UDC 666.2.01

MATERIALS WITH CONTROLLED DIELECTRIC CONSTANTS

BASED ON A GLASS CERAMIC OF LITHIUM

ALUMINA-SILICATE COMPOSITION

E. I. Suzdal’tsev1 and T. I. Rozhkova1

Translated from Ogneupory i Tekhnicheskaya Keramika, No. 5, pp. 19 – 21, May, 2003.

Materials based on a heat-resistant radio transparent glass ceramic of lithium alumina-silicate composition are

developed. Mixing the ceramic with silicon oxide provides a route to materials with controlled dielectric con-

stants (within 4.5 – 6.7 units), mechanical strength (60 – 110 MPa), and porosity (0 – 11%). The optimum

concentration range for SiO2 is found to be 5 – 40%.

The OTM 357-type ceramic developed at the Tekhno-

logiya Research and Production Association Federal State

Unitary Enterprise is a heat-resistant radio transparent mate-

rial with potential applications in various branches of indus-

try. The practical use of this material is determined by its

strength, dielectric, thermal, and physical properties. For use

in radio engineering, materials with a range of properties are

required, of which the dielectric properties are the most im-

portant. The traditional route toward materials with different

values of the dielectric constant is variation of the material’s

chemical composition. However, this route is technically

complicated and costly. It is felt that a basically new ap-

proach to solving the problem is needed.

In this work, we were concerned with preparing materi-

als based on OTM 357 glass ceramic with a wide range of di-

electric properties. For that purpose, a material with a lower

dielectric constant was used as the additive. Some of the

best-studied materials are silicon dioxide and SiO2-based ce-

ramics [1]. The dielectric constant of quartz glass is about

3.7 at a temperature of 20°C and a frequency of 1010 Hz; that

of OMT 357 glass ceramic measured under the same condi-

tion is about 7.0. The techniques by which additives are in-

troduced may be quite different, for example, adding a pow-

der to the slip composition during grinding [2], or blending

two slip compositions at a definite proportion. Each slip

component should exhibit good casting properties to ensure

the preparation of dense preforms suitable for further heat

treatment and production of sintered materials.

In our work, OMT 553 lithium alumina-silicate glass of

the following chemical composition was used (%): SiO2,

62.5 – 65.5; Al2O3, 24 – 26; TiO2, 4.3 – 5.5; Li2O, 3.6 – 3.9;

ZnO, 0.8 – 1.0; BaO, 0.9 – 1.1, and quartz glass (material for

thin-walled tubes) with a SiO2 content � 99.7%. Distilled

Refractories and Industrial Ceramics Vol. 44, No. 4, 2003

2601083-4877/03/4404-0260$25.00 © 2003 Plenum Publishing Corporation

1 Tekhnologiya Research and Production Association Federal State

Unitary Enterprise, Obninsk, Kaluga Region, Russia.

TABLE 1. Properties of SiO2-Containing Slips

SiO2

content,

%

Density

�,

g�cm3

Viscosity

�,

Pa � sec

pH

Electro-

kinetic

potential

�E, mV

Moisture

content W,

%

Grinding

fineness

T63 , %

0 1.987 12.3 9.016 – 140.4 14.58 8.23

5 2.000 9.5 8.703 – 138.4 14.71 8.66

10 1.991 8.8 8.634 – 134.6 15.39 8.64

15 1.982 8.2 8.613 – 133.5 13.47 9.24

20 1.980 8.5 8.570 – 131.1 15.48 7.60

25 1.972 7.9 8.512 – 127.9 13.48 9.17

30 1.969 7.5 8.377 – 120.5 15.83 8.18

35 1.965 7.5 8.271 – 114.5 15.19 7.90

40 1.952 6.6 – – 14.51 7.58

45 1.947 6.9 8.240 – 97.5 14.72 8.25

50 1.944 7.1 8.488 – 111.0 15.05 7.00

55 1.942 5.7 7.964 – 97.4 13.94 6.74

60 1.934 5.9 7.916 – 94.6 14.39 7.00

65 1.925 5.3 7.907 – 94.2 14.74 5.80

70 1.920 5.3 7.749 – 85.4 14.50 6.78

75 1.914 5.0 7.693 – 82.4 14.57 6.87

80 1.899 6.1 7.675 – 81.4 14.55 6.41

85 1.900 5.3 7.197 – 54.5 14.42 6.96

90 1.886 6.4 7.134 – 51.1 14.31 5.97

95 1.893 6.4 7.007 – 44.0 14.12 6.34

100 1.892 12.0 5.900 33.6 14.05 7.10

water was used as the dispersion medium. OMT 553 glass

was ground in a ball mill lined with Al2O3 tile using

Al2O3 grinding bodies; crushed quartz glass tubes were

ground in a ball mill lined with SiO2 using fused-quartz

grinding bodies. The grinding was carried out to obtain slips

with superior casting properties. The OMT 553 glass-based

slip had a density of 1.98 – 2.04 g�cm3, a viscosity of

8.6 – 24.7 Pa � sec, pH 7.4 – 9.2, and a grinding fineness of

7 – 13.5% (material retained on sieve 063). The quartz

glass-based slip had a density of 1.87 – 1.90 g�cm3, a viscos-

ity of 5.2 – 12.3 Pa � sec, pH 4 – 6.5, and a grinding fineness

of 4 – 7.6%. The quartz-glass slip at concentrations of 5 to

95 vol.% was added to the lithium alumina-silicate glass slip.

The slip mixture parameters are given in Table 1. The slip

mixtures were cast in plaster molds to prepare green pre-

forms; these were heat treated at 1200 – 1240°C for 4 h to

prepare sintered ceramics. The properties (density, porosity,

water uptake) of sintered ceramics with different content of

SiO2 were measured; relevant data are shown in Figs. 1 – 3.

The ceramic density, starting from 2.47 – 2.49 g�cm3, shows

a decrement of 0.02 – 0.05 g�cm3 for each 5% increase in

SiO2 content to reach a minimum level of 2.08 ± 0.02 g�cm3

at 55 – 90% SiO2. Simultaneously, the ceramic porosity in-

creases to a maximum of 11% (at 1200°C and 55 – 60%

SiO2 ), 11.4% (at 1220°C and 65 – 70% SiO2 ), and 11.3% (at

1240°C and 65 – 70% SiO2 ). A similar trend is observed in

the ceramic water uptake: maxima attained are 5.4%

(1200°C), 5.6% (1220°C), and 5.5% (1240°C). This charac-

teristic behavior may be due to the fact that, within the tem-

perature range specified and at a SiO2 concentration of

55 – 70% in the mixture, silicon dioxide undergoes cristo-

balitization, which results in decompaction of the material

structure. At quartz glass concentrations in the slip mixture

higher than 90%, the sintered material behaves as an ordi-

nary quartz ceramic. The fact that the corresponding speci-

mens’s characteristics have close values for heat-treatments

temperatures 1220 and 1240°C indicates that a sufficient de-

gree of densification has been attained.

The materials prepared were tested for dielectric proper-

ties (according to the State Standard GOST 8.544–86) and

for static bending strength (according to the All-Union Stan-

dard OST 11 0309–86) (Figs. 4 and 5). With increase in SiO2

content, the bending strength decreases substantially: from

120 to 42 MPa (at 1240°C), from 113 to 49 MPa (at

1220°C), and from 109 to 61 MPa (at 1200°C), that is, the

Materials with Controlled Dielectric Constants 261

2.50

2.45

2.40

2.35

2.30

2.25

2.20

2.15

2.10

2.05

2.000 10 20 30 40 50 60 70 80 90 100

1240°C

1200°C

1220°C

SiO content, %2

Den

sity

,g

cm�3

Fig. 1. Ceramic density measured as a function of the SiO2 content

at temperatures 1200, 1220 and 1240°C.

0 10 20 30 40 50 60 70 80 90 100

1240°C

1200°C

1220°C

SiO content, %2

11

10

9

8

7

6

5

4

3

2

1

Po

rosi

ty,

%

Fig. 2. Ceramic porosity measured as a function of the SiO2 content

at temperatures 1200, 1220 and 1240°C.

0 10 20 30 40 50 60 70 80 90 100

1240°C

1200°C

1220°C

SiO content, %2

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Wat

eru

pta

ke,

%

Fig. 3. Ceramic water uptake measured as a function of the SiO2

content at temperatures 1200, 1220 and 1240°C.

higher the heat-treatment temperature, the lower the bending

strength. The dielectric constant decreases smoothly with in-

creasing SiO2 concentration and has roughly the same value

for specimens treated at different temperatures (Fig. 5).

Based on the data obtained, one can specify a SiO2 con-

centration range (5 – 40%) within which materials can be

prepared with a sufficiently high strength (75 – 120 MPa)

that makes them suitable for radio engineering applications.

For these ceramics, the dielectric constant characteristically

varies from 5.0 to 6.7, and the dielectric loss tangent, from

0.009 to 0.007. As the SiO2 content increases above 40%, the

material strength shows a sharp decreases, which narrows the

range of potential applications of the ceramic material.

Thus, we have shown in this work that ceramic materials

with a high mechanical strength and dielectric properties vary-

ing in a wide range can be prepared. Materials exhibiting the

greatest promise for preparation of radio transparent ceramics

are lithium alumina-silicate glasses containing 5 – 40% SiO2.

REFERENCES

1. Yu. E. Pivinskii and A. G. Romashin, Quartz Ceramics [in Rus-sian], Metallurgiya, Moscow (1974).

2. F. Ya. Borodai, E. I. Suzdal’tsev, and M. A. Suslova, “Rheologi-cal properties of aqueous quartz glass suspensions containing ad-ditions of high-melting oxides and compounds,” in: Heat-Resis-tant Inorganic Materials. Collection of Research Papers, Issue 5[in Russian], ONTI, NITS, Moscow (1977), pp. 83 – 86.

262 E. I. Suzdal’tsev and T. I. Rozhkova

0 10 20 30 40 50 60 70 80 90 100

1240°C

1200°C

1220°C

SiO content, %2

120

110

100

90

80

70

60

50

40

Str

eng

th,

MP

a

Fig. 4. Ceramic bending strength measured as a function of the

SiO2 content at temperatures 1200, 1220 and 1240°C.

0 10 20 30 40 50 60 70 80 90 100

1240°C

1200°C

1220°C

SiO content, %2

7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

Die

lect

ric

const

ant

Fig. 5. Ceramic dielectric constant measured as a function of the

SiO2 content at temperatures 1200, 1220 and 1240°C.