Negative temperature coefficient (NTC) thermistors, characterized by their resistance decreasing with increasing temperature, are widely used in various fields, due to their high sensitivity, thermal stability and cost-effectiveness. Among these, the Y-Cr-Mn-O system perovskite NTC ceramics have been recognized for their precision and heat resistance, making them suitable for high-temperature exhaust gas detection systems. However, achieving consistency in electrical properties during industrial production of Y₂O₃-YCr_0.5Mn_0.5O₃ ceramics remains challenging, significantly affecting yield rates. This study is aimed to study the effects of densification, conductive phase particle size and dispersant on the consistency of electrical properties of the Y₂O₃-YCr_0.5Mn_0.5O₃ ceramics.

A two-step sintering method was employed to prepare 0.6Y₂O₃-0.4YCr_0.5Mn_0.5O₃ composite NTC ceramics. The initial step involved mixing raw materials, including Y₂O₃, Cr₂O₃, and Mn₃O₄, followed by ball milling and subsequent calcination to obtain YCr_0.5Mn_0.5O₃ powder. In the second stage, the powder was mixed with Y₂O₃ and other additives before being pressed into pellets and sintered at high temperatures. Various factors influencing consistency were examined, such as densification through Zr⁴⁺/Ca²⁺ co-doping, reduction of conductive phase particle size by re-milling and the application of different dispersants [ (NH₄)₂SO₄, C₆H₁₁NO₇, Sago 9080]. Consistency was evaluated based on the distribution of resistances measured from cut samples, using statistical methods to quantify how closely the resistance values clustered around the mean.

It is indicated that improving densification via Zr⁴⁺/Ca²⁺ co-doping could enhance consistency by 5%-15%, as compared with the single Ca²⁺ doping. Reducing the particle size of the conductive phase YCr_0.5Mn_0.5O₃ and ensuring uniform distribution contributed to higher consistency as well. Furthermore, the introduction of ammonium sulfate and ammonium citrate dispersants facilitated a more homogeneous dispersion of Y₂O₃ and YCr_0.5Mn_0.5O₃, thereby enhancing overall consistency. Notably, among the tested dispersants, ammonium citrate showed superior performance, achieving highest level of consistency across different tolerance ranges.

It is demonstrated that several strategies can be employed to improve the consistency of electrical properties in Y₂O₃-YCr_0.5Mn_0.5O₃ NTC ceramics. Enhancing densification through Zr⁴⁺/Ca²⁺ co-doping, optimizing the particle size of the conductive phase and utilizing effective dispersants all contributed positively to achieving greater consistency. These findings provided valuable insights for improving industrial production processes, ultimately leading to higher yields and more reliable NTC ceramic products. Future work should focus on further refining these approaches and exploring additional methods to optimize the electrical properties of NTC ceramics for practical applications.

Silicon carbide (SiC) ceramics, with their superior physical properties, are widely utilized in various industrial sectors, particularly under harsh conditions such as high temperatures and pressures. Reaction-bonded silicon carbide (RBSC), favored for its ease of manufacturing and cost-effectiveness, has traditionally required vacuum sintering, which limits production capacity and increases costs. This study is aimed to study the feasibility of pressureless reaction sintering of SiC through infiltration of silicon melt and molybdenum-silicon (Mo-Si) alloy under argon (Ar) protection, in order to enhance process efficiency and reduce costs.

In this study, the effects of different melt infiltrations on the sintering behavior of SiC ceramics were systematically examined. Mo-Si alloy was prepared by mixing and melting molybdenum and silicon powders according to the phase diagram, followed by crushing into small pieces under Ar at 1500 ℃ in BN crucible. Ceramic samples were fabricated using α-SiC powder, carbon black, phenolic resin, and other additives, which were mixed and pressed into rectangular bars. After pre-firing to remove binders, the green bodies were then infiltrated with either silicon or Mo-Si alloy melts at temperatures between 1530 ℃ and 1600 ℃. Density, microstructure, flexural strength, and hardness of the sintered samples were analyzed by using the Archimedes method, scanning electron microscopy (SEM), electronic universal testing machine, and Vickers hardness.

It is found that the infiltration effect of silicon melt on physical properties of the sintered samples was highly sensitive to the processing temperature. For instance, the flexural strength was increased from 58.6 MPa at 1530 ℃ to 317.0 MPa at 1570 ℃ for the silicon-infiltrated samples. Meanwhile, Mo-Si alloy-infiltrated samples achieved a relative density exceeding 94% at 1530 ℃, due mainly to the higher polarizability of the Mo-Si alloy, reducing the contact angle between silicon and carbon. The mechanical properties of the Mo-Si alloy-infiltrated samples remained stable across the tested temperature range but were lower than those of silicon-infiltrated samples at 1570 ℃ due to the segregation of MoSi₂.

It has been demonstrated that pressureless reaction sintering of SiC is feasible under Ar protection, especially when using Mo-Si alloy as the infiltration material, allowing for nearly full densification at relatively low sintering temperatures. However, the presence of MoSi₂ led to decreased mechanical properties as compared with silicon melt. While introducing Mo-Si alloy offers advantages in terms of reducing sintering temperature, further optimization is necessary to address the issue of MoSi₂ segregation and improve the overall performance of the SiC ceramics.