Crystalline products and their formation mechanism in thermal hydrolysis of ZrOSO4 solution

Solid products were prepared from mixtures of zirconyl carbonate and sulphuric acid solution under thermal hydrolysis at 240°C. These crystalline products and their formation mechanism were investigated by evaluating the amount of products, and by X-ray diffraction analysis and TEM observation. The solid products precipitated were zirconium oxide sulphates (ZOS, PZOS, ZrOSO₄·H₂O) and m-ZrO₂. The area and thickness of the fine plate-like crystals of ZOS increased with increasing concentration of the solution and hydrothermal treatment time. For thin starting solutions, the precipitation velocity and the amount of PZOS increased. For thick starting solutions, those of ZOS increased. For 0.5 mol/l^–1 ZrOSO₄ solution (ZS0.5) from the thin solution, PZOS crystallized early and m-ZrO₂ crystallized by releasing sulphuric acid ions in PZOS with treatment time. For ZS2.5 from the thick solution, ZrOSO₄·H₂O was crystallized by increasing the proportion of sulphuric acid ions to zirconium ions in the solution, which was caused by ZOS precipitation.     

Hydrothermal deposition and characterization of ultrafine ZrO2 thin films

Ultrafine ZrO₂ thin films (0.6 μm) have been prepared by a hydrothermal method on polycrystalline alumina substrates (α-Al₂O₃). The hydrothermal treatment of zirconium hydroxide sol in the temperature range of 100–240 °C for 6–72 h results in the formation of monoclinic single-phase films. The resulting films have a white appearance and are homogeneous, without visible pores and defects, and the average grain size in the film is 18 nm.     

Bulk and surface structures of iron doped zirconium oxide systems: Influence of preparation method

Three different Fe-Zr oxide systems were prepared using firstly classical impregnation of iron nitrates on calcined ZrO₂ (Fe_x/ZrO₂, x represents Fe/Zr ratio = 0.01 and 0.11), secondly impregnation of iron nitrates on dried zirconium hydroxide ZrO(OH)₂ (Fe_x/ZrO(OH)₂) and finally hydrolysis of aqueous suspension of iron and zirconium salts to coprecipitate iron and zirconium hydroxides (Fe_x-Zr). Thermal decomposition study of dried samples evidenced a delay in the temperature crystallization of zirconia for Fe_x-Zr and Fe_x/ZrO(OH)₂, the more the iron content in the sample, the more important the delay. For these samples, the formation and the stabilization of different phases were evidenced by several characterization techniques : X-Ray Diffraction (XRD), Raman spectroscopy and Electron Paramagnetic Resonance (EPR).The interaction of iron species with zirconia was different in accordance with different preparations. A bulk dispersion of the coprecipitated sample was observed and as a consequence Zr^{3 +} defects in the solid were not produced. In the case of Fe_x/ZrO₂ sample, production of surface Zr^{3 +} ions was established at low temperature of calcination (up to 600^∘C) and explained by the reaction of NO₃⁻ with Zr^{4 +} on the zirconia surface. However such interaction did not occur for Fe_x/ZrO(OH)₂ since a low dispersion of iron species was observed by X-ray Photoelectron Spectroscopy (XPS), deposited phase (Fe₂O₃) forming preferentially blocks. Temperature Programmed Reduction (TPR) showed that the reduction of small particles of Fe₂O₃ and bulk Fe₂O₃ present in the impregnated samples was easier than that of iron species well dispersed in the bulk of the coprecipitated solid.     

Purification of hot-pressed ZrCO into ZrC by a laser treatment

Pellets of zirconium oxycarbide ZrC_0.82O_0.14, obtained by hot pressing of zirconia and zirconium carbide, were irradiated by an ytterbium-doped fibre laser in argon atmosphere (incident power density?=?24.7?kW?cm^?2, beam diameter 0.7?mm). The surface of the samples, heated at 3300???C and more, i.e. near the melting, released its oxygen, leading to the carbide ZrC_0.75 despite the presence of traces of oxygen in the cell of treatment ( $ P_{{{\text{O}}_{2} }} $ estimated around 1?Pa). A mechanism is proposed for explaining this result, based on the thermodynamical stability of the carbide, higher at these temperature and oxygen pressure than the oxide. Oxygen of the oxycarbide evolved in the form of the gaseous species ZrO, while the grains of the oxycarbide, converted into the carbide, grew from 2 to 20???m. Similarly, the traces of dioxygen present inside the treatment cell react with the carbide, giving ZrO (gas), and do not form any oxidised solid phase. This opens interesting future prospects in the field of the production of oxygen-free zirconium carbide powders.     

Preparation and properties of uniform mixed and coated colloidal particles

Colloidal dispersions of uniform spherical particles of zirconium basic sulphate and zirconium oxy-basic carbonate were prepared by ageing zirconium sulphate solutions at elevated temperatures in the presence of urea. Different chemical compositions of the above products resulted when the ageing temperature was altered. Depending on the nature of the original solids, calcination at 800° C resulted in the formation of tetragonal or monoclinic zirconia. Under certain conditions a mixed phase, including cubic zirconia, has also been identified. The particle morphology was retained during these transformations. Coprecipitation in mixed solutions of zirconium and yttrium salts aged at 80° C yielded composite spherical particles of basic carbonate with a zirconium to yttrium ratio of the solid similar to that used in the initial solution. Zirconium basic sulphate particles coated with yttrium basic carbonate were prepared by ageing, in the presence of urea, a Zr₂(OH)₆SO₄ dispersion containing yttrium nitrate.     

Effect of urea on formation of hydroxyapatite through double-step hydrothermal processing

The effect of urea on the formation of hydroxyapatite (HAp) was studied by employing the double-step hydrothermal processing of a powder mixture of beta-tricalcium phosphate (β-TCP) and dicalcium phosphate dihydrate (DCPD). Co-existence of urea was found to sustain morphology of HAp crystals in the compacts under an initial concentration of 2 mol dm^-3 and less. Homogenous morphology of needle-like crystals was observed on the compacts carbonated owing to decomposition of urea. Carbonate ions (CO₃^2-) was found to be substituted in both the phosphate and hydroxide sites of HAp lattice. The synthesized HAp was calcium deficient, as it had a Ca/P atomic ratio of 1.62 and the phase was identified as calcium deficient hydroxyapatite (CDHA). The release of CO₃^2- ions from urea during the hydrothermal treatment determined the morphology of the CDHA in the compacts. The usage of urea in the morphological control of carbonate-substituted HAp (CHAp) employing the double-step hydrothermal method is established.     

Hydrothermal synthesis of improved ZnO crystals for epitaxial growth of GaN thin films

ZnO single crystals with thickness up to 12 mm, 2 inches in “diameter” and weight of about 150 g have been grown from KOH, NaOH, and K₂CO₃ based hydrothermal solutions on the seeds of (0001) orientation. The addition of LiOH up to 3.0–4.5 mol/L allowed to decrease the growth rate of ZnO crystals along the 〈0001〉 crystallographic direction. For positive and negative monohedra, it was achieved 0.12 and 0.01 mm/day, respectively, at temperature 340 °С and ΔТ = 10 °С. The best ZnO etching agent was found to be the solutions 25 mol% HCl + 3 mol% NH₄F at room temperature, and etching time 5 min. The dislocation density of ZnO crystals varied from 240 cm⁻² to 3,200 cm⁻² in the case of growth rates 0.04 mm/day to 0.11 mm/day, respectively. It was also found that ZnO crystals grown are stable in air, oxygen, nitrogen, and argon atmosphere as well as in vacuum at the temperatures up to 1,000 °C under thermal treatment during 4 h.     

Zirconia-based nanocrystals in the ZrO2-In2O3 system

Zirconia-based ZrO₂-In₂O₃ nanocrystals are prepared by hydrothermal treatment of coprecipitated zirconium oxyhydroxide and indium hydroxide. Indium oxide is shown to dissolve predominantly in cubic zirconia nanocrystals. Its solubility in nanocrystalline zirconia notably exceeds the equilibrium In₂O₃ solubility in ZrO₂ single crystals.     

氢化锆在高温水蒸气中的氧化行为

通过恒温恒压氧化实验研究了氢化锆在300~700℃高温水蒸气中的氧化行为。结果表明,氢化锆的质量增重随着氧化温度的升高而增大。在氧化过程进行15h以后,OH-/O在氧化膜中的内扩散成为氧化反应的控速步骤。水蒸气中的H抑制了氢化锆在高温氧化条件下的氢损失。氧化膜主要由单斜相M-ZrO2组成,氧化膜的最外层由四方相T-ZrO2和立方相C-ZrO2组成。     

Synthesis of acicular α-Bi2O3 microcrystals by microwave-assisted hydrothermal method

ABSTRACT

Materials based on bismuth(III) oxide are candidate to be used in optical and electronic devices because of their properties such as a variable band gap, photoconductivity, photoluminescence, high refractive index, and dielectric permittivity. These properties are dependent of several factors, e.g., present phases and crystal morphology. The microwave-assisted hydrothermal method (MAH) is a fast and efficient approach of synthesis to obtain semiconductor powders. However, the synthesis of monoclinic bismuth oxide (α-Bi₂O₃) with acicular morphology by MAH was not found in literature. In this paper, microcrystals of acicular α-Bi₂O₃ (monophasic) were successfully obtained by MAH using a synthesis temperature of 80°C for 0.5?h. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron, and transmission electron microscopies showed the formation of a monoclinic structure (space group P21/c) with acicular morphology that grew along the [001] direction. The temperature and time necessary to synthetize acicular microcrystals were significantly lower than those found for acicular microcrystals obtained by conventional hydrothermal method.     

Synthesis of high-surface-area complex zirconium phosphates via mechanochemical activation route

 High-power ball mill activation of the mixture of hydrated zirconium and lanthanum salts (oxonitrates, oxochlorides) with ammonium phosphate followed by hydrothermal treatment at temperatures not exceeding 200°C and a nearly neutral pH was found to yield crystalline dispersed phase of a cubic NH₄Zr₂(PO₄)₃ type along with admixtures of disordered orthorhombic compounds of a zirconium orthophosphate type. In the same conditions and at the same Zr/P ratio, hydrothermal treatment of gels obtained by reacting mixed zirconium and lanthanum nitrates solutions with ammonium phosphates yields no crystalline products, and only treatment in acid media generates a phase of the α-ZrPO₄(OH) type coexisting with the NH₄Zr₂(PO₄)₃ phase if polyethylene oxide is present. X-ray powder diffraction, transmission electron microscopy, ³¹MAS-NMR, FTIRS and thermal analysis were applied to elucidate factors affecting crystallization of complex zirconium phosphates in the hydrothermal conditions. The most essential factor appears to be generation of some nuclei of zirconium phosphates under high pressures developed in the course of mixed solids mechanical activation. These nuclei are embedded into matrix of such well-crystallized solid products as ammonium nitrate or chloride. Hence, metastable cubic or orthorhombic structure of the phases obtained via mechanical activation route can be assigned to the nuclei-matrix orientation relationship. Due to easily scaled-up synthesis procedure, these results appear to be very promising for manufacturing of dispersed framework zirconium phosphates as acid catalysts or fast proton conductors. Received: 18 November 1998 / Reviewed and accepted: 2 December 1998     

Reflux synthesis and hydrothermal processing of ZrO2 nanopowders at low temperature

In this paper, we report on the reflux synthesis at 90 °C and hydrothermal processing at 120 °C for obtention of zirconium oxide (ZrO₂) nanopowders under several conditions. These nanopowders were characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman) spectroscopy, adsorption–desorption N₂-isotherms, Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA) and field-emission scanning electron microscopy (FE-SEM). XRD patterns and Raman spectra indicated that ZrO₂ nanopowders present a monoclinic structure. In addition, the hydrothermal processing promoted an increase in crystallinity of ZrO₂ nanopowders. FT-IR spectra revealed a small shoulder on the ν (Zr–O) bands in transmittance spectra of the ZrO₂ nanopowders. The decomposition of precursor was accompanied by evolution of TGA curves. The morphology of ZrO₂ nanopowders was observed by FEG-SEM. Also, the FEG-SEM micrographs revealed that the presence of H₂O₂ in systems reduced the particle size, while the absence of promoted an increase in particle size.     

Preparation of Mn2SnO4 nanoparticles as the anode material for lithium secondary battery

The ultrafine Mn₂SnO₄ nanoparticles with diameters of 5-10 nm have been prepared by thermal decomposition of precursor MnSn(OH)₆. The MnSn(OH)₆ nanoparticles precursor was synthesized by a hydrothermal microemulsion method. X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy and electron diffraction have been employed to characterize the crystal structures and morphologies of the as-prepared samples. High-resolution transmission electron microscopy observations revealed that the as-synthesized nanoparticles were single crystals. The thermal characterization was studied by differential thermal analysis and thermogravimetry analysis measurements. Electrochemical test showed that the Mn₂SnO₄ nanoparticles exhibited a high initial charge-discharge capacity of 1320 mAh/g.     

Synthesis of ultrafine ZrB<Subscript>2</Subscript> powders by sol-gel process

Ultrafine zirconium diboride (ZrB₂) powders have been synthesized by sol-gel process using zirconium oxychloride (ZrOCl₂·8H₂O), boric acid (H₃BO₃) and phenolic resin as sources of zirconia, boron oxide and carbon, respectively. The effects of the reaction temperature, B/Zr ratio, holding time, and EtOH/H₂O ratio on properties of the synthesized ZrB₂ powders were investigated. It was revealed that ultrafine (average crystallite size between 100 and 400 nm) ZrB₂ powders can be synthesized with the optimum processing parameters as follows: (i) the ratio of B/Zr is 4; (ii) the solvent is pure ethanol; (iii) the condition of carbothermal reduction heat treatment is at 1550°C for 20 min.     

The orthorhombic/monoclinic transition in single crystals of zeolite ZSM-5

X-ray photographs of single crystals of zeolite H-ZSM-5 (Si_11.96Al_0.04H_0.04O₂₄) at different temperatures are presented. The reversible orthorhombic/monoclinic transition, previously observed with XRD and ²⁹Si m.a.s. n.m.r. on powder samples of H-ZSM-5, is confirmed. Upon cooling, the orthorhombic H-ZSM-5 single crystal changes into an aggregate of monoclinic twin domains. The directions of a and c in the orthorhombic single crystal and in the monoclinic twinned crystal (a ≈ 90.5°) are identical. The deviation of α from 90° can be ascribed to a mutual shift of successive (010) pentasil layers along c, induced by a distortion or rotation of the T₄ and T₆ rings interconnecting the layers. The space group of the monoclinic phase most probably is P₁²n11.     

Anchoring and Upgrading Ultrafine NiPd on Room‐Temperature‐Synthesized Bifunctional NH2‐N‐rGO toward Low‐Cost and Highly Efficient Catalysts for Selective Formic Acid Dehydrogenation

Hydrogen is widely considered to be a sustainable and clean energy alternative to the use of fossil fuels in the future. Its high hydrogen content, nontoxicity, and liquid state at room temperature make formic acid a promising hydrogen carrier. Designing highly efficient and low‐cost heterogeneous catalysts is a major challenge for realizing the practical application of formic acid in the fuel‐cell‐based hydrogen economy. Herein, a simple but effective and rapid strategy is proposed, which demonstrates the synthesis of NiPd bimetallic ultrafine particles (UPs) supported on NH₂‐functionalized and N‐doped reduced graphene oxide (NH₂‐N‐rGO) at room temperature. The introduction of the ? NH₂? N group to rGO is the key reason for the formation of the ultrafine and well‐dispersed Ni_0.4Pd_0.6 UPs (1.8 nm) with relatively large surface area and more active sites. Surprisingly, the as‐prepared low‐cost NiPd/NH₂‐N‐rGO dsiplays excellent hydrophilicity, 100% H₂ selectivity, 100% conversion, and remarkable catalytic activity (up to 954.3 mol H₂ (mol catalyst)^?1 h^?1) for FA decomposition at room temperature even with no additive, which is much higher than that of the best catalysts so far reported.     

Synthesis and characterization of γ-zirconium arsenate

The hydrothermal synthesis of a novel layered zirconium arsenate, Zr( AsO₄) (H₂AsO₄) · 2H₂O, with γ-type structure and preliminary data on its characterization by chemical and thermal analysis, X-ray diffraction (XRD) and ion exchange are reported. This XRD powder pattern is indexed on a monoclinic cell: a = 5.5699(2) Å, b = 6.8175(4) Å, c= 12.1203(6) Å and β= 103.15(1)°.     

Phases in copper-stabilized zirconia solid oxide fuel cells anode material

Solid oxide fuel cells (SOFC) are emerging as an alternate source of energy. Anodes form one of the components of the fuel cells. Ni/Yttrium stabilized zirconia is a classic anode material for SOFC when hydrogen is used as the fuel source, but it is not that effective when methane is used as fuel source due to carbon deposition on the anode. Recently, copper stabilized zirconia has been investigated as anode material for SOFC for its self-cleaning properties. We have tried to investigate phases in copper stabilized zirconia for better understanding of its properties. Copper stabilized zirconia (CSZ) with different CuO loading was prepared by the solid-state reaction method. X-ray diffraction studies on these samples reveal only monoclinic zirconia phase in those samples loaded with less than 5 mol% of CuO. Traces of monoclinic CuO along with monoclinic ZrO₂ is observed in the samples when loading of CuO is between 5 and 20 mol%. Orthorhombic copper zirconium oxide and monoclinic zirconia phases were observed when CuO loading was greater than 20 mol%. Scanning and back-scattered electron micrographs reveal a clear two-phase structure only in the samples with greater than 20 mol% of CuO loading. Atomic force microscopy carried out on 33 mol% loaded zirconia shows a three-phase structure with flattened seven-fold-coordination of Zr⁴⁺ with oxygen.     

Formation of chromium rich oxide scales for protection against metal dusting

Abstract

Metal dusting is a disintegration of metals and alloys into graphite and metal particles, caused by strongly carburizing gas mixtures mainly in the temperature range 400–700°C. Protection of steels against metal dusting is possible through the formation of dense chromium rich oxide scales but it is not guaranteed that such scales are formed at low temperatures, even on high Cr-steels.

Surface analytical studies have been conducted on the formation and composition of the oxide scales on 9–20%Cr steels. The growth of oxide films was followed by AES for 3 hours at 10^–7 mbar O₂ great differences were observed in dependence on surface finish. On ground samples, Mn and Si appeared early and Cr-rich oxide was formed, whereas on chemically etched samples Fe-rich oxides grew.

After long term exposures (240 h) under metal dusting conditions, i.e. in CO–H₂–H₂O mixtures at 600°C, thin Cr-rich scales were observed on ground steels which were impermeable to carbon whereas on chemically etched steels thick Fe-rich scales had grown and carbon penetration was detectable. Accordingly, the oxide formation on Cr-steels at relatively low temperatures strongly depends on the surface treatment. Any surface working such as grinding and sand-blasting etc. introduces dislocations and causes a fine-grained microstructure near the surface, and the dislocations and grain boundaries act as rapid diffusion paths for supply of Cr to the surface in the first minutes of exposure, which leads to the formation of a protective oxide scale.