1  Introduction of Varistor Ceramics 1 

1.1  ZnOVaristors 1 

1.2  FabricationofZnOVaristors 3 

1.2.1  PreparationofRawMaterials 4 

1.2.2  SinteringofZnOVaristors 5 

1.3  Microstructure 6 

1.4  TypicalParametersofZnOVaristors 7 

1.5  HistoryofZnOVaristors 9 

1.6  ApplicationsofZnOVaristors 12 

1.7  AlternativeVaristorCeramics 17 

1.8  Ceramic每PolymerCompositeVaristors 18 References 22 

2  Conduction Mechanisms of ZnO Varistors 31 

2.1  Introduction 31 

2.2  BasicConceptsinSolid-StatePhysics 33 

2.2.1  AtomicEnergyLevelandEnergyBandofCrystal 33 

2.2.2  Metal,Semiconductor,andInsulator 35 

2.2.3  CharacteristicsofFermi每DiracFunction 37 

2.2.4  ImpurityandDefectEnergyLevel 38 

2.3  EnergyBandStructureofaZnOVaristor 39 

2.3.1  EnergyBandStructureofaZnOGrain 39 

2.3.2  DSBofaZnOVaristor 40 

2.3.3  MicroscopicOriginofDSB 41 

2.3.4  Asymmetric I每V CharacteristicsoftheDSB 43 

2.4  ConductionMechanismofaZnOVaristor 45 

2.4.1  ConductionModelBasedonThermionicEmissionProcess 46 

2.4.2  MinorityCarrierGenerationProcess 49 

2.4.3  TheBypassE?ectModel 51 

2.5  DielectricCharacteristicsofaZnOVaristor 51 

2.5.1  ExplanationtoDielectricPropertiesofaZnOVaristor 52 

2.5.2  E?ectofInterfacialChargeRelaxationonConductingBehaviorofZnOVaristorsUnderTime-VaryingElectricFields 54 

2.5.3  DeterminationofBarrierHeightandRelatedParameters 58 

2.5.4  DeterminationofDeepDonorLevelintheZnOVaristor 59 

2.5.5  DeterminationofGrainandGrainBoundaryConductivity 60 References 62 

3  Tuning Electrical Characteristics of ZnO Varistors 67 

3.1  Introduction 67 

3.2  Liquid-PhaseFabrication 68 

3.2.1  MicrostructureofZnOVaristor 68 

3.2.2  PolymorphofBismuthOxide 71 

3.2.3  In?uenceofBi2O3Concentration 72 

3.2.4  VolatilizationofBismuthOxide 72 

3.3  PreparingandSinteringTechniques 74 

3.3.1  Fabrication 74 

3.3.2  FabricationStages 75 

3.3.3  E?ectofPores 76 

3.4  RoleofOxygenattheGrainBoundary 78 

3.5  DopantE?ects 79 

3.5.1  E?ectsofAdditives 79 

3.5.2  DonorDopants 82 

3.5.3  AcceptorDopants 86 

3.5.4  AmphotericDopants 87 

3.5.4.1  MonovalentDopants 88 

3.5.4.2  TrivalentDopants 89 

3.5.5  E?ectsofRareEarthOxides 92 

3.5.6  DopantsforImprovingtheStability 93 

3.5.7  EvidenceforHydrogenasaShallowDonor 95 

3.6  RoleofInversionBoundaries 95 

3.7  HighVoltageGradientZnOVaristor 98 

3.8  LowResidualVoltageZnOVaristor 101 

3.8.1  ResidualVoltageRatio 101 

3.8.2  LowResidualVoltageZnOVaristorsbyDopingAl 103 

3.8.3  LowResidualVoltageZnOVaristorsbyDopingGa 106 

3.8.4  LowResidualVoltageZnOVaristorswithHighVoltageGradient 108 References 110 

4  Microstructural Electrical Characteristics of ZnO Varistors 125 

4.1  Introduction 125 

4.2  MethodstoDetermineGrainBoundaryParameters 126 

4.2.1  TheIndirectMethod 126 

4.2.2  TheDirectMicrocontactMethods 126 

4.3  StatisticalCharacteristicsofGrainBoundaryParameters 129 

4.3.1  NonuniformityofBarrierVoltages 129 

4.3.2  DistributionofBarrierVoltage 131 

4.3.3  DistributionofNonlinearCoe?cient 132 

Contents 

4.3.4  DistributionofLeakageCurrentThroughGrainBoundary 133 

4.3.5  DiscussiononMicrocontactMeasurement 133 

4.4  Classi?cationofGrainBoundaries 134 

4.5  OtherTechniquestoDetectMicrostructurallyElectricalPropertiesofZnOVaristors 137 

4.5.1  ScanningProbeMicroscopy-BasedTechniques 137 

4.5.2  GalvanicDeterminationofConductiveAreasonaVaristor Surface 139 

4.5.3  LineScanDeterminationofDi?erencesinBreakdownVoltageWithinaVaristor 141 

4.5.4  CurrentImagesinSEM 141 

4.6  TestonFabricatedIndividualGrainBoundary 142 

4.6.1  ThinFilmApproach 143 

4.6.2  SurfaceIn-Di?usionApproach 143 

4.6.3  BicrystalApproach 143 References 145 

5  Simulation on Varistor Ceramics 149 

5.1  Introduction 149 

5.2  GrainBoundaryModel 151 

5.2.1  I每V CharacteristicModelofGrainBoundary 151 

5.2.2  GBModelConsideringConductionMechanism 154 

5.3  SimulationModelof I每V Characteristics 159 

5.3.1  Simple2DSimulationModel 159 

5.3.2  2DSimulationModelsBasedontheVoronoiNetwork 161 

5.3.3  ConsiderationonPoresandSpinels 164 

5.3.4  AlgorithmtoSolveEquivalentCircuit 165 

5.3.5  ModelVeri?cation 169 

5.4  SimulationModelforThermalCharacteristics 170 

5.4.1  ThermalConductionAnalysis 171 

5.4.2  Pulse-InducedFractureAnalysis 173 

5.5  SimulationsonDi?erentPhenomena 174 

5.5.1  SimulationonMicrostructuralNonuniformity 174 

5.5.2  SimulationonCurrentLocalizationPhenomenon 175 

5.5.3  In?uenceofMicrostructuralParametersonBulkCharacteristics 179 

5.5.3.1  In?uenceofZnOGrainParameters 180 

5.5.3.2  In?uenceofGrainBoundaryParameters 183 

5.5.4  In?uentialFactorsonResidualVoltageRatio 186 References 188 

6  Breakdown Mechanism and Energy Absorption Capability of ZnO Varistor 193 

6.1  Introduction 193 

6.2  ImpulseFailureModesofZnOVaristors 194 

6.3  MechanismsofPunctureandFractureFailures 197 

6.3.1  MechanismsofPunctureFailure 197 

6.3.2  MechanismofFractureFailure 201 

6.4  SimulationofPunctureandFractureFailures 204 

6.4.1  PunctureDestructionSimulation 204 

6.4.1.1  PunctureSimulationinMicrostructure 206 

6.4.2  CrackingFailureSimulationinMicrostructure 208 

6.5  Thermal Runaway 209 

6.5.1  PowerLossofZnOVaristor 210 

6.5.2  ThermalRunawayMechanism 210 

6.5.3  TeststoEnsuretheThermalStabilityCharacteristics 213 

6.6  In?uencesofDi?erentFactorsonFailuresofZnOVaristors 213 

6.6.1  In?uenceofMicrostructuralNonuniformity 213 

6.6.2  In?uenceofElectricalNonuniformityinMicrostructure 216 

6.6.3  SimulationAnalysisonBreakdownModes 217 

6.7  In?uentialFactorsonEnergyAbsorptionCapability 218 

6.7.1  In?uenceoftheAppliedCurrent 218 

6.7.2  In?uenceofVaristorCross-sectionalArea 221 

6.7.3  SimulationAnalysisonSurgeEnergyAbsorptionCapability 221 

6.8  DiscussionsonEnergyAbsorptionCapability 225 

6.8.1  EnergyAbsorptionCapabilityDeterminedbyFractureFailure 225 

6.8.2  EnergyAbsorptionCapabilityDeterminedbyPunctureFailure 226 

6.8.3  DiscussiononNonuniformityofEnergyAbsorptionCapability 228 

6.8.4  AdditivesE?ectonEnergyAbsorptionCapability 229 

6.8.5  OtherMeasurestoImproveEnergyAbsorptionCapability 230 References 230 

7  Electrical Degradation of ZnO Varistors 235 

7.1  Introduction 235 

7.2  DegradationPhenomenaofZnOVaristors 237 

7.2.1  DegradationPhenomenaoftheVaristorBulk 237 

7.2.2  DegradationofGrainBoundary 242 

7.2.3  PulseDegradationCharacteristics 245 

7.2.4  TopographicInformationforDegradationAnalysis 247 

7.3  MigrationIonsfortheDegradationofZnOVaristors 249 

7.3.1  GrainBoundaryDefectModel 249 

7.3.2  ExperimentalProofofIonMigration 251 

7.3.3  Identi?cationofDominantMobileIons 252 

7.3.4  Three-DimensionalExtension 256 

7.4  DegradationMechanismofZnOVaristors 257 

7.4.1  DCDegradationMechanism 258 

7.4.2  ACDegradationMechanism 258 

7.4.3  NonuniformDegradationMechanism 260 

7.4.4  PulseDegradationofZnOVaristors 262 

7.4.4.1  DegradationMechanismUnderImpulseCurrent 263 

7.4.4.2  SuperimposingDegradation 264 

7.5  RoleofInteriorMicrocracksonDegradation 266 

7.6  AntidegradationMeasures 267 

7.6.1  Speci?cPreparationProcedures 268 

7.6.2  OptimizationofFormula 269 

Contents 

7.6.2.1  DopantE?ectsonImprovingACDegradationCharacteristics 270 

7.6.2.2  DopantE?ectsonImprovingImpulseDegradationProperty 271 References 272 

8  Praseodymium/Vanadium/Barium-Based ZnO Varistor Systems 281 

8.1  PraseodymiumSystem 281 

8.1.1  DopingE?ects 281 

8.1.2  E?ectofSinteringProcesses 285 

8.1.3  High-VoltageApplications 288 

8.1.4  Low-VoltageApplications 288 

8.2  VanadiumSystem 289 

8.2.1  DopingE?ects 290 

8.2.2  ElectricalCharacteristics 291 

8.2.3  MicrostructuralCharacteristics 292 

8.2.4  E?ectsofVanadiumOxideonGrainGrowth 294 

8.3  BariumSystem 295 

8.3.1  PreparationandElectricalCharacteristics 295 

8.3.2  MicrostructuralCharacteristics 296 

8.3.3  ImprovingStabilityAgainstMoisture 298 

8.4  ZnO每GlassVaristor 298 References 300 

9  Fabrications of Low-Voltage ZnO Varistors 307 

9.1  Introduction 307 

9.2  ExaggeratingGrainGrowthbySeedGrains 308 

9.3  SynthesisofNanocrystallineZnOVaristorPowders 309 

9.3.1  Gas-PhaseProcessingMethods 309 

9.3.2  CombustionSynthesis 311 

9.3.3  Sol每GelMethods 311 

9.3.4  Solution-CoatingMethod 315 

9.4  Nano?llersinZnOVaristorCeramics 320 

9.5  SinteringTechniquestoControlGrainGrowth 321 

9.5.1  Step-sinteringApproach 321 

9.5.2  MicrowaveSinteringMethod 322 

9.5.3  SparkPlasmaSinteringTechnique 324 References 327 

10  Titanium-Based Dual-function Varistor Ceramics 335 

10.1  SrTiO3 Varistors 335 

10.1.1  Introduction 335 

10.1.2  MicrostructureofSrTiO3Varistors 336 

10.1.3  PreparationofSrTiO3Varistors 336 

10.1.4  PerformanceofSrTiO3 338 

10.1.5  ConductionMechanismofSrTiO3 339 

10.2  TiO2-BasedVaristors 341 

10.2.1 Introduction 341 

10.2.2  PreparationofTiO2-BasedVaristors 342 

10.2.3  MechanismofTiO2Capacitor每VaristorCeramics 342 

10.2.4  DopingofTiO2-BasedVaristors 343 

10.2.4.1 Acceptor-DopedTiO2-BasedVaristors 343 

10.2.4.2 Donor-DopedTiO2-BasedVaristors 344 

10.2.4.3 CodopingE?ectsofAcceptorandDonorDopants 345 

10.2.4.4 SinteringAdditivesinTiO2-BasedVaristors 347 

10.2.5  DevelopmentofTiO2-BasedVaristors 348 

10.3  CaCu3Ti4O12 Ceramics 348 

10.3.1  Introduction 348 

10.3.2  StructureofCCTO 349 

10.3.2.1 CrystalStructure 349 

10.3.2.2 PhaseandMicrostructure 350 

10.3.3  PerformancesofCCTOCeramics 352 

10.3.3.1 NonohmicCurrent每VoltageCharacteristic 352 

10.3.3.2 ColossalPermittivity 354 

10.3.3.3 DielectricLoss 357 

10.3.4  Mechanism 358 

10.3.4.1 IBLCModel 358 

10.3.4.2 ConductingMechanism 362 

10.3.4.3 PolarizationMechanismofGrains 364 

10.3.4.4 APolaronicStackingFaultDefectModel 365 

10.3.5  RoleofDopants 366 

10.3.5.1 RoleofDopingCuO 366 

10.3.5.2 DopingMechanismstoTuneCCTOPerformances 368 

10.4  BaTiO3VaristorsofPTCRE?ect 375 

10.4.1  Introduction 375 

10.4.2  DopingE?ects 377 

10.4.3  PreparationofBaTiO3Ceramics 379 

10.4.4  PTCRE?ectofBaTiO3Ceramics 381 

10.4.5  VaristorCharacteristicsofBaTiO3Ceramics 384 References 386 

11  Tin Oxide Varistor Ceramics of High Thermal Conductivity 407 

11.1  PreparationofSnO2-BasedVaristors 407 

11.2  ElectricalPerformancesofSnO2-BasedVaristors 410 

11.3  MechanismofSnO2-BasedVaristors 414 

11.3.1  FormationofGrainBoundaryPotentialBarrier 414 

11.3.2  AtomicDefectModel 415 

11.3.3  AdmittanceSpectroscopyAnalysis 417 

11.3.4  Capacitance每VoltageAnalysis 420 

11.3.5  E?ectofThermalTreatment 421 

11.4  RoleofDopantsinTuningSnO2-BasedVaristors 423 

11.4.1  DopantsforDensifyingSnO2-BasedVaristors 423 

11.4.2  AcceptorDoping 424 

Contents 

11.4.3  DonorDoping 427 

11.5  ThermalPerformances 429 

11.6  DegradationBehaviors 431 

11.7  DevelopmentofSnO2-BasedVaristors 432 References 434 

12  WO3-Based Varistor Ceramics of Low Breakdown Voltage 441 

12.1  Introduction 441 

12.2  TungstenOxide 442 

12.3  PreparationofWO3-BasedVaristors 444 

12.4  ElectricalPerformances 446 

12.5  ImprovingtheElectricalStability 448 

12.6  MechanismModelofWO3-BasedVaristors 449 

12.7  DopingE?ects 452 

12.7.1  TheAdditionofRareEarthOxides 452 

12.7.2  TheAdditionofCuO 453 

12.7.3  TheAdditionofAl2O3 454 

12.7.4  TheAdditionofTiO2 455 

12.7.5  TheAdditionofOtherAdditives 455 References 456 

Index 461