Fabrication And Electrical Properties’ Investigation Of ACu₃Ti₄O₁₂(A=Sr,La_2/3,Ca,Y_2/3) Thermistor Ceramics

NTC（negative temperature coefficient）ceramics are used as temperature sensors,whose characteristic is that the resistance decreases with the increase of temperature.It has the advantages of high temperature sensitivity,small volume,fast response,low price and good interchangeability,so it is widely used in temperature measurement,temperature control and temperature compensation.Transition metal oxides with spinel（AB₂O₄）,perovskite（ABO₃）and pyrochlore（A₂B₂O₇）structure usually exist in the classical NTC thermistor ceramic system.Because of its sensitivity to temperature,it is widely used in aerospace,industry and household appliances.In recent years,there are many researches on the behavior of NTC in new materials for thermistor applications.ACu₃Ti₄O₁₂（ACTO）is a kind of multifunctional ceramic material,which is composed of semiconductor grains with insulating grain boundaries and always twins.The variable valence properties of Cu and Ti ions in ACTO ceramics are necessary for NTC materials.Related to this,it is believed that electron concentration and Cu O precipitation play an important role in the change of electrical properties of Ca Cu₃Ti₄O₁₂（CCTO）ceramics,which seems to be the most widely accepted explanation at present.However,previous researches on ACTO mainly focused on improving the electrical properties of materials by single ion doping.In this paper,the conductive mechanism of ACTO series ceramics was studied by the substitution of different A-site ions,and the effects of traditional solid-state method and pechini method on thermal sensitive ceramics were compared.The main results are as follows:ACTO ceramics were prepared by conventional solid-state sintering at 1000℃.The main phase of ACTO ceramics is Ca Cu₃Ti₄O₁₂with body centered cubic structure.The change of A-site ions leads to different micro morphology.XPS analysis confirmed the coexistence of Cu⁺/Cu²⁺and Ti³⁺/Ti⁴⁺,which is considered to be the intrinsic conduction mechanism of NTC characteristics.The increase of CCTO grain size leads to the decrease of resistivity.The electrical properties can be adjusted by changing the A-site ions.The concentration of Cu⁺and Ti³⁺increases with the decrease of ion radius of a site.The increase of electron concentration will promote the electron jump,which leads to the decrease of resistivity.The increase of electron concentration is attributed to the increase of oxygen vacancies.Secondly,CCTO ceramics have semiconductor grains and insulating grain boundaries.We speculate that the electrical properties of these ceramics are mainly related to electron concentration and grain size.The effect of grain size on resistivity is greater than that of electron concentration,which leads to the decrease of CCTO resistivity.Theρ₂₅,B_300/500and Ea values of the obtained thermistors are in the range of 9.36×10⁶～1.47×10⁹Ω·cm,6012～10633 K and 0.51～0.91 e V,respectively.Y_2/3Cu₃Ti_1-xMn_xO₁₂（YCTMO）ceramics were prepared by conventional solid-state sintering at 980℃.The main phase of the ceramics is YCTO with body centered cubic structure.With the increase of Mn content,the resistivity,B constant and activation energy decrease,which is attributed to the increase of Cu⁺/Cu²⁺,Ti³⁺/Ti⁴⁺,Mn⁴⁺/Mn³⁺proved by XPS analysis.The values ofρ₂₅,B and Ea of NTC thermistor are in the range of 1.70×10⁴～2.20×10⁷Ω·cm,3579～4302 K and 0.30～0.37 e V,respectively.After annealing in air at 125℃ for800 hours,the aging coefficient（ΔR/R）of these materials is less than 0.4%,showing excellent stability.On the one hand,the precipitation of Cu O results in the copper defect Cu_1-xO in the grains.The distribution and migration of cations are easier,which leads to higher aging coefficient of Mn⁴⁺doped YCTO ceramics.On the other hand,with the increase of Mn content from 0.7 to 1,Mn ions occupy Cu sites,which promotes the conduction of Mn ions.At the same time,it can also lead to the transition of electrons between copper ions and manganese ions,which eventually leads to the degradation of the stability of YCTMO ceramics.