Steatite Ceramics with Specified Electrical Properties for the Stator of an Electrostatic Generator
Abstract Generated abstract
This study examines steatite ceramics as a mechanically stronger and more stable replacement for lime-sodium glass in the weakly conducting stator of an electrostatic generator. The authors prepared several steatite bodies by varying talc, clay, and barium carbonate contents, with particular attention to how the calculated composition of the barium aluminosilicate glass phase affects volume resistivity and related dielectric properties. Measurements of density, strength, electrical breakdown, resistivity, dielectric losses, permittivity, and thermal expansion showed that changing the glassy phase can regulate the electrical properties over the required range. Composition C-III most closely met the specified stator requirements, with suitable resistivity, dielectric behavior, mechanical strength, and thermal expansion.
Full Text
STEATITE CERAMICS WITH SPECIFIED ELECTRICAL PROPERTIES FOR THE STATOR OF AN ELECTROSTATIC GENERATOR
P. G. USOV, V. N. GURINA, Yu. I. ALEKSEEV
(Presented by the scientific seminar of the Department of Silicate Technology)
One of the principal working elements of an electrostatic generator (ESG) is a weakly conducting stator [1, 2], which serves to impose a definite law of voltage distribution between the inductors. The properties of the stator material must meet specific requirements (Table 1). As the material for such a stator, ordinary lime–sodium glass is most often used [2]; however, the presence of glass parts in the design of the unit does not ensure sufficiently reliable operation: glass has increased surface electrical conductivity at an ambient temperature below \(100\text{–}120^\circ\mathrm{C}\) and is readily subject to destruction under machine vibrations. In connection with these and a number of other factors, this work investigated the possibility of replacing the glass-like stator material with a ceramic one that would provide the necessary set of stable electrical properties and possess high mechanical strength.
Steatite ceramics were chosen as the object of investigation; according to the lower limit permitted by GOST 5458–64, they meet the requirements imposed on the electrical properties. At present, however, the radio-ceramics industry produces ceramics with improved electrical properties, exceeding the GOST requirements in all indicators (Table 1).
Thus, the aim of the work was to obtain steatite ceramics with specified values of the electrical parameters.
It is known that the properties of a ceramic material composed of crystalline and glassy phases are determined mainly by the composition
Table 1
Basic requirements for the stator material and properties of steatite ceramics
| Properties | Required values | Steatite ceramics according to GOST 5458–64 | Steatite ceramics, actual |
|---|---|---|---|
| Specific volume resistance, \(\rho_v\), \(\Omega\cdot\mathrm{cm}\) at \(20^\circ\mathrm{C}\) | \(10^{12}\div10^{14}\) | not less than \(10^{12}\) | \(10^{16}\) |
| Electric strength, \(E_{\mathrm{pr}}\), kV/mm | 20–25 | not less than 20 | 25–30 |
| Dielectric permittivity, \(\varepsilon\) | 6 | not more than 7.5 | 6.9 |
| Compressive strength limit, \(\sigma_{\mathrm{comp}}\), kg/cm\(^2\) | 2000 | 3000 | |
| Coefficient of linear thermal expansion, \(\alpha\), \(1/\mathrm{deg}\cdot10^6\) | 5–8 | 6–8 | 6 |
Table 2
Chemical composition of the starting materials, wt %
| Component | SiO₂ | MgO | Fe₂O₃ | Al₂O₃ | CaO | BaO | TiO₂ | K₂O | Na₂O | LOI | Σ |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Onot talc, “Extra” grade | 62.02 | 31.64 | 0.45 | 0.45 | 0.42 | — | — | — | — | 4.78 | 99.76 |
| Raw Onot talc with elevated impurity content | 60.12 | 31.56 | 1.09 | 0.86 | 0.77 | — | — | — | — | 5.56 | 99.96 |
| Chasov-Yar clay | 50.76 | 0.63 | 1.16 | 32.38 | 0.84 | — | 1.16 | 3.86 | — | 9.26 | 100.05 |
| BaCO₃ | — | — | — | — | — | 77.66 | — | — | — | 22.34 | 100 |
and the amount of amorphous phase; the properties of the crystalline phase are practically constant and, as a rule, considerably higher than the properties of the glass.
The glassy phase of steatite ceramics consists of the decomposition products of the clay mineral and a flux, for which barium carbonate is used (Table 2). The electrical properties of glasses are in a complex dependence on composition and are difficult even to calculate approximately; therefore, selection of glass-phase compositions for obtaining ceramics with specified properties is experimental in nature and is based on general ideas about the influence of the composition and ratio of oxides of different groups in the glass. Several batch compositions with different contents of the clay component and barium carbonate were investigated. “Extra” grade talc, used in the production of steatite ceramics, was replaced by talc with an increased content of impurities (Table 2). The batch compositions selected for investigation are given in Table 3.
Table 3
Batch compositions of the investigated bodies, wt %
| Component name | Industrial composition | C-I | C-II | C-III |
|---|---|---|---|---|
| Raw Onot talc | 38 | 80 | 80 | 75 |
| Fired Onot talc | 42 | — | — | — |
| Chasov-Yar clay | 5 | 5 | 10 | 15 |
| Barium carbonate | 15 | 15 | 10 | 10 |
As the principal controlled electrical property of the body, the value of the specific volume resistivity \((\rho_v)\) was adopted. The volume resistivity of a dielectric is less subject to the influence of random factors (the presence of hydrate films on the surface, the possibility of oxidation–reduction reactions occurring with the surrounding gaseous medium) than the surface resistivity \((\rho_s)\).
In barium aluminosilicate glasses, which constitute the glass phase of steatite ceramics, electrical conductivity is caused mainly by the presence of impurities, among which the most significant role belongs to alkalis and oxides of variable valence \((\mathrm{FeO}, \mathrm{Fe_2O_3})\). Cations of the alkaline-earth group \((\mathrm{Ba}^{++})\) practically do not participate in the transfer of current in solids; their positive effect consists in “suppressing” the activity of alkalis, which provide ionic conductivity in glasses. In alkali-free glasses, a sharp decrease in volume resistivity is caused by the presence of iron oxides as a result of strongly pronounced electronic conductivity. When alkalis and iron oxides are present together, electronic conductivity decreases; however, in the presence of \(\mathrm{Al_2O_3}\) this action of the alkalis is “neutralized” [3].
The data presented indicate a complex mutual influence of the components on the electrical properties of glasses, one that is not amenable to precise prediction. Electrons and ions participate in the mechanism of current transport in glasses; the main factor in this is the concentration of ions and their ionic radius. In this connection, for analysis of the glass-phase composition of the investigated bodies it was decided to express it in mole percent (Table 4).
From the data given in Table 4 it follows that the change in the content of oxides responsible for the electrical conductivity of the glassy phase occurs mainly at the expense of the alkali ion \(\mathrm{K}^+\). The content of iron oxides is maintained practically constant, since under the existing technology the ceramics are fired in an oxidizing atmosphere, and
Table 4
Chemical composition of the glass phase of the investigated batches, mol. % (calculated)
| Composition | SiO₂ | TiO₂ | CaO | Fe₂O₃ | Al₂O₃ | K₂O | BaO | Total |
|---|---|---|---|---|---|---|---|---|
| Industrial | 68.79 | 0.24 | 0.22 | 0.45 | 6.12 | 0.57 | 23.57 | 99.96 |
| C-I | 63.7 | 0.24 | 3.61 | 1.72 | 6.88 | 0.95 | 23.00 | 100.1 |
| C-II | 68.8 | 0.38 | 3.36 | 1.65 | 10.5 | 1.65 | 13.66 | 100.0 |
| C-III | 68.2 | 0.50 | 2.90 | 1.46 | 12.8 | 2.26 | 11.90 | 100.02 |
To create electronic conductivity due to iron oxides, the joint presence of Fe₂O₃ and FeO in an Fe⁺⁺⁺/Fe⁺⁺ ratio of less than 4.6 [3] is necessary, i.e., firing of the ceramic in a reducing atmosphere.
The ceramic was produced by sintering technology; specimens for investigation were pressed in steel press molds under a specific pressure of 750 kg/cm² from powder plasticized with paraffin with the addition of surface-active substances. The specimens were subjected to biscuit (in packing) and final firings. The microstructure of the ceramics of all compositions is characterized by sufficient homogeneity.
The crystalline phase is represented by protoenstatite with an average grain size of 2–6 μm. The glass uniformly surrounds the protoenstatite grains and does not form separate accumulations. The ratio of the crystalline and glassy phases of the investigated ceramic compositions lies within the range (6.5–7):(3–3.5). The properties of the ceramics, studied on standard specimens by methods corresponding to the requirements of GOST 5458–64, are given in Table 5.
Table 5
Properties of the ceramics of the investigated compositions
| Properties | Industrial | C-I | C-II | C-III |
|---|---|---|---|---|
| Firing temperature, °C | 1280 | 1280 | 1270 | 1260 |
| Bulk density, g/cm³ | 3.1 | 3.0 | 2.8 | 2.7 |
| Water absorption, % | 0 | 0 | 0 | 0 |
| Compressive strength limit, δcomp, kg/cm² | 3000 | 2200 | 2600 | 2500 |
| Electrical strength, $E_{\mathrm{br}}$, kV/mm | 25–30 | 23 | 22 | 21.5 |
| Specific volume resistivity, $\rho_v$, ohm·cm, at ($t^\circ$C) 100 | $0.9 \cdot 10^{16}$ | $2.3 \cdot 10^{15}$ | $2.0 \cdot 10^{14}$ | $3.1 \cdot 10^{13}$ |
| Specific volume resistivity, $\rho_v$, ohm·cm, at ($t^\circ$C) 200 | $6.1 \cdot 10^{15}$ | $0.5 \cdot 10^{14}$ | $1.4 \cdot 10^{13}$ | $3.0 \cdot 10^{12}$ |
| Dielectric loss tangent, $\tan\delta \cdot 10^{-4}$, at a frequency of 1 MHz, at ($t^\circ$C) 20 | 4 | 5 | 4.8 | 5.1 |
| Dielectric loss tangent, $\tan\delta \cdot 10^{-4}$, at a frequency of 1 MHz, at ($t^\circ$C) 100 | 7 | 6 | 7.5 | 6.2 |
| Dielectric loss tangent, $\tan\delta \cdot 10^{-4}$, at a frequency of 1 MHz, at ($t^\circ$C) 200 | 8 | 7 | 9 | 8 |
| Dielectric constant, $\varepsilon$, $f = 1$ MHz, at 20°C | 6.0 | 5.9 | 6.1 | 6.0 |
| Coefficient of thermal expansion CTE $\cdot 10^{-6}$, 1/deg, ($t^\circ$C) 20–200 | 7.2 | 6.1 | 6.0 | 6.2 |
From an analysis of the data presented in Table 5, it follows that the ceramic composition C-111 most fully satisfies the stated requirements. The value of the volume resistivity parameter of this ceramic composition, obtained experimentally, agrees satisfactorily with that calculated by formula [3]:
\[ \rho_v = A_\rho \cdot e^{\frac{U_\rho}{2KT}} \]
where \(\rho_v\) is the volume resistivity, ohm·cm;
\(K\) is the Boltzmann constant;
\(T\) is the absolute temperature, °K.
\(A_\rho, U_\rho\) are constants for the given property and the given glass. For composition C-111 the temperature dependence is determined by the constants \(A_\rho = 10^7\); \(U_\rho = 15.1 \cdot 10^{-20}\).
Thus, the possibility has been established of regulating the electrical properties of steatite ceramics by varying the composition of the glassy phase. A ceramic has been obtained whose composition and properties satisfy the requirements for the stator of an electric generator.
REFERENCES
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A. P. Kononov. Investigation of a disk-type electrostatic generator with conducting conveyors. Dissertation, Tomsk, 1967.
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V. I. Levitov, A. G. Lyapin. Electrostatic Generators with a Rigid Rotor. Parts I and II, TsINTI Elektroprom, Moscow, 1963, 1965.
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A. A. Appen. Chemistry of Glass. Leningrad, Khimiya Publishing House, 1970.