Negative temperature coefficient (NTC) thermistors are widely applied in various industries such as automotive electronics, communications, military, and medical fields due to their high sensitivity and fast response. Conventional nonlinear thermistors exhibit a resistivity that varies as an exponential function of temperature, thereby necessitating the use of complex circuitry to achieve linearized output. However, such compensation circuits increase the complexity of use and result in a decrease in the accuracy, limiting the applications. It is thus of great significance for the practical application of the materials to investigate linear NTC thermistors. In this study, the effect of the component ratio on the material properties was investigated via introducing the high-resistance phase CdWO₄ with the low-resistance phase CdSnO₃ to form a three-dimensional conductive network and inducing the acquisition of linear electrical properties. The prepared xCdWO₄-(1-x)CdSnO₃-based linear NTC thermosensitive materials could exhibit good negative temperature coefficient properties at 300-450 K, i.e., the resistivity of the samples could decrease linearly with increasing temperature.

The linear thermistor material of xCdWO₄-(1-x)CdSnO₃ (0.1≤x≤0.35) was synthesized by a solid-phase reaction method. CdCO₃, WO₃ and SnO₂ were used as raw materials and weighed according to the stoichiometric ratio. After grinding, drying and calcination, CdWO₄ and CdSnO₃ powders were obtained. The two powders were weighed according to a certain ratio, ground and dried to obtain the precursor powder. Based on the precursor powders prepared above, the precursor powders were pressed at 200 MPa by a molding method to obtain plain discs with a diameter of 10 mm and a thickness of approximately 3 mm, and sintered in a muffle furnace at 1423 K for 2 h to obtain the target ceramic samples.

The phase structure of the sintered samples was determined by a model XRD-6000 X-ray diffractometer (Shimadzu Co., Japan). The microscopic morphology and grain size of the powders and ceramic block sections were characterized by a model SIGMA500 scanning electron microscope (SEM, Zeiss Co., Germany), and the elemental characterization and distribution of the sections were analyzed by an energy dispersive spectroscope (EDS). The elemental content and valence were determined by a model K-Alpha X-ray photoelectron spectroscope (XPS, Thermo Fisher Co., USA). After the silver electrodes were prepared via applying silver paste on the upper and lower surfaces of the ceramic disc specimens, the temperature characteristics of the resistance were measured by a model MS2008-10T thermistor R/T test system (Weihong Technology Co., Shenzhen, China) at 300-450 K. The impedance spectra of the ceramic disc specimens at 300-450 K and 100-20 MHz were determined by a model WK6500B impedance analyzer (Wayne Kerr Co., UK).

The thermosensitive ceramics of xCdWO₄-(1-x)CdSnO₃ (0.1≤x≤0.35) system are prepared by a solid-phase method. The XRD patterns show that the samples consist of two main crystalline phases in the precursor powders, and no other phases occur. The peak intensity of the crystalline plane CdSnO₃ (121) gradually decreases and shifts to a low angle as CdWO₄ addition increases, indicating that the cell parameters of the phase CdSnO₃ increase and the cell volume increases. The SEM images and EDS spectra show that all the samples have the coexistence of two phases CdWO₄ and CdSnO₃. The XPS analysis shows that the Sn peaks move toward a higher binding energy with the increase of CdWO₄ addition, and the value of Sn⁴⁺/Sn²⁺ decreases with the increase of x, which is due to the fact that there is more Sn⁴⁺ converted to Sn²⁺ in CdSnO₃ alone. According to the curves of resistivity ρ versus temperature (T), the resistivity of all the samples exhibits a linear decrease with increasing temperature at 300-450 K, showing the typical NTC characteristics. When x = 0.1, the linearity is the optimum, and R² reaches 0.996. The Cole-Cole plots show a single quarter-circle arc, and the diameter decreases with increasing temperature, indicating that it is dominated by the grain effect, and the relaxation of grain boundaries is not obvious. The R_g and R_gb in the equivalent circuits both decrease with increasing temperature, further confirming the NTC characteristics.

xCdWO₄-(1-x)CdSnO₃ (0.1≤x≤0.35) linear thermistors were prepared by a solid-phase reaction method, having good linear resistance-temperature characteristics at 300-450 K. The room temperature resistivity and resistance temperature coefficient of the composite thermosensitive ceramics decreased, while the material constant B and activation energy increased as CdWO₄ doping increased. When x = 0.1, the thermistor had the optimal resistance-temperature linearization with R² of 0.996. Based on the impedance spectroscopy analysis, the conductive behavior of the xCdWO₄-(1-x)CdSnO₃ system was mainly dominated by the mechanism of the valence jump of Sn ion. The resistance-temperature linearized conductive mechanism originated from the unique microstructure formed by the composite of the high-resistance phase CdWO₄ and the low-resistance phase CdSnO₃. The microstructural units consisting of grain resistance, grain boundary resistance, and grain boundary capacitance could function through synergistic effects.