Synthesis and microwave dielectric properties of CaO-MgO-SiO2 submicron powders doped with Li2O-Bi2O3 by sol-gel method

Using Ca(NO₃)₂·4H₂O, Mg(NO₃)₂·6H₂O, Si(OC₂H₅)₄, LiNO₃ and Bi(NO₃)₃·5H₂O as raw materials, CaO-MgO-SiO₂ submicron powders were prepared at low temperature by sol-gel method. The crystallization temperature was decreased enormously by the introduction of Li-Bi liquid phase sintering aids into Ca-Mg-Si sol, and the powders with average particle sizes of 80-100 nm and 200-400 nm were obtained at the calcining temperature of 750 °C and 800 °C, respectively. The sintering characteristic and dielectric properties of powders calcined at 750 °C with different content of powders calcined at 800 °C were studied. When the content of powders calcined at 800 °C was 10 wt%, the dielectric ceramic sintered at 890 °C had compact structure, and possessed excellent microwave dielectric properties: ?_r = 7.16, Q × f = 25630 GHz, τ_f = −69.26 ppm/°C.     

Preparation and characterization of Y3Al5O12 (YAG) nano-powder by co-precipitation method

YAG precursor was synthesized by a co-precipitation method from a mixed solution of aluminum and yttrium nitrates with aqueous ammonia as the precipitator. The structure, phase evolution and morphology of YAG precursor and the sintered powders were studied by means of IR, TG/DTA, XRD, TEM methods. It was found the precursor with approximate composition of Al(OH)₃·0.3[Y₂(OH)₅·(NO₃)₂·2H₂O] directly transformed to pure-YAG phase at 800 °C and no intermediate phases were detected. YAG nanocrystalline powders from sintering the precursor at different temperatures were less-aggregated and the diameters of the grains were about 40-100 nm. BET surface area of the particles decreased with increase of calcination temperature and the powder sintered at 800 °C can be used for fabrication of transparent YAG ceramics.     

Preparation of LiFePO4/C composite powders by ultrasonic spray pyrolysis followed by heat treatment and their electrochemical properties

LiFePO₄ powders could be successfully prepared from a precursor solution, which was composed of Li(HCOO)·H₂O, FeCl₂·4H₂O and H₃PO₄ stoichiometrically dissolved in distilled water, by ultrasonic spray pyrolysis at 500 °C followed by heat treatment at sintering temperatures ranging from 500 to 800 °C in N₂ + 3% H₂ gas atmosphere. Raman spectroscopy revealed that α-Fe₂O₃ thin layers were formed on the surface of as-prepared LiFePO₄ powders during spray pyrolysis, and they disappeared after sintering above 600 °C. The LiFePO₄ powders prepared at 500 °C and then sintered at 600 °C exhibited a first discharge capacity of 100 mAh g⁻¹ at a 0.1 C charge-discharge rate. To improve the electrochemical properties of the LiFePO₄ powders, LiFePO₄/C composite powders with various amounts of citric acid added were prepared by the present method. The LiFePO₄/C (1.87 wt.%) composite powders prepared at 500 °C and then sintered at 800 °C exhibited first-discharge capacities of 140 mAh g⁻¹ at 0.1 C and 84 mAh g⁻¹ at 5 C with excellent cycle performance. In this study, the optimum amount of carbon for the LiFePO₄/C composite powders was 1.87 wt.%. From the cyclic voltammetry (CV) and AC impedance spectroscopy measurements, the effects of carbon addition on the electrochemical properties of LiFePO₄ powders were also discussed.     

Synthesis of single-crystalline Ce(CO3)(OH) with novel dendrite morphology and their thermal conversion to CeO2

Single-crystalline cerium carbonate hydroxide (Ce(CO₃)(OH)) with dendrite morphologies have been successfully synthesized by hydrothermal method at 150 °C using Ce(NO₃)₃·6H₂O as the cerium source, aqueous carbamide as both an alkaline and carbon source and poly(vinyl pyrrolidone) (PVP) as surfactant. Ceria (CeO₂) with dendrite morphologies have been fabricated by a thermal decomposition-oxidation process at 500 °C for 6 h using single-crystalline Ce(CO₃)(OH) dendrites as the precursor. The dendrite morphologies of Ce(CO₃)(OH) was sustained after thermal decomposition-oxidation to CeO₂. The as-prepared products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), and thermogravimetric analysis (TG).     

Microwave-assisted preparation of Ca6Si6O17(OH)2 and β-CaSiO3 nanobelts

Xonotlite (Ca₆Si₆O₁₇(OH)₂) nanobelts were synthesized by a microwave-assisted hydrothermal method at 180 °C for 90 min independent of the feeding molar ratio of Ca(NO₃)₂·4H₂O to Na₂SiO₃·9H₂O in the range of 0.8-3.0. Crystalline wollastonite (β-CaSiO₃) nanobelts were obtained by microwave thermal transformation of Ca₆Si₆O₁₇(OH)₂ nanobelts at 800 °C for 2 h. Ca₆Si₆O₁₇(OH)₂ nanobelts were used as both the precursor and the template for the preparation of β-CaSiO₃ nanobelts. The morphology and size of Ca₆Si₆O₁₇(OH)₂ nanobelts could be well preserved during the microwave thermal transformation process. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED).     

Comparative study of nanocrystalline Zr0.85Ce0.15O2 powders synthesised by spray-pyrolysis and gel-combustion methods

Zr_0.85Ce_0.15O₂ nanopowders synthesised by gel-combustion and spray-pyrolysis methods were comparatively studied by means of X-ray diffraction, Raman spectroscopy, thermogravimetric and differential thermal analyses, specific surface area measurements, scanning and transmission electron microscopies and chemical analysis. Fully tetragonal powders were obtained by both methods, as determined by X-ray diffraction and Raman spectroscopy. Both materials exhibited extremely small crystallite sizes (about 6 nm) and high specific surface areas (93 m²/g and 42 m²/g for gel-combustion and spray-pyrolysis powders, respectively). In both cases, no tetragonal-to-monoclinic transition was observed in the whole temperature range up to 1300 °C by differential thermal analysis. The amounts of the expected impurities (Si, B, C) were acceptable and comparable in both cases.     

Synthesis of hydroxyapatite using phosphate-rich glasses in the system CaO-P2O5-H2O and acoustic waves

Phosphate-rich glass powders in the CaO-P₂O₅ system were prepared by the sol-gel method using aqueous solutions of Ca(NO₃)₂·4H₂O and H₃PO₄. Glass powders were subjected to reaction with Ca(OH)₂ and water under varied experimental conditions, such as temperature (25–90 °C), particle size (1–5 and 40 m), and ultrasound waves (20 kHz). Parallel experiments were also conducted without ultrasound for comparison. The reaction products were composed of Ca(H₂PO₄)₂·H₂O (monocalcium phosphate monohydrate), CaHPO₄ (monatite), CaHPO₄ · 2H₂O (brushite), Ca₅(PO₄)₃OH (hydroxyapatite) and amorphous calcium phosphate. It is interesting to note that hydroxyapatite and brushite were obtained at a low temperature of 60 °C in a very short period of time (30 min). All samples were characterized by X-ray diffraction and scanning electron microscopy.     

Influence of Mg-substitution on the physicochemical properties of calcium phosphate powders

Tricalcium phosphate based ceramics (TCP) are bioresorbable and thereby considered to be promising bone replacement materials. The differences in crystal structure between α and β-TCP phases gives rise for different dissolution rates in vitro and in vivo, which may alter the bioresorbable behavior of TCP ceramics. It is suggested that the addition of magnesium ions, which are also present in biological tissues, stabilizes β-phase to higher temperatures and thus enables the sintering of β-TCP at elevated temperatures compared to Mg free TCP.In this paper, Mg-substituted TCP, with the general formula (Ca_1−xMg_x)₃(PO₄)₂ and 0.01 ≤ x ≤ 0.045, were produced by wet chemical synthesis from Ca(OH)₂, H₃PO₄ and MgO, after calcinations at three different temperatures between 750 and 1050 °C. The influence of different amounts of Mg substitution on the physical properties, microstructure, and sintering behavior of calcium phosphate powders was evaluated. Thermal analytical techniques, together with X-ray diffraction analysis, were successfully combined in order to characterize the occurring phase transformations during annealing of the powders. The results show that the addition of small amounts of Mg (up to 1.5 mol%) are adequate to postpone the β-α TCP phase transformation to 1330 °C and to accelerate the densification process during sintering of β-TCP ceramics.     

High-temperature chemical and microstructural transformations of an organic-inorganic nanohybrid captopril intercalated Mg-Al layered double hydroxide

The thermal evolution of a crystalline organic-inorganic nanohybrid captopril intercalated Mg-Al layered double hydroxide (LDH) [Mg_0.68Al_0.32(OH)₂] (C₉H₁₃NO₃S)_0.130(CO₃)_0.030·0.53H₂O obtained by coprecipitation method is studied based upon in situ high-temperature X-ray diffraction, in situ infrared and thermogravimetric analysis coupled with mass spectroscopy analysis. The results reveal that a metastable quasi-interstratified layered nanohybrid involving carbonate-LDH and reoriented less ordered captopril-LDH was firstly observed as captopril-LDH heat-treated between 140 and 230 °C. The major decomposition/combustion of interlayer organics occur between 270 and 550 °C. A schematic model on chemical and microstructural evolution of this particular drug-inorganic nanohybrid upon heating in air atmosphere is proposed.     

Formation and structure of zinc-substituted calcium hydroxyapatite

Zn-substituted Ca hydroxyapatites were synthesized by precipitation method under the specific conditions (pH 8, 90 °C) and their structural properties were investigated. The substituting limit of Zn was estimated at about 15 mol%. The lattice parameter a decreased up to 5 mol% Zn, and started to increase over 5 mol% Zn. The lattice parameter c monotonously decreased with increase in Zn fraction. The increase in lattice parameter a for higher Zn fraction was ascribed to increasing amount of lattice H₂O which substituted for OH sites in the apatite structure. The lattice H₂O in the Zn-substituted apatites was lost by the heat treatment at 400 °C. As a result, both the lattice parameters a and c of the heat-treated apatites decreased with increasing Zn fraction. Only the difference in ionic radius between Zn²⁺ (0.074 nm) and Ca²⁺ (0.099 nm) was decisive to the change in both lattice parameters after the heat treatment at 400 °C.     

Synthesis and crystallographic study of Pb-Sr hydroxyapatite solid solutions by high temperature mixing method under hydrothermal conditions

The solid solutions in the system of Pb and Sr hydroxyapatite, Sr_10−xPb_xHAp (x = 0-10), were successfully synthesized by high-temperature mixing method (HTMM) at 160 °C for 12 h under hydrothermal conditions. The samples were characterized by X-ray diffraction, chemical analysis and electron microscopic observation, and the site of the metal ions in the solid solutions was analyzed with the Rietveld method. The lattice constants, both a and c, of the solid solutions varied linearly with Pb content. It was found that Pb ions in the solid solutions preferentially occupied the M(2) site in the apatite structure. HTMM gives Sr-Pb HAp solid solutions much better crystallization. However, due to the formation of intermediate compound of Pb₃O₂(OH)₂ in the Pb(NO₃)₂·4H₂O solution before mixing with (NH₄)₂HPO₄ solution at 160 °C, HTMM causes the decrease of crystallization of the samples with high Pb content.     

Phase transformation of calcium phenyl phosphate in calcium hydroxyapatite

Calcium phenyl phosphate (CaPP) was synthesized from a mixture of Ca(OH)₂ and phenyl phosphate (C₆H₅PO₄H₂) in an aqueous media. XRD pattern of CaPP exhibited five diffraction peaks at 2θ = 6.6, 13.3, 20.0, 26.8 and 33.7°. The d-spacing ratio of these peaks was ca. 1:1/2:1/3:1/4:1/5. The molar ratios of Ca/P and phenyl/P of CaPP were 1.0 and 0.92, respectively, and the chemical formula of the material was expressed as (C₆H₅PO₄)_0.92(HPO₄)_0.08Ca·1.3H₂O, similar to that of dicalcium phosphate dihydrate (CaHPO₄·2H₂O: DCPD). These results allowed us to infer that CaPP is composed of a multilayer alternating bilayer of phenyl groups of the phosphates and DCPD-like phase. The structure of the material was essentially not altered after aging at pH 9.0-11.0 and 85 °C in an aqueous media. While, after aging at pH ≤8.0, the diffraction peaks of CaPP were suddenly weakened and disappeared at pH 7.0. Besides, new peaks due to calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂: Hap) appeared and their intensity was strengthened with decreasing the solution pH. TEM observation revealed that the Hap particles formed at pH 6.0 are fibrous with ca. 1.5 μm in length and ca. 0.2 μm in width. From these results, it is presumed that the layered CaPP was dissolved, hydrolyzed and reprecipitated to fibrous Hap particles at pH ≤8.0 and 85 °C in aqueous media. This phase transformation of CaPP in Hap resembled to the formation mechanism of Hap in animal organism.     

Hydrothermal synthesis attempts of dawsonite-type hydroxymetalocarbonate precursor compounds for catalytic Ho, Sm, and La oxides

Chemical interactions in mixed, aqueous solutions of NH₄HCO₃ and M(NO₃)₃·9H₂O, where M stands for Ho, Sm, or La, were facilitated under various hydrothermal treatment conditions (pH 8-12 and temperature = 75-135 °C). The solution chemistry established did not make available necessary concentrations of soluble HCO₃⁻ and MO(OH)₂⁻ species for the formation of dawsonite-type ammonium hydroxymetalocarbonates, NH₄M(CO₃)(OH)₂, but, alternatively, high concentrations of soluble CO₃²⁻, and M(H₂O)_n³⁺ or M(H₂O)_n−1(OH)²⁺ facilitating, respectively, precipitation of corresponding hydrated carbonate, M₂(CO₃)₂·2H₂O, or carbonate hydroxide, MCO₃(OH). X-ray powder diffractometry, infrared spectroscopy, and thermal analyses proved alternative formation of Ho₂(CO₃)₃·2H₂O or LaCO₃(OH) under the whole set of hydrothermal treatment conditions probed, and Sm₂(CO₃)₃·2H₂O at pH < 10 or SmCO₃(OH) at pH ≥ 10, thus implying dependence of the composition of the product carbonate compound on the hydrolysability of the initial M(H₂O)_n³⁺ species and, hence, the metal ionic size (La > Sm > Ho). Calcination of the various hydrothermal treatment products at ≥600 °C resulted in the thermal genesis of the corresponding sesqui-oxides (M₂O₃). Bulk and surface characterization studies of the product oxides, employing N₂ sorptiometry and scanning electron microscopy, in addition to the above analytical techniques, revealed overall strong crystallinity, large average crystallite size, and well-defined particle morphology. They revealed, moreover, surfaces, though of limited accessibilities (≤13 m²/g), exposing OH groups of various coordination symmetries and, hence, acid-base properties, thus furnishing promising surface catalytic attributes.     

Phase transformation of calcium phenyl phosphate in calcium hydroxyapatite using alkaline phosphatase at body temperature

Layered calcium phenyl phosphate ((C₆H₅PO₄)_0.92(HPO₄)_0.08Ca·1.3H₂O: CaPP), which is composed of a multilayer alternating bimolecular layer of phenyl groups and amorphous calcium phosphate phase, was treated in aqueous media including different amounts of enzyme such as alkaline phosphatase (ALP) at pH = 9.6 and 37 °C for 48 h. Treating the CaPP in the absence of ALP took place only the dissolution of this material. When the CaPP particles were treated in the presence ALP, calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂: Hap) was formed. The yielded Hap contained no phenyl group and the molar ratio Ca/P of this material was 1.66, almost corresponding to 1.67 of the stoichiometric Ca/P ratio of Hap. TEM observation revealed that the irregular-shaped CaPP particles disappeared and rod-shaped Hap nano-particles were generated. The particle length and crystallite size of Hap were slightly increased on increasing the additive amount of ALP. These facts allow us to infer that the CaPP particles are dissolved, hydrolyzed and recrystallized to Hap by treating with ALP in aqueous media at body temperature of 37 °C and that the ALP plays as a catalyst for hydrolysis of phenyl phosphate ions. This phase transformation of CaPP in Hap in the presence of ALP resembles to the formation mechanism of Hap in animal organism.     

Solid state and solution synthesis of Ba(Mg1/3Ta2/3)O3: A comparative study

Ba(Mg_1/3Ta_2/3)O₃ [BMT] dielectric ceramics are prepared by solid state (one step, two step and molten salt synthesis) and wet chemical methods (precipitation, citrate gel and sol-gel). The formation mechanism of BMT in each synthesis technique is discussed. The formation temperature and particle size of the formed BMT were found to be much lesser (in nanometer range) for solution synthesized powders. It is found that synthesis by sol-gel method resulted in the formation of ultra pure nanopowders of BMT at about 600 °C with average crystallite size of about 18 nm where as in solid state synthesis the formation of BMT was formed at about 1100 °C with average crystallite size of 220 nm. On sintering these powders, densification and grain growth of the chemically derived powders were found to be lower than that of solid state synthesized BMT powder. This has resulted in a slight decrease in density and microwave dielectric properties of the solution synthesized BMT samples. It is found that the microwave dielectric properties improved with increase in the average grain diameter of the sintered BMT ceramics.     

Synthesis of perovskite-type (La1−xCax)FeO3 (0 ≤ x ≤ 0.2) at low temperature

Gel formation was realized by adding citric acid to a solution of La(NO₃)₃·5H₂O, Ca(NO₃)₂·4H₂O, and Fe(NO₃)₂·9H₂O. Perovskite-type (La_1−xCa_x)FeO₃ (0 ≤ x ≤ 0.2) was synthesized by firing the gel at 500 °C in air for 1 h. The crystallite size (D_1 2 1) decreased with increasing x, while the specific surface area was 6.8-9.4 m²/g and independent of x. The XPS measurement of the (La_1−xCa_x)FeO₃ surface indicated that the Ca²⁺ ion content increased with increasing x, while the Fe ion content was independent of x. Catalytic activity for CO oxidation increased with increasing x.     

Influence of thermal treatment on the structure and adsorption properties of layered zinc hydroxychloride

Layered zinc hydroxychloride (Zn₅(OH)₈Cl₂·H₂O) synthesized by hydrolyzing the ZnO particles in aqueous ZnCl₂ solutions at 100 °C for 48 h was outgassed at different temperatures ranging from 100 to 250 °C for 2 h and the structure and adsorption properties of the products were examined by various means. Outgassing at 100-150 °C eliminated the H₂O molecules in interlayer of zinc hydroxychloride. The layered structure of zinc hydroxychloride was disintegrated at 175 °C by breaking the OH?Cl hydrogen-bond in interlayer to form curled thin films composed of poorly crystallized β-Zn(OH)Cl and ZnO, leading to the increment of the specific surface area from 4 to 39 m²/g. The β-Zn(OH)Cl was decomposed at 225 °C to form ZnO. The crystallinity of ZnO was increased on elevating the outgassing temperature, giving rise to the UV absorption property. The H₂O and CO₂ adsorption measurements revealed that the zinc hydroxychloride outgassed at 100-150 °C possessed a high H₂O and CO₂ adsorption selectivity, and the selectivity diminished by the formation of thin films of ZnO above 175 °C.     

Synthesis of BaZrO3 by a solid-state reaction technique using nitrate precursors

High phase purity barium metazirconate powders have been synthesized from a modified solid-state reaction. Reactive powders consisting of submicron particles and narrow particle size distribution were obtained by heating a 1:1 molar mixture of barium nitrate and zirconyl nitrate at 800°C up to 8 h. Simultaneous thermal analysis (TG-DTA) assisted in elucidating the probable reaction pathways leading to the formation of the target compound in the BaO-ZrO₂ system. Systematic structural and microstructural characterization on the green powders and the compacts sintered up to 1700°C were carried out. A two-stage sintering schedule consisting of a 6 h soak at 1600°C followed by slow heating up to 1700°C with no dwell, led to highly dense microstructural features.     

Order-disorder phase transitions and high-temperature oxide ion conductivity of Er2+xTi2−xO7−δ (x = 0, 0.096)

The Er_2+xTi_2−xO_7−δ (x = 0.096; 35.5 mol% Er₂O₃) solid solution and the stoichiometric pyrochlore-structured compound Er₂Ti₂O₇ (x = 0; 33.3 mol% Er₂O₃) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was performed in the temperature range 650-1690 °C. The amorphous phase exists below 700 °C. The crystallization of the ordered pyrochlore phase (P) in the range 800-1000 °C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the range 1000−1200 °C. Heat-treatment at T ≥ 1600 °C leads to the formation of an oxide ion-conducting phase with a distorted pyrochlore structure (P2) and an ionic conductivity of about 10⁻³ S/cm at 740 °C. Complex impedance spectra are used to separately assess the bulk and grain-boundary conductivity of the samples. At 700 °C and oxygen pressures above 10⁻¹⁰ Pa, the Er_2+xTi_2−xO_7−δ (x = 0, 0.096) samples are purely ionic conductors.     

Influence of calcium hydroxide on the fate of perfluorooctanesulfonate under thermal conditions

To explore the potential fate and transport of perfluorochemicals in the thermal treatment of sludge, perfluorooctanesulfonate (PFOS), a perfluorochemical species commonly dominant in wastewater sludge, was mixed with hydrated lime (Ca(OH)₂) to quantitatively observe their interaction under different temperatures. The phase compositions of the mixtures after the reactions were qualitatively identified and quantitatively determined using X-ray diffraction technique. The results of the thermogravimetry and differential scanning calorimetry analyses indicate that PFOS gasified directly during the thermal treatment process when the temperature was increased to around 425 °C. However, the formation of CaF₂ at 350 °C suggests that the presence of Ca(OH)₂ in the mixture can lead to the decomposition of PFOS at 350 °C, which is lower than the decomposition temperature of PFOS alone (425 °C). The increase of temperature promoted a solid state reaction between PFOS and Ca(OH)₂, and also enhanced the interaction between the gaseous products of PFOS and CaO (or Ca(OH)₂). The preferred Ca/F molar ratio to achieve fluorine stabilization by Ca(OH)₂ was above 1:1 in the experiment involving 400 °C and 600 °C treatment. It also showed that equilibrium efficiency is achieved within 5 min at 400 °C and within 1 min above 600 °C.