Influence of gas pressure on low-temperature preparation and film properties of nanocrystalline 3C-SiC thin films by HW-CVD using SiH4/CH4/H2 system

We investigated an influence of gas pressure on low-temperature preparation of nanocrystalline cubic silicon carbide (nc-3C-SiC) films by hot-wire chemical vapour deposition (HW-CVD) using SiH₄/CH₄/H₂ system. X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectra revealed that the films prepared below 1.5 Torr were Si-nanocrystallite-embedded hydrogenated amorphous SiC. On the other hand, nc-3C-SiC films were successfully prepared at gas pressure above 2 Torr. The high gas pressure plays two important roles in low-temperature preparation of nc-3C-SiC films: (1) leading to sufficient decomposition of CH₄ molecules through a gas phase reaction and an increase in the incorporation of carbon atoms into film and (2) promoting a creation of H radicals on the heated filament, allowing the sufficient coverage of growing film surface and a selective etching of amorphous network structure and/or crystalline-Si phase. It was found that total gas pressure is a key parameter for low-temperature preparation of nc-3C-SiC films.     

Amorphous Si1−xCx:H films prepared by hot-wire CVD using SiH3CH3 and SiH4 mixture gas and its application to window layer for silicon thin film solar cells

B-doped hydrogenated amorphous silicon carbon (a-Si_1−xC_x:H) films have been prepared by hot-wire CVD (HWCVD) using SiH₃CH₃ as the carbon source gas. The optical bandgap energy and dark conductivity of the film are about 1.94 eV and 2 × 10^− 9 S/cm, respectively. Using this film as a window layer, we have demonstrated the fabrication of solar cells having a structure of the textured SnO₂(Asahi-U)/a-Si_1−xC_x:H(p)/a-Si_1−xC_x:H(buffer)/a-Si:H(i)/μc-Si:H(n)/Al. The conversion efficiency of the cell is found to be 7.0%.     

N2 decomposition by hot wire and N2 post-deposition treatment on hydrogenated microcrystalline silicon thin films

Post-deposition treatment of hydrogenated microcrystalline silicon (µc-Si:H) was carried out using a hot wire in atmospheres of N₂, N₂/H₂ or H₂ and the states of the bonds in the µc-Si:H films were investigated using X-ray photoelectron spectroscopy. For the µc-Si:H film treated in N₂ at the filament temperature (T_f) of 1600 °C, a weak N1s peak was observed. It increased slightly with increasing T_f from 1600 to 1900 °C and increased dramatically with increasing T_f from 1900 to 2000 °C. The N1s peak of the µc-Si:H film treated in N₂/H₂ at T_f = 2000  °C was one order of magnitude lower than that in N₂ at T_f = 2000 °C. These findings indicate that N₂ molecules decompose on the heated filament and that the addition of H₂ prevents N₂ decomposition.     

Formation and structural characterization of nanocrystalline Si/SiC multilayers grown by hot filament assisted chemical vapor deposition using CH3SiH3 gas jets

We report on the formation and the structural characterization of nanocrystalline Si/SiC (nc-Si/SiC) multilayers on Si(100) by hot filament assisted chemical vapor deposition using CH₃SiH₃ gas pulse jets. Si rich amorphous SiC (a-Si_1 − xC_x, x ~ 0.33) was initially grown at the substrate temperature (T_s) of 600 °C with heating a hot filament at ~ 2000 °C. The following crystalline SiC layers were grown at T_s = 850 °C without utilizing a hot filament. When the a-Si_1 − xC_x layer was ultrathin (< 2 nm) on Si(100), this a-Si_1 − xC_x layer was transformed to a single epitaxial SiC layer during the subsequent SiC growth process. The Si{111} faceted pits were formed at the SiC/Si(100) interface due to Si diffusion processes from the substrate. When the thickness of the initial a-Si_1 − xC_x layer was increased to ~ 5 nm, a double layer structure was formed in which this amorphous layer was changed to nc-Si and nc-SiC was grown on the top resulting in the considerable reduction of the {111} faceted pits. It was found that nc-SiC was formed by consuming the Si atoms uniformly diffused from the a-Si_1 − xC_x layer below and that Si nanocrystals were generated in the a-Si_1 − xC_x layers due to the annealing effect during further multilayer growths.     

Tantalum influence on physical properties of (K0.5Na0.5)(Nb1−xTax)O3 ceramics

Lead-free (K_0.5Na_0.5)(Nb_1−xTa_x)O₃ ceramics with x = 0.00-0.30 were prepared by the solid-state reaction technique. The effects of Ta on microstructure, crystallographic structure, phase transition and piezoelectric properties have been investigated. It has been shown that the substitution of Ta decreases Curie temperature T_C and orthorhombic-tetragonal phase transition temperature T_O-T, while increasing the rhombohedral-orthorhombic phase transition temperature T_R-O. In addition, piezoelectric activity is enhanced with the increase of Ta content. The ceramics with x = 0.30 have the high value of piezoelectric coefficient d₃₃ = 205 pC/N. Moreover, k_p shows little temperature dependence between −75° C and 75 °C, and d₃₃ exhibits very good thermal stability over the range from −196 °C to 75 °C in the aging test.     

Phase relations and microwave dielectric properties of vanadium modified Ba5Nb4O15 ceramics

Microwave dielectric ceramics of Ba₅Nb_4−xV_xO₁₅ (x = 0-1) were prepared by a solid-state reaction method. Vanadium substitution can markedly lower the sintering temperature of Ba₅Nb₄O₁₅ from 1450 to 1100 °C. The X-ray powder diffraction analysis reveals the multiphase nature of this system. A hexagonal-to-orthorhombic phase transition was also observed for the BaNb₂O₆ secondary phase. The microwave dielectric properties, such as τ_f, ε_r and Q × f value, decreased with increasing vanadium content for samples sintered at 1100 °C. There was an apparent increase in τ_f and Q × f value for samples (x ≥ 0.5) sintered at 1200 °C due to the hexagonal-to-orthorhombic phase transition of the BaNb₂O₆ phase. These results suggested that the microwave dielectric properties of multiphase ceramics strongly depended on the phase compositions and the phase transitions.     

Synthesis of (1 − x)Ca2/5Sm2/5TiO3-xLi1/2Nd1/2TiO3 ceramic powder via ethylenediaminetetraacetic acid precursor

(1 − x)Ca_2/5Sm_2/5TiO₃-xLi_1/2Nd_1/2TiO₃ (CSLNT) ceramic powder was prepared by a liquid mixing method using ethylenediaminetetraacetic acid (EDTA) as the chelating agent. TG, DTA, XRD and TEM characterized the precursors and derived oxide powders. When x = 0.3, perovskite CSLNT was synthesized at 1000 °C for 3 h in air. The CSLNT (x = 0.3) ceramics sintered at 1200 °C for 3 h show excellent microwave dielectric properties of ?_r = 99, Q_f = 6200 GHz and τ_f = 9 × 10⁻⁶ °C⁻¹.     

New dielectric material system of x(Mg0.95Zn0.05Ti)O3-(1 − x)Ca0.8Sm0.4/3TiO3 at microwave frequency

The microstructures and the microwave dielectric properties of the x(Mg_0.95Zn_0.05)TiO₃-(1 − x) Ca_0.8Sm_0.4/3TiO₃ ceramic system were investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. Ca_0.8Sm_0.4/3TiO₃ has dielectric properties of dielectric constant ε_r ~ 120, Q × f value ~ 13,800 GHz and a large positive τ_f value ~ 400 ppm/°C. (Mg_0.95Zn_0.05)TiO₃ possesses high dielectric constant (ε_r ~ 16.21), high quality factor (Q × f value ~ 210,000 at 9 GHz) and negative τ_f value (− 59 ppm/°C). Sintering at 1300 °C with x = 0.9, 0.9(Mg_0.95Zn_0.05Ti)O₃ − 0.1 Ca_0.8Sm_0.4/3TiO₃ has a dielectric constant (ε_r) of 22.7, a Q × f value of 124,000 GHz and a temperature coefficient of resonant frequency (τ_f) of − 6.3 ppm/°C.     

Dielectric properties of (1−x)(Mg0.95Co0.05)TiO3-xCaTiO3 ceramic system at microwave frequency

The microwave dielectric properties and the microstructures of the (1−x)MgTiO₃-xCaTiO₃ ceramic system were investigated. With partial replacement of Mg by Co, dielectric properties of the (1−x)(Mg_0.95Co_0.05)TiO₃-xCaTiO₃ ceramics can be promoted. The microwave dielectric properties are strongly correlated with the sintering temperature. At 1275°C, the 0.95(Mg_0.95Co_0.05)TiO₃-0.05CaTiO₃ ceramics possesses excellent microwave dielectric properties: a dielectric constant ε_r of 20.3, a Q×f value of 107 000 ( at 7 GHz) and a τ_f value of −22.8 ppm/°C. By appropriately adjusting the x value in the (1−x)(Mg_0.95Co_0.05)TiO₃-xCaTiO₃ ceramic system, zero τ_f value can be achieved. With x=0.07, a dielectric constant ε_γ of 21.6, a Q×f value of 92 000 (at 7 GHz) and a τ_f value of −1.8 ppm/°C was obtained for 0.93(Mg_0.95Co_0.05)TiO₃-0.07CaTiO₃ ceramics sintered at 1275°C for 4 h.     

Microwave dielectric properties of La1−xBix(Mg0.5Sn0.5)O3 ceramics

The La_1−xBi_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the La_0.97Bi_0.03(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. An apparent density of 6.50 g cm⁻³, a dielectric constant (?_r) of 20.2, a quality factor (Q × f) of 58,100 GHz and a temperature coefficient of resonant frequency (τ_f) of −84.2 ppm °C⁻¹ were obtained for La_0.97Bi_0.03(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h.     

Structure and thermal conductivity of Gd2(TixZr1 − x)2O7 ceramics

The Gd₂(Ti_xZr_1 − x)₂O₇ (x = 0, 0.25, 0.50, 0.75, 1.00) ceramics were synthesized by solid state reaction at 1650 °C for 10 h in air. The relative density and structure of Gd₂(Ti_xZr_1 − x)₂O₇ were analyzed by the Archimedes method and X-ray diffraction. The thermal diffusivity of Gd₂(Ti_xZr_1 − x)₂O₇ from room temperature to 1400 °C was measured by a laser-flash method. The Gd₂Zr₂O₇ has a defect fluorite-type structure; however, Gd₂(Ti_xZr_1 − x)₂O₇ (0.25 ≤ x ≤ 1.00) compositions exhibit an ordered pyrochlore-type structure. Gd₂Zr₂O₇ and Gd₂Ti₂O₇ are infinitely soluable. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ increases with increasing Ti content under identical temperature conditions. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ first decreases gradually with the increase of temperature below 1000 °C and then increases slightly above 1000 °C. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ is within the range of 1.33 to 2.86 W m^− 1 K^− 1 from room temperature to 1400 °C.     

Dielectric properties of a new ceramic system (1 − x)Mg4Nb2O9-xCaTiO3 at microwave frequency

The microstructures and the microwave dielectric properties of the (1 − x)Mg₄Nb₂O₉-xCaTiO₃ ceramic system were investigated. In order to achieve a temperature-stable material, CaTiO₃ (τ_f ∼ 800 ppm/°C) was chosen as a τ_f compensator and added to Mg₄Nb₂O₉ (τ_f ∼ −70 ppm/°C) to form a two phase system. It was confirmed by the XRD and EDX analysis. By appropriately adjusting the x-value in the (1 − x)Mg₄Nb₂O₉-xCaTiO₃ ceramic system, near-zero τ_f value can be achieved. A new microwave dielectric material, 0.5Mg₄Nb₂O₉-0.5CaTiO₃ applicable in microwave devices is suggested and possesses the dielectric properties of a dielectric constant ?_r ∼ 24.8, a Q × f value ∼82,000 GHz (measured at 9.1 GHz) and a τ_f value ∼−0.3 ppm/°C.     

Thermal and mechanical properties of sintered machinable LaPO4-ZrO2 composites

Monazite-type LaPO₄ was synthesized using the wet precipitation and mechanochemical reaction methods. Mixtures of xLaPO₄-(1−x)ZrO₂ (x=0-1) were dry-pressed to disks or plates and cold isostatically pressed (CIP) at 100 MPa for 10 min and then sintered at temperature between 1500 and 1600°C for 1, 3, and 5 h in air, respectively. Relative densities larger than 96.8% (x≤0.4) and 92% (x=0.5-1) were achieved. It was found that these composites with x≥0.25 and single-phase LaPO₄ were machinable, that is, they could be cut and drilled using conventional tungsten carbide metal-working tools. The drilling rates were measured by applying a fixed load of 49 N to the drill at 6400 rpm. X-ray diffraction results showed that the LaPO₄ did not react with ZrO₂, at least at 1600°C in air. The linear thermal expansion coefficient, thermal conductivity, bending strength, and Young’s modulus of the sintered composites were measured.     

Synthesis of nanocrystalline xCuFe2O4-(1 − x)BiFeO3 magnetoelectric composite by chemical method

xCuFe₂O₄-(1 − x)BiFeO₃ spinel-perovskite nanocomposites with x = 0.1, 0.2, 0.3 and 0.4 were prepared using citrate precursor method. X-ray diffraction (XRD) analysis showed phase formation of xCuFe₂O₄-(1 − x)BiFeO₃ calcined at 500 °C. Transmission electron microscopy (TEM) shows formation of nanocrystallites of xCuFe₂O₄-(1 − x)BiFeO₃ with an average particle size of 40 nm. Variation of dielectric constant and dielectric loss with frequency showed dispersion in the low frequency range. Coercivity, saturation magnetization and squareness have been found to vary with concentration of ferrite phase and annealing temperature due to the increase in crystallite size. Squareness and coercivity increased with an increase in annealing temperature up to 500 °C and then decreased with a further increase in temperature to 600 °C. Magnetoelectric effect of the nanocomposites was found to be strongly depending on the magnetic bias and magnetic field frequency.     

Bismuth-modified BiScO3-PbTiO3 piezoelectric ceramics with high Curie temperature

New piezoelectric ceramics, 0.15BiScO₃-0.85(Pb_{1 − 3x / 2}Bi_x)(Ti_0.98Mn_0.02)O₃ (x = 0.04~0.10), were prepared by using conventional solid phase processing. The results of X-ray diffraction (XRD) show that the ceramics have a single phase tetragonal perovskite structure. The ceramics, poled by normal poling process, have piezoelectric coefficient d₃₃, planar electromechanical coupling factor kp and thickness electromechanical coupling factor k_t of 50~60 pC/N, ~ 11% and ~ 30%, respectively. An extremely high mechanical quality factor Q_m of 1540 was obtained at the composition x = 0.08. The Curie temperature (T_C) is in the range of 520-550 °C, higher than 490 °C of pure PbTiO₃. The combination of good piezoelectric properties and high T_C makes these ceramics suitable for elevated temperature piezoelectric devices.     

Phase evolution and microwave dielectric characteristics of Ti-substituted Mg2SiO4 forsterite ceramics

In the present paper, the phase evolution and microstructures of Ti-substituted Mg₂SiO₄ forsterite ceramics with nominal composition Mg₂(Si_1 − xTi_x)O₄ were investigated together with their microwave dielectric characteristics. MgSiO₃ secondary phase was observed in Mg₂SiO₄ ceramics, and it could be suppressed by Ti-substitution. However, Mg₂TiO₄ and MgTi₂O₅ appeared gradually with increasing Ti-substitution amount. The dielectric constant slightly increased from 6.8 to 8.1 with Ti-substitution, and the Qf value was improved significantly and reached the maximum at x = 0.1 where the optimum combination of microwave dielectric characteristics were achieved: ε_r = 7.4, Qf = 73,760 GHz at 15 GHz, τ_f = − 60 ppm/°C.     

Morphotropic phase boundary in high temperature ferroelectric xBi(Zn1/2Ti1/2)O3 − yPbZrO3 − zPbTiO3 perovskite ternary solid solution

A bismuth and lead oxide based perovskite ternary solid solution xBi(Zn_1/2Ti_1/2)O₃ − yPbZrO₃ − zPbTiO₃ (xBZT − yPZ − zPT) was investigated as an attempt to develop a high T_C ferroelectric material for piezoelectric sensor and actuator applications. A morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases was determined through an XRD study on a pseudobinary line 0.1BZT − 0.9[xPT − (1 − x)PZ] for composition 0.1Bi(Zn_1/2Ti_1/2)O₃ − 0.5PbZrO₃ − 0.4PbTiO₃. Enhanced piezoelectric and ferroelectric activities were observed for MPB composition with dielectric constant ε_r′ ~ 23,000 at Curie temperature (T_C) ≈ 320 °C, remanent polarization (P_r) = 35 μC/cm², piezoelectric coefficient (d₃₃) = 300 pC/N, unipolar strain = 0.15%, and electromechanical coupling coefficient (k_P) = 0.45.     

Microstructure and microwave dielectric characteristics of (1 − x)Ba(Zn1/3Ta2/3)O3-xBaTi4O9 ceramics

(1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ (0.1 ≤ x ≤ 0.85) composites are prepared by mixing 1150 °C-calcined BaTi₄O₉ with 1150 °C-calcined Ba(Zn_1/3Ta_2/3)O₃ powders. The crystal structure, microwave dielectric properties and sinterabilites of the (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics have been investigated. X-ray diffraction patterns reveal that BaTi₄O₉, ordered and disordered Ba(Zn_1/3Ta_2/3)O₃ phases exist independently over the whole compositional range. The sintering temperatures of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics are about 1240 - 1320 °C and obviously lower than those of Ba(Zn_1/3Ta_2/3)O₃ ceramics. The dielectric constants (?_r) and the temperature coefficient of resonant frequency (τ_f) of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics increase with the increase of BaTi₄O₉ content. Nevertheless, the bulk densities and the quality values (Q × f) of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics increase with the increase of Ba(Zn_1/3Ta_2/3)O₃ content. The results are attributed to the higher density and quality value of Ba(Zn_1/3Ta_2/3)O₃ ceramics, the better grain growth, and the densification of sintered specimens added a small BaTi₄O₉ content. The (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramic with x = 0.1 sintered at 1320 °C exhibits a ?_r value of 31.5, a maximum Q × f value of 68500 GHz and a minimum τ_f value of 4.1 ppm/°C.     

Chemical diffusion coefficient of lithium ion in Li3V2(PO4)3 cathode material

The kinetic properties of monoclinic lithium vanadium phosphate were investigated by potential step chronoamperometry (PSCA) and electrochemical impedance spectroscopy (EIS) method. The PSCA results show that there exists a linear relationship between the current and the square root of the time. The D?_Li values of lithium ion in Li_3-xV₂(PO₄)₃ under various initial potentials of 3.41, 3.67, 3.91 and 4.07 V (vs Li/Li⁺) obtained from PSCA are 1.26 × 10^− 9, 2.38 × 10^− 9, 2.27 × 10^− 9 and 2.22 × 10^− 9 cm²·s^− 1, respectively. Over the measuring temperature range 15-65 °C, the diffusion coefficient increased from 2.67 × 10^− 8 cm²·s^− 1 (at 15 °C) to 1.80 × 10^− 7 cm²·s^− 1 (at 65 °C) as the measuring temperature increased.     

Thermal stability and transport studies of (100 − 2x)TeO2-xAg2O-xWO3 (7.5 ≤ x ≤ 30) glass system

Differential scanning calorimetry (DSC), infrared (IR) and direct current (DC) conductivity studies have been carried out on (100 − 2x)TeO₂-xAg₂O-xWO₃ (7.5 ≤ x ≤ 30) glass system. The IR studies show that the structure of glass network consists of [TeO₄], [TeO₃]/[TeO₃₊₁], [WO₄] units. Thermal properties such as the glass transition (T_g), onset crystallization (T_o), thermal stability (ΔT), glass transition width (ΔT_g), heat capacities in the glassy and liquid state (C_pg and C_pl), heat capacity change (ΔC_p) and ratios C_pl/C_pg of the glass systems were calculated. The highest thermal stability (237 °C) obtained in 55TeO₂-22.5Ag₂O-22.5WO₃ glass suggests that this new glass may be a potentially useful candidate material host for rare earth doped optical fibers. The DC conductivity of glasses was measured in temperature region 27-260 °C, the activation energy (E_act) values varied from 1.393 to 0.272 eV and for the temperature interval 170-260 °C, the values of conductivity (σ) of glasses varied from 8.79 × 10⁻⁹ to 1.47 × 10⁻⁶ S cm⁻¹.