| Manganite-based thermistors with negative temperature coefficient(NTC)shows an exponential reduction in resistance with temperature and are suitable for the circuit protection,temperature monitoring and temperature compensation.NTC thermistors are widely applied in household appliances,consumer electronics products,automotive electronics,and medical equipment,owing to their reliability,low manufacturing cost,fast response,easy replacement and wide range of temperature-availability.Spinel structure NTC compounds with the formula AB2O4(A,B=Mn,Ni,Cu,Co,Fe,Cr,and so on)are the most widely studied so far.In these compounds,transition cations are located on tetrahedral sites and octahedral sites.Moreover,one common equation is usually used to represent the relationship between their electrical properties,R=R0exp(Ea/KbT),B=Ea/Kb,where R0 is the resistance at the positive infinity temperature,T is the absolute temperature,Ea is the electrical conduction activation energy,and Kb is the Boltzmann constant.The B-value is the ratio of the activation energy Ea and the Boltzmann constant Kb.These important electronic constants such as R25,B value constant,activation energy and response time can be effectively employed to represent the characteristics of NTC thermistors.In this paper,the effects of composition and preparation process on the electrical properties of Mn-based NTC materials were studied NTC thermistor materials such as MnCoNi,MnCoNiCu,MnCoCu,MnCoCuRu were prepared by solid-state reaction method.The phase structure,microstructure,element distribution,valence state distribution,resistivity,activation energy and temperature resistance characteristics of the above NTC materials were studied by changing the composition and preparation process.In addition,thin film NTC sensors were fabricated by a series of semiconductor processes,such as coating,photolithography and etching.The research results are as follows:(1)Mn2-2xCo3xNi1-xO4 samples were prepared with the same conditions etc.forming pressure,sintering temperature and atmosphere.The influence of a continuous elements content change on the crystallographic evolution,microstructure,cations valence,elemental distribution,theoretical and measured densities,and electrical properties was investigated using proper analytical techniques.With the increase of Co content,the B value of the material decreases slightly,and the room temperature resistivity also decreases.The stability and reliability of mnconi ternary NTC materials are verified by the results of high temperature aging at 280?.By studying the effect of annealing temperature on Cu doped mnconi,it is found that the characteristic peaks of XRD images of samples are clear when annealing temperature is 12 00 ?.The b value is 3184 K,and the resistivity at 25? is 260 ?·cm.The doping of Cu and the increase of annealing temperature can reduce the B value and the resistivity of 25 ?.(2)Bulk MCN thermistors and an MCN ceramic target,both with a single molecular ratio of Mn1.5Co1Ni0.5O4,were synthesised via the solid state reaction method.Furthermore,thin-film MCN thermistors with varying film thicknesses were fabricated by employing a semiconductor process;and the effects of thickness on the surface topography,crystal structure,elemental distribution,response time,and the electrical properties of the prepared film were investigated.The thermal time constant of the bulk chip was 5.13 s.The time constant values for the thin-film sensors with thicknesses 528 nm,610 nm,and 677 nm were 0.43 s,0.46 s,and 0.52 s,respectively.This indicates that the response times for the thin-film temperature sensors were approximately an order of magnitude lower than that for the bulk chip sensors.(3)Mn1.6Co0.4CuO4 compounds doped with RuO2(0.1,0.2,and 0.3)were prepared via a solid-state reaction method,which is a conventional technique of ceramic powder synthesis.The band structure and the density of states(DOS)of RuO2 were simulated with computer program,these simulation analysis of RuO2 could help to explain the influence of RuO2 on the low resistivity of MCCR,the valence band maximum(VBM)and the conduction band minimum(CBM)are partially overlapped,through which Fermi level(EF=0)passes.This implies that RuO2 exhibits metallic characterization..The effects of RuO2 doping on the crystallographic evolution,microstructure,elemental distribution,cation valence states,and electrical properties were investigated using various analytical techniques.X-ray photon spectroscopy results demonstrated conversion of the valence state in Cu(Cu2+to Cu+)and Mn(Mn2+to Mn3+and Mn4+)cations.With increasing RuO2 content from 0 to 0.3,the resistivity of the material decreased to 7.43 and 0.13 ?·cm,the thermal sensitivity(B)constant of the materials were 2378 K to 1329 K.(4)Mn1.6Co0.4CuO4/0.1RuO2(MCCR)ceramic target was first prepared via solid state reaction.Then the MCCR thin film thermistor were fabricated via a series of semiconductor processes,including magnetron sputtering,photoresist coating,exposing,cleaning,and wet etching.The surface and cross section SEM images revealed the thickness of MCCR thin films decreased as the annealing temperature raising.The X-ray diffraction(XRD)patterns of MCCR compound show the main peaks of spinel structure and tiny peaks of other impurities.The valence states of metal elements were investigated via XPS spectroscopy,revealing the conversion of the valence state in Cu and Mn cations.The temperature-depended resistance of thin film devices was measured in a thermostatic oil bath,the resistance of those sensors at 25? were 47.3 1,27.4,18.9,and 13.56 ?.Meanwhile the thermal time constant for those sensors were 0.78 s,0.97 s,1.05 s,and 1.24 s,which were faster than the bulk thermistor... |
PDF Full Text Request