Effect of annealing process on IR transmission and mechanical properties of spark plasma sintered Yttria

In this study, fully dense Yttria ceramics were successfully fabricated by spark plasma sintering (SPS) at temperatures of 1300 and 1350 $℃$. The effects of post-annealing on IR transmission were investigated by Fourier transform infrared spectroscopy (FTIR) at various temperatures ranging from 1050 to 1250 $℃$. It was found that the optimum annealing temperature depends strongly on the sintering temperature. Annealed samples showed white opaqueness mainly due to the increase and coalescence of pores after annealing and showed an absorption band around 6.6 $\mu m$ which limits usage of yttria in IR applications. Sintering at 1350 $℃$ and annealing at 1250 $℃$ led to the maximum IR transmittance above 80% at wavelength of 5 $\mu m$ for a 3.5-mm-thick sample. The hardness and the fracture toughness of the samples were analyzed in detail and hardness of 9.2 GPa and fracture toughness of 1.65 MPa m^1/2 were obtained for the above sample.     

Twinning and charge compensation in Nb2O5–doped SnO2–CoO ceramics exhibiting promising varistor characteristics

We investigated the effects of dual doping of SnO₂ varistor ceramics with 1 mol% CoO and different amounts of Nb₂O₅ (0.1–2 mol%) on the formation of twin boundaries, microstructure development and electrical properties. Nb₂O₅ addition shifts densification to higher temperatures (up to 1430 °C), producing microstructures composed of twinned SnO₂ grains. Already 0.1 mol% Nb₂O₅ triggers a three-fold increase in growth rate via the diffusion induced grain boundary mobility (DIGM). At 0.5 mol% of Nb₂O₅ chemical equilibrium is achieved and SnO₂ grains undergo normal grain growth. Electron back-scatter diffraction (EBSD) has shown that the prevailing type of twins is {101}. Cyclic twins are common. High-angle annular dark-filed scanning transmission electron microscopy (HAADF-STEM) image analysis revealed non-uniform segregation of Nb along the twin boundaries, indicating that they are not directly triggered by Nb₂O₅, but are a result of yet unexplained sequence of topotaxial replacement reactions. Energy dispersive spectroscopy (EDS) has shown that by dual doping of SnO₂ with CoO and Nb₂O₅ the amount of Co dissolved in SnO₂ grains is always ~4x lower compared to the amount of incorporated Nb and propose the following mechanism of tin out-diffusion: $6Sn{\left(IV\right)}_{Sn(IV)}^{\times }?Sn{\left(II\right)}_{Sn(IV)}^{\text{'}\text{'}}+Co{\left(II\right)}_{Sn(IV)}^{\text{'}\text{'}}+4Nb{\left(V\right)}_{Sn(IV)}^{?}$. Optimal electrical properties were achieved at 1 mol% Nb₂O₅ addition displaying high nonlinearity (α=50), moderate break-down voltage (571±12 V/mm) and low leakage current (I_L = 4.2 µA). The addition of 2 mol% of Nb₂O₅ has an inhibiting effect on densification and SnO₂ grain growth, resulting in a collapse of nonlinearity and increase of leakage current.     

High-Q microwave dielectric properties of Li(Zn0.95Co0.05)1.5SiO4 ceramics for LTCC applications

This study investigated the effects of Li₂O-MgO-ZnO₂-B₂O₃-SiO₂ (LMZBS) glass on the microstructure, sintering behaviour and microwave dielectric properties of Li(Zn_0.95Co_0.05)_1.5SiO₄ ceramics. Li(Zn_0.95Co_0.05)_1.5SiO₄ powders were synthesised by a traditional solid-state route and added with different amounts of LMZBS glass (0–4 wt%) to decrease the sintering temperature of the ceramics to approximately 900 °C. The XRD patterns showed that no chemical reactions occurred between the Li(Zn_0.95Co_0.05)_1.5SiO₄ ceramics and the LMZBS glass within the doping range. The SEM images indicated that the sample added with 1.5 wt% glass and sintered at 900 °C exhibited a compact and uniform microstructure. In particular, the microwave dielectric properties of the products were related to LMZBS glass content and sintering temperature. The sample with 1.5 wt% LMZBS glass exhibited excellent microwave dielectric properties, namely, ${\epsilon }_r$=6.12, $Q\times f$=83,600 GHz and ${\tau }_f=-39.1\mathrm{ppm}/\mathrm{℃}.$     

Effect of wall surface materials on deposition of particles with the aid of negative air ions

This work studied how wall surface materials influence the removal of airborne particles with negative air ions (NAIs) in indoor environments. Five wall surface materials—stainless steel, wood, PVC (polyvinyl chloride), wallpaper and cement paint—were applied to the inner surface of a test chamber. Two monodispersed solid NaCl particle sizes, 300 and 30 nm, were tested. The NAIs in the chamber were generated by negatively electric discharge in the range 3000–5000 ions cm${}^{-3}$. Experiments on the natural decay and application of NAIs were conducted at an air exchange rate of 1 h${}^{-1}$. The decay coefficient, the removal efficiency, the time to halve the concentration ($T_{50}$) and the effective cleaning rate (ECR) were taken into account. The experimental data revealed that NAIs enhanced the removal of both 300 and 30 nm particles for each wall surface material. The decay coefficients of the NAI applications ($k_{\mathrm{a}}$) were 2.9–7.4 and 2.5–4.0 times higher than those of natural decay ($k_{\mathrm{n}}$) for 300 and 30 nm particles, respectively. However, the effect of the wall surfaces on the removal of particles was observed in both natural decay and NAI application experiments. The order of natural decay for 300 and 30 nm particles under different wall surfaces was cement paint $>\mathrm{PVC}\cong \mathrm{wallpaper}\cong \mathrm{wood}>$ stainless steel, as perhaps determined by the roughness of the wall materials. The overall removal efficiencies of 300 and 30 nm particles during 30 min of NAI emission were over 60% and 80%, respectively, in the chamber with wood and PVC wall surfaces. The half concentration times ($T_{50}$) of NAI applications for various wall surfaces were less than 20 min, except for stainless steel and cement paint walls. The ECR demonstrated that the net effects of the NAIs for 300 nm particles followed the order wood (34.6 Lpm) $>$ PVC (33.3 Lpm) $>$ wallpaper (27.0 Lpm) $>$ stainless steel (16.6 Lpm) $>$ cement paint (14.8 Lpm). The ECR for 30 nm particles followed the order wood (41.4 Lpm) $>$ PVC (30.4 Lpm) $\cong$ cement paint (30.3 Lpm) $>$ wallpaper (27.7 Lpm) $>$ stainless steel (20.1 Lpm). The NAI could remove particles from the wood and PVC wall surfaces substantially more effectively than from other wall materials. The various electrical characteristics and roughness of the wall materials may have been responsible for the associated of the various ECRs with the various wall surface materials.     

Enhanced low-field magnetoresistance in LSMO/AZO composites prepared by plasma activated sintering

To improve the low-field magnetoresistance of La_0.7Sr_0.3MnO₃ (LSMO) ceramics, Al (3?wt%)-doped ZnO (AZO) was added to LSMO as the second phase, and (1???x) LSMO/xAZO (x?=?0, 0.1, 0.2, 0.3 and 0.4) composites were first prepared by plasma activated sintering. The effect of the AZO content on the structural, electrical and magnetic properties of the composites was studied. The results of XRD, SEM, EDS and TEM analyses indicated that the LSMO phase coexisted with the AZO phase without any reaction, and that all of the samples had a high relative density (>?96.9%). The magnetization of the (1???x) LSMO/xAZO composites decreased with the increase of the AZO content, while the Curie temperature remained at approximately 360?K. With an increase of the AZO content, both the resistivity and metal-insulator transition temperatures decreased. The resistivity (ρ) – temperature (T) data were well fitted to the equation of $\rho ={\rho }_0+{\rho }_2{T}^2+{\rho }_{4.5}{T}^{4.5}+{\rho }_5{T}^5$ in the low-temperature region, and to the non-adiabatic small polaron hopping model in the high-temperature region. The maximum value of the low-field magnetoresistance at room temperature was obtained for the 0.8LSMO/0.2AZO composite, which reached 16.2%, indicating the addition of AZO effectively enhanced the low-field magnetoresistance of LSMO ceramics.     

Large magnetocaloric and magnetoresistance effects in metamagnetic Sm0.55(Sr0.5Ca0.5)0.45MnO3 manganite

The metamagnetic transition, magnetocaloric and magnetoresistance effects are investigated in polycrystalline Sm_0.55(Sr_0.5Ca_0.5)_0.45MnO₃ (SSCMO) manganite. A sharp magnetization jump at Curie temperature (T_C) 73.5 K with large thermal hysteresis is observed. Magnetic measurements and Arrott plots analysis indicate that the transition is first order in nature. Under a low magnetic field change of 1 T, the magnetic entropy change exhibits a peak value of $-\Delta S_M^{\max }(T)$=4.01 J/kg K with relative cooling power (RCP) value of 44.1 J/kg in the vicinity of T_C. The mechanism of charge conduction in insulator phase is polaron transport. Large negative magnetoresistance ratio with value of ~99% is obtained within a broad temperature range below metal insulator transition temperature under 1 T magnetic field. These results indicate the potential applications of SSCMO in magnetic refrigeration and spintronics devices.     

Theoretical analysis and synthesis of Al4O4C and Al2CO phase in the resin bonded Al-Al2O3 refractory in N2-flowing

In flowing nitrogen, Al₄O₄C and Al₂CO bonded Al₂O₃-based composite was successfully prepared by a gaseous phase mass transfer pathway at 1600 °C for 3 h after an Al-AlN core-shell structure was formed in the resin bonded Al-Al₂O₃ refractory at 580 °C for 8 h. The formation mechanism of Al₄O₄C and Al₂CO phase is as follows. An Al-AlN core–shell structure is built at 580 °C for 8 h and broken at higher temperatures, and then, Al(g) reacts with C from the resin and N₂ to form Al₄C₃ and AlN, respectively. Owing to the exothermic reaction of the Al₄C₃ and AlN formation, the reaction temperature in the resin bonded Al-Al₂O₃ refractory is above the practical environmental temperature; for instance, the reaction temperature is above 1715 °C at 1600 °C in this work. Therefore, Al₄C₃ reacts with Al₂O₃ to generate Al₄O₄C and then Al₄O₄C is transformed into Al₂OC by the reaction ${\mathrm{Al}}_4{\mathrm{O}}_{\mathrm{4}}{C(s)+\text{Al}}_4{\mathrm{C}}_{\mathrm{3}}(s)\to {\mathrm{4Al}}_{\mathrm{2}}\text{OC}(s)$ at elevated temperatures. Al₂OC solid solution is finally formed through the dissolution of AlN into Al₂OC.     

High-Q dielectrics using ZnO-modified Li2TiO3 ceramics for microwave applications

The microwave dielectric properties of the (1 ? x)Li₂TiO₃–xZnO (x = 0.1–0.5) ceramic system prepared by mixed oxide route have been investigated. The rock-salt structured (1 ? x)Li₂TiO₃–xZnO were characterized by using X-ray diffraction spectra, scanning electron microcopy (SEM). The dielectric properties are strongly dependent on the compositions, the densifications and the microstructures of the specimens. The decrease of Q × f value at high-level ZnO addition (x > 0.3) was owing to the intensity of the (0 0 2) superstructure reflection decreased and became disordered rock-salt structure. For practical applications, a new microwave dielectric material 0.7Li₂TiO₃–0.3ZnO is suggested and it possesses a good combination of dielectric properties with an ${\epsilon }_r$ of ～22.95, a Q × f of ～99,800 GHz (measured at 8.91 GHz), and a ${\tau }_f$ of ～0 ppm/°C. A low-loss dielectric resonant antenna using aperture-coupled cylindrical dielectric resonant was designed and fabricated using the proposed dielectric to study its performance.