The Preparation And Properties Of NTC Thermistor Ceramic And Its Application In The Temperature Monitoring Of Concrete

Negative temperature coefficient（NTC）thermistors have been widely used for temperature measurement,controlling and compensation in various fields.With the development of science and technology,the requirements for their applied temperature range,measurement accuracy and stability have become more and more rigorous on one hand,and their application fields also needs to be further expanded on the other.Therefore,it has significant theoretical and practical values to explore the NTC ceramic materials with highly controllable and adjustable electrical properties and high stability.Given the properties limitations of two kinds of the most commonly used NTC ceramics,spinel and perovskite structure NTC ceramics,this thesis has tried to improve their properties by selecting different valence-stated Ni sources for spinel structure NTC ceramic and doping different ions at the B-sites of perovskite structure NTC ceramics first.And then attempted to further optimize their electrical properties through uniform composite and layered composite,respectively.Furthermore,the adjustable ability and action mechanism of different methods to the electrical properties has been also explored.Finally,based on the preparation and performance study of the NTC ceramic materials,the NTC thermistor temperature sensors suitable for concrete temperature monitoring have been designed and fabricated,and the corresponding temperature measurement system has been also established.Hence,the application field of NTC thermistor has been extended to the temperature monitoring of concrete with improving its performance.The specific work and results are as follows:（1）The preparation and properties investigation of the spinel-structured NiMn₂O₄ NTC ceramics with different Ni sourcesThe spinel-structured NiMn₂O₄ NTC ceramics were prepared by the conventional solid-state reaction and two-step sintering（TSS）methods using Ni₂O₃ and Ni O as two different Ni sources.The effects of different Ni sources on the phase composition,microstructure,electrical properties and stability of NiMn₂O₄ceramics were investigated.The results show that using Ni₂O₃ as Ni source can significantly increases the resistivity and thermal constant B without affecting the crystal structure and microstructure.Compared with Ni O-based NiMn₂O₄,the resistivity at 25°C(ρ₂₅)of Ni₂O₃-based NiMn₂O₄increased dramatically from 3109Ω·cm to 106958Ω·cm,the thermal constant(B_25/50)increased from3264 K to 4473 K,and the resistance shiftsΔR/R₀decreased from 0.80%to 0.74%after annealing for 1000 h at 150°C,indicating that using Ni₂O₃ as Ni source can improve its stability.The conductive mechanism of NiMn₂O₄ NTC ceramics can be explained by the hopping conduction,and the influence of different valence Ni sources on the electrical properties of NiMn₂O₄ was realized indirectly by changing the concentration of different valence-stated Mn ions at the B-sites.Therefore,the electrical properties of spinel NiMn₂O₄can be adjusted effectively by different Ni sources,which provides a new pathway to the design of spinel-structured NTC materials.（2）The preparation and properties investigation of perovskite-structured LaCr_1-xFe_xO₃（0≤x≤0.7）NTC ceramicsThe perovskite-structured LaCr_1-xFe_xO₃（0≤x≤0.7）NTC ceramics were prepared by the traditional solid-state method and TSS method using Fe as the substitution for Cr at the B-sites of LaCrO₃.The crystal structure,microstructure,electrical properties and stability of this system had been thoroughly investigated.The results show that the Fe doping can totally enter the B-sites of LaCrO₃ crystal structure by replacing Cr and form a uniform solid solution when x<0.4.And tiny La Fe O₃ phase gradually appeared when x>0.4.Furthermore,Fe doping also increased the grain size and inhibited the volatilization of Cr.Investigation of the electrical properties indicated that the LaCr_1-xFe_xO₃（0≤x≤0.7）ceramics had exhibited typical NTC effect and good linear relationship between Lnρand 1000/T.Theρ₂₅ decreased initially and then increased within a wide range of 634.4-83915.9Ω·cm with increasing the Fe content,whereas the B values varied within a relatively narrow range of 3651-4301 K,and these two important parameters had high adjustability.The resistance shiftsΔR/R₀ were less than 0.65%after aging for 1000 h at 125°C,which demonstrates the high stability of Fe doped LaCrO₃ system.Such combination of parameters enabled it to be a potential candidate for NTC thermistors in the temperature range from-50℃to 300℃.The aging behavior could be related to the cation vacancies/cations migration.（3）The preparation and properties investigation of the perovskite-structured LaCr_1-xMn_xO₃（0≤x≤0.7）NTC ceramicsThe LaCr_1-xMn_xO₃（0≤x≤0.7）NTC ceramics with highly controllable electrical properties and high stability were prepared by the solid-state reaction and TSS sintering method using Mn as the doping ions.The phase composition was indexed to be orthorhombic perovskite structure.When x<0.7,Mn can completely enter the B-sites by replacing Cr,while x=0.7,excessive Mn cannot entirely enter the B-sites with a tiny amount of Mn₂O₃ remaining.All the LaCr_1-xMn_xO₃ceramics had a high relative density,uniform element distribution,and accurate stoichiometric ratio,enabling them to have high chemical stability.In the whole tested temperature range（100-200℃）,the curve of Lnρvs.1000/T exhibited a good linear relationship,and the resistivity and B value decreased with increasing Mn concentration.Theρ₂₅,B_25/50 and resistance shiftΔR/R₀ varied in the range of 15.0-51505.3Ω·cm,3191-4301 K,and less than 0.57%,respectively.This combination of parameters made such system an outstanding candidate for NTC thermistors applied to a lower temperature range from-100°C to 200°C,which successfully extended the application temperature of LaCrO₃ to a point as low as-100°C.（4）The preparation and properties investigation of the NiMn₂O₄-LaMn_1-xTi_xO₃ uniform composite NTC ceramicsThe NiMn₂O₄-LaMn_1-xTi_xO₃（0≤x≤0.7）uniform composite system was prepared by the solid-state reaction and the conventional sintering method.The composites were composed of cubic spinel NiMn₂O₄ phase and rhombohedral perovskite LaMn_1-xTi_xO₃ phase.There was ions migration between the B-sites of the two structure phases during the high temperature sintering process,which mainly happened at the grain boundaries of different grains:the polygonal and lamellar grains.These two grains corresponded to NiMn₂O₄ and LaMn_1-xTi_xO₃phase,respectively and distributed uniformly,which was confirmed by the SEM/EDS results.Moreover,the Ti doping also inhibited the growth of grain,which was benefit for the high density.The NiMn₂O₄-LaMn_1-xTi_xO₃（0≤x≤0.7）uniform composite system had good NTC effect and its resistivity and B value increased with increasing the Ti concentration,decreased with increasing the content of LaMn_1-xTi_xO₃ phase.Theρ₂₅ and B_25/50could be adjusted to the desired values in the range of 3.2-53200.0Ω·cm and 1300-4008 K respectively by tuning the concentration of Ti doping,the degree of composite,and the sintering temperature,and it is even available for the antidromic combination of resistivity and B value.This enabled such composite system to be used in different fields.The conductive behavior of this composite system was related to the percolation theory and the ions migration,among them,the percolation theory had played a dominant role.（5）The preparation and properties investigation of the NiMn₂O₄-LaCr_0.7Fe_0.3O₃ layered composite NTC ceramicsThe layered composite NTC ceramic system NiMn₂O₄-LaCr_0.7Fe_0.3O₃ with a sandwich-like structure was prepared by the solid-state reaction and layer-by-layer compression molding approach.This composite exhibited single-mode sintering behavior.After TSS sintering,the layered composite ceramics were composed of orthorhombic perovskite LaCr_0.7Fe_0.3O₃,cubic spinel NiMn₂O₄phases,and a small amount of orthorhombic structure La Mn O₃.There was no obvious cracking,warping or delamination at the interlayer interface,indicating that the desirable microstructure was obtained by the designed method.This composite system had integrated the advantages of each component:the highly sensitive spinel NiMn₂O₄ contributed the high B value（3941-4167 K）,the highly stable LaCr_0.7Fe_0.3O₃provided the high stability（theΔR/R₀ at 25℃were still less than 0.55%after aging for1500 h at 150℃）,the transitional interlayer ensured the good interface bonding and high-quality co-sintering.And theρ₂₅ can be adjusted to the desired values in the range of1182-110233Ω·cm by adjusting the degree of composite.The conductive behavior was a combined action of the small-polaron jumping mechanism mainly occurred in the two basic layers and the ions migration mainly happened in the transitional interlayer.The investigation of such composite system has provided a new method and theoretical basis for designing composite NTC materials.（6）The application of the NTC ceramic materials in the temperature monitoring of concreteOn the basis of the preparation of NTC ceramic materials,the compositions suitable for wide temperature range application from the uniform composite system NiMn₂O₄-LaMn_1-xTi_xO₃（0≤x≤0.7）were selected to fabricate the temperature sensors by combining the glass and stainless steel metal sealing.The NTC thermistor temperature sensors suitable for concrete temperature monitoring had high measurement accuracy,high efficiency,and no requirement on the temperature compensation.Further,a wire/wireless temperature measurement system based on this NTC thermistor temperature sensor was established.This temperature measurement system consisted of the NTC thermistor temperature sensors,data collection module,data transfer module,data storage module,power supply module,and other modules.Specifically,data transmission adopted two modes,the USB connection port and the Zig Bee wireless communication,which supported the interconversion of RS-232,RS-485,and Zig Bee wireless signals,exhibiting good applicability.The NTC thermistor temperature sensors and data collection module can be operated parallelly,which made the simultaneous measurement of multiple test points possible.The temperature measurement system can effectively improve the quality and efficiency of the temperature monitoring of concrete due to its stable performance,fast response（wired 10-30 ms,wireless 70 ms）,high accuracy（±0.05℃）,and low cost.