SnO2–Zn2SnO4 composite ceramics with a colossal dielectric permittivity and varistor behavior are prepared by traditional ceramic processing. By increasing bias voltage from 0 to 10 V at a low frequency (~103 Hz) and at room temperature, the relative permittivity decreased rapidly from about 20 000 to several thousand, whereas the radius of the semicircle in the complex impedance decreased and the tail gradually disappeared. However, the peak height and the position of the imaginary part of the complex modulus in the spectra were independent of the applied DC voltage. The slope deduced from the bias voltage‐dependent straight lines of the double‐logarithmic imaginary permittivity spectra were constant with a value of ?0.63 at high frequencies and they decreased to ?1 at low frequencies. The results strongly indicate that a number of weekly trapped charges existed in the ceramic bulk. From the temperature‐dependent dielectric and electric modulus spectra, the trapped charge activation energy was about 0.32 eV, which may be associated with the oxygen vacancies. Based on the results, a modified equivalent circuit related to the colossal dielectric permittivity and varistor properties was proposed, in which a Warburg impedance was added in parallel with the resistance and capacitance. 相似文献
Large thermal residual stresses in coatings during the coating deposition process may easily lead to coating delamination of coated carbide tools in machining. In order to reduce the possibility of coating delamination during the tool failure process, a theoretical method was proposed and a numerical method was constructed for the coating design of WC-Co cemented carbide cutting tools. The thermal residual stresses of multi-layered coatings were analytically modeled based on equivalent parameters of coating properties, and the stress distribution of coatings are simulated by Finite element method (FEM). The theoretically calculated results and the FEM simulated results were verified and in good agreement with the experimental test results. The effects of coating thickness, tool substrate, coating type and interlayer were investigated by the proposed geometric and FEM model. Based on the evaluations of matchability of tool substrate and tool coatings, the basic principles of tool coating design were proposed. This provides theoretical basis for the selection and design of coatings of cutting tools in high-speed machining.
The electrical and dielectric properties of (1 ? x)(0.94Bi1/2Na1/2TiO3–0.06BaTiO3)–x(K0.5Na0.5NbO3) with x = 0, 0.03, 0.09, 0.18 have been investigated by impedance spectroscopy over a wide temperature range. The dc conductivity of the ceramics follows the Arrhenius law with an activation energy ranging from ~1.20 to 1.50 eV. Measurements under different atmospheres show the materials exhibit n‐type semiconducting behavior at elevated temperatures. The presence of a highly polarizable phase for all compositions is revealed by electric modulus (M″) spectra. The Burns temperature decreases with increasing KNN content. The change in temperature‐dependent permittivity with composition is explained by the difference in thermal evolution of polar nanoregions induced by the addition of KNN. 相似文献
To solve the problem of heavy metal pollution and agricultural wastes reclamation, spent substrate of pleurotus ostreatus (SSPO) was used as adsorbent to remove Zn2+ from aqueous solution. The biosorption of zinc ions on SSPO was studied as a function of the solution pH, temperature and initial Zn2+ concentration. The equilibrium sorption data were well represented by linear Langmuir isotherm models with R2 value of 0.9955 and non-linear Freundlich with R2 value of 0.9973. The BET surface area of SSPO can reach 51.16m2g?1. SEM-EDX and XRD revealed that (NH4)2Zn·H2O and Zn2PO4(OH) were the main compounds in metal-loaded SSPO. FTIR analysis indicated the governing functional groups such as O-H, N-H and P=O played an important role in biosorption. The desorption studies showed the reversibility of SSPO. The results indicate that SSPO is a potential adsorbent in wastewater treatment due to its great sorption capacity and low cost. 相似文献