Crystallization and microstructural evolution of MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript>–La<Subscript>2</Subscript>O<Subscript>3</Subscript> glass-ceramics

Crystallization and microstructure of glasses with the molar compositions 1MgO·1.2Al₂O₃·2.8SiO₂·1.2TiO₂·xLa₂O₃ (x = 0.1 and 0.4) were thermally treated at different temperatures in the range from 950 to 1250 °C and then analyzed by X-ray diffraction and scanning electron microscopy, in combination with energy-dispersive X-ray spectroscopy and electron backscatter diffraction. It was found that the microstructure is first homogeneous with the precipitation of randomly distributed crystals and then indialite domains with embedded perrierite and rutile crystals are formed. For higher temperatures or prolonged times, more domains appear and expand into the bulk of the sample. Finally, the entire sample consists of the indialite domains and the boundaries that are enriched in rutile, perrierite, and magnesium aluminotitanate. Nevertheless, very distinct differences are observed between the samples with different La₂O₃ concentrations. For the sample with x = 0.4, the domains were detected at lower temperatures, while the quantity and size of the domains increase faster due to the promoted precipitation of indialite. For the sample with x = 0.1, in addition to the domain boundaries, secondary boundaries between the “regions” (assemblages of the domains) are observed in a larger length scale. The average size of the crystalline phases found between the “regions” is larger than that typically observed at the domain boundaries. The sizes of the crystals at the boundaries decrease with higher concentrations of La₂O₃, and the crystals (especially perrierite) within the domains become larger, resulting in a more homogeneous microstructure. This results in better dielectric properties, i.e., much higher quality factor for the sample with x = 0.4 in comparison to that with x = 0.1 after heat-treatment at 1150 or 1250 °C.     

A new niobate-based CaO–2CuO–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> microwave dielectric ceramic composite for LTCC applications

A novel CaO–2CuO–Nb₂O₅ (CCN) ceramic composite was prepared by the solid-state reaction method in the temperature range of 810–890 °C. Typically, the CCN sintered at 870 °C exhibited the excellent microwave properties of ε _r ?=?15.7, Q?×?f?=?28,700 GHz, τ _f = ? 38.4 ppm/°C. The τ _f of CCN was turned to be near zero by adding TiO₂, while the ε _r increased slightly and the Q?×?f decreased. The 0.91CCN–0.09TiO₂ ceramic sintered at 920 °C showed modified properties of ε _r ?=?16.9, Q?×?f?=?21,500 GHz, τ _f = ? 1.6 ppm/°C, which shows potential in LTCC applications.     

Effect of sintering temperature on microstructures and microwave dielectric properties of Ba<Subscript>2</Subscript>MgWO<Subscript>6</Subscript> ceramics

The microwave dielectric properties of Ba₂MgWO₆ ceramics were investigated with a view to the use of such ceramics in mobile communication. Ba₂MgWO₆ ceramics were prepared using the conventional solid-state method with various sintering temperatures. Dielectric constants (? _r ) of 16.8–18.2 and unloaded quality factor (Q _u  × f) of 7000–118,200 GHz were obtained at sintering temperatures in the range 1450–1650 °C for 2 h. A maximum apparent density of 6.76 g/cm³ was obtained for Ba₂MgWO₆ ceramic, sintered at 1650 °C for 2 h. A dielectric constant (? _r ) of 18.4, an unloaded quality factor (Q _u  × f) of 118,200 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?34 ppm/°C were obtained when Ba₂MgWO₆ ceramics were sintered at 1650 °C for 2 h.     

Low temperature sintering and microwave dielectric properties of Zn<Subscript>3</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramic

Crystal structure and dielectric properties of Zn₃Mo₂O₉ ceramics prepared through a conventional solid-state reaction method were characterized. XRD and Raman analysis revealed that the Zn₃Mo₂O₉ crystallized in a monoclinic crystal structure and reminded stable up to1020 °C. Dense ceramics with high relative density (~ 92.3%) were obtained when sintered at 1000 °C and possessed good microwave dielectric properties with a relative permittivity (ε _r ) of 8.7, a quality factor (Q?×?f) of 23,400 GHz, and a negative temperature coefficient of resonance frequency (τ _f ) of around ??79 ppm/°C. With 5 wt% B₂O₃ addition, the sintering temperature of Zn₃Mo₂O₉ ceramic was successfully lowered to 900 °C and microwave dielectric properties with ε _r ?=?11.8, Q?×?f?=?20,000 GHz, and τ _f = ??79.5 ppm/°C were achieved.     

Crystal structure,microstructure and microwave dielectric properties of novel MgAl<Subscript>2</Subscript>Ti<Subscript>3</Subscript>O<Subscript>10</Subscript> ceramic

In the present work, a novel MgAl₂Ti₃O₁₀ ceramic was obtained using a traditional solid-state reaction method. X-ray diffraction and energy dispersive spectrometer showed that the main MgAl₂Ti₃O₁₀ phase was formed after sintered at 1300–1450 °C. With rising the sintering temperature from 1300 to 1450 °C, the bulk density (ρ), relative permittivity (ε _r ) and Q?×?f value firstly increased, reached the maximum values (3.61 g/cm³, 14.9, and 26,450 GHz) and then decreased. The temperature coefficient of resonator frequency (τ _f ) showed a slight change at a negative range of ??94.6 to ??83.7 ppm/°C. When the sintering temperature was 1400 °C, MgAl₂Ti₃O₁₀ ceramics exhibited the best microwave dielectric properties with Q?×?f?=?26,450 GHz, ε _r ?=?14.9 and τ _f ?=???83.7 ppm/°C.     

High-temperature crystal chemistry of Na<Subscript>6</Subscript>(UO<Subscript>2</Subscript>)<Subscript>2</Subscript>O(MoO<Subscript>4</Subscript>)<Subscript>4</Subscript>

Thermal deformations of Na₆(UO₂)₂O(MoO₄)₄ were studied by high-temperature powder X-ray diffraction. The compound crystallizes in the triclinic system, space group Р\(\bar 1\), a = 7.636(7), b = 8.163(6), c = 8.746(4) Å, α = 72.32(9)°, β = 79.36(4)°, γ = 65.79(5)°, V = 472.74(4) ų. It is stable in the temperature interval 20–700°С. The thermal expansion coefficients (TECs) are α₁₁ = 25.5 × 10^–6, α₂₂ = 7.8 × 10^–6, and α₃₃ = 1.1 × 10^–6 (°C)^–1. The orientation of the TEC pattern relative to the crystallographic axes is a₃₃^Z = 45°, a₃₃^X = 122°, a₂₂^Z = 59°, and a₂₂^X = 66°. The anisotropy of the thermal expansion is due to specific features of the crystal structure of the compound.     

Compounds WAl<Subscript>4</Subscript>, WAl<Subscript>3</Subscript>, W<Subscript>3</Subscript>Al<Subscript>7</Subscript>, and WAl<Subscript>2</Subscript> and Al-W-N combustion products

X-ray diffraction data are presented for combustion products in the Al-W-N system. New, nonequilibrium intermetallic compounds have been identified, their diffraction patterns have been indexed, and their unit-cell parameters have been determined. The phases α-and β-WAl₄ are shown to exist in three isomorphous forms, differing in unit-cell centering. The phases α′-, α″-, and α?-WAl₄ are monoclinic, with a ₀ = 5.272 Å, b ₀ = 17.770 Å, c ₀ = 5.218 Å, β = 100.10°; point groups C12/c1, A12/n1, I12/a1, respectively. The phases β′-, β″-, and β?-WAl₄ are monoclinic, with a ₀ = 5.465 Å, b ₀ = 12.814 Å, c ₀ = 5.428 Å, β = 105.92°; point groups A112/m, B112/m, I112/m, respectively. The compounds WAl₂ and W₃Al₇, identified each in two isomorphous forms, differ in cell metrics (doubling) but possess the same point group: P222. WAl ₂ ^′ : orthorhombic, a ₀ = 5.793 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. WAl ₂ ^″ : orthorhombic, a ₀ = 11.586 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. W₃Al ₇ ^′ : orthorhombic, Pmm2, a ₀ = 6.225 Å, b ₀ = 4.806 Å, c ₀ = 4.437 Å. W₃Al ₇ ^″ : orthorhombic, Pmm2, a ₀ = 12.500 Å, b ₀ = 4.806 Å, c ₀ = 8.874 Å. The new phase WAl₃: triclinic, P1, a ₀ = 8.642 Å, b ₀ = 10.872 Å, c ₀ = 5.478 Å, α = 104.02°, β = 64.90°, γ = 107.15°.     

Good electrical performances and impedance analysis of (1 − <Emphasis Type="Italic">x</Emphasis>)KNN–<Emphasis Type="Italic">x</Emphasis>BMM lead-free ceramics

(1 ? x)(K_0.5Na_0.5)NbO₃–xBi(Mg_0.75Mo_0.25)O₃ [(1 ? x)KNN–xBMM] (x?=?0.005, 0.01, 0.02) ceramics were prepared via a solid-state reaction method. X-ray diffraction patterns (XRD) and Raman spectrum showed that a solid solution was formed between the BMM and KNN, which improved the electrical properties of KNN. With increasing the BMM content, the grain firstly increased and then decreased. When x?=?0.01, the ceramics exhibited the optimized microstructure, indicating that there exits an optimal doping component. Temperature dependence of relative permittivity also increases firstly and then decreases. The relative permittivity (ε_r) of ~?1418 in stabilization zone, ε_max?~?4861 at the Curie temperature T _C ~ 394 °C, good temperature stability ?ε/ε_123 °C?≤?±?15% from 123 °C to 348 °C, and the dielectric loss tanδ?≤?0.036 from 109 to 348 °C were obtained for 0.99KNN-0.01BMM ceramics. Conductivity behavior of the (1 ? x)KNN–xBMM was investigated as a function of temperature from 420 to 520 °C and frequency from 40 to 106 Hz, showing that the basic mechanisms of conduction and relaxation processes were thermally activated, and oxygen vacancies were the possible ionic charge transport carriers at higher temperatures.     

High-temperature behavior and degradation mechanism of SiC fibers annealed in Ar and N<Subscript>2</Subscript> atmospheres

The thermal and mechanical stability of SiC fibers at elevated temperature is an important property for the practical application of SiC fiber-reinforced ceramic matrix composites and is related to the heat-treating atmosphere. In this study, the high-temperature behavior of KD SiC fibers with low oxygen content was investigated in both Ar and N₂ at temperatures from 1400 to 1800 °C through scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Auger electron spectroscopy, resistivity measurements, and tensile tests in order to understand the effects of atmospheres on the degradation of the fibers. The results show that high-temperature treatment caused more severe strength degradation in Ar than in N₂. In particular, the fibers heat treated in N₂ at 1700 °C retained a relatively high strength of 1.52 GPa, 60 % of their original strength, while the fiber strength was completely lost after heat treatment in Ar. Fiber strength degradation was mainly caused by a combination of crystal growth and surface flaws. The formation of huge grains and porosity in the fiber surfaces, owing to the thermal decomposition of the SiC _x O _y N _z and SiC _x O _y phases, significantly degraded the strength for fibers heat treated in Ar. However, the suppressing effect of N₂ on the decomposition of the SiC _x O _y N _z phase in the fiber surfaces and nitrided case on the decomposition of the SiC _x O _y phase in the fiber cores, led to higher SiC fiber temperature stability in N₂ rather than Ar.     

Effect of annealing on exchange bias in Mg<Subscript>0.2</Subscript>Fe<Subscript>x</Subscript>Cr<Subscript>1.8−x</Subscript>O<Subscript>3</Subscript> nano oxides

Magnesium-iron chromium oxides (Mg_0.2Cr_1.8?x Fe _x O₃ with x varying from 0.3 to 0.9) produced by hydrothermal process in a stirred pressure reactor from pure metal chlorides have been annealed at 700 °C. Single phase corundum structure and nanophase structure of the as-synthesized samples were confirmed by X-ray diffraction (XRD). Instead of the correlation between H _EB and D _XRD observed at T _A = 600 °C, we find significant changes. The H _EB increases with decreasing particle size reaches a maximum at ~43 nm (x = 0.5) then decreases.     

Low temperature sintering and microwave dielectric properties of Li<Subscript>6</Subscript>Mg<Subscript>7</Subscript>Ti<Subscript>3</Subscript>O<Subscript>16</Subscript> ceramics with LiF additive for LTCC applications

Li₆Mg₇Ti₃O₁₆ ceramics were prepared by the conventional solid-state method with 1–5 wt% LiF as the sintering aid. Effects of LiF additions on the phase compositions, sintering characteristics, micro-structures and microwave dielectric properties of Li₆Mg₇Ti₃O₁₆ ceramics were investigated. The LiF addition could effectively lower the sintering temperature of Li₆Mg₇Ti₃O₁₆ ceramics from 1550 to 900 °C. For different LiF-doped compositions, the optimum dielectric permittivity (ε _r ) and quality factor (Q·f) values first increased and then decreased with the increase of LiF contents, whereas the temperature coefficient of resonant frequency (τ _f ) fluctuated between ??14.39 and ??8.21 ppm/°C. Typically, Li₆Mg₇Ti₃O₁₆ ceramics with 4 wt% LiF sintered at 900 °C exhibited excellent microwave dielectric properties of ε _r ?=?16.17, Q·f?=?80,921 GHz and τ _f ?=???8.21 ppm/°C, which are promising materials for the low temperature co-fired ceramics applications.     

Preparation,structure and microwave dielectric properties of 3MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–3TiO<Subscript>2</Subscript> ceramics

3MgO–Al₂O₃–3TiO₂ (MAT) ceramics were prepared by a conventional solid-state reaction method. The crystal structure, sintering behavior and microwave dielectric properties of ceramics were investigated using X-ray diffraction, scanning electron microscopy and network analyzer. MAT ceramics contained the coexistence of three phases, including MgAl₂O₄, MgTiO₃ and MgTi₂O₅. The ceramics sintered at 1350 °C for 4 h presented excellent comprehensive performances with relative permittivity (ε _r ) of 15.4, quality factor (Q × f) of 91,000 GHz and temperature coefficient of resonant frequency (τ _f ) about ?55.1 ppm/°C.     

Mechanical properties of amorphous silicon carbonitride thin films at elevated temperatures

The mechanical properties of amorphous silicon carbonitride (a-SiC _x N _y ) films with various nitrogen content (y = 0–40 at.%) were investigated in situ at elevated temperatures up to 650 °C in inert atmosphere. A SiC film was measured also at 700 °C in air. The hardness and elastic modulus were evaluated using instrumented nanoindentation with thermally stable cubic boron nitride Berkovich indenter. Both the sample and the indenter were separately heated during the experiments to temperatures of 300, 500, and 650 °C. Short duration high temperature creep tests (1200 s) of the films were also carried out. The results revealed that the room temperature hardness and elastic modulus deteriorate with the increase of the nitrogen content. Furthermore, the hardness of both the a-SiC and the a-SiCN films with lower nitrogen content at 300 °C drops to approx. 77 % of the corresponding room temperature values, while it reduces to 69 % for the a-SiCN film with 40 at.% of nitrogen. Further increase of temperature is accompanied with minor reduction in hardness except for the a-SiCN film with highest nitrogen content, where the hardness decreases at a much faster rate. Upon heating up to 500 °C, the elastic modulus of the a-SiCN film decreases, while it increases at 650 °C due to the pronounced effect of short-range ordering. The steady-state creep rate increases at elevated temperatures and the a-SiC exhibits slower creep rates compared to the a-SiCN films. The value of the universal constant x = 7 relating the W _p/W _t and H/E ^* was established and its applicability was demonstrated. Analysis of the experimental indentation data suggests a theoretical limit of hardness to elastic modulus ratio of 0.143.     

Structure and electrical properties of MnO doped (Ba<Subscript>0.96</Subscript>Ca<Subscript>0.04</Subscript>)(Ti<Subscript>0.92</Subscript>Sn<Subscript>0.08</Subscript>)O<Subscript>3</Subscript> lead free ceramics

In this work, (Ba_0.96Ca_0.04)(Ti_0.92Sn_0.08)O₃–xmol MnO (BCTS–xMn) lead-free piezoelectric ceramics were fabricated by the conventional solid-state technique. The composition dependence (0 ≤ x ≤ 3.0 %) of the microstructure, phase structure, and electrical properties was systematically investigated. An O–T phase structure was obtained in all ceramics, and the sintering behavior of the BCTS ceramics was gradually improved by doping MnO content. In addition, the relationship between poling temperature and piezoelectric activity was discussed. The ceramics with x = 1.5 % sintering at temperature of 1330 °C demonstrated an optimum electrical behavior: d ₃₃ ~ 475 pC/N, k _p ~ 50 %, ε _r ~ 4060, tanδ ~ 0.4 %, P _r ~ 10.3 μC/cm², E _c ~ 1.35 kV/mm, T _C ~ 82 °C, strain ~0.114 % and \(d_{33}^{*}\) ~ 525 pm/V. As a result, we achieved a preferable electric performance in BaTiO₃-based ceramics with lower sintering temperature, suggesting that the BCTS–xMn material system is a promising candidate for lead-free piezoelectric ceramics.     

Effects of sintering temperature on the properties of Mn/Y codoped Ba<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>TiO<Subscript>3</Subscript> ceramics for tunable application

(Ba_0.67Sr_0.33)_1?3x/2Y _x Ti_1?y/2Mn _y O₃ [BST(Mn + Y), x = 0.006, y = 0.005] ceramics were fabricated by using citrate–nitrate combustion derived powder. Microstructure and dielectric properties of the BST(Mn + Y) ceramic samples were investigated within the sintering temperature ranged from 1220 to 1300 °C. Sintering temperature has a great influence on the microstructure and electrical properties of the ceramic samples. The dielectric properties, ferroelectric properties, and tunability are enhanced by optimizing sintering temperature. The relatively high tunability of 40 % (1.5 kV/mm DC field, 10 kHz) was obtained, and relatively low dielectric loss, <0.0052 (at 10 kHz, 20 °C) was acquired for BST(Mn + Y) samples sintered at 1275 °C for 3 h. Both the low dielectric loss and enhanced tunable properties of BST(Mn + Y) are useful for tunable devices application.     

Doping mechanisms and electrical properties of bismuth tantalate fluorites

Phase-pure bismuth tantalate fluorites were successfully prepared via conventional solid-state method at 900 °C in 24–48 h. The subsolidus solution was proposed with the general formula of Bi_3+x Ta_1?x O_7?x (0 ≤ x ≤ 0.184), wherein the formation mechanism involved a one-to-one replacement of Ta⁵⁺ cation by Bi³⁺ cation within ~4.6 mol% difference. These samples crystallised in a cubic symmetry, space group Fm-3 m with lattice constants, a = b = c in the range 5.4477(± 0.0037)–5.4580(± 0.0039) Å. A slight increment in the unit cell was discernible with increasing Bi₂O₃ content, and this may attribute to the incorporation of relatively larger Bi³⁺ cation in the host structure. The linear correlation between lattice parameter and composition variable showed that the Vegard’s law was obeyed. Both TGA and DTA analyses showed Bi_3+x Ta_1?x O_7?x samples to be thermally stable as neither phase transition nor weight loss was observed within ~28–1000 °C. The AC impedance study of Bi₃TaO₇ samples was performed over the frequency range 5–13 MHz. At intermediate temperatures, ~350–850 °C, Bi_3+x Ta_1?x O_7?x solid solution was a modest oxide ion conductor with conductivity, ~10^?6–10^?3 S cm^?1; the activation energy was in the range 0.98–1.08 eV.     

Microwave dielectric properties of ZnO–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>–<Emphasis Type="Italic">x</Emphasis>TiO<Subscript>2</Subscript> ceramics prepared by reaction-sintering process

The ZnO–Nb₂O₅–xTiO₂ (1 ≤ x ≤ 2) ceramics were fabricated by reaction-sintering process, and the effects of TiO₂ content and sintering temperature on the crystal structure and microwave dielectric properties of the ceramics were investigated. The XRD patterns of the ceramics showed that ZnTiNb₂O₈ single phase was formed as x ≤ 1.6 and second phase Zn_0.17Nb_0.33Ti_0.5O₂ appeared at x ≥ 1.8. With the increase of TiO₂ content and sintering temperature, the amount of the second phase Zn_0.17Nb_0.33Ti_0.5O₂ increased, resulting in the increase of dielectric constant, decrease of Q × f value, and the temperature coefficient of resonant frequency (τ _f ) shifted to a positive value. The optimum microwave dielectric properties were obtained for ZnO–Nb₂O₅–2TiO₂ ceramics sintered at 1075 °C for 5 h: ε _r  = 45.3, Q × f = 23,500 GHz, τ _f  = +4.5 ppm/°C.     

Reactive sintering and particle morphology control of β″-alumina-based water purification filters

This paper reports on the preparation of porous membranes consisting of plate-like β″-alumina grains and the evaluation for microfiltration properties. Porous β″-alumina-based ceramics were prepared by the solid-state reactive sintering of Na₂CO₃ and α-Al₂O₃ at 1100–1300 °C. To study the effect of impurities in the starting powder mixtures, LiF-doped membranes were also prepared. As for the water filtration test, the turbidity before and after the vacuum filtration was measured using sintered porous membranes. To simulate bacteria-contaminated water, a suspension of a commercial boehmite powder (D ₅₀ = 0.7 μm) in distilled water was used. The non-doped samples sintered at 1200 °C were composed of β″-alumina (84 wt%) and β-alumina (16 wt%) grains and showed a good microfiltration performance; the turbidities before and after filtration were 894.4 NTU and 1.46 NTU, respectively.     

Influence of Nd doping on microwave dielectric properties of SrTiO<Subscript>3</Subscript> ceramics

Sr_1?x Nd _x TiO₃ (x?=?0.08–0.14) ceramics were prepared by conventional solid-state methods. The analysis of crystal structure suggested Sr_1?x Nd _x TiO₃ ceramics appeared to form tetragonal perovskite structure. The relationship between charge compensation mechanism, microstructure feature and microwave dielectric properties were investigated. Trivalent Nd³⁺ substituting Sr²⁺ could effectively decrease oxygen vacancies. This reduction and relative density were critical to improve Q?×?f values of Sr_1?x Nd _x TiO₃ ceramics. For ε _r values, incorporation of Nd could restrain the rattling of Ti⁴⁺ cations and led to the reduction of dielectric constant. The τ _f values were strongly influenced by tilting of oxygen octahedral. The τ _f values decreased from 883 to 650 ppm/°C with x increasing from 0.08 to 0.14. A better microwave dielectric property was achieved for composition Sr_0.92Nd_0.08TiO₃ at 1460 °C: ε _r ?=?160, Q?×?f?=?6602 GHz, τ _f ?=?883 ppm/°C.     

Microstructural and surface morphological studies on Co doped ZnS diluted magnetic semiconductor thin films

Microstructural and surface morphological studies of Co (2.5%) doped ZnS thin films deposited at different substrate temperatures (T_S) of 200, 400 and 600 °C by means of pulsed laser deposition are presented. The deposited films are in wurtzite-hexagonal crystal structure as confirmed by X-ray diffraction and Raman spectroscopy techniques. The films deposited at higher T_S show columnar morphology, as evidence by transmission electron microscopy measurements. Images of the surface topography have been taken by atomic force microscopy (AFM) for the film deposited at different T_S. The film deposited at T_S of 200 °C shows cone-like structures while deposited at T_S of 400 and 600 °C show columnar structures. A fractal analysis has been performed on AFM images to understand the microstructure and surface morphology of thin film at different T_S. Fractal analysis also reveals the morphological changes in the film with increasing T_S. The observed ferromagnetism is correlated with columnar growth of the film which can be used as diluted magnetic semiconductor for spintronic applications.