DD241895A1 - CERAMIC MASS AND A CERAMIC MATERIAL MADE FROM HERB - Google Patents

Abstract

The subject of the invention is a ceramic mass based on glass and quartz and a dense ceramic material produced therefrom which satisfies in its properties the essential requirements for electroceramic materials of the type KER 220 according to TGL 7838. The object of the invention is to develop an electroceramic material with the properties of the type KER 220 according to TGL 7838 at low cost and from readily available raw materials. It is the object of the invention to find a ceramic composition based on relatively low-melting glass, readily available quartz and temporary auxiliaries, from which a dense material can be produced without addition of fluoride and without crystallization of the glass phase, by sintering below 1 300 K, fulfilling the essential property requirements Electroceramic materials of the type KER 220 according to TGL 7838 corresponds. According to the invention, the ceramic mass after burning out the auxiliaries consists of 40 to 60% by weight of a quartz powder of a certain grain size and of the mass fraction of a crystallization-stable silicate glass powder with a certain half-value grain size and specific physical properties to be supplemented in each case up to 100%.

Description

Field of application of the invention

The invention relates to a ceramic mass based on glass and quartz and a made of it by sintering at a temperature below 1300 K material whose properties meet the essential requirements of electroceramic materials of the type KER 220 according to TGL 7838.

Characteristic of the known technical solutions

Ceramic materials for the production of materials according to KER 220 (TGL 7838) based on natural magnesium silicates, clay and feldspar are known in the art . The use of synthetic magnesium silicates has also been considered

By a firing process at temperatures above 1600 K from these ceramic masses a shard is available, which contains protoenstatite as a crystalline structural constituent in addition to a glass phase. By various additions to the basic offset or by a suitable exchange of mass components, the structure of the cullet and thus its properties can be specifically influenced / 1 /; However, the firing temperature is not lowered fundamentally.

Ceramic compositions based on Erdalkalialumoborosilicatglas and the crystalline phases corundum or zirconium, from which dense materials are obtained at firing temperatures below 1300 K, are in DD-PS 120.008; DD-PS 116.815 and DD-PS 119.200 described. They fulfill the properties required in the TGL 7838 for the material type KER 220. For their preparation, however, relatively expensive crystalline components and refractory glasses are required.

In DE-PS 2,706,659 a composition based on the starting materials of low-melting alkaline glass and quartz and certain fluorides is proposed. From this, a material can be produced by firing below 1300 K, which essentially meets the property requirements of the TGL 7838 on the materials of the type KER 220. Furthermore, DD-PS 132.262 discloses a ceramic mass based on glass and quartz, from which the material according to KER 220 is obtained when the glass phase recrystallizes to a considerable extent after densification. The disadvantage of the compositions proposed in the two last-mentioned patents is that the achievement of a flexural strength of 120 MPa requires the addition of certain fluorides and / or the recrystallization of the glass phase. Both are with an additional technological

-2- Ζ4Ί »SO

Effort, e.g. increased demands on the management of the firing process, measures to avoid fluorine emission and higher raw material costs associated.

Object of the invention

The aim of the invention is to develop an electroceramic material with the properties of the type KER 220 according to TGL 7838 inexpensively and from readily available raw materials.

Explanation of the essence of the invention

It is an object of the invention to find a ceramic composition based on relatively low-melting glass, readily available quartz and temporary auxiliaries, from the

- without addition of inorganic fluorides

- Without crystallization of the glass phase

a dense material can be produced by a substantially non-reactive sintering process at a temperature below 1300 K, which corresponds to the essential property requirements of electroceramic materials of the type KER 220 according to the TGL 7838 due to its substantial composition and a specific microstructure. According to the invention, after the auxiliary materials have burned out, the ceramic mass consists of 40 to 60% by weight of a quartz powder whose grain size is less than 0.010 mm for more than 99.9% of the particles and whose particle size distribution is preferably less than 0.005 mm, and from US Pat in each case up to 100% to additional mass fraction of a crystallization-stable silicate glass powder. According to the invention, the silicate glass powder has a half-value grain size d ₅ o equal to or less than 0.005 mm and moreover has the following properties

T ₉ equal to or less than 840K

C1300 ... 700K equal to or greater than 8 · 10 " ⁶ K" ¹

Gleichοο equal to or greater than 550 K

tan δ equal to or smaller than 35 x 10 ^"4 (tan δ at 1 MHz)

The quartz powder preferably consists of a quartz-rich raw material with at least 97Ma .-% quartz, preferably from a glass sand, a quartz sand, a residue of rock treatment, such as kaolin treatment, or a mixture of the components mentioned, or it can be prepared by crushing it in a known manner getting produced. The silicate glass preferably has the following chemical composition (in mol%)

SiO ₂ 65 to 77

Al ₂ O ₃ 3.0 to 5.5

B ₂ O ₃ 0 to 12

BaO 1 to 5

Alkaline earth metal oxides 2.5 to 8.5

Alkali metal oxides 11 to 16

PbO 0bis2

According to the invention, by treating the ceramic mass, shaping and substantially non-reactive sintering at a temperature below 1300 K, a dense ceramic material having a glass-quartz-pore structure, the microstructure of which contains quartz particles of which more than 99.9% has a particle size have smaller than 0.001 mm, wherein the grain size distribution in the material substantially equal to the ceramic mass, the quartz particles are almost completely wetted and enclosed by the glass phase, the pores are dispersed and rounded mainly in the glass phase and the shards the essential for electroceramics of the type KER 220 according to TGL 7838 required mechanical and electrical properties.

As a result of an investigation into the influence of the core size distribution of quartz used in the mass, an unexpectedly high increase in strength was observed when the grain size of all quartz particles in the ceramic mass and thus also in the material a value of 0.010mm and the passage grain size d _{M of} the particle size distribution a value of 0.005mm falls below. This increase in strength goes far beyond the level which, as is known, can be expected with a gradual reduction in the average particle size of the disperse phase and was found on the examined glass-quartz sintered material in a particle size range above the stated limit values. It is also surprising that the unexpectedly high increase in strength occurs just when the ratio of mean quartz and average glass particle size reaches or falls below the value 1 and thus leaves that range which is claimed in DD-PS 115.104.132.262 and 114.250 or is marked as preferential. Microstructural and acoustic emission studies indicate the following phenomena that could be responsible for the unexpected effect: A gradual reduction of the mean diameter of the disperse phase particles leads to an approximation of the critical diameter for the spontaneous microcracking or its undershoot. The probability of spontaneous as well as age-related microcracking decreases until finally there is an almost micro-crack-free microstructure of particularly high strength and fracture toughness / 3 /.

In this view known to those skilled in the art, however, a size which also influences the strength, the degree of the binding of the quartz particles into the matrix material, is disregarded.

It is always assumed that the degree of integration is already sufficiently high, so there is practically no possibility of an additional increase in strength due to a further increase in the degree of integration. For the fluoride-free glass-quartz sintered material, however, a generally insufficient and dependent on the composition of the composition and sintering temperature quartz inclusion is characteristic. Only with the quartz grain size distribution according to the invention is this disadvantage overcome and, at the same time, the above-discussed grain size effect significantly exceeds the increase in strength achieved. The improvement in the quartz inclusion can thus be taken as the cause of this additional increase in strength.

For the time being, the meaning of the ratio of quartz and glass grain size can only be assumed. It is clear, however, that assuming the ratio Ö _{Q / aa,} the sintering temperature required to reach the compression maximum increases and thus the conditions for the wetting of the quartz particles by the matrix glass improve.

The structure of the material with almost complete quartz inclusion is characterized by a low pore number and size as well as by pores dispersed in the glass and rounded. The number of cracking nuclei and their tendency to crack are thus significantly lower than in the fluoride-free glass-quartz sintered material of the prior art.

As a glassy starting material for the inventive composition can all serve a transformation temperature below the high-low quartz transition itself crystallization-stable silicate having an average linear expansion coefficient of at least 8 × 10 ^"6 K" ¹ and.

The electrical properties of the glass must ensure that the material according to the invention meets the corresponding requirements for materials of the type KER 220 according to TGL7838. A suitable glass composition range is

SiO ₂ 65 to 77

Al ₂ O ₃ 3.0 to 5.5

B ₂ O ₃ 0 to 12

BaO Ibis 5

alkaline earth metal oxides

(except BaO) 2.5 to 8.5

Alkali metal oxides 11 to 16

PbO 0bis2

The grain size distribution of the glass is to be chosen so that the highest possible green density and a uniform distribution of quartz particles in the glass matrix of the sintered body is achieved. A glass powder whose half-value grain size d _{50 is} less than 0.005 mm is advantageous.

The preparation of the ceramic composition according to the invention is carried out in a conventional manner from quartz powder having a particle size below 0.010 mm and the pre-shredded glass according to conventional non-plastic mass ceramic techniques.

The use of temporary excipients, preferably for plasticization, which can be removed by burning at temperatures below 830 K, is provided. Advantageously, a Mischmahlung the offset. Preferred variants of the shaping are isostatic pressing with possibly subsequent white machining, dry pressing and hot spraying. The moldings are densely sintered in units known per se at a temperature below 1300 K.

embodiments

The invention will be explained in more detail by the following embodiments, wherein it is not limited to these examples.

Starting materials:

1. Silicate glass of chemical composition in mol .-%: SiO ₂ 75 CaO 3.7

Al ₂ O ₃ 3.1 BaO 4.6

B ₂ O ₃ 1.9 Na ₂ O 3.2

Fe ₂ O ₃ 0.1 K ₂ O 8.5

and with the following physical properties α3οο ... 7οοκ = 8.6 × 10 K ~ T ₉ = 840K ρ = 2.61 g / cm ³

31 kg of said glass are ground in a 100-l ball mill with 79 kg flint stones as grinding media with the addition of 0.25% ethanol (based on the dry matter) for 168 hours. The milled product has the grain transmission values da * = 0.0084 mm d ₅₀ = 0.0029 mm d ₁₆ = 0.0007 mm and a specific surface area of 3.6 m ² / g. The SiO ₂ milling abrasion is 2.5Ma .-%.

2. Quartz sand Weferlingen variety B

16 kg of the quartz sand are milled in a 40-l ball mill with a Mahlkörperfüllung of 30 kg flint stones with the addition of 0.25% ethanol for 96 hours. From the ground product by air classification becomes a fraction with the grain size passage values

ds4 = 0.0039 mm "

d ₅₀ = 0.0027 mm

d ₁₆ = 0.0016mm

and a specific surface of 3.5m ² / g separated. The fraction contains no quartz particles with a particle size above 0.006 mm.

Mass preparation and test specimen production:

200 g portions of the separated quartz grain fraction and the glass powder with the glass-quartz volume ratios 60/40, 45/55 and 40/60 are premixed for about one hour in a turbulator and then with the addition of 0.05 η acetic acid ( Solid-liquid mass ratio 1: 1) by intensive stirring in a slip whose pH is kept by 8. A special polyvinyl alcohol (PVA, 2g PVA per 100g offset) serves as setting and pressing aid. The spray granules prepared from the slurry are pressed dry uniaxially with a pressure of 100 MPa to test specimens. The burning out of the auxiliary materials (up to approx. 770 K) is followed by the sintering process at 1145 K. The burning time is 15 min.

Structure and properties:

The test specimens obtained show the disclosed structure with the components glass phase, quartz and pores. The closed porosity reaches values of up to 7% by volume. With a mean flexural strength value of 135MPa, the test specimens meet the strength requirements for KER 220 electroceramics. The necessary electrical properties are also achieved with the exception of the volume resistivity at 873 K.

Comparative Example:

Starting materials, mass preparation and test specimen production:

The starting materials described in the exemplary embodiments and the conditions of the mass preparation and test specimen production are maintained. By way of derogation, the ground quartz sand having the grain size passage values ds4 = 0.0160 mm dso = 0.0065 mm d ₁₆ = 0.0018 mm

and a specific surface of 3m ² / g without prior classification by means of air classification for mass preparation.

Structure and properties:

Due to the coarse grain content of the quartz, the microstructure has on average larger pores than the materials described in the exemplary embodiments. In particular, the larger quartz particles are partially incomplete enveloped by the matrix glass or surrounded by intrinsic stress-induced ring cracks. The average bending strength of only 106 MPa does not reach the value required for KER 220 electroceramic materials and, with a probability of error of less than 0.1%, is below the bending strength of the materials described in the working examples. literature

/ 1 / Hecht.A. - Electroceramics, 2nd edition, Springerverlag, Berlin 1976, p.334 / 2 / Krahl, K., Palatzk, A. - Hermsdorfer technical communications, 2 (1962) 5,117 ff. / 3 / Storch, M., et al. - cfi / Ber. Dt. Keram. Ges. 61, (1984) 7,335 ff.

Claims (4)

Invention claim: 1. Ceramic mass on the basis of glass, quartz and temporary auxiliaries, preferably plasticizers, characterized in that they after burning out the excipients from 40 to 60 wt .-% of a quartz powder whose grain size for more than 99.9% of the particles below 0.010 mm and its d ^ value of the particle size distribution is preferably below 0.005 mm, and from each to 100% to be supplemented mass fraction of a crystallization-stable silicate glass powder having a half-grain size d ₅₀ equal to or less than 0.005 mm and the following physical properties T ₉ equal to or less as 840 K T _x ioo equal to or greater than 550 K α3οο ... 7οοκ equal to or greater than 8 · 10 " ⁶ K" ¹ tanö equal to or less than 35 -10 " ⁴ (tanö at 1 MHz) consists. 2. Ceramic composition according to item 1, characterized in that the quartz powder to point Taus a quartz-pure raw material with at least 97 wt .-% quartz, preferably from a glass sand, a quartz sand, a residue of the rock treatment, for example kaolin treatment, or a mixture of consists of components mentioned or is made by crushing it in a conventional manner. 3. Ceramic composition according to item 1 or 2, characterized in that the silicate glass according to item 1 or 2, characterized in that the silicate glass according to item 1 has the following chemical composition in mol .-%: SiO ₂ 65 to 77 Al ₂ O ₃ 3.0 to 5.5 B ₂ O ₃ 0 to 12 BaO Ibis 5 Alkaline earth metal oxides. 2.5 to 8.5
Alkali metal oxides 11 to 16 PbO 0bis2 4. A dense ceramic material having a glass-quartz-pore structure, which is prepared by substantially non-reactive sintering at a temperature below 1300 K from the ceramic composition according to item 3, characterized in that the structure of the cullet contains quartz particles whose grain size to more than 99.9% is less than 0.010mm, where - The grain size distribution of the quartz in the material substantially equal to the ceramic mass; - The quartz particles are almost completely wetted by the glass phase and enclosed; - The pores are predominantly dispersed in the glass phase and rounded; - the shards have the essential mechanical and electrical properties required for electroceramic type KER 220 according to TGL7838.