stability is improved and sintering can take place at lower temperatures, 850 - 950 °C, to arrive at the same degree of final crystallization. This makes the dielectric glass ceramic materials of this invention compatible with structures that incorporate silver or silver alloy conductor materials. Another benefit of using partially crystallized glass as a filler in glass ceramics is that shrinkage during sintering is minimized. Since crystallization implies a density increase, and an attendant shrinkage, the use of a smaller fraction of the uncrystallized glass ceramic thus has the effect of reducing shrinkage. Finally, the sintered glass ceramic products formed with the partially crystallized glass have a very fine crystalline structure due to the presence of initiators of crystallization with a very developed surface area. The invention also extends to unfired green bodies comprising the above dielectric glasses dispersed in an organic system. The invention also extends to multilayered ceramic substrates comprising layers of the dielectric glasses set out above and interconnected conductor layers of precious metals therebetween. The multilayered ceramic circuit structure is formed of a plurality of laminated dielectric layers with patterned electrical conductor layers to form a predetermined wiring circuit. The laminated layers comprise a sintered dielectric glass ceramic of compositions set out above. The invention further extends to a multilayered ceramic capacitor comprising layers of the above dielectric glasses with conductive layers of highly conductive metals therebetween, the assembly having been fired to form a dense hermetic structure. A still further aspect of the invention relates to a multilayered substrate having one or more layers comprised of the dielectric glasses set out above in the form of buried capacitors, the assembly having been fired to form a dense, hermetic structure. The invention further extends to a multilayered filter comprising layers of the dielectric glasses set out above with conductive metals therebetween, the assembly having been fired to form a dense, hermetic structure. Medium dielectric constant materials can function as the dielectric for capacitors or capacitors buried within low dielectric constant multilayered structures. Due to the close match of the CTE of these glass ceramics and GaAs semiconductors, miniaturized three- dimensional circuits can be made with the incorporation of active and passive elements. This can provide many options for system integration and high performance and accelerate the successful adoption of LTCC in a number of different applications. The higher effective dielectric permittivity typical for high e_r materials contributes to the reduction of radiation loss, and as a result, circuits employing such materials can provide both selfshielding and radiation emission protection. These materials also enable the development of non-reflective wide band loads by employing multilayered resistive structures. It will be understood that the additive B₂0₃, when included in the compositions of the present invention, does not result in a separate boron-containing phase such as is present in boron glasses of the prior art. Furthermore, the additive B₂0₃ is included only in a minor quantity, excluding the possibility of a boron glass phase forming. It should also be understood that the compositions given for the dielectric glasses of the invention are bases on the initial weight percentages of the formulations. Some of the components, particularly PbO will have a decreased percentage in the final glass composition by about 10 - 20% (based on that ingredient initial amount) due to volatilization. Definitions The following terms as used herein and in the claims have the following meanings: By the term "major proportion", as used herein and in the claims, is meant a weight percent greater than about 50. By the term "minor proportion", as used herein and in the claims, is meant a weight percent less than about 50. By the term "dielectric glass" is meant all compositions of the present invention in the form of a glass, whether as a frit, flakes, or powder form, the powders of which can be sintered as green bodies to form a dense dielectric glass ceramic material. By the terms "sintered" or "sintering" or "fire" or "firing" is meant heat treatment to convert a green body containing dielectric glass powders, with or without partially crystallized glass powders, of the present invention from its initial amorphous or partially amorphous condition to a predominantly crystalline condition (i.e. generally greater than 90% crystalline), at temperatures above the softening temperature for the glass. By the term "host glass" is meant a glass frit, glass flakes, or glass powder to which additives such as the partially crystallized glass filler can be mixed to produce a base for sintering to glass ceramics. Generally, within this application and in the claims, the term host glass is used to describe an uncrystallized glass to which partially crystallized glass is added prior to converting the mixture to a glass ceramic. This is to distinguish it from the dielectric glasses having the novel compositions of the present invention, which can be converted to glass ceramics without the addition of partially crystallized glasses. By the term "partially crystallized glass" is meant that the glass frit or glass flakes have undergone a heat treatment step sufficient to partially but not fully crystallize the material. Partially crystallized glass contains crystalline phase on the surface of the glass frit or flakes, and amorphous phase within the glass frit or flake particles, with the crystalline/amorphous phase ratio varying depending on the crystallization temperature and the holding time at that temperature. By the term "crystallization temperature" is meant the highest temperature at which glass frit or glass flakes undergo heat treatment in order to produce partially crystallized glass. By the term "sintering temperature" is meant the highest temperature at which green bodies made from dielectric glasses or host glass/partially crystallized glass mixtures of this invention undergo heat treatment to develop dense glass ceramic bodies, containing predominantly crystalline structure (i.e. generally greater than 90% crystalline). By the terms "bulk or volume type of crystallization" is meant that the particles of glass frit or flakes when heated at temperatures above their softening point crystallize simultaneously throughout the bulk of the particles and the crystalline/amorphous phase composition and ratio are the same on the surface and throughout the bulk of the particles. By the term "surface type of crystallization" it is meant that when the dielectric glasses of the invention undergo heat treatment at temperatures in the range of 850 to 1100°C to form partially crystallized glass, crystallization is predominantly of a surface nature, that is crystallization starts from favourably disposed atoms or groups of atoms that lie on the surface the glass frit or flakes and propagates inwards into the bulk of the particles. The particles contain crystalline layers on the surface and amorphous internal layers. The volume ratio of crystalline and amorphous phases varies depending on the crystallization temperature and holding time. By the term "titanium-based perovskite glass ceramics" is meant those glass ceramics which, when sintered, have predominantly a crystalline phase based on perovskite structures AB0₃, including for example, SrTi0₃, PbTi0₃, CdTi0₃, and LaTi0₃₀₆, and the like, and having different substitutions for the A and B sites which allow for developing different solid solutions still of the perovskite structure. Description of the Drawings Figures 1 , 2 and 3 are Differential Thermal Analysis (DTA) curves generated for 40g samples of powders (2-4μm) of the glass compositions 1, 3 and 7 respectively from Example 1, using ramp rate of 4°C/min. The curves show the first exothermic peak (T), for each composition, from which the temperature of crystallization for partially crystallized glass can be determined. Description of the Preferred Embodiments Dielectric glasses and sintered glass ceramics in accordance with the present invention are formed with the preferred compositions as set forth in the Tables in the Examples, to provide the therein listed dielectric constant ranges and the properties. In general, to achieve dielectric glass ceramic compositions with dielectric constants in the ranges set out below, compositions should be prepared with the ratios of ingredients as follows: I) Dielectric Compositions with e_r 15 to 19: Si0₂ 22-29; A1,0₃ 7-10; SrO 20-30; La₂0₃ 10-18; TiO_: 10-19; PbO 0-6; MgO 0-7. LI) Dielectric Materials with e_r 20 to 30: a)Si0₂ 18-27; Al,O₃ 7-10; SrO 20-30; La₂O₃ 0-15; TiO, 8-28; PbO 0-20; MgO 0-7; B₂0₃ 0 - 10, or b) Si0₂ 18-27; Al₂O₃ 7-10; SrO 20-30; La₂0₃ 3-15; Ti0₂ 8-20; PbO 0-20; MgO 2-7. IH.) Dielectric Materials with e_r 31 to 50: a) Si0₂ 15-23; Al₂O₃ 5-10; SrO 24-31; La,O₃ 0-7; TiO, 17-28; PbO 3-13; MgO 0-5; Nb₂0₅ 0-15; CdO 0-6; B,O₃ 0 - 10, or b) Si0₂ 15-23; Al₂O₃ 5-10; SrO 24-31; La₂0₃ 0-7; TiO, 17-21: PbO 3-10; MgO 0-5; Nb₂O₅ 0-15; CdO 0-6. IV) Dielectric materials with e_r 51 to 100: a) Si0₂ 14-22; A1,0₃ 5-10; SrO 25-35; La₂O₃ 0-10; TiO, 15-28; PbO 7-20; MgO 0-5; Nb₂O₅ 0-20; CdO 0-6; B₂0₃ 0-10, or b) SiO, 14-22; A1₂0₃ 5-10; SrO 25-35; La,O₃ 0-10; TiO, 15-25; PbO 7-20; MgO 0-5; Nb,O₅ 0-20; CdO 0-6; B,O₃ 0-5 The dielectric glasses of the present invention may contain small amounts of other common glass constituents which do not adversely affect the desired glass ceramic properties, but preferably the glasses contain no more than 2% by weight of other additives or impurities. The following impurities are preferably included in only very small amounts so as not to significantly alter the properties of the composition: • alkali metal oxide < 0.5% • barium oxide < 1 % • calcium oxide < 0.5% • cerium oxide < 1 % • zirconia < 1 % • bismuth oxide < 0.3% In respect of alkali metal oxides, the raw materials are preferably tightly controlled for alkali metal oxide impurities such as Na₂O, K,O and Li,O. The raw materials should not provide in excess of 0.2 wt% for dielectric materials with e_r 16-25 and 0.3 wt% for dielectric materials with e_r >25. If the amount of alkali impurities exceeds 0.3 wt%, this can affect tanδ values which will be significantly higher than that pointed out herein. Similarly the addition of 2.5 % Bi,O₃ to the composition has been found to result in a 10-times increase in tanδ. All given compositions have specific ratios of components, which provides the desired properties. If the amount of SiO, or Al,O₃ exceeds these limits, the dielectric constants of the materials are generally lower than 15. If the content of SiO, and Al,O₃ is less than 15%, sintering does not occur, and sintered crystalline bodies are porous. When TiO,, MgO, Nb,0₅, La,O₃, CdO and PbO exceed their limits, porosity also may occur. When the amounts of TiO,, and the preferred components, CdO, PbO and Nb,0₅ are less than their limits, dielectric constants may be reduced below the desired range. Preparation of Glass Frit The percentages of each component in the dielectric glass may be varied within the ranges set out below depending on the final desired properties of the fired ceramic material. It will be understood that the raw materials are not necessarily provided in their oxide forms listed above, but may be provided in more readily available or less expensive precursor forms of the metal oxides, such as strontium and magnesium carbonates. Such components as SiO,, TiO,, La₂0₃, CdO, Nb,O_s, and A1₂0₃ preferably should be introduced in their oxide forms. Lead oxide can be employed in an oxygen enriched form such as Pb₃0₄. SrO is preferably provided as strontium carbonate. Since Sr-compounds quite often contain barium oxide (BaO) as an impurity, the content of BaO in SrCO₃ should not exceed 1 percent, i.e. should be less than 0.2 - 0.4 wt % in the glass. The glass melting procedure to form glass frit is known in the art. Generally, the glass ingredients are melted in a platinum with 10% rhodium crucible at 1450 - 1550°C for about 2 hours until it is fully melted and homogenized. The glass is poured into a container with de- ionized water to obtain frit glass particles or through a set of rollers and rolled into flakes. The frit particles or flakes are then milled by known techniques to reduce their particle size to a powder range, generally in the range of 1 - lOμm and most preferably 2 - 4 μm. Preferably the dielectric glass powders are tape cast in order to form multilayered components, as set out below. Alternatively, the glass ceramic powders may be molded, cast, sprayed or roll coated to form substrates or other multilayered circuit components, all as is known in the art. In each case, the composition in the form of glass frit is milled or ground to reduce the particle size, is generally mixed with known organic systems to form a paste or slurry suitable for tapecasting, molding, spraying, etc., to form a green body. The green body is thereafter sintered at a temperature of about 950°C or less. Preferred glass compositions can be sintered at temperatures of about 850°C, as set out in the examples. In accordance with known techniques to form substrates or multilayer dielectric components, the dielectric glass powders are mixed with an organic system, which will typically include known organic binders, plasticizers, solvents, surfactants and deflocculating agents. Any inert materials can be used as the organic system in the practice of the invention. Water or any one of a variety of organic liquids, with or without thickening or stabilizing agents or other additives as are known in the art, such as surfactants and plasticizer, can be used. Typical binders include acrylic binders and polyvinyl compounds. Typical of organic solvents are the aliphatic alcohols, their esters, for example the acetates and propionates; terpenes such as pine oil, terpineol and the like, solutions of resins such as the polymethacrylates of lower alcohols, or solutions of ethyl cellulose, in solvents such as pine oil, the monobutyl ether of ethylene glycol monoacetate, and Carbitol. Plasticizers such as dimethylphthalate, dibutyl phthalate, and various other phthalates can be employed. Exemplary surfactants are sold under the trade marks Triton. Solsperse and Duomeen. Additionally, viscosity modifiers, antiskining agents and the like can by used, as is known in the art. Tape Casting A slurry of the dielectric glass powders can be prepared by using known organic systems which include known binders, plasticizers, solvents and deflocculating agents. Preferred binders include acrylic and poly vinyl butyral binders. Preferred plasticizers include phthalate plasticizers. Preferred solvents include toluene or ketone solvents. Preferred deflocculating agents include known fish and pine oils. The glass powder is added to the organic system so that the volume ratio of solids to liquid is most preferably about 60/40. The mixture is milled together in a plastic container with zirconia balls until a homogeneous slurry is obtained (approximately 2 to 24 hours). The resultant mixture is then degassed and cast into a thin tape to the desired thickness using a doctor blade, all by techniques known in the art. The tape is air dried, then cut and via holes may be punched through the tape at desired locations. One or more metal conductor paths may be formed on the punched tape. Suitable metals for the conductors include gold, silver-palladium and platinum-gold. The green body, so formed is subsequently sintered at low temperatures (not greater than about 950 °C), typically after two or more sections have been laminated and pressed together to form a multilayered circuit substrate or discrete component. Alternatively, the tape can be used without conductor lines to form solid substrates. The crystalline structure of the sintered bodies has been found to be very fine, with average grain size close to lμm. This feature provides excellent surface smoothness (Ra 0.05 to 0.15μm) in an as-fired condition. With additional polishing, surface smoothness can be further improved to Ra 0.0 lμm. This feature facilitates fabrication of conductor lines with exact dimensions, reduced conductor losses and errors due to line impedance. Use of Fillers/Precrystallized Glass In order to achieve high dimensional stability, the dielectric glass powders described above, but now referred to as host glass powders, may be mixed with a partially crystallized glass powder, as set out hereinbelow, or with other compounds having thermal expansion coefficients lower than 7 ppm/°C (hereinafter "low thermal expansion coefficient fillers") such as one or more of lead, strontium, magnesium, or aluminum titanates, titanium dioxide, niobium pentaoxide, cordierite and the like. These fillers may be mixed in varying proportions, depending on the desired end properties of the dielectric glass ceramic product being produced. SrTiO₃ can be used as a filler, but only in an amount not exceeding 1 - 2% weight. Low thermal expansion coefficient filler should have a particle size in the range of 1 - 5 μm, and, with the exception of SrTiO₃, may be included in an amount in the range of about 1 and 20 % by weight (or preferably 1 and 10% by weight). To preserve dimensional stability of the fired dielectric compositions, it is preferable to add to the host glass powder, a powder of the same host glass composition in a partially crystallized form, in an amount of about 2 to 50% by weight. To prepare the partially crystallized glass end, glass frit prepared as above, is partially crystallized prior to addition to the host glass. The partially crystallized glass will generally have substantially the same composition as the uncrystallized glass, that is it will generally include the same major components and most preferably the same minor additives as the host glass composition to prevent reaction in the crystalline phase during firing. However, the components might be included in slightly different proportions. The glass frit of the size 1 to 2 mm is partially crystallized near or above its first exothermic peak temperature (DTA curves) but below a temperature which results in full crystallization. The peak temperature used for crystallization is between T, and T,, wherein T, is a temperature which is not more than 40°C, and preferably not more than 30°C, below a first exothermic peak on a differential thermal analysis curve for the glass, and T, is a temperature at least 20°C, and preferably 50°C, below a temperature which causes complete crystallization of the glass. DTA curves are generated by standard known techniques using a small sample of a powder of the particular glass composition. The crystallization temperature is held for a time sufficient to produce a partially crystallized glass having crystals and residual amorphous phase in a volume ratio between 5:95 and 95:5. Preferred peak temperatures for the compositions set out herεinabove and in the examples are generally from 850 - 1100°C held for 0.5 - 5 hours. The partially crystallized frit is then milled under conditions similar to the host glass to obtain an average particle size of 1 - 10 μm, more preferably 2 - 4 μm. The amount of partially crystallized glass added to the host glass will vary widely with the particular compositions used, the degree of crystallization, and the desired end properties for the sintered product. In general, in adding partially crystallized glass to the host glass, the lower the crystallization temperature used, the higher the amount of partially crystallized glass is needed in order to maintain dimensional stability. The term dimensional stability includes an absence of sagging, warpage, distortion, as well as low shrinkage. The dimensional stability of the glass ceramic compositions of the present invention can be conveniently evaluated by measuring sample shrinkage after sintering, by techniques known in the art. However, the dimensional stability can also be evaluated qualitatively by visual inspection of the sintered compositions. Samples having poor dimensional stability are found to have a visible raised dimple on the surface after sintering. This dimple, as an indicator of poor dimensional stability, can be avoided even without the addition of fillers or partially crystallized glass, simply by reducing the temperature ramp rate during sintering of about 1 - 2°C/min. Alternatively, partially crystallized glass or filler addition eliminates the dimple formation, providing sintered samples with very high dimensional shape and dimensional stability at temperature ramp rates up to about 15°C/min. When preparing the partially crystallized glass, it should be partially crystallized at a temperature above the ultimate sintering temperature of the composition, usually in the range of 850 - 1100°C. At crystallization temperatures of 850°C, the addition of partially crystallized glass has been found to eliminate dimple formation with the amount of the partially crystallized glass being at the higher end of the addition range. However, when crystallization temperatures greater than 900 °C were used, the partially crystallized glass was found to include a higher degree of crystallization and the amount of partially crystallized glass needed to eliminate any dimple was much less. Also improved was the shrinkage of the samples containing partially crystallized glass. Shrinkage upon sintering was reduced by 5-6% by the addition of partially crystallized glass over values obtained without the partially crystallized glass. These lower shrinkage values are very important for providing structural integrity of the microcircuit conductor lines. The tape can also be used without conductor lines to form solid substrates. The crystalline structures of the sintered bodies are very fine with average particle size close to lμm. This feature gives them excellent surface smoothness (Ra 0.05 to 0.15 μm)in as fired condition. With additional polishing surface smoothness can be further reduced to less than 0.01 μm. This feature facilitates fabrication of conductor lines with exact dimensions. reduced conductor losses and errors due to line impedance. Measurements The measurement of dielectric constants and other properties set out above can be conducted by known techniques, but in the examples, measurements were conducted as follows: Dielectric properties of the materials were measured at room temperature at the three frequencies ( 1MHz, 3.25 GHz and 10 GHz) set out in Table 2. An HP4284A precision LCR meter with a Dielectric Test Fixture HP 1645 IB (Hewlett Packard) was used for measurements at 1 MHz of samples with dimensions 25 x 25 x 1 mm with silver-platinum metallization. The measurements at 3.25, 9 and 10 GHz were performed on samples 1 x 1 x 25 mm and 1 x 1 x 50 mm by a resonance technique developed by W. Tinga et al., University of Alberta. Edmonton, Canada. This method was specifically developed for materials with medium e_r and is an improved approach to the theoretical formulae of the perturbation method, employing very small size samples and high cavity to sample volume ratio allowing for obtaining reliable data for e_r up to 100. A scalar network analyzer was used to record perturbation effects on a rectangular wave guide resonator. Uncertainty in the data was reduced by applying corrections to the resonator perturbation equations while measurement resolution was improved by averaging techniques. Uncertainty of the dielectric constant was estimated to be less than 2% while that of the measured loss tangent data was less than 10%. Loss tangent resolution was improved by nearly 2 orders of magnitude to 3 x 10^"5 by applying both short and long time interval averaging to the frequency data. To reduce error in volume calculations, the linear dimensions of samples were measured by using a Mitutoyo PJ311 Profile Projector. Thermal expansion coefficients are measured in the temperature range of 20 to 300 °C using a Theta Dilatronic Dilatometer in differential mode against an ASTM Standard sapphire sample. Porosity was controlled qualitatively by ink penetration or quantitatively by xylene impregnation and SEM analysis. The materials of the present invention have porosity usually less than 1% according to SEM evaluations. Dimensional stability is evaluated by measurements of shrinkage in two perpendicular directions in 3-4 points on the sample edges. The shape stability is controlled visually by observing the sample surface for dimple formation, as discussed above, and for warpage. No warpage of samples formed in accordance with the present invention was observed. Examples The present invention is further illustrated in the following non-limiting examples. Example 1 Batches (300 g) containing all the ingredients shown in Table 1 (below) were dry mixed in a Turbula-type mixer for 1 hour to provide a homogeneous mixture. The batch was then melted in a platinum-rhodium crucible at 1450 - 1550°C for 2 hours until fully melted and homogenized. The glass melt was then poured into a container of deionized water to form frit. The frit (250 g) was wet milled with 100 - 120 ml of isopropanol and 800-1100 g of zirconia balls for 24-64 hours. The obtained powder had reduced particle size average of 2 - 4 μM. The glass powder (200g) was slurried with an organic system of: 6 g of poly vinyl butyral binder, 1 g fish oil, 25 g methyl ethyl ketone solvent and 16 g hexane solvent, and 4 g butyl benzyl phthalate plasticizer. The slurry was milled together with zirconia balls in a ball mill until a homogeneous slurry was obtained (2 to 24 hours). The resulting mixture was then degassed and cast into a tape on Mylar strip using a doctor blade technique. The tape thickness was 100-200 μm. The tape was dried and cut into laminates with dimensions 25 x 25, 50 x 50 or 75 x 75 mm. The number of layers varied from 8 to 50. The laminates were pressed in a laminating die at room temperature at 1000 kg/cm² for 5 minutes or at 70 °C for 5 - 10 minutes at 216 kg/cm². The organic burnout and sintering were performed either in two separate operations or in one step. The burnout step was performed at ramp rate 1-2° C/min up to 480 °C with holding time at this temperature of 1 - 2 hours, depending on the sample thickness. The sintering was performed at a ramp rate of 1 - 8° C/min (for some materials 8 - 10°C/min was possible) up to peak temperatures with holding time from 1 to 2 hours depending on the sample thickness. The properties of the materials are given in Tables 2, 3 and 4 below. Table 1: Compositions

Table 3: Shrinkage of Samples of Composition #3

Table 4: Effect of Temperature Ramp Rate on Dimple Characterization

The appearance of dimples in the sintered product was used as an indication of dimensional instability of the compositions. Dimples were characterized by their depth in accordance with the following technique. A Dektak 3030 ST Surface Texture Analysis System was used to measure the dimple depth for a number of samples with the most typical shapes. The rest of the samples were evaluated visually by comparison with the tested samples. Dimples were evaluated by illuminating the sample surface with visible light at an angle of about 30 degrees from normal incidence and examining variations in the reflected light from the sample at a similar angle. The tested samples were placed in one of the following four categories: Category 0 defined the samples which did not have any change in the surface roughness more than 0.15μm; Category 1 defined samples with dimples having a depth less than 5μm and which were only slightly visually noticeable; Category 2 defined samples with dimples having a depth less than lOμm and which clearly visually noticeable; and Category 3 defined samples with dimples having a depth of 10 to 30μm, visible with light at normal incidence. Discussion of Properties As can be seen from the Tables 1 and 2, the dielectric constants of the sintered glass ceramic materials of the present invention ranged from about 15 to about 100 at 1 MHz. The dissipation factor had extremely low values at lower and radio frequencies for most of the materials with values far below the resolution ability of the testing machine. In the gigahertz band tanδ increased, but still maintained low loss level (1 x 10³ to 5 x 10³). There is an obvious trend of the tanδ increasing with increasing e_r that is a typical feature of perovskite containing materials. The CTEs measured were in the range of 6.8 to 7.8 ppm/K. These values are close enough to the CTE of both gallium arsenide and ceramics such as alumina and BeO to suggest that the materials of this invention can be used as 'transfer' tape-on-substrates. The materials had a fine crystalline structure, giving them excellent surface smoothness in the as-fired condition. The shrinkage repeatability of the same composition # 3 is given in Table 3. The shrinkage of samples having the same composition and the same firing profile varied in the range of ± 0.05%. The shape description given in Table 4 shows that there were some small dimples observed for samples having been sintered at ramp rate of from 3 - 8 °C/min. The samples with e_r higher than 30 were less prone to developing dimples and retained the shape at any ramp rate. Example 2 This example is included to demonstrate the invention utilizing partially crystallized glass powders to improve the dimensional stability of the sintered glass ceramic products. Glass frit was prepared as set forth in Example 1 , using the compositions identified as 1 , 3 and 7. The glass frit particles were partially crystallized at temperatures in the range of 850 to 1150°C, using a ramp rate of 5°C/min up to peak temperature, and holding at the peak temperature for a period of 2 - 4 hours. The temperature of crystallization, the ramp rate and the holding times were varied depending on the desired degree of crystallization, as can be seen in the Tables 5 - 7 below. The partially crystallized glass frit particles of 1 - 2 mm were ground (220 g) in a ball mill with lOOOg of zirconia balls for 24 to 48 hours with the addition of 100-120 ml of isopropanol, to give an average particle size of 2 - 4 μm. The glass powders (host glass powder and partially crystallized powder) total 200g were weighed according to the formulations of Table 5 - 7 and were then mixed with an organic system including - 2 g of fish oil, 50 ml of methyl ethyl ketone, 32 ml hexane, 8 g of butyl benzyl phthalate and 12 g of polyvinyl butyral for 4 - 24 hours. The slurry was tape cast (thickness 100-200μm), laminated and sintered as set out in Example 1. The sintering temperatures used were 850 and 950°C. The results are set out in Tables 5 - 7 below.

Table 5: Properties of Glass-Ceramics with Addition of Different amount of

Partially crystallized Glass (Base Composition #1)

Table 6: Properties of Glass-Ceramics with Addition of Different amount of

Partiall cr stallized Glass (Base Com osition #3)

Discussion of Results

The three host glass compositions from Table 1 were selected to show the impact of partially crystallized glass on the material properties. The dielectric constants of the host glasses without partially crystallized glass were as follows: a. Glass composition #1, e_r 17 at 10 GHz b. Glass composition #3, e_r 27 at 10 GHz c. Glass composition #7, e_r 58 at 10 GHz

The impact of the partially crystallized glass on the properties of the fired glass ceramic is demonstrated in Tables 5 - 7 and discussed below with respect to the following variables: i. Crystallization temperature of partially crystallized glass (from 850 - 1150°C) ii. Amount of partially crystallized glass powder added to host glass powder iii. Peak temperature of firing profile of the glass ceramic made from the mixture of host glass and partially crystallized glass (850 and 950°C). A. Glass Composition #3 Looking first at Table 6, for composition #3, from the DTA curve of Figure 2, one can see that this glass has an exothermic peak at 866.25 °C. In order to see the effect of the ratio of crystalline to amorphous phase, the glass frit particles were crystallized at four temperatures, with the following result: a. 850°C - This temperature was slightly lower than the temperature of the exothermic peak. As a result, only a thin crystalline layer was formed on the surface of the frit particles. The crystalline and amorphous phase ratio, visually evaluated by microscopic analysis, was about 10:90. The crystalline layer consisted mainly of titanium-containing phases such as perovskite and some unidentified phases. b. 900°C - This temperature was slightly higher than the temperature of the exothermic peak. At this temperature, the crystalline layer formed on the particle surface during heat treatment occupied about a half of the particle volume. The smaller particles in the powder crystallized more intensively, however, their amount was relatively small (not more than 5- 10%). The approximate ratio of crystalline to amorphous phases was about 50:50. The crystalline phases were mainly represented by perovskite phase (major amount) and silicate phase (minor amount). c. 1050°C - This temperature was significantly higher than the first exothermic peak. The crystalline to amorphous phase ratio was about 95:5. The crystalline phase contained perovskite and two silicate phases. The two silicate phases were of the same composition but of more stable high temperature forms. d. 1150°C - This was the highest crystallization temperature when all particle volume was occupied by crystals. The crystalline phases were similar to that of the partially crystallized glass at 1050°C, with a slightly higher amount of the high temperature stable silicate phases. The properties of the sintered dielectric glass ceramic materials obtained from the above glass mixtures, with and without the partially crystallized glass, are given in Table 6. The glass crystallized at 850 °C with the lower crystalline/amorphous phase ratio could be introduced in the host glass in an amount more than 50% by weight. About 30% by weight of partially crystallized glass at 850°C was needed to eliminate the dimples. When the crystallization temperature was 900°C, 10% by weight of the partially crystallized glass was sufficient to avoid dimples. With the increased amount of crystals at 1050°C. only 5% by weight of partially crystallized glass was needed to avoid the dimples. However, the addition of even 3% by weight of the glass crystallized at 1150°C caused the sintered glass ceramic to develop with no dimple, but with open porosity. The porosity resulted because the crystallization process intensified to such a degree that it overlapped with the sintering process. By decreasing the amount of the glass crystallized at 1150°C to 1 - 1.5% by weight, it was possible to achieve a fully dense structure on sintering. However, it was very difficult to achieve a homogeneously distributed mixture of the fully partially crystallized and host glass (host glass constituted 98.5 - 99%). Use of partially crystallized glass which is crystallized to such a complete extent would require the additional steps of using a "Master Blend", which is costly and not as effective. The glass partially crystallized at 850 °C created mainly perovskite phases that could be seen by the increased values of e_r, tanδ, and CTE in comparison with that of glass ceramics which included no partially crystallized glass. However, sintering of the glass was possible at a lower temperature. As seen in Table 6, the glass ceramic sintered at 850 °C provided about the same property values as were obtained for the glass ceramic fired at 950°C. This example shows that sintering temperatures can be significantly reduced by the addition of partially crystallized glass. Lower sintering temperatures (850 - 870°C) for these glass ceramics allows for the utilization of not only gold conductors, but also of low temperature conductors such as silver and its alloys. The glass partially crystallized at higher temperatures (900 to 1050°C) stimulated the formation of both perovskite and silicate phases, and as a result, affected e_r , tanδ, and CTE values less than did the glass partially crystallized at 850°C. It should also be noted that the addition of partially crystallized glass provides a reduction in shrinkage of the sintered glass ceramic, by 5% or greater. Shrinkage gradually decreased with an increase in the crystallization temperature. B. Glass Composition #1 This host glass composition was chosen in order to demonstrate the use of crystallization temperatures which were much lower than the temperature of the exothermic peak on its DTA curve, which as can be seen from Figure 1, is 884.41 °C. When this glass was partially crystallized at 850 °C, it developed only a very thin crystalline layer. By visual evaluation, the ratio of crystalline to amorphous phase was less than 5:95. As a result, the properties of the glass ceramics sintered from these mixtures remained almost unchanged and dimples were not eliminated. The addition of glass partially crystallized at temperatures higher than 884°C showed the behavior similar to that described above for the composition #3. One difference between the compositions was that composition #1 had a higher amount of silica. Thus significant decreasing of the sintering temperature was only possible at the expense of such properties as tanδ and CTE because the silicate phases, which provided the glass ceramic with low values of tanδ and CTE, developed only at the higher temperatures (900-950 °C). C. Composition #7 This host glass composition was selected to show the effect of partially crystallized glass when the temperature of the exothermic peak was quite low, for composition #7, as can be seen from Figure 3, it was 836.16°C. The addition of the glass partially crystallized at 850°C, which was slightly higher than the exothermic peak temperature, had a major impact on the properties of the sintered glass ceramic, even when the amount of partially crystallized glass added was only 3 - 6% by weight. In this case, perovskite and silicate phases crystallized simultaneously and the values of e_r , tanδ and CTE were not very different from the same parameter for the glass ceramic which did not contain the partially crystallized glass. Here again, the behavior of the partially crystallized glass was practically the same as for the glass composition #1 and #3 above. Considering the effects of partially crystallized glass shown by this example, it can be seen that partially crystallized glass was capable of improving dimensional stability (eliminated dimple), decreasing shrinkage (see Table 8), lowering sintering temperature, and regulating the crystalline structure formation. By variation of the crystallization temperatures it is possible to tailor the properties of the sintered glass ceramic resulting from glass powder mixtures containing partially crystallized glass to the required needs or applications of the glass ceramic. In general, the amount of partially crystallized glass to be added to the host glass may vary and is preferably between 2 to 50 % and more preferably 5 to 30%, depending on the initial composition. In general, the higher the temperature for crystallization that was used, and the smaller the particle size used (preferably 2 - 4 μm), the smaller was the amount of partially crystallized glass required to achieve full density and high stability of the sample dimensions.

Table 8: Shrinkage of Samples of Composition # 3 With/Without Partially

Cr stallized Glass

The next example is included to illustrate glass ceramics produced from dielectric glasses of the invention, with and without the inclusion of low thermal expansion coefficient ceramic fillers. Example 3 - Dielectric Glasses and Glass Ceramics With/Without Low CTE Ceramic Fillers

Glasses were prepared with the compositions set out in Table 8A (amounts being in percent weight), and sintered to glass ceramics having the properties set out in Table 8B, the procedures being generally as set out in Example 1. The low thermal expansion coefficient fillers were added to the dielectric glass powder in a percent weight based on the dielectric glass powder. When cordierite was used as the ceramic filler, a low cordierite material was used, this being a form of cordierite having an orthorhombic crystal structure and a low thermal expansion coefficient. Measurement of the properties of the sintered materials was performed on a laminate of 10 layers with a sintering profile of 1 ° C/min to 450 °C with holding times of 30 min. at each of 130°C, 230°C and 350°C, and of 120 min. at 450°C, followed by 10°C/min to 850°C, holding for 24 min. Table 8A: Compositions

The example demonstrates the ability to engineer the thermal expansion coefficient of the materials with the addition of a low thermal expansion coefficient ceramic filler such as cordierite. Furthermore, the adverse effects on the dielectric properties of the materials can be minimized by also adding aluminum titanate. The materials retained acceptable values of dielectric constant and dissipation factor, while the thermal expansion coefficient was adjusted through the addition of the low CTE fillers. The above materials were tested for compatability with commercially available LTCCs with low dielectric constants (DuPont 951 Green Tape™). Laminated green tapes of the materials (one layer of Composition A, B or C sandwiched between four layers of the DuPont 951 ) gave no cracks after sintering at 850 °C for 15-30 minutes with a fast firing profile. Better properties (lower tan δ down to about 3 X 10^"3) could be obtained with a holding time at the final temperature. Similar glass compositions without the ceramic fillers were produced. Composition A was formulated as set out above without the inclusion of the ceramic fillers (formulation Al) and, on sintering in the manner set out above, produced a glass ceramic having ε_r of 30.4 and tan δ of 36.2 X 10^, at 9 GHz and room temperature. Composition A was further adjusted by increasing TiO₂ to 25 wt. % and B,O₃ to 4 wt. %, and omitting the La₂O₃ and the ceramic filler. This dielectric glass formulation (A2) produced a glass ceramic, after sintering at 870 °C, having ε_r of 31.6 and tan δ of 48.1 X 10^"4. In comparing these dielectric glass ceramic materials (Al and A2) with those of Example 1, it will be noted that Al and A2, with the higher amounts of TiO₂ and B₂O₃ and the lower amounts of La₂O₃, could be fired at temperatures of 850 or 870°C.

The next four examples are included to illustrate glass ceramics produced from the dielectric glasses of the invention, without the inclusion of partially crystallized glasses or refractory fillers, using the procedures set out in Example 1. Example 4 - Dielectric Compositions with e_r 15 to 19

Glass compositions were prepared as set out in Example 1 with the compositions set out in Table 9, and were sintered to glass ceramics with the properties set out in Table 10. The compositions all fell in the following ranges: SiO, 22-29; A1,O₃ 7-10; SrO 20-30; La,O 10-18; TiO, 10-19; PbO 0-6; MgO 0-7. Experiments showed that there was no significant dispersion of dielectric constant within the range of frequencies 3.25 GHz to 10 GHz. Therefore, the data for e_r is given only for the frequency 3.25 GHz. The LTCC procedure outlined above did not produce properties different from that in the Tables. Table 9: Compositions of Materials with ε_r 15 to 19

Table 10: Main Properties of Materials With ε_r 15-19

Example 5 - Dielectric Materials with e_r 20 to 30

Glass compositions of materials were formulated as set out in Table 11 within the ratios as follows: SiO, 18-27; A1₂0, 7-10; SrO 20-30; La,O₃ 0-15; TiO, 8-28; PbO 0-20; MgO 0-7; B₂O₃ 0-10. The properties of glass ceramics produced from these compositions are set out in Table 12.

Table 11: Compositions of Dielectrics with ε_r 20 to 30

Table 12: Properties of Dielectrics with ε_r20 to 30

Example 6 - Dielectric Materials with e_r 31 to 50

Glass compositions were prepared are as set out in Table 13, all in the ratios as follows: SiO, 15-23; A1₂O₃ 5-10; SrO 20-30; La₂O₃ 0-7; TiO2 17-28; PbO 3-13; MgO 0- 5; Nb,O<j 0-15; CdO 0-6; B₂O₃ 0-10. The properties of the glass ceramics generated from these glasses are materials set out in Table 14. Table 13: Compositions of Materials with ε_r 31 to 50

Example 7 - Dielectric materials with e_r 51 to 75

Glass compositions were prepared in accordance with the amounts set out in Table 15, all in the ratios: SiO₂ 14-22; A1₂O₃ 5-10; SrO 25-35; La₂O₃ 0-10; TiO, 15-28; PbO 7-20; MgO 0-5; Nb,O₅ 0-20; CdO 0-6; B,O₃ 0-10. The properties of glass ceramics generated from these glasses are set out in Table 16.

Example 8

This example shows compositions and properties of glass ceramics which can be made with mixtures of one or more host glasses, with and without one or more partially crystallized glasses of the same or substantially the same compositions as the host glasses.

The glass compositions and the weight percents, along with the resulting dielectric constant of the sintered glass ceramic, are set out in Table 17, with composition numbers referring back to Example 1.

Table 17: Glass Compositions, Crystallization and Sintering Parameters and Dielectric Constants of Sintered Materials

Example 9 - Dielectric materials with ε_r 76-100

A dielectric glass composition can be prepared in accordance with this invention to yield a glass ceramic having a dielectric constant in the range of 76-100.

In accordance with the procedures set out in Example 1, a dielectric glass is prepared having the following composition: SiO, 15.5; Al,O₃ 5.5; SrO 30.5; TiO, 24.0; PbO 18.0;

CdO 5.5; B,O₃ 1.0. Glass ceramics formed by the procedures of Example 1 and fired at

935 °C have the following properties:

Dielectric constant, ε_r 90 at 1 MHz, RT

Dissipation factor (tanδ), at 1.3 x 10^" 1 MHz, RT

CTE, ppm/l ^oC, 20-300°C 7.8

Porosity no Advantages of Dielectric Glasses of Invention • Most of the dielectric glasses of the invention are compatible with some commercially available conductor pastes, including for example DuPont 5717D and Electro-Science Laboratories D801CT. • The dielectric glasses of the invention are compatible with each other and certain commercially available low dielectric constant materials. This feature allows the fabrications of gradient e_r structures having several layers with different dielectric constants. Similar thermal expansion coefficients and chemical compositions enable these materials to create different types of structures tailored to specific needs. • The materials can be incorporated in multilayered structures with low dielectric constant materials such as buried capacitors, filters, resonators, thus providing miniaturization of complex multicomponent devices and simplifying the fabrication process. Advantages of Partially Crystallized Glasses of Invention • By using the same or substantially the same composition of the host and partially crystallized glasses, no reaction between the glass matrix and the partially crystallized glass is allowed. • Wettability of partially crystallized glass is much greater than for refractory fillers, so a larger amount may be included in the host glass without interfering with the properties of the sintered glass ceramic. • The partially crystallized glass promotes both sintering and crystallization during the formation of the glass ceramic. • The partially crystallized glass provides a supporting crystalline frame and thereby prevents shape distortion during formation of glass ceramics. • Glass ceramics formed with the partially crystallized glass have a very fine crystalline structure due to a very developed surface area. • In some cases, the presence of the partially crystallized glass can significantly decrease the sintering temperature of the glass ceramics. • The inclusion of partially crystallized glass in host glasses allows one to tailor the sintering and dielectric properties of the glass ceramics formed therefrom to the required needs or applications of the glass ceramics. For instance by varying the temperature and duration of crystallization and changing the ratio of crystalline to amorphous phases in the partially crystallized glass, together with the amount of partially crystallized glass added to the host glass, the properties of the glass ceramic may be varied, as has been shown in the above examples. All publications mentioned in this specification are indicative of the level of skill of those skilled in the art to which this invention pertains. All publications are herein incorporated by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference. Throughout this specification, certain definitions are set out for specific terms and expressions and are intended to be applied to those terms and expressions when use in the appended claims. The terms and expressions in this specification are used as terms of description and not of limitation. There is no intention, in using such terms and expressions, of excluding equivalents of the features illustrated and described, it being recognized that the scope of the invention is defined and limited only by the claims which follow. References 1. A.J. Pilot, D.P. Partlow, and K. Zaki, "Low Loss, Low Temperature Cofired Ceramic for Microwave Multichip Modules:, Proceedings of the 1994 International Symposium on Microelectronics, Boston, Massachusetts, November 15-17, pp. 318- 323, 1994. 2. Kyocera Corp. Catalogue, Electronic Fine Ceramics, pp. 11-12, 1996. 3. Intertec Southwest LLC Catalogue, 4201 S. Santa Rita Ave, Tucson, AZ 85714. 4. Electro-Science Laboratories, Inc., "High Technology Materials", King of Prussia, PA, U.S.A., see Trade Name D-41210. 5. I. Zimmer, R. Raether, and G. Miller, "In-situ Investigations of Sintering and Crystallization of Aluminosilicate Glass Ceramics:, Glasstech. Ber. Glass. Sci. Technol., (10) No. 6, pp 186-188, 1997.

Claims

Claims: 1. A dielectric glass comprising: (a) a major proportion of a glass composition comprising 14 to 30% by weight SiO,, 5 to 1 1% by weight Al₂O₃, 20 to 35% by weight SrO, and 8 to 28% by weight TiO₂; and (b) a minor proportion of one or more additives selected from the group consisting of 0 to 7% by weight MgO, 0 to 20% by weight La,O₃, 0 to 20% by weight PbO, 0 to 20% by weight Nb,O₅, 0 to 8% by weight CdO, and 0 to 10% by weight B,O₃.

2. The dielectric glass as set forth in claim 1, wherein the components (a) are included in a total amount between 70 and 80% by weight and the components (b) are included in a total amount between 20 to 30% by weight, and wherein the amount of TiO, is 8 to 25% by weight and the amount of B₂O₃ is 0 to 5% by weight.

3. The dielectric glass as set forth in claim 1, having the composition in % by weight: SiO, 22-29; A1,O₃ 7-10; SrO 20-30; La,O₃ 10-18; TiO, 10-19; PbO 0-6; MgO 0-7, and which when sintered at between 850 and 950┬░C to form a glass ceramic, has a dielectric constant of 15 to 19 at 1 MHz.

4. The dielectric glass as set forth in claim 1, having the composition in % by weight: SiO, 18-27; A1,O₃ 7-10; SrO 20-30; La₂O₃ 0-15; TiO, 8-28; PbO 0-20; MgO 0-7; B₂O₃ 0- 10, and which when sintered at between 850 and 950┬░C to form a glass ceramic, has a dielectric constant of 20 to 30 at 1 MHz.

5. The dielectric glass as set forth in claim 1, having the composition in % by weight: SiO, 15-23; Al,O₃ 5-10; SrO 24-31; La,O₃ 0-7; TiO, 17-28; PbO 3-13; MgO 0-5; Nb₂O₅ 0-15; CdO 0-6; B,O₃ 0-10, and which when sintered at between 850 and 950┬░C to form a glass ceramic, has a dielectric constant of 31 to 50 at 1 MHz.

6. The dielectric glass as set forth in claim 1, having the composition in % by weight: SiO, 14-22; A1₂O₃ 5-10; SrO 25-35; La,O₃ 0-10; TiO, 15-28; PbO 7-20; MgO 0-5; Nb,O₅ 0-20; CdO 0-6; B₂O₃ 0-10, and which when sintered at between 850 and 950┬░C to form a glass ceramic, has a dielectric constant of 51 to 100 at 1 MHz.

7. The dielectric glass as set forth in claim 1 in the form of a glass powder having an average particle size of 1 - 10 ╬╝m.

8. The dielectric glass as set forth in claim 1 in the form of a glass powder having an average particle size of 2-4 ╬╝m.

9. The dielectric glass as set forth in claim 8, in admixture with one or more low thermal expansion coefficient ceramic fillers added to the dielectric glass in an amount less than about 20% by weight.

10. The dielectric glass as set forth in claim 9, wherein the ceramic filler is selected from the group consisting of lead titanate, strontium titanate, magnesium titanate, aluminum titanate, titanium dioxide, niobium pentaoxide and cordierite, with the proviso that if the ceramic filler is strontium titanate, it is included in an amount less than 2% by weight.

11. The dielectric glass as set forth in claim 1 or 10, which includes cordierite as a ceramic filler in an amount less than about 10% by weight, and less than about 2% by weight of other additives or impurities.

12. The dielectric glass as set forth in claim 1 or 10, which includes less than about 0.5% by weight of either calcium oxide or alkali metal oxides, less than 1% by weight of any one of barium oxide, cerium oxide, and zirconia, and less than 0.3% by weight of bismuth oxide.

13. A method of producing a dielectric glass, comprising: (a) producing a glass frit or glass flakes having the composition comprising: (i) a major proportion of a glass composition comprising 14 to 30% by weight SiO,, 5 to 11% by weight Al,O₃, 20 to 35% by weight SrO, and 8 to 28% by weight TiO,; and (ii) a minor proportion of one or more additives selected from the group consisting of 0 to 7% by weight MgO, 0 to 20% by weight La,O₃, 0 to 20% by weight PbO, 0 to 20% by weight Nb,O₅. 0 to 8% by weight CdO. and 0 to 10% by weight B,O₃; and (b) grinding the glass frit or flakes to produce a finely divided glass powder.

14. The method as set forth in claim 13, which includes forming a dense dielectric glass ceramic from the dielectric glass by the additional steps of: (c) mixing the glass powder from step (b) with an organic system; (d) forming a green body from the mixture of step (c); and (e) sintering the green body at a temperature not greater than about 950┬░C to produce a dense dielectric glass ceramic material.

15. The method as set forth in claim 14 wherein the components (i) are included in a total amount between 70 and 80% by weight and the components (ii) are included in a total amount between 20 to 30% by weight, and wherein the amount of TiO, is 8 to 25% by weight and the amount of B,O₃ is 0 to 5% by weight.

16. The method as set forth in claim 14, wherein the glass frit or glass flakes have the composition in % by weight: SiO₂ 22-29; A1₂O₃ 7-10; SrO 20-30; La,O₃ 10-18; TiO, 10- 19; PbO 0-6: MgO 0-7, and wherein sintering at between 850 and 950 ┬░C forms a glass ceramic material having a dielectric constant of 15 to 19 at 1 MHz.

17. The method as set forth in claim 14, wherein the glass frit or glass flakes have the composition in % by weight: SiO, 18-27; A1₂O₃ 7-10; SrO 20-30; La,O₃ 0-15; TiO, 8-28; PbO 0-20; MgO 0-7; B,O₃ 0-10, and wherein sintering at between 850 and 950┬░C forms a glass ceramic material having a dielectric constant of 20 to 30 at 1 MHz.

18. The method as set forth in claim 14, wherein the glass frit or glass flakes have the composition in % by weight: SiO, 15-23; A1,O₃ 5-10; SrO 24-31; La,O₃ 0-7; TiO, 17-28; PbO 3-13; MgO 0-5; Nb,O₅ 0-15; CdO 0-6; B,O₃ 0-10. and wherein sintering at between 850 and 950┬░C forms a glass ceramic having a dielectric constant of 31 to 50 at 1 MHz.

19. The method as set forth in claim 14, wherein the glass frit or glass flakes have the composition in % by weight: SiO, 14-22; Al,O₃ 5-10; SrO 25-35; La,O₃ 0-10; TiO, 15-28; PbO 7-20; MgO 0-5; Nb,O, 0-20; CdO 0-6; B,O₃ 0-10, and wherein sintering at between 850 and 950 ┬░C forms a glass ceramic material having a dielectric constant of 51 to 100 at 1 MHz.

20. The method as set forth in claim 14, wherein the glass powder has an average particle size of 1 - 10 ╬╝m, preferably 2-4 ╬╝m.

21. The method as set forth in claim 20, further comprising, after step (b). adding one or more low thermal expansion coefficient ceramic fillers to the glass powder in an amount less than about 20% by weight.

22. The method as set forth in claim 21 ,wherein the ceramic filler is selected from the group consisting of lead titanate, strontium titanate, magnesium titanate, aluminum titanate, titanium dioxide, niobium pentaoxide and cordierite. with the proviso that if the ceramic filler is strontium titanate, it is included in an amount less than 2% by weight.

23. The method as set forth in claim 14 or 21, which includes, after step (b), adding cordierite as a ceramic filler in an amount less than about 10% by weight, and wherein the glass frit or glass flakes include less than about 2% by weight of other additives or impurities.

24. The method as set forth in claim 14 or 22, wherein the glass frit or glass flakes include less than about 0.5% by weight of either calcium oxide or alkali metal oxides, less than 1 % by weight of any one of barium oxide, cerium oxide, and zirconia. and less than 0.3% by weight of bismuth oxide.

25. A method of producing partially crystallized glass, comprising: (a) producing a glass frit or glass flakes having the composition comprising: (i) a major proportion of a glass composition comprising 14 to 30% by weight SiO₂, 5 to 11% by weight Al,O₃, 20 to 35% by weight SrO, and 8 to 28% by weight TiO,; and (ii) a minor proportion of one or more additives selected from the group consisting of 0 to 7% by weight MgO, 0 to 20% by weight La,O₃, 0 to 20 % by weight PbO, 0 to 20 % by weight Nb,0₅, 0 to 8 % by weight CdO, and 0 to 10 % by weight B,O₃; (b) heating the glass frit or glass flakes at a temperature between T, and T,, wherein T, is a temperature which is not more than 40 ┬░C below a first exothermic peak on a differential thermal analysis curve for the glass, and T, is a temperature at least 20 ┬░C below a temperature which causes complete crystallization of the glass frit or glass flakes, for a time sufficient to produce a partially crystallized glass frit or flakes having crystalline and amorphous phases in a volume ratio between 5:95 and 95:5; and (c) grinding the partially crystallized glass frit or glass flakes to produce a finely divided, partially crystallized glass powder.

26. The method as set forth in claim 25, wherein the crystallization temperature in step (b) is not more 30 ┬░C below a first exothermic peak on a differential thermal analysis curve for the glass, and is at least 50 ┬░C below a temperature which causes complete crystallization of the glass frit or glass flakes.

27. The method as set forth in claim 25, wherein the crystallization temperature in step (b) is between 850 and 1100┬░C, and is held for 0.5 to 5 hours.

28. The method as set forth in claim 27 wherein the components (i) are included in a total amount between 70 and 80% by weight and the components (ii) are included in a total amount between 20 to 30% by weight, and wherein the amount of TiO₂ is 8 to 25% by weight and the amount of B,O₃ is 0 to 5% by weight.

29. The method as set forth in claim 28. wherein the glass frit or glass flakes have the composition in % by weight: SiO, 22-29; Al₂O₃ 7-10; SrO 20-30; La₂O₃ 10-18; TiO, 10- 19; PbO 0-6; MgO 0-7.

30. The method as set forth in claim 28. wherein the glass frit or glass flakes have the composition in % by weight: SiO, 18-27; A1,O₃ 7-10; SrO 20-30; La,O₃ 0-15; TiO, 8-28; PbO 0-20; MgO 0-7; B,O₃ 0-10.

31. The method as set forth in claim 28, wherein the glass frit or glass flakes have the composition in % by weight: SiO, 15-23; Al,O₃ 5-10; SrO 24-31; La₂O₃ 0-7; TiO, 17-28; PbO 3-13; MgO 0-5; Nb,O₅ 0-15; CdO 0-6; B,O₃ 0-10.

32. The method as set forth in claim 31, wherein the glass frit or glass flakes have the composition in % by weight: SiO, 14-22; A1,O₃ 5-10; SrO 25-35; La,O₃ 0-10; TiO, 15-28; PbO 7-20; MgO 0-5; Nb₂O_s 0-20; CdO 0-6; B,O₃ 0-10.

33. The method as set forth in claim 31, wherein the partially crystallized glass powder has an average particle size of 1 - 10 ╬╝m.

34. The method as set forth in claim 31, wherein the partially crystallized glass powder has an average particle size of 2-4 ╬╝m.

35. The method as set forth in claim 25, wherein the glass frit or glass flakes include less than about 2% by weight of other additives or impurities.

36. The method as set forth in claim 25. wherein the glass frit or glass flakes include less than about 0.5% by weight of either calcium oxide or alkali metal oxides, less than 1% by weight of any one of barium oxide, cerium oxide, and zirconia. and less than 0.3% by weight of bismuth oxide.

37. A mixture of a host glass with a partially crystallized glass, comprising: (a) a host glass in a finely divided form; and (b) 2 to 50 % by weight, based on the host glass, of a partially crystallized glass in a finely divided form and having a composition which is substantially the same as that of the host glass, said partially crystallized glass having crystalline and amorphous phases in a volume ratio between 5:95 and 95:5, and having been crystallized at a temperature between T_j and T,, wherein T, is a temperature which is not more than 40 ┬░C below a first exothermic peak on a differential thermal analysis curve for the glass, and T, is a temperature at least 20 ┬░C below a temperature which causes complete crystallization of the glass.

38. The mixture as set forth in claim 37. wherein the host glass and the partially crystallized glass have compositions which forms predominantly a titanium-based perovskite crystal phase on sintering.

39. The mixture as set forth in claim 37, wherein the host glass and the partially crystallized glass have compositions which forms predominantly a strontium titanate based glass ceramic on sintering.

40. The mixture as set forth in claim 39, wherein the crystallization temperature is not more 30┬░C below a first exothermic peak on a differential thermal analysis curve for the glass and is at least 50 ┬░C below a temperature which causes complete crystallization of the glass.

41. The mixture as set forth in claim 39, wherein the crystallization temperature is between 850 and 1 100┬░C, and is held for 0.5 to 5 hours.

42. The mixture as set forth in claim 41. wherein the host glass and the partially crystallized glass have compositions comprising: (i) a major proportion of a glass composition comprising 14 to 30% by weight SiO,, 5 to 11 % by weight Al,O₃, 20 to 35% by weight SrO, and 8 to 28% by weight TiO,; and (ii) a minor proportion of one or more additives selected from the group consisting of 0 to 7% by weight MgO, 0 to 20% by weight La,O₃, 0 to 20% by weight PbO, 0 to 20% by weight Nb,O₅. 0 to 8% by weight CdO, and 0 to 10% by weight B,O₃.

43. The mixture as set forth in claim 42 wherein the components (i) are included in a total amount between 70 and 80% by weight and the components (ii) are included in a total amount between 20 to 30% by weight, and wherein the amount of TiO, is 8 to 25% by weight and the amount of B,O₃ is 0 to 5% by weight.

44. The mixture as set forth in claim 42, wherein the host glass and the partially crystallized glass have the composition in % by weight: SiO, 22-29; A1,O₃ 7-10; SrO 20-30; La,O₃ 10-18; TiO, 10-19; PbO 0-6; MgO 0-7.

45. The mixture as set forth in claim 42, wherein the host glass and the partially crystallized glass have the composition in % by weight: SiO, 18-27; A1,O₃ 7-10; SrO 20-30; La,O₃ 0-15; TiO, 8-28; PbO 0-20; MgO 0-7; B,O₃ 0-10.

46. The mixture as set forth in claim 42, wherein the host glass and the partially crystallized glass have the composition in % by weight: SiO, 15-23; Al,O₃ 5-10; SrO 24-31; La,O₃ 0-7; TiO, 17-28; PbO 3-13; MgO 0-5; Nb,O₅ 0-15; CdO 0-6; B,O₃ 0-10.

47. The mixture as set forth in claim 42, wherein the host glass and the partially crystallized glass have the composition in % by weight: SiO, 14-22; A1,O₃ 5-10; SrO 25- 35; La,O₃ 0-10; TiO, 15-28; PbO 7-20; MgO 0-5; Nb,O_s 0-20; CdO 0-6; B,O₃ 0-10.

48. The mixture as set forth in claim 42, wherein the host glass and the partially crystallized glass have an average particle size of 1 - 10 ╬╝m.

49. The mixture as set forth in claim 42, wherein the host glass and the partially crystallized glass powder has an average particle size of 2-4 ╬╝m.

50. The mixture as set forth in claim 49, wherein the host glass and the partially crystallized glass include less than about 2% by weight of other additives or impurities.

51. The mixture as set forth in claim 49, wherein the host glass and the partially crystallized glass include less than about 0.5% by weight of either calcium oxide or alkali metal oxides, less than 1 % by weight of any one of barium oxide, cerium oxide, and zirconia, and less than 0.3% by weight of bismuth oxide.

52. A method of producing dense glass ceramic bodies from a host glass with a partially crystallized glass, comprising: (a) producing a glass frit or glass flakes of a host glass; (b) grinding the glass frit or glass flakes of the host glass to produce a finely divided host glass powder; (c) heating a glass frit or glass flakes having a composition which is substantially the same as the host glass at a temperature between T, and T,, wherein T, is a temperature which is not more than 40 ┬░C below a first exothermic peak on a differential thermal analysis curve for the glass, and T, is a temperature at least 20┬░C below a temperature which causes complete crystallization of the glass, for a time sufficient to produce a partially crystallized glass frit or glass flakes having crystals and residual amorphous phase in a volume ratio between 5 : 95 and 95 : 5 ; (d) grinding the partially crystallized glass frit or glass flakes to produce a finely divided, partially crystallized glass powder; (e) mixing the host glass powder and the partially crystallized glass powder, the partially crystallized glass powder being included in an amount of 2 to 50% weight, based on the host glass powder; (f) forming a green body from the mixture of step (e); and (g) sintering the green body to produce a dense glass ceramic body.

53. The method as set forth in claim 52, wherein the host glass and the partially crystallized glass have compositions which forms predominantly a titanium-based perovskite crystal phase on sintering.

54. The method as set forth in claim 52, wherein the host glass and the partially crystallized glass have compositions which forms predominantly a strontium titanate based glass ceramic on sintering.

55. The method as set forth in claim 54, wherein the crystallization temperature of step (c) is not more 30┬░C below a first exothermic peak on a differential thermal analysis curve for the glass and is at least 50 ┬░C below a temperature which causes complete crystallization of the glass.

56. The method as set forth in claim 54, wherein the crystallization temperature is between 850 and 1100┬░C, and is held for 1 to 5 hours.

57. The method as set forth in claim 56, wherein the host glass and the partially crystallized glass have compositions comprising: (i) a major proportion of a glass composition comprising 14 to 30% by weight SiO,, 5 to 11 % by weight Al,O₃, 20 to 35% by weight SrO, and 8 to 28% by weight TiO,; and (ii) a minor proportion of one or more additives selected from the group consisting of 0 to 7% by weight MgO, 0 to 20% by weight La,O₃, 0 to 20 % by weight PbO, 0 to 20 % by weight Nb,O₅, 0 to 8 % by weight CdO, and 0 to 10 % by weight B,O₃.

58. The method as set forth in claim 57 wherein the components (i) are included in a total amount between 70 and 80% by weight and the components (ii) are included in a total amount between 20 to 30% by weight, and wherein the amount of TiO, is 8 to 25% by weight and the amount of B,O₃ is 0 to 5% by weight.

59. The method as set forth in claim 57, wherein the host glass and the partially crystallized glass have the composition in % by weight: SiO, 22-29; Al,O₃ 7-10; SrO 20-30; La,O₃ 10-18; TiO, 10-19; PbO 0-6; MgO 0-7.

60. The method as set forth in claim 57, wherein the host glass and the partially crystallized glass have the composition in % by weight: SiO, 18-27; Al,O₃ 7-10; SrO 20-30; La,O₃ 0-15; TiO, 8-28; PbO 0-20; MgO 0-7; B₂O₃ 0-10.

61. The method as set forth in claim 57, wherein the host glass and the partially crystallized glass have the composition in % by weight: SiO, 15-23; Al,O₃ 5-10; SrO 24-31; La,O₃ 0-7; TiO, 17-28; PbO 3-13; MgO 0-5; Nb,O₅ 0-15; CdO 0-6; B,O₃ 0-10.

62. The method as set forth in claim 57, wherein the host glass and the partially crystallized glass have the composition in % by weight: SiO, 14-22; Al₂O₃ 5-10; SrO 25- 35; La,O₃ 0-10; TiO, 15-28; PbO 7-20; MgO 0-5; Nb,O₅ 0-20; CdO 0-6; B,O₃ 0-10.

63. The method as set forth in claim 57, wherein the host glass powder and the partially crystallized glass powder have an average particle size of 1 - 10 ╬╝m, and wherein the sintering step (g) is conducted at a temperature not greater than about 950┬░C.

64. The method as set forth in claim 57, wherein the host glass powder and the partially crystallized glass powder has an average particle size of 2-4 ╬╝m, and wherein the sintering step (g) is conducted at a temperature not greater than about 950┬░C.

65. The method as set forth in claim 64. wherein the host glass powder and the partially crystallized glass powder include less than about 2% by weight of other additives or impurities.

66. The method as set forth in claim 64, wherein the host glass powder and the partially crystallized glass powder include less than about 0.5% by weight of either calcium oxide or alkali metal oxides, less than 1% by weight of any one of barium oxide, cerium oxide, and zirconia, and less than 0.3% by weight of bismuth oxide.