Study On The Properties Of Alkali Metal(Li/Na/K) Doped Mn-Ni-Co-O Based NTC Thermistor Ceramics

Temperature sensors are widely used in aerospace,marine,automotive,home appliances,medicine and other fields.The core component of the negative temperature coefficient thermistor(NTC)is an important part of temperature measurement,control and suppression of inrush current.The widely used NTC thermistor ceramic materials are transition metal oxides,such as Mn-Co-Cu-O,Mn-Co-Ni-O and Mn-Cu-Ni-O systems,with a typical AB2O4 spinel structure,This is because the NTC thermistor material has good reproducibility.This material has been extensively studied by researchers due to its advantages of mixed valence states and special crystal structure.The spinel material of Mn-Co-Ni-O system has the advantages of environmental friendliness,high activity and stability,low natural abundance,and low cost of manganese and cobalt,which have been widely reported.The manganese-based oxides rich in alkali metals(Li,Na,K)are widely used in battery materials,electrocatalysis,highly sensitive sensor devices,infrared detectors and other fields.With the revolution of renewable energy technologies,smart grids and electric vehicles Sexual development,low cost and light quality of alkali metals have gradually become the "new darling" of this industry.The alkali metal ceramic has a compact structure,light weight,good thermal stability,low cost,excellent performance,low sintering temperature(<960°C)and good dielectric properties to meet the needs of the modern integrated electronics industry.However,there are few reports on the doping research in this thermal ceramic.In this thesis,the preliminary study of alkali metal(Li/Na/K)doped Mn-Ni-Co-O based NTC thermal ceramics is divided into three parts.The specific research results are divided into the following three parts:(1)Synthesize the negative temperature coefficient(NTC)thermosensitive ceramic Mn0.6Ni0.9Co1.5-x Lix O4(x = 0,0.05,0.1,0.2)used in the low temperature region by co-precipitation method.The low-temperature thermistor material with low B value and high resistance value is a thermosensitive ceramic material Mn0.6Ni0.9Co1.5-x Lix O4 prepared by co-precipitation method with lithium ion doped Mn-Co-Ni group.XRD indicated the formation of single-phase Li Mn2O4 spinel.SEM shows that the incorporation of lithium reduces the sintering activity and increases the grain size by 2to 6.8 μm.XPS shows that the significant changes in the content of Mn3+/Mn4+ and Co2+/Co3+ confirm the mechanism of variable-valent ion transition conduction.The relationship between resistivity and temperature indicates that after Li doping,the Mn0.6Ni0.9Co1.5O4 material increases the resistance value and decreases the material constant B value.(2)Using fast liquid phase sintering to obtain highly efficient and dense ceramic samples,the idea of this scheme is to achieve efficient and fast liquid phase sintering through the addition of Li F sintering aids.It reduces the volatilization of low melting point lithium under traditional high temperature and long time sintering,and saves a lot of energy,production cost and researcher time during large-scale production.The addition of sintering aid Li F uses the principle of rapid liquid phase sintering at 1000°C for 5 minutes to effectively increase the sintering density.XRD and Raman also showed that the addition of Li F did not produce other impurity phases than cubic spinel,and the addition of Li F significantly affected the crystallinity of the crystal.SEM showed that the density becomes higher,and the electrical properties reflect the trend of the overall lower resistivity and confirm the increase in density.(3)The alkali-metal(Li/Na/K)doped Mn-Co-Ni-O system was prepared by the sol-gel method to obtain the thermally sensitive ceramic material Mn1.2Ni0.3Co1.5-x(Li/Na/K)x O4,and respectively The traditional solid phase sintering method and the two-step sintering method are used to obtain the heat-sensitive ceramics.XRD phase structure analysis,SEM surface morphology test analysis,XPS material internal ion valence state analysis and electrical performance test analysis were performed on the samples.XRD results show that Li/Na/K doping does not introduce any impurity phases other than the Mn Co2O4 cubic spinel phase.According to the SEM image,it was found that Li doping increased the grain size,Na doping decreased the grain size,and K doping had a small effect on the grain size.The main reason is the combined effect of the grain boundary effect and the degree of grain boundary order.The grain size of the two-step sintered ceramic sample is relatively uniform,and the problem of low-melting material volatilization caused by sintering at high temperature for a long time is improved.Its conduction mechanism can be explained by the small polaron hopping conduction model.Electrons are mainly transferred between Mn3+ and Mn4+ ions to complete the transition.