Effect of B2O3 addition on the sintering temperature and microwave dielectric properties of Zn2SiO4 ceramics

B₂O₃ (25.0 mol%) was added to Zn_2?xSiO_4?x ceramics (0.0 ≤ x ≤ 0.5) to decrease the sintering temperature. Specimens with 0.0 ≤ x ≤ 0.3 sintered at 900 °C were well sintered with a high density due to the formation of a B₂O₃ or B₂O₃–SiO₂ liquid phase. The Q × f value of the Zn₂SiO₄ ceramic was relatively low, 32,000 GHz, most likely due to the presence of a ZnO second phase. A maximum Q × f value of 70,000 GHz was obtained for the specimens with x = 0.2–0.3, and their ?_r and τ_f values were approximately 6.0 and ?21.9 ppm/°C, respectively. Ag metal did not interact with the 25.0 mol% B₂O₃-added Zn_1.8SiO_3.8 ceramic, indicating that Zn_2?xSiO_4?x ceramics containing B₂O₃ are a good candidate materials for low temperature co-fired ceramic devices.     

Influence of SiO2 on electrical properties of the highly nonlinear ZnO-Bi2O3-MnO2 varistors

The ZnO-Bi₂O₃-MnO₂-xSiO₂ (ZBMS) varistor was prepared at a low sintering temperature of 880 °C via the conventional solid state method. The phase transformation, microstructure, and electrical properties of the ZBMS varistor were studied as a function of doping amount of SiO₂. It is showed that the growth of ZnO grain is restrained by the introducing of SiO₂; and the grain size decreases from 4.68 μm to 2.98 μm. The breakdown voltage E_1mA exhibits a simultaneous variation from 608.11 V/mm to 1232.88 V/mm. It is also revealed that SiO₂ has a significant effect on the Schottky barrier structure. As a result, the highest barrier height φ_b of 5.34 eV is attained at a composition of x = 2.0 wt%, which contributes to the highest nonlinear coefficient α of 73.68. Moreover, all the samples show low leakage current of I_L < 0.1 μA.     

Densification and mechanical properties of cBN–TiN–TiB2 composites prepared by spark plasma sintering of SiO2-coated cBN powder

cBN–TiN–TiB₂ composites were fabricated by spark plasma sintering at 1773–1973 K using cubic boron nitride (cBN) and SiO₂-coated cBN (cBN(SiO₂)) powders. The effect of SiO₂ coating, cBN content and sintering temperature on the phase composition, densification and mechanical properties of the composites was investigated. SiO₂ coating on cBN powder retarded the phase transformation of cBN in the composites up to 1873 K and facilitated viscous sintering that promoted the densification of the composites. Sintering at 1873 K, without the SiO₂ coating, caused the relative density and Vickers hardness of the composite to linearly decrease from 96.2% to 79.8% and from 25.3 to 4.4 GPa, respectively, whereas the cBN(SiO₂)–TiN–TiB₂ composites maintained high relative density (91.0–96.2%) and Vickers hardness (17.9–21.0 GPa) up to 50 vol% cBN. The cBN(SiO₂)–TiN–TiB₂ composites had high thermal conductivity (60 W m⁻¹ K⁻¹ at room temperature) comparable to the TiN–TiB₂ binary composite.     

Structural,thermal and electrical properties of Ti4+ substituted Bi2O3 solid systems

In the present study, the effect of TiO₂ doping on (1 ? x) Bi₂O₃ (x)TiO₂ (x = 0.05, 0.10, 0.15, 0.20) materials is investigated using X-ray diffraction (XRD), differential thermal analysis (DTA), ac conductivity, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). XRD results show the formation of single phase Bi₁₂TiO₂₀ at x ≥ 0.15 concentration of TiO₂. It is observed that, the lower concentration of TiO₂ leads to the formation of mixed phase. The x = 0.15 and x = 0.20 samples exhibit regular and uniform distribution of the grains as compared to x = 0.10 sample. The highest conductivity is observed for x = 0.15 specimen, e.g., 9 × 10^?7 S cm^?1.     

Solubility and microstructural development of TiO2-containing 3Al2O3·2SiO2 and 2Al2O3·SiO2 mullites obtained from single-phase gels

The interdependence of the titanium oxide amount and the anisotropic growth of mullites prepared from single-phase gels were investigated. Gels with stoichiometries 3(Al_2−xTi_xO₃)·2(SiO₂) and 2(Al_2−xTi_xO₃)·(SiO₂), with 0 ≤ x ≤ 0.15 were prepared by the semialkoxide method. Gels and specimens heated at temperatures between 1200 and 1600 °C were characterized by using infrared spectroscopy (IR), X-ray diffraction (XRD) and transmission and field emission scanning electron microscopies (TEM and FESEM). Al₂TiO₅ as minor impurity was detected in both series of mullites for gel precursor compositions x = 0.10 and x = 0.15, obtained at temperatures between 1200 and 1600 °C. Variations of lattice parameters of mullite, processed at temperatures from the range between 1400 and 1600 °C, with the starting nominal amount of titanium oxide indicated that the solubility limit of titanium oxide was in ranges 3.8–4.1 and 4.1–4.4 wt% TiO₂ for 3:2 and 2:1 mullites series, respectively. The anisotropic growth of titanium-doped mullite crystalline grains was significant only when the nominal amount of titanium oxide exceeded the limit of solubility into the mullite structure (for both mullite series). Stronger anisotropy occurred for the 3:2 series specimens, i.e. for the SiO₂-richer mullites. In both series of mullites, the anisotropic grain growth was observed for the process temperatures higher than 1400 °C; the crystalline grains of mullites processed at lower temperatures were equiaxials and of almost the same size.     

Effect of Zn2+ substitution on the microwave dielectric properties of LiMgPO4 and the development of a new temperature stable glass free LTCC

The LiMg_(1?x)Zn_xPO₄ ceramics have been prepared by the solid state ceramic route. The LiMg_(1?x)Zn_xPO₄ ceramic retains the orthorhombic structure up to x = 0.2. The compositions with 0.3 ≤ x ≤ 0.8 exist as a mixture of orthorhombic and monoclinic phases. When Mg²⁺ is fully replaced with Zn²⁺ (x = 1.0) complete transition to monoclinic phase occurs. The ceramic with x = 0.1 (LiMg_0.9Zn_0.1PO₄) sintered at 925 °C exhibits low relative permittivity (?_r) of 6.7, high quality factor (Q_u × f) of 99,700 GHz with a temperature coefficient of resonant frequency (τ_f) of ?62 ppm/°C. The slightly large τ_f is adjusted nearly to zero with the addition of TiO₂. LiMg_0.9Zn_0.1PO₄–TiO₂ composite with 0.12 volume fraction TiO₂ sintered at 950 °C shows good microwave dielectric properties: ?_r = 10.1, Q_u × f = 52,900 GHz and τ_f = ?5 ppm/°C. The ceramic is found to be chemically compatible with silver.     

Preparation and magnetoelectric properties of NiFe2O4–PZT composites obtained in-situ by gel-combustion method

Magnetoelectric composites of xNiFe₂O₄–(1 ? x)Pb(Zr,Ti)O₃ with x = 2, 5, 10, 20, 30% were prepared by citrate–nitrate combustion using PZT-based template powders. In order to ensure a better connectivity of dissimilar phases, we have used chemical methods for preparation in situ composites, followed by adequate sintering procedure. The structural, microstructural and functional properties of di-phase magnetoelectric composites of NiFe₂O₄–PZT are reported. The XRD analysis is demonstrating the synthesis of pure ferrite phase directly on the ferroelectric templates. An excellent mixing was obtained in the composite powders, as proved by a detailed SEM analysis.The magnetic and dielectric behaviors of the ceramic composites vary with the ratio of the two phases. The dielectric behavior is greatly influenced by the magnetic phase. The magnetoelectric (ME) coefficient was measured as a function of applied DC magnetic field. The maximum ME coefficient (dE/dH) varies from 0.0011 mV/(cm Oe) to 0.5 mV/(cm Oe) with increasing of NF addition.     

Self-organized nano-crystallisation of BaF2 from Na2O/K2O/BaF2/Al2O3/SiO2 glasses

In glasses with the compositions (100 ? x)(2Na₂O·16K₂O·8Al₂O₃·74SiO₂)xBaF₂ (with x = 0 to 6), the glass transition temperature decreases with increasing BaF₂-concentration. Samples with x = 6 were thermally treated at temperatures in the range from 500 to 600 °C for 5–160 h. This leads to the crystallisation of BaF₂. The quantity of crystalline BaF₂ increases with increasing time of thermal treatment, while the mean crystallite size remains constant within the limits of error. The glass transformation temperature of partially crystallised samples increases with increasing crystallisation time and approaches a value equal to the temperature, at which the samples were treated. This is explained by the formation of a highly viscous layer enriched in SiO₂ which is formed during crystallisation. This layer acts as a diffusion barrier and hinders further crystal growth.     

Improved ionic conductivity in NdGdZr2O7: Influence of Sc3+ substitution

Aliovalent Sc³⁺ doped pyrochlores NdGdZr_2?xSc_xO_7?δ (0.0 ≤ x ≤ 0.15) have been prepared by gel-combustion method followed by high temperature sintering. Detailed structural characterization of these compounds has been carried out using X-ray diffraction and Raman spectroscopy, which together have established the retention of pyrochlore structure of these compositions till x = 0.15 Sc³⁺ incorporation. Both X-ray diffraction and Raman results have also indicated the presence of local lattice distortion with net cell contraction upon Sc³⁺ incorporation in these solid solutions. Micro structural studies have revealed highly dense end products with uniform grain distribution and homogeneous compositions at grain size level. Ionic conductivity characterization of system been carried out from 473 to 573 K. A significant improvement in the total ionic conductivity of this series has been observed which has been explained in terms of additional oxygen vacancies created upon Sc³⁺ incorporation and enhanced lattice disorder.     

Investigation of the structure and activity of VOx/CeO2/SiO2 catalysts for methanol oxidation to formaldehyde

The effect of ceria on the partial oxidation of methanol to formaldehyde over VO_x/CeO₂/SiO₂ catalysts was investigated. A two-dimensional layer of ceria on silica was prepared by grafting cerium (IV) t-butoxide (Ce(OC₄H₉)₄) onto high surface area, mesoporous silica, SBA-15, and then calcining the resulting product in air at 773 K. Ce surface concentrations obtained this way ranged from 0.2 to 0.9 Ce nm^?2. Next, V was introduced by grafting VO(OⁱPr)₃ onto CeO₂/SiO₂ in order to achieve a surface concentration of 0.6 V nm^?2. XANES spectra indicate that all of the V is in the 5+ oxidation state and Raman spectra show that vanadia exist as pseudo-tetrahedra bonded to either silica or ceria. Data from Raman spectroscopy and temperature-programmed desorption of adsorbed methanol indicate that with increasing Ce surface density, most of the V becomes associated with the deposited ceria. The turnover frequency for methanol oxidation is nearly two orders of magnitude higher for VO_x/CeO₂/SiO₂ than for VO_x/SiO₂, whereas the apparent activation energy and apparent first-order pre-exponential factor are 17 kcal/mol and 1.4 × 10⁶ mol CH₂O (mol V atm s)^?1, respectively, for VO_x/CeO₂/SiO₂ and 23 kcal/mol and 2.3 × 10⁷ mol CH₂O (mol V atm s)^?1, respectively, for VO_x/SiO₂.     

Catalytic performance of P-modified MoO3/SiO2 in oxidative desulfurization by cumene hydroperoxide

Phosphorous-modified MoO₃/SiO₂ catalysts with various P/Mo molar ratios (MoP_xO/SiO₂, x = 0.3 − 3.0) were prepared by co-impregnation and compared with MoO₃/SiO₂ in the oxidation of dibenzothiophene by cumene hydroperoxide. The addition of P significantly enhanced the catalytic performance, and MoP_1.0O/SiO₂ exhibited the highest oxidation activity. XRD characterization indicated that the active phase of MoP_1.0O/SiO₂ was amorphous, whereas ³¹P-NMR measurements revealed that MoP_1.0O/SiO₂ had a similar connectivity of Mo-O-P to H₃PMo₁₂O₄₀. The total sulfur of a hydrotreated diesel was reduced from 298 to 5 ppmw by oxidation with CHP and subsequent dimethylformamide extraction.     

Dielectric properties and microstructures of non-reducible high-temperature stable X9R ceramics

New X9R dielectrics based on the precursors of (1 ? x) BaTiO₃–xLiTaO₃ (BT–xLT) compositions and containing no lead and bismuth constituents were successfully prepared in this study. After sintering at 1150–1250 °C in a reducing atmosphere of 97%N₂–3%H₂, major phases including BaTiO₃ and Ba₂LiTa₅O₁₅ and a minor phase of Li₂TiO₃ were identified in the sintered ceramics. The intensities of the XRD peaks corresponding to the Ba₂LiTa₅O₁₅ phase rose with the increasing x value in the precursors. The grain sizes of the sintered ceramics appeared to fall in the range between 0.3 and 0.5 μm and slightly increased with the x value of the BT–xLT ceramics. The best composition in term of dielectric properties was found in the BT–0.25LT ceramic, which was marked with a dielectric constant of 895, tan δ of 1.01% and 7.1%, TCCs of ?3.04% and ?14.80%, and electrical resistivities of 9.9 × 10¹² and 1.6 × 10¹² Ω cm, respectively at 25 °C and 200 °C.     

Molecular structure of SiOx-incorporated diamond-like carbon films; evidence for phase segregation

Silicon-oxide incorporated amorphous hydrogenated diamond-like carbon films (SiO_x–DLC, 1 ≤ x ≤ 1.5) containing up to 24 at.% of Si (H is excluded from the atomic percentage calculations reported here) were prepared using pulsed direct current plasma-enhanced chemical vapour deposition (DC-PECVD). Molecular structure, optical properties and mechanical properties of these films were assessed as a function of Si concentration. The spectroscopic results indicated two structural regimes. First, for Si contents up to ~ 13 at.%, SiO_x–DLC is formed as a single phase with siloxane, O–Si–C₂, bonding networks. Second, for films with Si concentrations greater than 13 at.%, SiO_x–DLC with siloxane bonding and SiO_x deposit simultaneously as segregated phases. The variations in mechanical properties and optical properties as a function of Si content are consistent with the above changes in the film composition.     

Direct epoxidation of propylene to propylene oxide over RuO2-CuO-NaCl-TeO2-MnOx/SiO2 catalysts

Multi-metallic RuO₂-CuO-NaCl-TeO₂-MnO_x/SiO₂ catalysts were investigated for the epoxidation of propylene to propylene oxide (PO) using O₂ under atmospheric pressure. The metallic weight ratio and the total metal loading on SiO₂ support were optimized at Ru/Cu/Na/Te/Mn = 7.14/3.57/1.79/0.175/1.00 and 25.0 wt%, respectively. RuO₂ and CuO were determined to exhibit a critical bifunctional role for PO synthesis. These active sites were assisted by MnO_x and TeO₂ to increase PO formation rate and by NaCl to reduce CO₂ formation. With using the Box-Behnken design, a maximized PO formation rate of 1258 g_PO h^− 1 kg_cat^− 1 was obtained by varying O₂/propylene feed ratio and reaction temperature.     

Ionic conductivity across the disorder–order phase transition in the SmO1.5–CeO2 system

Samples of Sm_xCe_1 ? xO_2 ? δ (0.05 ≤ x ≤ 0.55) were prepared by solid-state reactions and the disorder–order phase transition and grain ionic conductivity were investigated using XRD and ac impedance spectroscopy technique, respectively. For 0 ≤ x ≤ 0.35 the material has a fluorite structure and gradually stabilizes into a C-type rare-earth structure at 0.40 ≤ x ≤ 0.55 because of oxygen-vacancy ordering. The highest grain ionic conductivity observed is 0.0565(37) S cm^?1 at 700 °C for Sm_0.20Ce_0.80O_2 ? δ with an associated activation energy (E_A) of 0.791(7) eV. The slopes for E_A and pre-exponential factor change during phase transition and the conductivity decreases monotonically. Upon comparison of the E_A between the SmO_1.5–CeO₂ and NdO_1.5–CeO₂ systems, it is seen E_A for the SmO_1.5–CeO₂ system is lower than NdO_1.5–CeO₂ system at compositions with less than 25% trivalent rare earth element while higher E_A is observed for the SmO_1.5–CeO₂ system at Nd/Sm concentrations above 25%.     

Densification of SiO2–cBN composites by using Ni nanoparticle and SiO2 nanolayer coated cBN powder

Cubic boron nitride (cBN) powder was coated with Ni nanoparticle and SiO₂ nanolayer (abbreviated as cBN/Ni and cBN/SiO₂, respectively) by rotary chemical vapor deposition (RCVD), and compacted with SiO₂ powder by spark plasma sintering at 1473–1973 K for 0.6 ks. The effects of Ni and SiO₂ coatings on the densification, phase transformation of cBN and hardness of SiO₂–cBN composites were compared. The phase transformation of cBN to hBN was identified at 1973 K in SiO₂–cBN/SiO₂ composites, 300 K higher than that in SiO₂–cBN/Ni composites, indicating that SiO₂ retarded the transformation of cBN. The relative density of SiO₂–cBN/SiO₂ with 50 vol% cBN sintered at 1873 K was 99% with a hardness of 14.5 GPa.     

Nitrate–citrate combustion synthesis of Ce1−xGdxO2−x/2 powder and its characterization

The structure, thermal expansion coefficients, and electric conductivity of Ce_1−xGd_xO_2−x/2 (x = 0–0.6) solid solution, prepared by the gel-combustion method, were investigated. The uniform small particle size of the gel-combustion prepared materials allows sintering into highly dense ceramic pellets at 1300 °C, a significantly lower temperature compared to that of 1600–1650 °C required for ceria solid electrolytes prepared by traditional solid state techniques. XRD showed that single-phase solid solutions formed in all the investigated range. The maximum conductivity, σ_600 °C = 5.26 × 10⁻³ S/cm, was found at x = 0.2. The thermal expansion coefficient, determined from high-temperature X-ray data, was 8.125 × 10⁻⁶ K⁻¹ at x = 0.2.     

Physicochemical and electrochemical performance of LiFe1−xNixPO4 (0≤x≤1.0) solid solution as potential cathode material for rechargeable lithium-ion battery

In this work, olivine-type LiFePO₄ (LFP) and nickel (Ni) substituted LiFe_1-xNi_xPO₄ (0≤ x≤1.0) solid solutions were synthesized using the wet-milling route. Synthesized samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XRD results show that all the samples were crystallized in the orthorhombic phase with Pnma space group and no secondary phase was detected even at higher Ni-concentration. Rietveld refinement results reveal that average <Li-O> distance increases up to x=0.03 (LFNP3) and decrease thereafter monotonically on further increase in Ni-concentration. Moreover, the LFNP3 sample shows highest electrical conductivity as compared to other solid solutions. Among the synthesized samples for x≤0.1, LFNP3 shows highest discharge capacity at all C-rates. LFNP3 exhibits a discharge capacity of 158 (±5) mAh g⁻¹ at 0.1 C (almost 93% of the theoretical capacity) and displays the high and stable discharge capacity of 145 (±5) mAh g⁻¹ at 1 C rate for 150 charge/discharge cycles. Among high Ni content (0.1< x≤1.0) samples, LiFe_0.7Ni_0.3PO₄ (LFNP30) delivers best charge/discharge capacity at all C-rates. LFNP30 exhibits a discharge capacity of 90 (±5) mAh g⁻¹ at 0.1 C rate.     

Effect of CaO addition on the structure and electrical conductivity of the pyrochlore-type GdSmZr2O7

Polycrystalline GdSm_1?xCa_xZr₂O_7?x/2 (0 ≤ x ≤ 0.20) ceramics have been prepared by the solid-state reaction method. The effects of CaO addition on the microstructure and electrical properties of the pyrochlore-type GdSmZr₂O₇ ceramic were investigated. GdSm_1?xCa_xZr₂O_7?x/2 (x ≤ 0.05) ceramics exhibit a pyrochlore-type structure; however, GdSm_1?xCa_xZr₂O_7?x/2 (0.10 ≤ x ≤ 0.20) ceramics consist of the pyrochlore-type structure and a small amount of CaZrO₃. The total conductivity of GdSm_1?xCa_xZr₂O_7?x/2 ceramics follows the Arrhenius relation, and gradually increases with increasing temperature from 723 to 1173 K. GdSm_1?xCa_xZr₂O_7?x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10^?4–1.0 atm at each test temperature. The highest total conductivity is about 1.20 × 10^?2 S cm^?1 at 1173 K for the GdSm_0.9Ca_0.1Zr₂O_6.95 ceramic.     

Influence of addition of copper cadmium ferrite on the dielectric and electrical behavior of BaSrTiO3 ceramics

Polycrystalline composites of (1 ? x) (Ba_0.7Sr_0.3TiO₃)–x(Cu_0.5Cd_0.5Fe₂O₄) (x = 0.15, 0.30, 0.45) were prepared by a modified wet chemical method. X-ray diffraction studies at room temperature confirmed the formation of polycrystalline composite with perovskite–spinel structure. Scanning electron micrographs also suggest polycrystalline microstructure with the grains of unequal size distributed throughout the pellet samples. Dielectric studies of the composites reveal that the effect of ferrite in the composites is to shift the ferroelectric–non-ferroelectric phase transition to higher temperature side. Complex impedance spectroscopy analysis indicated negative temperature coefficient of resistance behavior identical to semiconductors. The activation energy calculated from the temperature dependence of conductivity pattern shows that the conduction process can occur due to the hopping of electrons between Fe³⁺ and Fe²⁺, leading to the contribution of both single and doubly-ionized oxygen vacancies.