Steatite Ceramics with Tunable Dielectric Properties
Abstract Generated abstract
This study examines steatite ceramic compositions intended for high-voltage devices, particularly electrostatic generators requiring dielectric materials with controlled volume resistivity. The authors modified talc-based ceramic batches by changing the glass-phase composition, replacing barium oxide with calcium oxide and adding alkali oxides, then evaluated sintering behavior, phase composition, microstructure, and electrical and mechanical properties. The fired materials were found to contain protoenstatite crystals of about 2 to 5 micrometers with uniformly distributed glassy interlayers, and changes in the glass phase produced substantial differences in specific volume resistivity while largely preserving other useful properties. The results indicate that steatite ceramics with tunable conductivity can be obtained through raw-material selection and alkali oxide additions, with tested compositions showing promise for electrostatic generator components and other high-voltage applications.
Full Text
Steatite Ceramics with Tunable Dielectric Properties
P. G. Usov, V. N. Gurina, Yu. I. Alekseev
The development of dielectric materials with tunable electrophysical properties is one of the current scientific and technical problems /1/. The efforts of researchers engaged in obtaining new materials are directed toward achieving either maximum values of electrophysical parameters \((\rho_v, \rho_s)\) or minimum values \((\tg \delta)\) /2/. At the same time, a number of technical problems can be solved on the basis of dielectric materials that, in terms of their properties, occupy an intermediate position.
For example, rotary electrostatic generators (ESG), known as sources of high direct voltage, have two basic elements: a rotor and a stator. To ensure normal operation of the machine, they must differ in volume resistivity by two to three orders of magnitude, with the volume resistivity of the stator material being within the range \(10^{10} - 10^{13}\ \Omega\cdot\text{cm}\) /3/.
At present, organic polymer insulation (epoxy resins) and inorganic insulation (glass) are used for manufacturing the rotor and stator. These dielectric materials possess fairly high electrophysical properties at a temperature of \(20^\circ\text{C}\). When the temperature rises, which may sometimes occur in the working volume of the machine, their properties can differ significantly from the initial ones. In addition, the design of high-power ESGs (1–10 kW) is associated both with improving the materials used and with searching for new materials that stably retain their initial properties during operation in an environment, for example, of corona discharge.
In connection with this, the present work considers the problem of obtaining ceramic materials with increased values of electrical conductivity for their use in the design of ESGs. The object of the study chosen was steatite ceramics, which have become widely used as an insulating material in high-frequency engineering and in high-voltage apparatus. In the production of steatite ceramics, a relatively inex—
deficient raw material (talc, clay); they contain a small amount of oxides of the alkali group and of metal oxides of variable valence (\(Fe_2O_3\), \(TiO_2\)) (Table I), which ensures a high value of volume resistivity and low dielectric losses in the products.
Table I
Chemical composition of the starting materials, wt.%
| Component | \(SiO_2\) | \(MgO\) | \(Al_2O_3\) | \(TiO_2\) | \(Fe_2O_3\) | \(CaO\) | \(BaO\) | \(Na_2O\) | \(K_2O\) | \(SO_3\) | L.O.I. |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Onot talc, grade “Extra”* | 62,02 | 31,64 | 0,45 | – | 0,45 | 0,42 | – | – | – | – | 4,78 |
| Raw Onot talc | 59,64 | 27,51 | 5,25 | – | 1,05 | 0,91 | – | – | – | – | 6,10 |
| Enriched Onot talc | 63,21 | 29,16 | 5,56 | – | 1,11 | 0,98 | – | – | – | – | – |
| Chasov-Yar clay | 50,72 | 0,63 | 32,38 | 1,16 | 1,11 | 0,86 | – | – | 3,86 | – | 9,26 |
| Oglanly bentonite | 71,28 | 3,50 | 16,30 | 0,28 | 1,56 | 2,03 | – | 2,33 | 0,50 | 0,56 | 5,25 |
| \(CaO\) | – | – | – | – | – | 100 | – | – | – | – | – |
| \(Na_2CO_3\) | – | – | – | – | – | – | – | 58,50 | – | – | 41,50 |
| \(BaCO_3\) | – | – | – | – | – | – | 77,66 | – | – | – | 22,34 |
* taken for comparison
In the investigated steatite compositions, the raw-material composition was selected on the basis of increasing the amount of glass phase in the ceramic and changing its composition in comparison with industrial material. In this connection, barium oxide in the composition of the glass phase was replaced by \(CaO\), and an additional amount of \(Na_2O\) was introduced (Table 2).
According to literature data /4,5/, the electrical conductivity of steatite ceramics is mainly ionic in character and depends on the composition of the glassy phase. A theoretical analysis of the chemical composition of the investigated bodies (Table 3) shows that the content of oxides of variable valence, \(TiO_2\) and \(Fe_2O_3\), remains practically unchanged and, consequently, the ionic character of the electrical conductivity
may be due to an increase in the amount of the oxides \(Na_2O\) and \(K_2O\) in the initial batch.
Table 2
Batch composition of the materials studied, wt. %
| Component | Industrial | C-IV | C-U | C-UI | C-UP |
|---|---|---|---|---|---|
| Raw Onot talc | 38 | ||||
| Calcined Onot talc | 42 | 86 | 86 | 76,18 | 80 |
| Chasov-Yar clay | 5 | 10,75 | 10 | ||
| \(BaCO_3\) | 15 | 10 | |||
| \(CaO\) | 3,25 | 3,25 | 2,96 | ||
| Oglanlin bentonite | 10,75 | 9,76 | |||
| \(Na_2CO_3\) | 9,1 |
Table 3
Chemical composition of the glass phase of the investigated bodies
(calculated)
| Composition | \(SiO_2\) | \(TiO_2\) | \(CaO\) | \(Fe_2O_3\) | \(Al_2O_3\) | \(Na_2O\) | \(K_2O\) | \(BaO\) |
|---|---|---|---|---|---|---|---|---|
| Industrial | 48,50 | – | 1,14 | 0,86 | 7,35 | – | 0,64 | 42,50 |
| C-IV | 60,96 | 0,35 | 11,58 | 2,98 | 23,00 | – | 1,15 | – |
| C-U | 65,60 | 0,08 | 12,15 | 3,32 | 18,08 | 0,63 | 0,13 | – |
| C-UI | 51,76 | 0,08 | 11,64 | 2,98 | 16,84 | 16,56 | 0,13 | – |
| C-UP | 53,56 | 0,28 | 2,21 | 2,61 | 20,03 | – | 1,02 | 20,29 |
Analysis of the properties of the glass phase on the basis of the phase diagrams of the systems \(BaO - Al_2O_3 - SiO_2\), \(CaO - Al_2O_3 - SiO_2\), and \(Na_2O - Al_2O_3 - SiO_2\) showed that the glass phase of composition C-UI has the widest melting interval (about \(300^\circ C\)) /6/.
Articles and specimens were formed by slip-casting technology, by pressing powders plasticized with paraffin.
Microscopic studies of the fired materials, carried out jointly with X-ray phase analysis, made it possible to conclude that, during sintering of specimens from composition S–UI, active physicochemical processes occur already at a temperature of \(600^\circ\text{C}\). The sintering interval is in the range \(1150–1165^\circ\text{C}\).
The vacuum density at the maximum bulk density of specimens made from masses of the other compositions considered was attained at firing temperatures of \(1210–1310^\circ\text{C}\).
The crystalline form of the obtained materials is represented by protoenstatite with crystal size of \(2–5\ \mu\text{m}\). The glass is distributed uniformly, in the form of thin interlayers between the crystals.
The properties of the materials studied were determined on specimens by methods corresponding to the requirements of GOST 5458–64.
Vacuum-tight steatite ceramics based on Onot talc with an elevated impurity content, without additional introduction of oxides (composition S–UP), possess fairly good electrophysical properties and high mechanical strength, and may be used as a material for manufacturing an ESC horn, as well as in a number of other high-voltage devices. The other compositions (Table 4) are characterized by a noticeable increase in electrical conductivity with little change in other properties.
Ceramics of composition S–IV were tested in the design of an ESC; positive results were obtained.
Conclusions
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It has been shown experimentally that the value of the specific volume resistivity of ceramics can be regulated by varying the raw-material composition of the batch and by introducing an additional amount of oxides of the alkali group.
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Methods of experimental analysis have established that the principal crystalline phase of the completely sintered articles of the compositions considered is protoenstatite with crystal size of \(2–5\ \mu\text{m}\); consequently, the phase most sensitive to changes in specific volume resistivity is...
Table 4
Electrophysical and physicomechanical properties of the obtained ceramic materials.
| Property name | C-IV | C-V | C-VI | C-VII |
|---|---|---|---|---|
| Firing temperature, °C | 1300 | 1210 | 1160 | 1290 |
| Bulk density, g/cm³ | 2,68 | 2,74 | 2,58 | 2,8 |
| Water absorption, % | 0,01 | 0 | 0,01 | 0,02 |
| Compressive strength, kg/cm² | 4,700 | 4,780 | 3,600 | 4,400 |
| Coefficient of thermal expansion, CTE \(10^{-1}, 1/^\circ\mathrm{C}\), in the temperature range 20–800°C | 7,0 | 6,75 | 8,7 | 7,3 |
| Specific volume electrical resistivity \(\rho_v\), ohm·cm, at temperature (°C): 20 | \(6{,}15 \cdot 10^{13}\) | \(5{,}9 \cdot 10^{13}\) | \(2{,}3 \cdot 10^{11}\) | \(2{,}4 \cdot 10^{14}\) |
| Specific volume electrical resistivity \(\rho_v\), ohm·cm, at temperature (°C): 100 | \(6{,}15 \cdot 10^{13}\) | \(5{,}8 \cdot 10^{13}\) | \(4{,}8 \cdot 10^{10}\) | \(1{,}7 \cdot 10^{14}\) |
| Specific volume electrical resistivity \(\rho_v\), ohm·cm, at temperature (°C): 200 | \(1{,}94 \cdot 10^{12}\) | \(1{,}5 \cdot 10^{12}\) | \(7{,}4 \cdot 10^{8}\) | \(1{,}2 \cdot 10^{14}\) |
| Specific volume electrical resistivity \(\rho_v\), ohm·cm, at temperature (°C): 300 | \(7{,}7 \cdot 10^{11}\) | \(3{,}9 \cdot 10^{11}\) | \(6{,}2 \cdot 10^{6}\) | \(1{,}1 \cdot 10^{14}\) |
| Tangent of the dielectric loss angle \((\mathrm{tg}\,\delta \cdot 10^4)\) at a frequency of 1 MHz, at temperature (°C): 20 | 17,9 | 26 | 86 | 12 |
| Tangent of the dielectric loss angle \((\mathrm{tg}\,\delta \cdot 10^4)\) at a frequency of 1 MHz, at temperature (°C): 100 | 42,8 | 71 | 243 | 14 |
| Tangent of the dielectric loss angle \((\mathrm{tg}\,\delta \cdot 10^4)\) at a frequency of 1 MHz, at temperature (°C): 200 | 252 | 123 | 1310 | 23 |
| Relative dielectric permittivity \((E')\) at a temperature of 20°C and frequency \(f = 1\) MHz | 6,3 | 6,1 | 6,4 | 6,3 |
glassy phase of the ceramic.
-
A selection of compositions has been made, along with a theoretical and experimental assessment of the technological parameters and properties of the products obtained.
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The results of the work may be used in selecting materials for the design of high-voltage devices.
References
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I. A. Makarov. Synthesis of corundum ceramics with specified electrical properties. Author’s abstract of dissertation, Moscow, 1969.
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A. Yu. Borisova et al. Electrotechnical steatite ceramic materials. Department of VNIISM for Scientific and Technical Information, Standardization and Normalization in Electrical Engineering. Moscow, 1968.
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V. I. Levitov. Electric machines with a rigid rotor, Part I, II. Moscow, 1963, 1965.
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Kh. S. Valeev. Electrical properties of steatite materials at elevated temperatures. “Elektrichestvo,” 4, 1955.
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B. M. Tareev. Physics of dielectric materials. Energiya Publishing House, Moscow, 1973.
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V. Eitel. Physical Chemistry of Silicates. Foreign Literature Publishing House, Moscow, 1962.
AMALGAM–POLAROGRAPHIC DETERMINATION OF IMPURITIES
\(Zn, Cd, Pb, Cu\) AND \(Bi\) IN NIOBATES, TANTALATES OF ALKALI METALS AND PENTOXIDES OF NIOBIUM AND TANTALUM
Z. S. Mikhailova, L. F. Zaichko, A. A. Kaplin
Niobium, tantalum, and their compounds are among the most difficult analytical objects, especially when determining small amounts of impurities in them. The sensitivity of the spectral and chemical-spectral methods known in the literature for determining heavy metals in niobium, tantalum, and their compounds is \(1 \cdot 10^{-4} - 5 \cdot 10^{-5}\%\). In work /1/, for the first time, for the determination of lead, antimony, and bismuth in \(Nb_2O_5\) with a sensitivity of \(5 \cdot 10^{-5} - 5 \cdot 10^{-4}\%\), the method of amalgam polarography with accumulation (APA) was applied. The aim of the present work was to expand the range of elements (\(Zn, Cd, Cu\)) determined in niobium, tantalum, and their compounds by the APA method and to increase the sensitivity of determination of these elements.
In the work a polarograph OH-102 was used, a cell with inserted quartz cups, the indicator electrode was a mercury-film electrode, and the reference electrode was a saturated calomel electrode. As solvents, on the basis of literature /1,2/ and our data, mixtures of acids were used: \((HF + H_2SO_4)\) or \((HF + HNO_3)\). Increasing the sensitivity of determining the elements without separating the main component /1/ does not appear possible because of the exceptional