NTC (negative temperature coefficient) thermistors are semiconductor ceramics whose resistance decreases with increasing temperature, making them essential components in various temperature sensing applications. Typically, ceramic materials are sintered at high temperatures exceeding 1,150°C. However, in laminated devices incorporating internal electrodes, co-sintering can lead to cracking and mechanical failure due to mismatches in the thermal expansion coefficients between the ceramic layers and metal-based electrodes. Moreover, the use of noble metal electrodes increases production costs. To address these challenges, a low-temperature sintering approach is required. Previous studies have demonstrated that incorporating glass frit can reduce the sintering temperature of ceramics, although this often results in increased electrical resistance. In this study, NiMnCoO₄ (NMC) ceramics, as a representative NTC thermistor composition, were prepared with the addition of 10 wt% glass frit. To mitigate the resulting increase in resistivity, trace amounts (1 wt%) of various metal oxides, including CuO, ZnO, and MnO, were introduced. Among these, the addition of CuO notably decreased both the resistivity and B constant values. In contrast, MnO had little effect on resistivity, while ZnO led to an increase. With respect to the B25/85 constant, samples containing MnO and ZnO exhibited higher values than those without metal oxide additives. These findings indicate that the incorporation of 1 wt% CuO is effective in reducing the increased resistivity in NMC ceramics subjected to low-temperature sintering via glass frit addition.

NTC thermistors are essential components widely used for temperature sensing in various electronic sensor applications. However, conventional NTC thermistor ceramics typically require high sintering temperatures above 1150℃, necessitating the use of high-cost noble metal electrodes such as palladium (Pd) or platinum (Pt), which increases the overall manufacturing cost. In this study, low-melting-point oxides were successfully introduced as sintering aids to reduce the sintering temperature of NiMnCoO₄-based semiconducting ceramics. As the additive content increased, the B constant and average grain size exhibited an increasing trend, while the sample containing 5 wt% additives showed the lowest room-temperature resistivity. Furthermore, samples sintered at 1000℃ demonstrated slightly higher room-temperature resistivity and B constant values compared to those sintered at 1150℃. These results confirm that the addition of low-melting-point oxides is effective in lowering the sintering temperature of NiMnCoO₄ ceramics, suggesting the potential for reducing production costs and improving design flexibility in thermistor fabrication.

La_0.7Sr_0.3-xMg_xMnO₃ (LSMMO) (x=0.05~0.20) specimens are fabricated by a solid phase sintering method, and the sintering temperature and time are 1,300℃ and 2 hours, respectively. The dependence of the crystalline structure according to the amount of Mg²⁺ contents is not observed, and all specimens show a polycrystalline rhombohedral crystal structure, the X-ray diffraction (110) peaks move to the high angle side with increasing the amount of Mg²⁺ contents. LSMMO specimens exhibit a granule-shaped microstructure with an average grain size of 1 μm or less. Resistivity gradually decrease as the amount of Mg²⁺ contents increased. And in the La_0.7Sr_0.1Mg_0.2MnO₃ specimen, resistivity and B_25/65-value are 36.7 Ω-cm and 394 K at room temperature, respectively. LSMMO specimens show a variable-range hopping (VRH) electrical conduction mechanism, and the negative temperature of coefficient of resistance (NTCR) is approximately 0.37~0.38%/℃.

La_0.7Sr_0.3MnO₃ precursor solution were prepared by a sol-gel method. La_0.7Sr_0.3MnO₃ thin films were fabricated by a spin-coating method on a Pt/Ti/SiO₂/Si substrate. Structural and electrical properties with the variation of sintering temperature were measured. All specimens exhibited a polycrystalline orthorhombic crystal structure, and the average thickness of the specimens coated 6 times decreased from about 427 nm to 383 nm as the sintering temperature increased from 740℃ to 830℃. Electrical resistance decreased as the sintering temperature increased. In the La_0.7Sr_0.3MnO₃ thin films sintered at 830℃, electrical resistivity, TCR, B-value, and activation energy were 0.0374 mΩㆍcm, 0.316%/℃, 296 K and 0.023 eV, respectively.

(La_0.5Nd_0.2Sr_0.3)MnO₃ specimens were prepared by a solid-state reaction. In all specimens, X-ray diffraction patterns of an orthorhombic structure were shown. The fracture surfaces of (La_0.5Nd_0.2Sr_0.3)MnO₃ specimens showed a transgranular fracture pattern be possibly due to La ions (0.122 nm) as a perovskite A-site dopant substituting for Nd ions (0.115 nm) having a small ionic radius. The full-width at half maximum (FWHM) of the Mn 2p XPS spectra showed a value greater than that [8] of the single valence state, which is believed to be due to the overlapping of Mn²⁺, Mn³⁺, and Mn⁴⁺ ions. The dependence of Mn 2p spectra on the Mn³⁺/Mn⁴⁺ ratio according to sintering time was not observed. Electrical resistivity resulted in the minimum value of 100.7 Ω-cm for the specimen sintered for 9 hours. All specimens show a typical negative temperature coefficient of resistance (NTCR) characteristics. In the 9-hour sintered specimen, TCR, activation energy, and B25/65-value were -1.24%/℃, 0.19 eV, and 2,445 K, respectively.

The alumina substrates that Ni electrode was printed on and the multi-layered PTCR thermistors of which composition is (Ba_0.998Ce_0.002)TiO_3 + 0.001MnCO_3 + 0.05BN were fabricated by a thick film process, and the effect of re-oxidation temperature on their resistivities and resistance jumps were investigated, respectively. Ni electroded alumina substrate and the multi-layered PTC thermistor were sintered at l,150℃ for 2 h under PO_2= 10^-6 Pa and then re-oxidized at 600∼850℃ for 20 min. With increasing the re-oxidation temperature, the room temperature resistivity increased and the resistance jump (LogR_290/R_25) decreased, which seems to be related to the oxidation of Ni electrode. The small sized chip PTC thermistor such as 2012 and 3216 exhibits a nonlinear and rectifying behavior in I-V curve but the large sized chip PTC thermistor such as 4532 and 6532 shows a linear and ohmic behavior. Also, the small sized chip PTC thermistor such as 2012 and 3216 is more dependent on the re-oxidation temperature and easy to. be oxidized in comparison with the large sized chip PTC thermistor such as 4532 and 6532. So, the re-oxidation conditions of chip PTC thermistor may be determined by considering the chip size.

PTCR ceramics of (Ba0.998Sm0.002)TiO3 + 0.001MnCO3 + xSiO2 (x=1, 2, 3, 4, 5, 6 mol%) were fabricated by solid state method. Disk samples of diameter 5 mm and thickness about 1mm were sintered at 1,290℃ for 2 h in reduced atmosphere of 5%H2-95%N2 followed by re-oxidation at 600℃ for 30 min. in 20%O2-80%N2.and their microstructures and electrical properties were investigated with SEM and Multimeter. The color of sintered samples was strongly dependent on SiO2 content showing that the color of samples with SiO2 of 1∼2 mol% was gray but that of samples with SiO2 of 4∼6 mol% was changed from gray to blue, which seems to be related with the reduction of samples due to the oxygen vacancies created during the sintering in reduced atmosphere. SiO2 content had a great influence on the microstructure and the electrical properties. With increasing SiO2 content, the grain size of samples increased and the resistivity as well as the resistivity jump (R285/Rmin) decreased, which is considered to be attributed to the resistivity change at grain interior and grain boundary due to the fast mass transfer through SiO2 liquide phase during the sintering. Samples with 2 mol% SiO2 has the resistivity of 202 Ω cm and the resistivity jump of 3.28. It is expected that SiO2 doped BaTiO3 based PTC ceramics can be used for multilayered PTC thermistor due to the resistance to the sintering in reduced atmosphere.

Room temperature powder spray in vacuum process, so called Aerosol deposition (AD) is a room temperature (RT) process to fabricate thick and dense ceramic films, based on collision of solid ceramic particles. This technique can provide crack-free dense thin and thick films with thicknesses ranging from sub micrometer to several hundred micrometers with very fast deposition rates at RT. In addition, this technique is using solid particles to form the ceramic films at RT, thus there is few limitation of the substrate and easy to control the compositions of the films. In this article, we review the progress made in synthesis of piezoelectric thin/thick films, multi-layer structures, NTC thermistor thin/thick films, oxide electrode thin films for actuators or sensor applications by AD at Korea Institute of Materials Science (KIMS) during the last 4 years.

0.935BaTiO3-0.065(Bi0.5Na0.5)TiO3+xmol%MnO2 (BBNTM-x) ceramics with 0≤x≤0.05 were fabricated with muffled sintering by a modified synthesis process. Their microstructure and enhanced positive temperature coefficient of resistivity (PTCR) characteristics were systematically investigated in order to obtain lead-free high TC PTCR thermistors. All specimens showed a perovskite structure with a tetragonal symmetry and no secondary phase was observed. Grain growth was achieved when the doped MnO2 was increased above 0.02 mol%. This is due to the effect of positive Mn ion doping as an acceptor compensating a Ba vacancy occurred by the higher donor dopant concentration of Bi3+ ion. Especially, enhanced PTCR characteristics of the extremely low ρRT of 99 Ω·㎝, PTCR jump of 5.1×10(3), α of 15.5%/℃ and high TC of 167℃ were achieved for the BBNTM-0.04 ceramics.

(1-x)BaTiO3-x(Bi0.5Na0.5)TiO3 (0.01≤x≤0.10) ceramics were fabricated with muffled sintering by a modified synthesis process. Their positive temperature coefficient of resistivity (PTCR) characteristics were investigated systematically. All specimen showed a perovskite structure with a tetragonal symmetry. Both the lattice parameter of a and c axes were slightly decreased with increasing (Bi0.5Na0.5)TiO3 (BNT) content. Grain growth was achieved when the incorporated BNT was increased to 6 mol% and the inhibition of grain growth is considered to be due to the appearance of Ba vacancy (V"(Ba)) in the (1-x)BaTiO3-x(Bi0.5Na0.5)TiO3 (0.08≤x). With 4 mol% BNT addition, room temperature resistivity decreased to 48 Ω·㎝ and a resistivity jump (ρmax/ρmin) was as high as 1.1×10(4), respectively. Curie temperature was also increased to 171˚C with increasing BNT content.