Investigations On The Electrical Properties And Stabilities Of Negative Temperature Coefficient Thermistors

Some materials exhibit decreasing electrical resistance with increasingtemperature. Thermistors made from these negative temperature coefficient (NTC)materials have been widely used for temperature measurement and control, and forthe suppression of inrush current, due to their high sensitivity to temperature change,high stability and low price. Most of NTC thermistors are based on mixed oxides ofMn, Ni, Fe, Co and Cu which crystallize in spinel structure. Their specific resistivityfollows the well-known Arrhenius relation:ρ=ρ₀exp(E_a/kT), in whichρis the specificresistivity,ρ₀ the resistivity at infinite temperature, E_a the activation energy forelectronic conduction, k Boltzmann's constant and T absolute temperature. Twoparameters are used to characterize NTC thermistor materials,ρ₂₅, the specificresistivity at 25℃and the thermal constant B (unit in K) with B=E_a/k. The agingbehavior of the thermistors, often defined as the drift in electrical resistance withtime at elevated temperatures, is also vital to the application. The electrical propertiesand the aging behaviour of NTC thermistors are mainly determined by thedistribution of cations as well as the the microstructure. For practical applications,NTC thermistors with proper electrical resistivity and B value as well as low degreeof aging, are required. This thesis is to study the electrical properties and stability ofspinel-structured manganites in relation to the preparation method used, chemicalcomposition and the presence of high-conductivity second phase.Chapter 1 gives abrief introduction to the history, basic parameters, application and development trendof NTC ceramics. Studies on the preparation methods and the conduction mechanismof the ceramics are reviewed as well. Finally, the outline of this dissertation isdescribed.In Chapter 2, a modified solid-state reaction method has been adopted to prepareNi_0.75Mn_2.25O₄ powder. The effect of annealing temperature on the electricalproperties of the ceramic was investigated. Home-made nickel and manganeseoxalates were mixed with a molar ratio of Ni:Mn=0.75:2.25 and ball milled. Anultrafine powder with high sintering activity and narrow size distribution wasobtained by calcining the mixed oxalate in air at 950℃, and a relative density over 95ï¼…can be obtained at a relatively low sintering temperature of 1150℃. Theelectrical resistivity of the ceramic was found to increase with increasing annealingtemperatures in the range of 150-850℃. It was also found that the electrical stabilitycan be improved by addition of Zn to Ni_0.75Mn_2.25O₄.In Chapter 3, the effects of Cu and Zn co-doping on Ni-Mn-O spinel-structuredceramics are investigated. Dense ZnCu_xNi_0.5Mn_1.5-xO₄ ceramics were prepared frommixed oxalate-derived powders, XPS analysis revealed that in the co-doped materialCu_xZn_1.0Ni_0.5Mn_1.5-xO₄, majority of Cu ions reside at the B-sites due to the almostexclusive occupation of Zn ions at the A-sites, but in the material Cu_xNi_0.5Mn_2.5-xO₄,Cu ions are situated at both A and B sites. The co-doped material exhibites asignificant decrease in the electrical resistivity while without much decrease in thethermal constant. In comparison with the material doped with Cu alone, the co-dopedmaterial also shows much improved electrical stability upon annealing at elevatedtemperature of 150℃in air, which is attributed to its stable distribution of cations inthe spinel.In Chapter 4, the effects of Mg doping on the cationic distribution and electricalproperties of Ni-Mn-O spinel-structured NTC ceramics are investigated. All theMg_xNi_0.66Mn_2.34-xO₄ ceramics crystallize in the spinel structure at 0≤x≤1. The wavenumbersν₁ andν₂ of the infrared absorption spectra were found to change uponMg-doping, indicating that Mg²⁺ ions are situated at both A- and B-sites. The latticeparameters were calculated from the XRD results. With the relationship of the latticeparameter with cationic distribution, the distribution of Mg²⁺ ions at each sites wereestimated. It was shown that majority of Mg²⁺ ions occupy the A-sites, while 20-30ï¼…of Mg²⁺ ions occupy the B-sites, With increasing Mg doping, the electrical resistivityand B values of the ceramics increases, and the electrical stability is improved as well.The decrease of Mn³⁺/Mn⁴⁺ pairs induced by the doping of Mg²⁺ ions at B-sites isbelieved to be responsible for the increase in resistivity. This work shows that theelectrical properties of Ni-Mn-O NTC materials can be tuned by doping of low-priceMg to meet the reuirements of practical applications.Chapter 5 presents an alternative approach to tune the electrical properties of thespinel-structured NTC cermics by introduing a second phase of high electricalconductivity. Perovskite-structured LaMnO₃ is a good electronic conductor and it may not present a serious problem in terms of its compatibility with the spinelstructured Ni-Mn-O, thus the composites comprising these two components areinvestigated in this chapter. The electrical resistivity of the composites at 25℃wasfound to decrease by one to two orders of magnitude depending on the amount of theLaMnO₃ introduced, while the thermal constant B remains larger than 3000 K. Theresistivity drift after annealing at 150℃for 1000 h in air was relatively small (～1%).The general effective media percolation model was adopted to describe the electricalconduction behavior of the composite, and the percolation volume fraction for theperovskite phase was estimated to be～0.37. This work demonstrates that it ispossible to tune the electrical resistivity and thermal constant of the spinel-structuredoxide through making composite with low-resistivity perovskite-structured oxide.In Chapter 6, the effects of Bi₂O₃ additive on the densification behavior andelectrical properties of Cu_0.1Ni_0.66Fe_0.5Mn_1.74O₄ NTC ceramic are investigated. Thedensification temperature of the ceramic was lowered from 1100 to 900℃with 3wt% Bi₂O₃ additive, thus making it possible to co-fire the ceramic and metalelectrodes of a thermistor. The reduction of the sintering temperature is believed toresult from the presence of liquid phase originating from the low melting-point Bi₂O₃additive. The as-prepared dense ceramic exhibited acceptable electrical properties,especially its electrical resistivity drift with time was much smaller than that for theporous ceramic prepared without the sintering additive.