Ceramic and Glass Insulations

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15Ceramic and Glass Insulations

INTRODUCTION 419TECHNOLOGY 419CHEMISTRY 419GRADES 421PROCESSING 422ASTMSTANDARDS FOR ELECTRICAL INSULATING

CERAMICS 422PROPERTIES 426

Thermal Properties 426Physical Properties 428Dielectric Properties 429

SUMMARYOF PROPERTIES AND USES 430DEVELOPMENT PROGRAMS 432MARKET TRENDS 432

417


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Chap. 15 Ceramic and Glass Insulations 419

INTRODUCTION

Ceramics and glasses are among the oldest dielectrics used in electrical equipment andin the transmission and distribution ofpower. A variety of ceramic and glass materialsis available from several manufacturers, including the following:

MATERIALSAluminaAluminum nitrideAluminum silicateBerylliaBoron nitrideCordieriteDiamondMagnesiaPorcelainSapphireSilica (fused)SteatiteZircon

SUPPLIERSAlcoa, Kaiser, Ormet, Reynolds, BrushWellman, StarKaramontSaxonburgBrush WellmanGeneral Electric (BorazonTM), Carborundum (Combat?')Saxonburg, Star, D.M. StewartNortonDowChemicalSouthern Porcelain, Star, CoorsCrystal Systems, TycoLaboratories, Union CarbideAmersil, Corning Glass, General Electric, Raytheon, Thermal American,U.S. Fused QuartzD.M. Stewart, Saxonburg, StarCorning Glass, 3M

This chapter covers ceramics and glasses used for electrical insulation other thanglass-bonded mica (see "Glass-Bonded Mica," p. 412 of Chapter 14, Mica and MicaProducts) and substrates for printed circuits (see "Substrates for Printed Circuits,"p. 285 of Chapter 9, Unclad and Clad Structures).

TECHNOLOGYCeramic structures are characterized by outstanding resistance to normal aging and theability to withstand high operating temperatures, heat shock, and electrical degradation. However, it is to be noted that the structural strength of ceramics holds to muchhigher temperatures than do dielectric properties. The point at which there is a rapidincrease in dielectric loss is referred to as the dielectric transition temperature. Abovethis temperature, dielectric breakdown occurs rapidly, typically accompanying localized heating and the establishment of high thermal gradients within the ceramic body,which may cause structural destruction.

CHEMISTRYEarly ceramics were composed of natural clays (hydrated aluminum silicates) with thegeneralized formula A1203S i02 xH20. Several materials used as dielectrics are now. classed as ceramics. In general , they are crystalline structures bonded together by asmaller amount of a vitreous matrix fanned during the firing process. The ratio of crys-


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420 Handbook of Electrical and Electronic Insulating Materialstalline and amorphous phases significantly affects the dielectric and physical properties of the ceramic. The formation and rate of growth of the crystalline phase is determined by the temperature versus time profile of the fir ing operation. The fir ingtemperature for most ceramics is in the range of 1,200 to 1,700C (2,192-3,092F).By contrast, glass is technically an amorphous liquid of extremely high viscosity, composed of silica (75 percent), soda ash (20 percent), and lime (5 percent), often combined with metallic oxides such as those of calcium, lead, lithium, and cerium. Theprincipal ceramics used in electrical insulations include:

Alumina, or aluminum oxide (AI2 0 3) , occurs naturally as the mineral corundum and in hydrated form in bauxite. Virtually all commercially produced aluminainvolves a caustic leach of the bauxite at elevated temperature and pressure. Lime isadded to control the phosphorus content and to increase the solubility of alumina. Theresulting slurry, 40-50 percent solids, is pumped to pressurized digesters, where highpressure steam is used to raise the temperature. Alumina and some of the silica are dissolved during this step, soluble sodium aluminate is formed, and a complex sodiumaluminum silicate is precipitated. The resulting slurry containing sodium aluminate insolution is cooled to atmospheric boiling temperature, and a coarse sand fraction isremoved by gravity separators or wet cyclones. The liquor is clarified and cooled untilit becomes supersaturated, and then is seeded with crystals of alumina trihydrate. Alumina is precipitated as the trihydrate. It is then filtered and washed, after which it iscalcined (heated below the melting point) to drive of f the water or hydration. Highgrade ceramic shapes with melting point of2,040C (3,700F) are made from aluminacalcined at 1,480 to 1,590C (2,7000-2,900F).

Aluminum nitride (AIN) powders are sintered at very high temperatures innitrogen atmosphere to avoid its oxidation to alumina. Aluminum silicate ceramics contain varying amounts of alumina (AI2 0 3) andsilica (Si02) . They are made synthetically by heating aluminum fluoride at 1,000 to1,200C (1,8300-2,200F) with silica and water vapor. Beryllia, or beryllium oxide (BeO), is derived from beryl (Be3AI2Si601s), theore of beryllium. the ore is wet-ground and screened to form a slurry that is transportedto a processing plant, where it is leached with a 10 percent sulfuric acid solution. Theinsoluble residue is filtered and discarded. The remaining beryllium sulfate solution istreated by a solvent-extraction process using di-2-ethylhexyl phosphoric acid inkerosene. The beryllium in solution is subsequently converted to beryllium hydroxide,which is then calcined to form beryllia. Beryllium is the lightest structural metal

known. The Occupational Safety and Health Administration (OSHA) has designatedberyllium and its compounds highly toxic with a worker exposure limit of 0.002 milligram per cubic meter of air for an 8 hour time-weighted average. Boron nitride (BN) is formed at very high pressure (85,000 atmospheres) andtemperatures (l,800C, 3,270F) from mixtures of boron and nitrogen, or from hexagonal boron nitride in the presence of catalysts such as lithium, calcium, magnesium, ortheir nitrides. Boron occurs in nature as sodium borate (borax), Na2B40 , 10 H20 .Boron is derived by several processes, including reduction by heating boric oxide withpowdered metals (sodium, magnesium, aluminum); by vapor phase reduction of boron


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Chap. 15 Ceramic and Glass Insulations 421trichloride with hydrogen over hot filaments; and by electrolysis of fused salts. Borondust ignites spontaneously in air, presenting a severe fire and explosion hazard.

Cordierite is a naturally occurring magnesium aluminum silicate (2 MgO . 2A12 0 3 5 Si02) . Diamond films are produced by mixing methane and hydrogen in a plasmaenvironment and depositing pure diamond films on a substrate.

Magnesia, or magnesium oxide (MgO), is produced by calcining magnesiumcarbonate or magnesium hydroxide; by treating magnesium chloride with lime andheating; and from sea water via the hydroxide. Two forms are produced. A light, fluffymaterial is prepared by a relatively low-temperature dehydration of the hydroxide. Theother form is a dense material made by high-temperature furnacing of the oxide afterit has been formed from the carbonate or hydroxide. Magnesia is toxic by inhalationof fumes. OSHA has placed a worker exposure limit of 15 milligrams per cubic meterof air for an 8 hour time-weighted average.

Porcelain, or potassium aluminum silicate (4 K2 0 Al2 0 3 3 Si02) , is formedby sintering mixtures of clays, quartz, and feldspar (potassium alumino-silicate). Ultraporcelain, the highest mechanical and dielectric grade, is composed of aluminum oxide,barium oxide, and clay. Quartz, or crystallized silicone dioxide (silica, Si02) , is produced by crystalgrowth from molten silica under carefully regulated conditions. For a discussion ofcrystal growth, see "Semiconductor Silicon" (p. 286) in Chapter 9, Unclad and CladStructures. Fused silica is obtained by the electric-arc fusion of high-purity quartz sand.Ingots thus formed are then crushed to silica grains for casting and kiln bonding. Sapphire, or corundum (AI 20 3) , is a crystalline variety of alumina. Purified,synthetic sapphire is made by crystal-growing techniques. For a discussion of these

techniques, see "Semiconductor Silicon" (p. 286) in Chapter 9, Unclad and Clad Struc-tures. In a proprietary EFG process, Saphikon (a Division of Tyco Laboratories)produces continuous lengths of sapphire crystal in almost any cross-sectional configuration from furnaces operating at almost 2,200C (4,OOOOP). Steatite is a mixture of hydrous magnesium silicate, clay, and alkaline oxides. Zircon, or zirconium silicate (ZrSi04, or Zr02 Si02) , used in ceramics is apurified form of a naturally occurring mineral.A new process for making ceramic multilayer circuit boards is based on co-firingalumina layers. Du Pont offers a system called Green Tape. Mixtures of alumina, glass,and other powders are combined with a plastic and a solvent, producing an uncuredflexible tape. Tapes are cut, punched, and etched, and then holes are interconnected.The stack is pressed in a plastic press to make a multilayer laminate. The laminate isthen fired at 800C (1,472F) which fuses the ceramic and burns out the plastic.

GRADESGrades are available with varying degrees of basic mineral content with over 99.99 percent purity for single crystals such as quartz, and with up to 99.9 percent purity forother ceramic powders. Manufacturers of ceramic parts, including custom fabricators,


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422 Handbook of Electrical and Electronic Insulating Materialsoften formulate compositions to meet their own special requirements by blendingceramic powders with binders and other additives.

PROCESSINGCeramic powders are usually wet blended with enough water to wet the particles.Excess water causes the slurry to settle and should be avoided. It is important to usedeionized water and lined tanks to minimize powder contamination. The slurry maythen be vibro-energy-milled using ultrasonics, or ball-milled. This process breaks upagglomerates and produces a narrow size range of discrete particle,. After milling,lubricants and organic binders, such as sodium lignum sulfonate, may beadded duringagitation. Moisture may then be completely removed by spray drying. The resultingparticles flow freely to fill dies evenly with minimum voids and pits. The "green" partsthus obtained are then sintered, or fused, in high-temperature electric kilns with precise process controls. Organic components are burned out during this operation.A wet process commonly used to manufacture large ceramic pieces is similar tothat above, but omits the organic lubricants and binders. A portion of the water in theslurry is removed by filter press to make a put ty or paste. The put ty or paste is thenformed into the desired shape, and subsequently dried to a sufficiently low moisturecontent so that it can be kiln fired without cracking.In a simpler process, the ceramic powder is mixed with just enough water tomake a stiff paste or putty, after which the desired shape is formed, dried, and fired athigh temperature with exposure to air.A "dry" process uses less water and produces parts with good dimensional properties. Mechanical and dielectric properties, however, are inferior to those obtained by

the wet process.During the kiln firing, crystalline growth proceeds at a rate which depends on thebasic constituents, the temperature, and the duration of firing. Insulator porcelain isfired at 1,250C (2,280F), steatite at 1,400C (2,550F), and alumina and beryllia at1,700C (3,090F).

ASTM STANDARDS FOR ELECTRICALINSULATING CERAMICS

The American Society for Testing and Materials has developed the following standardsfor electrical insulating ceramics:

D 116, Vitrified Ceramic Materials for Electrical Appliances. This standardoutlines procedures for testing samples ofvitrified ceramic materials that are to be usedas electrical insulation. They are equally suited for both unglazed and glazed specimens. Test methods are described in Table 15-1. F 356, Beryllia Ceramicsfor Electronic and ElectricalApplications. This specification covers fabricated beryllia parts suitable for electronic and electrical applications. Specified are limits and methods of test for electrical, mechanical, thermal, and


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Chap. 15 Ceramic and Glass InsulationsTABLE 15-1 Property Tests and Test Methods forVitrified Ceramic Materials

(ASTM 0 116)

423

PROPERTYMEASUREDCompressive strengthDielectric constant and dissipation factorDielectric strengthElastic propertiesElectrical resistivityFlexural strengthHardnessPorositySpecific gravityT h e n n a l c o n d u c t i v i ~Thermal expansionThermal shock resistance

ASTMMETHODC 773D 2149, D 150, D 2520D 149C 623D 257, D 1829C 674, F 417C 730, E 18C 373C 329, C 20C 408, C 177F 228, C 539D 116

general properties of the bodies used for these fabricated parts. Three types ofmaterials are identified:

TYPEIIIIII

BERYLLIA CONTENTWEIGHT PERCENT

MINIMUM

959899

Requirements for beryllia ceramics are shown in Table 15-2. D 2442, Alumina Ceramics for Electrical and Electronic Applications. Thisspecification covers the requirements for fabricated alumina parts suitable for electronic and electrical applications, and ceramic-to-metal seals as used in electrondevices. Specified are limits and methods of test for electrical, mechanical, thermal,and general properties of the bodies used for these fabricated parts. Four types ofmaterials are identified:

TYPEIIIIIIIV

ALUMINA CONTENTWEIGHT PERCENT

MINIMUM

82939799


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424 Handbook of Electrical and Electronic Insulating MaterialsTABLE 15-2 Requirements for Beryllia Ceramics to beUsed as Electrical Insulations(ASTM F 356)

ASTMTYPE I TYPE II TYPE III METHODElectricalDielectric constant, max, 25C D 150at 1MHz 7.0 7.0 7.0at 10 GHz 6.8 6.8 6.9

Dissipation factor, max, 25C D 150at 1MHz 0.0005 0.0005 0.0004at 10 GHz 0.001 0.001 0.001

Volume resistivity, min, n . em D 257at 25C 1014 lOiS lOiS D 1829at 300C lOll 1012 1012at 500C 1010 lOll lOllat 700C 108 109 109at 900C 106 108 108

Dielectric strength, min, 125 mils D 149volts/mil 300 300 300

MechanicalDensity, g/cnr', min 2.80 2.80 2.85 F 77Flexural strength, min, avg psi 20,000 24,000 25,000 F 417Compressive strength, min, avg psi 200,000 222,000 240,000 C 773Modulus of elasticity, min, psi 44 x 106 45 )( 106 49 x 106 C 623ThermalMean coef. of linear thermalexpansion, J..1II1/mlC E 22825-1,OOOC 9.0 min 8.7 min 9.4 min

10.0 max 9.4 max 9.7 maxThermal conductivity, (min) C408cal/s/cm" C/cmat 25C 0.35 0.52 0.60at 100C 0.30 0.40 0.41at 400C 0.14 0.15 0.15at 800C 0.05 0.05 0.06

Specific heat at 100C C 351cal/gf'C 0.30 0.30 0.31

Thermal shock resistance" pass pass passTo be agreed upon between manufacturer and purchaser.(Reprinted with permission from the Annual Book ofStandards. Copyright, ASTM, 1916 Race Street,Philadelphia, PA 19103.)


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426 Handbook of Electrical and Electronic Insulating MaterialsTABLE 15-4 Requirements for Impervious Steatite Ceramics to BeUsed for Electrical and Electronic Applications(ASTM 0 2757)

ASTMTYPE I TYPE II TYPE III TYPE IV METHODPhysicalSpecific gravity, min 2.5 2.6 2.7 2.7 C20Flexural strength, D 116min avg psi 18,000 18,000 20,000 20,000Dye penetration none none none none D 116Thermal expansion, D 11625-7ooCem/ern C x 10-6 7-10 7-10 7-10 7-10Hardness, Rockwell 45N 45 45 50 SO E 18Impact strength,min, in : lb f 4.0 4.5 5.0 5.5 D 116ElectricalDielectric strength, D 1161/4 in thickvolts/mil min 220 230 230 230Volume resistivity, D 116minD t cmat 25C 1014 1014 1014 1014at 100C 1 x 1012 1 x 1013 5 x 1013 5 x 1013at 300C 1 x 10' I X 10JO 5 X 1010 5 x 1010at 500C 1 x lOS 5 x 10' 1 x 108 1 x 109at 700C 1 x 104 1 x 106 1 x 106 1 x 107at 900C 1 x 103 I x lOS 1 x lOS 1 x 106

Dielectric constant,at 25C, maxat 1 MHz 6.0 6.0 6.2 6.5 D 150at 4 GHz 5.8 5.8 6.0 6.3 D2520Loss index, 25C, maxat 1MHz 0.035 0.016 0.008 0.004 D 150at 4 GHz 0.038 0.018 0.015 0.007 D2520(Reprinted with permission from the Annual Book ofStandards. Copyright, ASTM, 1916 Race Street, Philadelphia,PA 19103.)

PROPERTIESThermal Properties

Thermal conductivity is the principal property that determines the dissipation of localheating. Of all electrical insulating ceramics, beryllia has by far the highest thermalconductivity. Where ceramics are used in conjunction with other materials, it is oftenimportant that rates of expansion be compatible. That is, the coefficients of linear thermal expansion should be as close as possible.Thermal properties of selected ceramic materials are shown in Tables 15-5-15-8.


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Chap. 15 Ceramic and Glass Insulations

TABLE 15-5 Thermal Conductivities of ElectricalInsulating CeramicsTEMPERATURE

427

OF C Btu/(h)(W)(OF/ft)*Beryllia 200 93 104Magnesia 200 93 18Alumina 212 100 16.4Sapphire 200 93 15-16Boron nitride 212 100 13.3-25.5Porcelain not stated 8.2Aluminum silicate 77 25 4.8Quartz 200 93 3.0-5.4Zircon 200 93 2.4Steatite 77 25 1.5Cordierite 77 25 1.2Silica (fused) 1,000 538 0.33*To convert to cal/(s)(cm2)(oC/cm), multiply by 0.00413.Note: Because of variance in composition and processing methods, the actual value for a specific materialin the above table may vary significantly from the value shown.Sources: Handbook ofPhysics and Chemistry and others.

TABLE 15-6 Coefficient of Linear Expansion ofElectrical Insulating Ceramics

MagnesiaAluminum silicateQuartzBeryllia (99% BeO)Alumina (99.9% A12 03)PorcelainSteatiteSapphireZirconCordieriteBoron nitrideSilica (fused)

TEMPERATURE RANGE C25-40025-4000-80not stated25-50025-40025-40025-40025-40025-40025-3500-800

12.88.18.07-97.46.96.64.5-7.04.53.10.18-3.10.55

*To convert to of, divide by 1.8.Note: Because of variance in composition and processing methods, the actual value for a specific materialin the above table may vary significantly from the value shown.Sources: Various.


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428 Handbook of Electrical and Electronic Insulating MaterialsTABLE 15-7 Maximum Operating Temperatures ofElectrical Insulating Ceramics

TEMPERATURE

MagnesiaSapphireZirconAluminaBerylliaAluminum silicateSilica (fused)SteatiteCordieritePorcelainBoron nitride (in oxidizing atmosphere)Quartz

2,2002,0001.8701,7001,7001,4251,3701,3151,3001.200985na

4,0003,6303,4003,1003,1002,6002,5002,4002,3802,2001,805na

Note: Because of variance in composition and processing methods, the actual value for a specific materialin the above table may vary significantly from the value shown.na-not available.Sources: Various.

TABLE 15-8 Thermal Shock Resistanceof Electrical Insulating CeramicsCERAMICAluminaAluminum silicateBerylliaBoron nitrideCordieriteMagnesiaPorcelainQuartzSapphireSilica (fused)SteatiteZircon

RATINGGoodExcellentExcellentExcellentGoodExcellentPoorPoornaExcellentPoorGood

Note: Because of variance in composition and processingmethods , the actual value for a specific material in theabove table may vary significantly from the value shown.na-not available.Sources: Various.

Physical PropertiesParts made of ceramics are characteristically hard, dense, and incombustible. Valuesfor compressive strength are eight-ten times tensile strength values, indicating betterperformance under compression than under tension. Most ceramic bodies have low


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Chap. 15 Ceramic and Glass Insulations 429impact strength and tend to fracture when struck sharply with a hard object, althoughthere are noteworthy exceptions. For example, beryllia can be fabricated into armor forboth vehicles and personnel.Ceramic parts are not affected by long exposure to the atmosphere, even at highoperating temperatures. Densities and mechanical properties of electrical insulatingceramics are shown in Table 15-9.

TABLE 15-9 Densities and Mechanical Properties ofElectrical Insulating CeramicsFLEXURAL COMPRESSIVE TENSILESTRENGTH STRENGTH STRENGTH

DENSITY psi psi psig/crrr'" (000)* (000)* (000)*

Alumina (99.9% A1203) 4.0 41 370 30Aluminum silicate 1.9 3 10 1.5Beryllia (99% BeO) 2.9 34 240 min 21Boron nitride 2.1 oa 34-45 oaCordierite 2.1-2.9 15 50 5.5Magnesia 2.3-2.5 2-10 2-10 oaPorcelain 3.0 12.5 75 8.5Quartz 2.7 na oa naSapphire 4.0 na 450 60Silica (fused) 2.0 1.5 5 naSteatite 2.9 21 90 9Zircon 3.6 4.6 27 2At room temperatureNote: Because of variance in composition and processing methods, the actual value for a specific materialin the above table may vary significantly from the value shown.na-not available.Sources: Various.

Dielectric PropertiesDielectric strengths ofmost electrical grade ceramics are in the range of 100-400 voltsper mil for 125mil specimens. Dielectric constants are in the 4-10 range, with aluminanear the high end of the range and fused silica at the low end. Dielectric constant values are often sensitive to temperature, and may rise, fall, or remain unaffected by temperature changes, depending on the type and density of the ceramic. The dissipation,or loss factor, also sensitive to temperature, is lowest for beryllia, followed by alumina.Moisture accumulating on the surface or penetrating into the body of a ceramicsignificantly degrades all dielectric properties, and may also degrade mechanical properties via surface tension phenomena which can enhance crack propagation in highlystressed ceramics. Dust, or other deposits (as on transmission line insulators), worsensdielectric degradation and becomes conducting with high moisture content. Glazesreduce the ability of dust or other deposits to adhere to ceramic bodies and make theirsurfaces more washable, but they may incur significant power losses. Dielectric greases


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43 0 Handbook of Electrical and Electronic Insulating Materialshave also been used to protect ceramic swfaces in contaminated environments, bu tgreases often require periodic cleaning and replacement.Values for dielectric properties of ceramics are shown in Table 15-10.

TABLE 15-10 Dielectric Properties of Electrical Insulating CeramicsDIELECTRICSTRENGTH DIELECTRIC DISSIPATION125 mils CONSTANT FACTORvolts/mil" I MHz 1 MHz

(ASTM D 149) (ASTM D 150) (ASTM D 150)Alumina (99.9% A12 03) 340 10.1 0.0002Aluminum silicate 150 4.1 0.0027Beryllia (99% BeO) 350 6.4 0.0001Boron nitride 950 4.2 0.00034Cordierite 200 4.8 0.0050Magnesia na 5.4


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Chap. 15 Ceramic and Glass Insu lations 431 Beryllia has the highest thermal conductivity and specific heat of any oxideceramic. Machined parts can be held to a tolerance of O.0002 inch. Exceptionally high

impact strength can be achieved by special processing. Dielectric strength is high anda dissipation factor of 0.0001 is even lower than alumina. Beryllia ceramics are usedextensively for semiconductor packages, and for heat sinks, ceramic-to-metal seals,waveguide windows, radomes, and electrical/electronic components.

Boron nitride is less britt le than most ceramics, has high thermal shock resistance, high thermal conductivity, and a very low rate of thermal expansion. Dielectricstrength is outstanding and the dielectric constant is among the lowest for all ceramics. Mechanical strength and moisture resistance are low unless compounded with40-60 percent silica. Uses include additives to other ceramics to increase thermal conductivity and reduce thermal expansion, transistor and rectifier mounting wafers, andnonabrading electronic parts.

Cordierite has excellent heat shock resistance and fair dielectric properties.Mechanical strength is low. It is suitable for applications where heat shock is severe. Diamond film is a material of exceptionally good properties for electronicsapplications. It has a thermal conductivity five times higher than copper, making it aunique heat sink to reduce chip temperatures. Dielectric properties are among the bestfor most ceramics.

Magnesia has the highest coefficient of thermal expansion of all ceramics.Dielectric properties are good, but mechanical properties are weak. Its main applicat ion is as insulat ion in sheathed heating units and thermocouples. A semiconductorgrade is available.

Porcelain is chemically inert and has satisfactory dielectric properties for applications at power frequencies, where it is widely used. Mechanical properties and thermal shock resistance are low. Glazed porcelain is used extensively for insulation onelectrical transmission lines, circuit breakers, cutouts, and spark plugs. Low voltageuses include switches, bushings, fuse blocks, and light sockets.

Quartz and fused silica possess the lowest dielectric loss properties of any inorganic material. Fused quartz is virtually immune to thermal shock. Thermal expansionis very low. The dissipation factor is among the lowest for ceramics. Physical properties are generally very low. Uses include radomes, nose cones, wave filters, and otherelectronic components. Sapphire has a single crystal surface desirable for integrated circuit and otherelectronic systems. A dielectric constant of 11.5 is the highest of all ceramics. Hightemperature stability is excellent. Uses include electron and microwave tubes, and substrates for integrated circuits. Steatite has good dielectric properties, average mechanical properties, and poorthermal shock resistance. It can be fabricated to close tolerances. Die wear is low.It is suitable for making electronic components where mechanical properties are not demanding. Zircon ceramics offer improved mechanical and dielectric properties over otherporcelains. Thermal shock resistance is also better. The material is abrasive and diewear is high. Uses include stand-ofT insulators, pole lightning arrestors, and applications requiring high arc resistance.


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432DEVELOPMENT PROGRAMS

Handbook of Electrical and Electronic Insulating Materials

Property values of ceramics are significantly affected by changes in the ratios of basicmaterials in the composition, sometimes with synergistic effects. Ofpromise is a blendof sil icon carbide and beryll ia which is being evaluated as an improved material forintegrated circuits.

Development continues on polymer compounds as replacements for ceramics ingeneral use as high-voltage transmission and distribution line insulators. In addition tobeing mechanically suitable, materials must have high resistance to forming carbonizing paths and to erosion under outdoor high-voltage service conditions. Surface degradation of polymer insulator is a combination of surface erosion and interaction withcontaminants. Surface contamination is caused by industrial pollution, wind-blownagricultural chemicals, and salt fog along coastal regions. Contaminated surfaces tendto wet and erode readily. Eventual failure of the insulator is caused by flashover and/ortracking encouraged by wet, contaminated surfaces, as well as by imperfect ionsincurred in the manufacture of the insulator.

Silicone rubber, one of the polymeric candidates for replacing ceramics, hasinherently low arc resistance which, however, is improved to the acceptable level byaddition of alumina trihydrate (ATH) fillers. Ethylene-propylene-diene copolymer(EPDM rubber) with ATH fil lers is also being used to replace ceramic high-voltagepower line insulators. ATH fillers may also be added to room-temperature-vulcanizing(RTV) silicone rubber dispersions as surface coatings for ceramic, fiberglass reinforcedresin, ethylene-propylene rubber, and epoxy insulators to provide enduring water repellency. These coatings also have high resistance to corona and arcing, and are more permanent than greases which require reapplication.

Thus, the selection of a suitable polymer in preference to porcelain includes sev-eral criteria involving the environment to which the insulator will be exposed: Temperature range Prevailing humidity and rainfall Salt fog and mist Industrial pollutants and agricultural chemicals Corona attack Arcing

MARKET TRENDSFor electrical applications, ceramics will lose market share and volume as the acceptance and use ofpolymeric materials becomes widespread. Ceramics used in electronicdevices and packages continue to keep pace with the dynamic growth of high-technology applications. Ceramics are especially important in meeting the military's needsfor materials with long-term environmental stability. Japanese ceramics for electronicuses already dominate world markets, and will undoubtedly continue to provide formidable competition to United States manufactured products.

Ceramic and Glass Insulations

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