SrCexNbxWO4+4x: A novel NTC ceramic for high-temperature thermistor with a wide temperature range

A series of novel negative temperature coefficient thermistor materials based on SrCe_xNb_xWO_4+4x (0.1 ≤ x ≤ 0.8) ceramics was synthesized by the solid-state route. X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy revealed that the SrCe_xNb_xWO_4+4x ceramics were composed of scheelite structural solid solution based on the SrWO₄ phase and a CeNbO_4.08 secondary phase that exhibited a fergusonite structure. The resistance-temperature analysis demonstrated that the resistivity ρ₈₀₀ of the SrCe_xNb_xWO_4+4x ceramics decreased from 8.25 ± 0.08 × 10⁶ to 2.52 ± 0.11 × 10² with the increase of x value, along with a decrease in the thermal constant B from 11,102 ± 97–4137 ± 37 K. It was observed that the SrCe_xNb_xWO_4+4x ceramics exhibited high resistivity and excellent aging characteristics as well as suitable B values at elevated temperatures, making it a promising candidate that can be used for the fabrication of high-temperature NTC thermistors with a wide operating temperature range.     

Oxygen barrier resistance of HfB2-MoSi2-TaB2 coatings in a wide temperature region

HfB₂-MoSi₂-based ultra-high temperature ceramic (UHTC) coatings have shown remarkable antioxidant effects owing to the formation of silicate glass layers with low oxygen permeability in high-temperature environments, which shows great potential in the antioxidation of carbon structural materials. To further enhance the oxidation resistance of the HfB₂-MoSi₂-based coating in a wide temperature region, the influence of volume ratio between HfB₂ and TaB₂ on the antioxidant capacity of the HfB₂-MoSi₂-TaB₂ coatings was investigated. The addition of 15 vol% TaB₂ in the 60HfB₂-40MoSi₂ coating delays the initial oxidation temperature of the 60HfB₂-40MoSi₂ sample from 300 °C to 500 °C, which decreased the oxidation loss by 75.85% during dynamic oxidation. In oxidation process at 900 °C and 1700 °C, the weight gains of the 45HfB₂-40MoSi₂–15TaB₂ coating reduced by 78.56% and 63.14%, respectively. Due to the coexistence of 45 vol%HfB₂ and 15 vol%TaB₂, the suitable Ta⁵⁺ promoted the homogenization and dispersion of Hf/Ta-oxides, which forms the coral-like Hf/Ta oxides skeleton in the glass layer, thus preventing the oxygen penetration and decreasing the inert factor of the HfB₂-MoSi₂ coating at 1700 °C by 51.19%. However, excessive TaB₂ weakened the self-healing ability of the Ta-Hf-Si-O glass layer and inhibited the oxygen barrier effect of the HfB₂-MoSi₂-TaB₂ coating.     

Excellent energy storage properties over a wide temperature range under low driving electric fields in NBT-BSN lead-free relaxor ferroelectric ceramics

To develop environment-friendly dielectric capacitors with low working electric field and wide useable temperature, in this work, we fabricate (1-x)Na_0.46Bi_0.54TiO₃- xBaSnO₃((1-x)NBT-xBSN) lead-free relaxor ferroelectric ceramics by adding BaSnO₃ into Na_0.46Bi_0.54TiO₃ matrix. BSN exhibits slim polarization-electric field (P-E) loops, small remnant polarization (P_r) and good temperature stability because of its room-temperature paraelectric characteristics, and has different cation ionic radii with Na_0.46Bi_0.54TiO₃. Therefore, when BSN is introduced into NBT, the relaxor behavior of the (1-x)NBT-xBSN ceramics is more pronounced and the P-E loops are much slimmer. Besides, because the substitution of Ba²⁺ ions with higher valence for Na⁺ ions neutralizes the hole carriers, which are caused by the volatilization of Na₂O, the resistivity and breakdown strength are improved with increasing BSN content. As a consequence, at x = 0.30, the ceramic exhibits simultaneously a large recoverable energy density (W_rec) of 1.51 J/cm³ and high energy efficiency (η) of 81.2% at a low driving electric field of 145.3 kV/cm because of the collaborative enhancement effect of the high breakdown strength and low remnant polarization. More interestingly, variations of the W_rec and the η for this kind of ceramic are respectively as small as 10% and 0.8% over a wide temperature range of 20–140 °C, demonstrating superior temperature stability. In this report, we provide a new and efficient way for designing and fabricating environment-friendly dielectric capacitors with good reliability and superior high-temperature energy storage capacity.     

MoSi2和(Mo,W)Si2涂层的宽温域氧化过程

采用包渗法在Mo及Mo?W基体上分别制备MoSi2及(Mo,W)Si2涂层,研究了W掺杂对MoSi2涂层抗氧化性能的影响规律和作用机理。结果表明,W元素固溶到MoSi2涂层中,形成(Mo,W)Si2固溶体,涂层微观结构更加致密化。在1600℃高温下静态氧化,(Mo,W)Si2涂层抗氧化失效时间长达70 h,1200℃下氧化1000 h仍具有良好的防护性能,抗氧化性能大幅提升。加入W元素阻碍了Si元素与基体间的扩散反应,降低了涂层中Si元素的消耗速率,显著增强了(Mo,W)Si2涂层抗高温氧化性能。在500℃低温下静态氧化50 h,与MoSi2涂层相比,(Mo,W)Si2涂层氧化产生明显的“Pest”现象,涂层严重粉化失效。加入W元素降低了涂层中Si元素的扩散速率,导致低温下涂层表面无法形成致密氧化层,加剧涂层的快速氧化。     

High pyroelectric response over a broad temperature range in NBT-BZT: SiO2 composites for energy harvesting

Pyroelectric energy harvesting is considered a highly promising technology for converting low-grade waste heat into electricity, but the practical applications of pyroelectric generators is limited by the their poor energy densities and instability. In this work, we construct SiO₂ networks with low heat capacities in NBT-BZT ceramics. These networks improve the heat transfer (dT/dt) and broaden the pyroelectric temperature region of the composites by reducing heat absorption capacity, thereby leading to high pyroelectric energy density and stability. The temperature range of the NBT-BZT composite with 0.1 wt% SiO₂ for pyroelectric coefficient higher than 20 × 10^?4 C m^?2 K^?1 is increased to 20 °C, This increase results in the high thermostability of energy harvesting. In addition, the NBT-BZT: 0.1 wt% SiO₂ composites show an optimized pyroelectric energy density of 110 u J cm^-3, nearly three times that of the pure NBT-BZT ceramics. This work is beneficial for the application of high-performance pyroelectric materials for devices used in energy harvesting.     

Preparation of high-entropy ceramic oxide powder at different calcination temperatures and electrochemical applications

High entropy ceramic oxide (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O (HEO) powders were synthesized by precipitation. X-ray diffraction (XRD), thermogravimetric analysis (TG-DSC), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were used to characterize the morphologies and structures of powders obtained at different calcination temperatures. The results showed that with increasing calcination temperature, the powder changed from a flocky precursor to a compact block shape. Due to the influence of the diffusion rates of different elements, the powder morphologies and structures differed for different calcination temperatures. Finally, the electrochemical properties of HEO powders prepared by the precipitation method were tested. The results showed that HEOs exhibited good electrochemical performance. The initial cyclic discharge capacity was 624.3 F/g; after cycling with different current densities, the discharge capacity still reached 591.3 F/g when cycled at 0.1 A/g, which was attributed to the good cycling performance and rate performance.     

Preparation and formation mechanism of Cr-free spinel-structured high entropy oxide (MnFeCoNiCu)3O4

In this work, two Cr-free high entropy oxides (HEOs), an equimolar (MnFeCoNiCu)₃O₄ and a non-equimolar (Mn_0.272Fe_0.272Co_0.272Ni_0.092Cu_0.092)₃O₄ have been synthesized by a solid-state reaction method. The reaction sequence and electrical conductivity were also studied for these two HEOs. It is demonstrated that a rock-salt phase containing a solid solution of NiO and CuO appears in the synthesizing process of (MnFeCoNiCu)₃O₄, which is ascribed to the incomplete solubilization of rock-salt phase in the spinel phase. For (Mn_0.272Fe_0.272Co_0.272Ni_0.092Cu_0.092)₃O₄, a single spinel phase (Fd-3m) is obtained at 750 °C, which is much lower than that of the (MnFeCoNiCu)₃O₄ sample. Furthermore, Mn, Fe, Co, Ni elements exist in the chemical states of +2 and + 3, and Cu exists in Cu²⁺ state. The electrical conductivity of (Mn_0.272Fe_0.272Co_0.272Ni_0.092Cu_0.092)₃O₄ is approximately 15.77 S cm^-1 at 800 °C, which is nearly three times higher than that of the (MnFeCoNiCu)₃O₄ sample.     

Lead-free high-temperature dielectrics with wide temperature stability range induced from BiFeO3-BaTiO3-based system

Lead-free BNSTNZ ((Bi,Na,Ba,Sr)(Ti,Nb,Zr)O₃)-modi?ed BF35BT (0.65BiFeO₃-0.35BaTiO₃) dielectrics were investigated by conventional solid-state reaction method. Dielectric permittivity of BFBT-BNSTNZ ceramics was suppressed through addition of BNSTNZ content, while dielectric temperature stability range was expanded from 105 °C to 412 °C as BNSTNZ content increases from 0.025 to 0.1, due to the ferroelectric-relaxor phase transition. In particular, x = 0.10 exhibits the widest stability temperature range from 88 °C to 500 °C having small variation of (Δε_m/ε_{m 150 °C} ≤ 15%) with high dielectric permittivity (> 1000) and low dielectric loss (tan? ≤ 0.1) in temperature range from 50 °C to 250 °C. Moreover, high room temperature energy storage density (W_store) of 0.75 and 0.57 J/cm³ with energy storage efficiency (?) of 57% and 78% for x = 0.03 and x = 0.10, respectively, was achieved. These results indicate that BFBT-BNSTNZ can be a promising system for high-temperature dielectric and energy storage applications.     

Improved temperature stability and high piezoelectricity in lead-free barium titanate-based ceramics

To achieve high piezoelectricity and the corresponding good temperature stability of BaTiO₃ (BT) ceramics, a new system of (1-x-y) BaTiO_3?–?yCaTiO_3?–?x(BaZr_1-zHf_z)O₃ (BT-yCT-xBZH_z) was designed. The O-T phase boundary was observed at 0.065?≤?x?≤?0.085, 0.08?≤?y?≤?0.12 and 0?≤?z?≤?1.0 near room temperature. Due to the O-T phase boundary, the enhancement of electrical properties (d₃₃?=?500 pC/N, k_p?=?0.52, P_r?=?14.0?μC/cm² and ε_r?=?2800), large strain (～0.23%) and d₃₃^*?=?1100?pm/V (10?kV/cm) can be obtained. Moreover, temperature-dependent electrical properties were investigated in detail, and a reliable performance was realized due to high T_C (>100?°C). The d₃₃^* was demonstrated to be temperature-insensitive from 16?°C to 60?°C (varying less than 7%), which is superior to the reported BT-based ceramics. Besides, d₃₃ and P_r fluctuate less than 22% in this temperature range, also showing an usable stability. We believe that this work can be beneficial to facilitate an increasing adoption of lead-free BT-based piezoceramics for practical applications.