FR2672588A1 - CRYSTALLIZABLE GLASS POWDER IN A FRITTE GLASS CERAMIC CONTAINING HEXAGONAL CORDIERITY, AS THE MAIN CRYSTALLINE PHASE. - Google Patents

Abstract

Disclosed is a glass powder crystallizable into a sintered glass ceramic containing hexagonal cordierite as the main crystalline phase, which is characterized by the following composition expressed as oxides (mol%): 48-61 SiO2, 10-16 Al2 O3 , 23-35 MgO, 0-4 B2 O3, 0-2.5 P2 O5, 0.5-5 SIGMA B2 O3 + P2 O5, 0-3 ZnO, 0-3 CaO, 0-1.5 BaO, 0 , 5-12 F instead of O. The glass powder may further contain in total up to 3 mol% of one or more of the following oxides: PbO, SrO or SnO2. The glass powder can be crystallized, at sintering temperatures below 97 ° C and in a relatively wide sintering temperature range, into a sintered glass ceramic having excellent and constant properties. It is particularly suitable for the manufacture of electrotechnical and electronic components, because it has very good electrical, dielectric and mechanical properties. The sintering conditions can be varied within relatively wide limits, without this having a negative effect on the product obtained.

Description

CRYSTALLIZABLE GLASS POWDER IN A GLASS CERAMIC

  FRITTE CONTAINING HEXAGONAL CORDIERITY AS A PHASE

MAIN CRYSTALLINE

  The present invention relates to a crystallizable glass powder of a sintered glass ceramic containing hexagonal cordierite as the main crystalline phase. Because of its good mechanical and electrical properties, cordierite is very suitable as a substrate for electronic components, particularly for multilayer printed circuit boards. Since pure cordierite has very high sintering temperatures, the conductive materials deposited before sintering are destroyed, we start from glass powder which, at relatively low temperatures, can be sintered and which, to a large extent, is transformed

  thus simultaneously in a crystalline phase.

  It is known that glass powders, the stoichiometric composition of which corresponds to pure cordierite, are only very slightly sinterable. By compression and firing at temperatures up to 12000 C, porous and low-sintered glass ceramics are obtained. mechanical strength The poor sinterability is due to premature surface crystallization of the glass particles, resulting in a dramatic increase in viscosity of several powers of ten, which prevents further sintering of the glass phase. It is also disadvantageous that mixed crystalline phases with a high quartz content, containing MgO, are formed. Representatives of this series of mixed crystals are g-cordierite, comprising the following oxides in the indicated ratio: Mg O / A 1203 / Si O 2 = 2: 2: 5, and a magnesium aluminosilicate (MAS) whose corresponding ratio is 1: 1: 4 The presence of these phases c Metastable crystallites further slows the

  crystallization in hexagonal cordierite.

  Already known glass powders of modified composition, with which the sinterability is improved Thus, according to US 3926648, the sinterability is improved by adding 0.5 to 2% by weight of K The sintered bodies produced from these glass powders, however, have deteriorated electrical or dielectric properties, which generally precludes their use for electrotechnical and electronic components, particularly in the case of particularly demanding conditions. In German patent DE 2602429, this modification takes place by the addition of 0.7 to 5.6 mol% of one or more of the following modifying oxides: Ba O, Pb O, Sr O or Ca O These The oxides are able to form mixed crystals in the structure of cordierite Due to the small proportion of vitreous phase remaining, the sinterability of the powder is however limited, so that the powders Their main applications are as coatings for ceramic, glass or glass ceramic articles. In German Patent 2901172, two different sintered glass ceramics are described, one being a glass ceramic and 0-spodumene base, such as main crystalline phase, and the other being a sintered glass ceramic & cordierite base, as main crystalline phase The structure of the latter sintered glass ceramic is formed, in addition to hexagonal cordierite, clinoenstatite and possibly gcordierite This complex structure requires that all the operating parameters, such as the chemical composition, the properties of the powders and the sintering schedule, are kept within close tolerance limits, in order to obtain always the same structure and, consequently, reproducible properties These sintered glass ceramics are therefore expensive to produce and are difficult to adapt The various operating requirements of the various users are described in US Pat. No. 4,540,671. A sinterable glass powder made of a sintered glass ceramic, in which sintered solid solution coexists cordierite and a mixed-crystal phase, are described. high quartz content But the existence of metastable mixed crystal phase with high quartz content leads to disadvantages already

previously exposed.

  The object of the present invention is to find a crystallizable glass powder of a sintered glass ceramic containing hexagonal cordierite, as the main crystalline phase, the composition of which is such that it is sintered in a hermetically sealed manner at 9700 temperatures. C or below, that it has good electrical and mechanical properties, and that it gives the user a great latitude of choice in the sintering conditions with regard to the heating rate, the sintering temperature and the time of

sintering.

  According to the present invention, this object is achieved by means of a glass powder having the following composition expressed in oxides (% by moles): 48 61 Si O 2

10 16 A 1203

23 35 Mg O

0 4 B 203

0 2.5 P 205

0.5 5 E B 203 + P 205

  In a particular embodiment of the invention, the glass powder has the following composition: 51 57 Si O 2 O 3 Zn O 0 3 Ca 0 0 1.5 Ba 0 0.5 12 F

12.5 15.5 to 1203

26 31 Mg O

0.5 2.5 P 205

: 0.3 2.0 B 203

  1.0 3.0 z B 203 + P 205 0.3 2.5 Zn O 0.5 11.0 F instead of O. To obtain a sealed sintered glass ceramic, that is to say without open pores, and with a high proportion of crystalline phase, the course of sintering and crystallization is important Unlike what happens for sealed glass ceramics, which, as a result of the addition of germ-forming agents, such as titanium dioxide or zirconium dioxide, undergo internal volume nucleation, the crystallization, during manufacture, of a sintered glass ceramic and glass powder base is made from

  primitive surfaces of the particles of the glass powder.

  The sealed sintering must be carried out, to a large extent, still in the vitreous state. If we observe a premature surface crystallization, the viscosity of the surface layer of the glass particles increases so much that the continuation of the sealed sintering is strongly disturbed, even even interrupted if on the other hand the ability of the glass to crystallize is removed, e.g. by adding larger quantities of modifying oxides, the desired finely crystalline homogeneous structure, which has a high crystalline crystal phase, can not be produced. Sintered crystal glass ceramics, which are weak, are then formed. strength, which are unsuitable for most applications. The SiO 2 content of the glass powder must be between 48 and 61 mol%. If the SiO 2 content falls below 48 mol%, it is not possible to carry out below of 9700 C, a sintering comprising a

  sufficient crystallization of hexagonal cordierite phase.

  If the content of SiO 2 exceeds 61 mol%, then a preferential crystallization of metastable magnesium aluminosilicate crystals is observed, with the drawbacks described above. Particular preference is given to an SiO 2 content of between 51 and 57% by mole The content of A 1203 is between 10 and 16% by mole If the upper limit of 16% by mole is exceeded, then the temperature necessary to manufacture a sealed sintered glass ceramic increases, while at a content of A 1203 of less than 10 mol%, the proportion in the crystalline phase of the desired hexagonal cordierite phase decreases and crystalline phases are formed, reinforced with MgO and SiO 2, which cause a rise in the constant dielectric A content of 12.5 to 15.5 mol% of A 1203 is preferred.

  Mg O content should be between 23 and 35 mol%.

  Exceeding the limit of 35% results in a decrease in the proportion of hexagonal cordierite, whereas a content of less than 23% results in a degradation of the sintering properties of the glass powder. A content of 26 to 31 is preferred. % moles of Mg O. In order to lower the sintering temperature, the glass powder contains up to 4 mol% of B 203 and up to 2.5 mol% of P 205, the total content of these two

  compounds to be between 0.5 and 5 mol%.

  The upper limits of these intervals must not be exceeded, otherwise the remaining vitreous phase increases

  too much, resulting in deterioration of properties.

  To contribute to lowering the sintering temperature, however, the total amount of B 203 and / or P 205 added should not be less than 0.5 mol%. Additions of zinc oxide accelerate the crystallization of hexagonal cordierite at low temperatures The zinc oxide content should not be more than 3 mol%, otherwise it runs the risk of having a lot of difficulty, as a result of premature surface crystallization, sintering to a sealed body A zinc oxide content of 0.3 to 2.5 mol% is preferred. Ca O also accelerates crystallisation at low temperatures, but it is necessary to avoid contents of more than 3 mol%, because, at such low levels, the appearance of anorthite, as additional and undesirable crystalline phase, is already observed Additions of BaO improve the homogeneity and stability of the glass mass However, a BaO content of 1.5 mol% must not be exceeded because, at higher contents, the crystallization of the glass powder is slowed down and occurs only at low temperatures.

temperatures far too high.

  An essential element of the invention is a fluorine content of 0.5 to 12 mol%, preferably 0.5 to 11 mol%, the fluorine ion replacing an equivalent amount of oxygen ions in the crystal lattice It has surprisingly been found that, by addition of fluorine, the crystallization temperature of the glass powder is shifted to lower temperatures, below 9000 C, without affecting the sintering process. This behavior is contrary to that observed. for crystallisation-promoting additives, such as zinc oxide or barium oxide, with which there must always be a compromise between sealed sintering and lower crystallization temperature The addition of fluorine accelerates the crystallization of the cordierite phases hexagonal and suppresses the precursor g-cordierite and magnesium aluminosilicate unwanted phases The structure remains stable over a wide temperature range for sintering and The fluorine content must not exceed 12 mol%, otherwise new fluorine-containing crystalline phases may occur, which affects the stability of the structure with respect to variations in the sintering temperature. There is also a deterioration in the homogeneity of the melted glass mass and in the hydrolytic stability of the sintered glass ceramic. Low additions of fluorine already have a significant effect on the crystallisation temperature, at the same time as lowering the sintering temperature or reducing the sintering times, but the fluorine content must not be less than 0.5 mol%, otherwise the effect of the addition of the

  fluoride is not always satisfactory.

  As other crystallization-promoting additives, it is possible to use PbO, SrO or SnO.sub.2. The maximum proportion of one of these oxides must not exceed 3 mol%. If several of these three oxides are used together, the The total amount used should then not be more than 3 mol%. Additions greater than 3 mol% lead to premature crystallization, which prohibits good sintering. When using Pb O, care should be taken that, at higher sintering temperatures, the lead oxide can be reduced, in a reducing sintering atmosphere, to metallic lead, which leads to an inadmissible lowering of the

electrical resistivity.

  In the manufacture of the glass from which the glass powder is obtained, the usual refining agents, such as, for example, the usual 1-0 concentrations of up to about 1% by weight can be added to the glass. Sb 2 O 3, As 2 O 3 or cerium compounds, without affecting its properties. However, they are not absolutely essential. The size of the grains of the sinterable glass powder plays a large role in the optimization of the sintering and the subsequent surface crystallization. In general, good results are obtained with a glass powder having

  an average grain size of between 1 and 12 gm.

  t; With grain sizes that are too small, the glass powder tends to premature surface crystallization due to the large surface area to volume ratio, which prevents good sintering. Large grain sizes, on the other hand, make crystallization difficult because lead to the formation of a relatively coarse crystalline structure, whose mechanical strength is deteriorated, and they provide sintered bodies with a high surface roughness Particularly favorable results are obtained with a mean grain size of the glass powder between 1.5 and 7) m. The glass powders are produced in a manner known per se, by melting, at temperatures of about 1450 to 1650 ° C., a glass of the composition described, obtained from the usual raw materials of the glass, then pouring, for soaking the molten glass in cold water or on cooled metal cylinders, which gives the cullet which is converted into powder having the indicated grain size, using known processes for making frit or grinding. The glass powder thus obtained is then converted into a shaped part, in a manner known per se, eg by conventional forming processes in the ceramic industry, such as dry pressing, extrusion, injection molding, sheet-drawing, etc. The conversion is carried out by adding conventional commercial organic auxiliaries and / or suitable suspending agents. Thus, for extrusion, a plasticized mass is used which has plastic properties. dry by adding pressing auxiliaries and by compression which provides a free-flowing granulate For the production of flexible green sheets, which are subsequently used for the production of electronic substrates, the drawing of sheets from a ceramic slip proved to be effective In the case of addition of suspending agents, drying is also necessary before sintering. During heating, the organic materials present are burnt by appropriate programming of the sintering schedule temperature before substantial sintering of the glass occurs. Sintering and crystallisation of the glass powder occur, depending on the composition. The sintering times required depend on the sintering temperature used and are between about 15 minutes and several hours. Lower sintering temperatures require a prolongation of the temperature of the sintering process. sintering time For a predetermined glass composition, it is possible, if necessary, to obtain, by lengthening the sintering time, at a desired lower sintering temperature, a structure similar to that which would be obtained with a high sintering temperature. and a shorter sintering time. Thus, the manufacture of a pore-free sintered glass ceramic is carried out. preparing a glass having the composition described, grinding said glass to a particle size of 1 to 12 μm, giving the glass particles the desired shape, by continuously heating the shaped glass particles; up to a temperature of up to 9700 C, keeping them at that temperature until the particles sinter and the glass is

  devitrify, and then cool the sintered objects.

  Before heating and sintering, it is possible to provide the shaped bodies: 1 E or their precursors with patterns of electrical circuits,

  for example by screen printing or any similar process.

  Although the composition of the glass powder of the invention differs from the stoichiometric composition of cordierite, however, substantial proportions of the crystalline phase having the hexagonal cordierite structure are produced by the formation of mixed crystals, the crystalline phases not consisting of cordierite or the remaining glassy phase being kept relatively small This formation of mixed crystals further allows the incorporation of over-stoichiometric proportions of Mg O and Si O 2, at the expense of A 1203, without affecting the crystallinity The ability the sintering of the glass powder is nevertheless improved and the indispensable proportion

  of modifying oxides is diminished.

  The sintered glass ceramics made from the glass powders of the invention are hermetically sealed and thus allow the encapsulation of sensitive electronic components. Because of the low dielectric constant, the metallized electronic substrates obtained from them have low delays in signal propagation time The low sintering temperature, up to 9700 ° C, allows metallizations to be carried out from good conducting metals, such as copper, silver, silver-palladium and gold The coefficient of thermal expansion sintered glass ceramic is comparable to that of silicon, so that the mechanical tensions

  between the substrate material and the semi-pellets

  Silicon conductors, due to the difference in thermal expansion coefficients, are minimal. Besides the sealed sintered glass ceramic substrates, it is also possible to manufacture cabling substrates.

  multilayer having internal conductive traces.

  Sintered glass ceramics made from the glass powders of the invention are distinguished by low heat conduction, high temperature resistance and good thermal shock behavior. Their high electrical resistivity and high dielectric strength allow their application in electrical insulation Glass powders are also suitable for coating or assembling ceramics and are

  otherwise stable at high temperature.

  The present invention is explained in more detail with reference to Examples The results are summarized in Tables 1 to 4 In Tables 1 and 3, the chemical composition is shown in Tables 2 and 4, the physical data of Examples Among the physical data, the average grain size (d 50) is measured by means of a laser granulometer. Sintering is carried out at the indicated temperatures, each time using a heating rate of 30 C / min. In the crystalline phase, the following X-ray reflections are used: reflection 102 for hexagonal cordierite, reflection 011 for MAS, reflection 120 for forsterite, reflection 610 for magnesium aluminosilicate (JCPDS 35-310). proportions of enstatite modifications are also present in sintered glass ceramics Due to the large number of possible modifications and their small percentages, it was necessary to undertake a qualitative evaluation. The hexagonal cordierite (JCPDS file no. 13-293), gordierite (JCPDS file # 14-249) and 1 forsterite (JCPDS file # 34-189) are dosed by reference. to fully crystallized standards The reference product for the MAS phase (JCPDS 27-716) still contains other crystalline phases, so that the quantitative values are tainted in this case with a slightly larger error (of the order of magnitude of about%) The dielectric constant ú and the loss angle tang d are measured at 250 C and at a frequency of 1 M Hz. For higher frequencies, in the gigahertz range, the dielectric constant and The angle of loss decreases significantly In order to evaluate the crystallization temperatures, differential thermal analysis is used. As for methodological reasons, differential thermal analysis works with high heating rates (here 5 OC / mid n), the actual crystallization temperature is systematically lower for normal sintering programs However, we can have an indication of the effect of the chemical composition on the crystallization temperature In the table, the beginning of the peak of crystallization and maximum crystallization These values make it possible to assess the crystallization temperatures of various sintered glass ceramics in relation to each other.

to others.

  Examples 1 to 4 (Tables 1 and 2) show the influence of the fluorine content on the sintering temperature and on the crystallization behavior, for an otherwise identical composition. Example 1 is used here to compare the powders of The compositions of which are given in Table 1 are made by melting the corresponding mixture at 16000 C. The melt is quenched between two water-cooled metal cylinders and the material is milled to the grain size indicated in FIG. Table 2 A free-flowing compressed granule is obtained from the glass powder by granulating the powder, in a manner known per se, in an intensely stirred mixer, with an aqueous solution of polyethylene glycol. dry at a pressure of 800 bar, shaped parts are made for sintering tests. The sintering time is 1 h at the specified sintering temperature, the heating rate is st 30 C / min At a temperature of 3100 C, 1 hour of stopping to eliminate

  the polyethylene glycol of the shaped part.

  Examples 2 to 4 show very clearly, compared to Example 1, the extraordinarily positive effect exerted by fluorine. It can be seen from Table 2 that fluorine moves the crystallization temperature to lower temperatures. The crystallization of the hexagonal cordierite is accelerated, the other crystalline phases are inhibited. The structure is stabilized, the crystalline phase portion is only slightly modified over a wide range of temperatures, and thus the hexagonal cordierite content in Example 3 is substantially constant for sintering temperatures ranging from 910 to 305 9700 C. Due to the stabilization of the structure by fluorine, the coefficient of thermal expansion is also little modified for the compositions of the invention, for sintering temperatures between 910 and 9700 C. Example 1, which serves as a reference material in the absence of fluorine, has, on the other hand, crystalline phase contents. All the sintered bodies of the invention are, at the specified sintering temperatures, sintered in a hermetically sealed manner. Examples 5 to 13, which correspond to other glass powders compositions of the invention, are summarized in Tables 3 and 4 Examples 11 & 13 show the influence of additives Sr O, Pb O and Sn O 2 sure

  the acceleration of the crystallization.

Example 14

  From the glass powder of Example 8, 200 m thick, green, flooded sheets were made, according to the following method. The glass powder was mixed in a ball mill with organic auxiliary agents and a mixture of organic solvents,

  homogenize everything and make a pourable slip.

  The mixture consists of 53% by weight of glass powder, 2 C of 37% by weight of a mixture of azeotrope-based solvents based on trichlorethylene and ethanol, 5% by weight of polyvinylbutyral as binder, 4% by weight of % by weight of dioctyl phthalate as plasticizer and 1% by weight of menhaden oil as a fluidizer The suspension is deaerated and poured onto a moving continuous strip Between the casting shoe and the band is a narrow slot

  adjustable to adjust the thickness of the sheet.

  After the course of a drying path, the flexible sheet

  can be detached from the casting tape.

  Several layers of these green sheets are stacked on top of each other and pressed into a laminating press at a pressure of 0.5 to 3 k N / cm 2 and a

  temperature of 900 C, to make it a unitary laminate.

  The laminates thus obtained are sintered on a plane firing support at 3 ° C. for 9300 ° C. and for a sintering time of 1 hour. Due to the high content of auxiliary organic agents, a heating rate is generally employed. At a temperature of 20 ° C / min in the vicinity of the combustion temperature, that is to say in the range Δ of temperatures 2200 C-3300 C, the heating rate is reduced to 1 OC / min. 3300 C for 2 hours in order to complete the removal of the organic auxiliaries Then the heating is continued at C / min until the desired sintering temperature is reached. The structure and properties of the sintered glass ceramic substrates thus obtained are in accordance with the values also obtained when sintering parts

  molded with the same temperature schedule.

  In all the examples, the electrical resistivity is greater than 1013 cm -1 at room temperature, which means that it is in the presence of a very good material.

electrical insulation.

  Table 1: Compositions of glass powders (X in moles) If O 2 A 1203 Mg OP 205 B 203 54 O 1571 28/4 2> O Oj 5 54 t 0 15/1 28/4 2) 0 0/5 54 ## EQU1 ##

  ______________________________________________ _____________

  F (instead of 0) 0 O 4; O 6; 6/6

Example:

____________

  Table 2: Crystalline phase contents and material properties of sintered glass ceramics obtained Temp of average T (o C) content crystalline phase (X by weight) grains of (other) p OIAS Cord hex Silicate Mg-AI Forsterite Other grits of (duration of P MAS r h.powder sintering: d 50 (pm) 1 hour)

2 8 890 59 <10

218 910 54 22 2 10 -

258 930 38 39 2 10 -

2/8 950 16 53 2 10 -

2 or 8 970 10 57 2 10 -

2 4 890 45 36 8 -

2 4 910 24 55

2 4 930 5 68 <5 -

2 4 950 <5 70

2/4 970 <5 76

___ ____ __ _________ _______

2 4 1890 18 61

2/4 910 5 70

2 4 930 5 71

2/4 950 <5 71

2 4 970 <5 75 _

2 3 890 31 45 -

2 3 910 17 53

2/3 930 11 58

2/3 950 5 70

2/3 970 <5 71

at _______-

Propr dielectr.

(I H Hz, 25 'C) tan'

(10-4)

52 16

51 16

_ _

/ 1 19

___ _ _ _

/ 1 28

  Ex: 3 ATD Start of peak ('C) Peak Max. ('C) Coeff of dilatat. thermal

(20-300 C)

  o (10-6 / K) 2/25 1/82 1 r 86 1/84 2/01 2 89

2 I 74915 994

-1

905,950

885,922

00 O 897

  Table 3: Compositions of glass powders (% by moles) Example: Sio 2 A 1203 Mg OP 205 B 203 Zn O Ca O Ba O Sr O Sn O 2Pb O l F 2, 2 32 5 2 2 (instead of 0) i Ii

': 3.5 13/8 290 140 1.2 1.5 4 8

Iitl

  6 52/7 13/6 28/6 1/2 0/9 1/5 1/5 to 3/0

II

  7 52/7 13/6 28/6> 6 1/5 1/5 1/5 3/0

I II

  8 152/7 13/6 28/6 1/2 0/9 1/2 1/5 03 2/9

  II O l 9 i 55 O 130 28/5 1 j 1 O 9 15 0/7 155 t O 13/0 28/5 1 1 0, ^ 91/5 OJ il 155, 13 O 28 5 1 1 0/9 1 5

I II

  12 t 55 O 13/0 28/5 1,1 0/9 1 5 t 0/7 It I

3 55/0 13.0 28.5 1.1 09 15 8

  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _I

  5.2 1 33028.5,, 30, 9 - 35, 155.0 13/0 28 5 1/1 0/9 1.5: 1 0/8 Table 1: Contents in crystalline nhases and material properties of sintered glass ceramics obtained Kiittaqe Size Temp Coeff

  Ex. Mean TF (OC) Crystalline Phase Content (Z by weight) Proprdiel dilatat.

  purified grains of gaM Cord hex Silicate mg-AIForsteriteOther (1H Hz, 25 'C) thermal powder Irita: taen C (20-300 C) d 50 (Lmn) I Fue d 50 C 1 pm) 1 hour (10- 4) d, (10-G / K)

3/1 910 <1 76 <1 1 2 13

  3 j 1 930 78 1 1 a) _ 3 _ 970 01 2 a) 5.4 17 2/25

G 3/5 910 2 63

3 '5 930 63 7 -

  597 o 70 3 54 38 3 O 7 3/8 910 58 3 a) 3/8 930 58 3 a) 3 Za 970 fi 4 a) 5.4 26 2 8 3/1 910 60 6 a) 3 1 930 60 5 a) 3 ll 970 72 2 a) 53 2 N 2 "G 9 3/3 93 (59 2 <5 b) 33/970 77 2 b) 52 22 2 J 97

31: 930 67 2 <5

3 Z 3 _ 70 74 2 <5 - _ 5 3 1 29

  11 3/2 930 15 44 3 <5 c) 3l 971 _ G 55 5 <5 c) 4 3-55

12 3 1 930 G 7 2 <5

19 <) 70 71 2 <5 5 24 2 71

13 3 3 930 22 42 10

/ 3,970 15 48 9 5 3 13 2/40

  a) very little enstatite b) = very little anorthitus c) = very little cldrhochsanridine ATD Beginning peak of peak max '

('C) (' C)

890,925

930 973

920 975

920 944

930,990

92 to 3 962

955 1013

935 973

9215 975

to "Il G ,, O O

Claims (5)

  1 Crystallizable glass powder of a sintered glass ceramic containing hexagonal cordierite, as the main crystalline phase, characterized by the following composition expressed as oxides (mole%): 48 61 Si O 2 16 A 1203 23 35 Mg O 0 4 B 203 0 2.5 P 205 0.5 5 E B 203 + P 205   0 3 Zn O O 3 Ca O 0 1.5 Ba 0 0.5 12 F & s place of O 2 Glass powder according to claim 1, characterized by the following composition: 51 57 Si O 2 12.5 15.5 to 1203 26 31 Mg O 0.5 2.5 P 205 0.3 2.0 B 203 1.0 3.0 E B 203 + P 205   0.3 2.5 Zn O 0.5 11.0 F instead of O 3 Glass powder according to claim 1 or 2, characterized by a total content of up to 3 mol%   in one or more oxides Pb O, Sr O, Sn O 2.   4 Glass powder according to any of the   claims 1 & 3, characterized by an average size grains of 1 & 12 lm.