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.     

熔盐镁热还原法合成ZrB2超细粉体

为了降低ZrB_2粉体的合成温度,并在此基础上合成粒径细小、纯度高的ZrB_2粉体,以ZrO2及B4C为原料,以Mg粉为还原剂,以NaCl-KCl为熔盐介质,研究熔盐镁热还原法低温合成ZrB_2超细粉体的工艺。探讨了反应温度、B/Zr物质的量比及Mg粉用量对合成ZrB_2超细粉体的影响,并对粉体的物相组成及显微结构进行了表征。结果表明合成ZrB_2的起始温度为1 173K,最佳合成温度为1 473K。合成纯相的ZrB_2粉体最佳工艺条件为:B/Zr物质的量比为2.2,Mg过量50%(质量分数),1 473K反应3h。所合成ZrB_2粉体的晶粒尺寸为30~300nm。     

Low-temperature synthesis of ZrB2 nano-powders using a sorbitol modified sol–gel processing route

High-purity zirconium diboride nano-powders were synthesized by carbothermal reduction reaction at a relatively low temperature from a novel sorbitol modified sol–gel method. Phase composition and morphology of the ZrB₂ nano-powders were characterized by X-ray diffraction and transmission electron microscopy, respectively. The effect of sorbitol on sol–gel process and the formation mechanism of ZrB₂ powders were investigated by fourier transform infrared spectroscopy and thermo gravimetric/differential thermal analysis. Sorbitol, used as bridging and chelating ligand, led to the formation of chelate complex through its polyhydroxy opening reaction with H₃BO₃, and then induced the condensation of zirconia forming Zr–O–C–B network, which promoted the carbothermal reduction reaction to complete at 1450 °C for 1 h. The synthesized powders exhibited near-spherical morphology with a small average crystalline size of about 100 nm. With respect to the conventional solid state method, the sorbitol modified sol–gel method route guaranteed a faster, easier and energy-saving process for obtaining single-phase nano-powders.     

Synthesis of Zirconium Diboride Films and ZrB<Subscript>2</Subscript>/BC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Heterostructures

Using mechanochemical synthesis, we have prepared zirconium borohydride, Zr(BH₄)₄, as a precursor for ZrB₂ film growth by chemical vapor deposition. We have carried out the thermodynamic modeling of phase formation processes in the Zr–B–(N)–H and Zr–B–(N)–H–O systems in a wide temperature range, from 100 to 2500°C, at various p_(H2)/p_(Zr(BH4)4) and p_(NH3)/p_(Zr(BH4)4) partial pressure ratios in the starting gas mixtures. A process has been proposed for the growth of zirconium diboride films by Zr(BH₄)₄ decomposition using two techniques: chemical vapor deposition and plasma-enhanced chemical vapor deposition. We also developed a process for the growth of multilayer ZrB₂-and BC _x N _y -based structures.     

Mechanical properties and microstructure of in situ synthesized ZrB2-ZrN1−x composites

ZrB₂-ZrN_1−x composites were in situ synthesized from Zr and BN powders by hot-pressing at high temperatures. Thermodynamic calculation indicates that ZrN will be formed preferentially than ZrB₂ in Zr-BN system. Three samples with Zr/BN molar ratios of 3:2, 3.5:2 and 2:1 were investigated at temperatures above 1650°C. All mixtures of Zr and BN transformed to ZrB₂-Zr_1−x composites completely without any other detectable phases. Nonstoichiometric zirconium nitride, Zr_1−x, is supposed to be formed in 3.5:2- and 2:1-samples. The microstructural morphology of well-sintered ZrB₂-Zr_1−x composites is characterized by quadrate column-shaped ZrB₂ distributed evenly in the interwoven acicular Zr_1−x matrix. A certain amount of hollow rectangular-shaped ZrB₂ with open ends is found in 3.5:2-sample hot-pressed at 1700°C, while some large spherical particles with lots of acicular Zr_1−x sticked on its surface are observed in 2:1-sample hot-pressed at 1800°C. Excessive Zr compared to the stoichiometric Zr/BN molar ratio of 3:2 will facilitate the densification process. Acicular Zr_1−x is apparently beneficial to the improvement of bending strength and fracture toughness of ZrB₂-Zr_1−x composites.     

High purity synthesis of ZrB2 by a combined ball milling and carbothermal method: Structural and magnetic properties

The present work provides a new insight into the high purity synthesis of zirconium diboride (ZrB₂) powders and a method of controlling impurity during the synthesis process. The single phase ZrB₂ nano-powder was synthesized by a combined ball milling and carbothermal method using zirconium oxide (ZrO₂), boron oxide (B₂O₃) and carbon (C) as starting materials. The reaction pathway, phase purity, and morphology of the ZrB₂ produced are elucidated from X-ray diffraction (XRD) and scanning electron microscopy studies. The details of the impure phases generated during synthesis were obtained from multi-phase Rietveld refinements of XRD data. Experiments revealed that the method of synthesis carried out at 1750?°C involving ZrB₂:B₂O₃:C at a molar ratio of 1:4.5:7.5 could produce highly pure ZrB₂ nano-powders of 67?nm average crystallite size. The magnetometry studies on such pure form of ZrB₂ nano-powders indicated that both paramagnetic and diamagnetic characteristics coexisted in ZrB₂, which could be attributed to its polycrystallinity.     

Effect of high dilution on the in situ synthesis of Ni–Zr/Zr–Si(B,C) reinforced composite coating on zirconium alloy substrate by laser cladding

High dilution of transition metals was employed as a new idea for in situ synthesis of Ni–Zr/Zr–Si(B, C) reinforced composite coatings by high power diode laser (HPDL) cladding Ni–Cr–B–Si powders on zirconium substrate. Microstructure, phase composition, the mechanism of in situ synthesis reinforcement and the microhardness of coatings were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and micro-sclerometer. The results reveal that the morphologies and phase constituents are related to the content of alloying elements in powders. In low alloy coatings, the matrix was mainly composed of intermetallic compounds including NiZr and Ni₁₀Zr₇, while the reinforcements consisted of Zr₅Si₄, β-ZiSi, α-ZrSi and ZrC. At the top of high alloy coatings, the matrix was partially comprised of Zr-based amorphous phase with the reinforcements containing ZrB₂. It is thermodynamically favorable for ZrB₂ ceramic reinforcement to form compared to ZrC phase. The microstructure evolution was dependent on the contribution of the high dilution zirconium alloy substrate to the in situ reinforcement synthesis. The microhardness of the coating showed clear improvement compared with zirconium alloy substrate, although high variability was also found.     

Reaction chemistry during self-propagating high-temperature synthesis (SHS) of H3BO3-ZrO2-Mg system

In the present investigation, the ZrB₂ powder is produced by SHS of mixture containing H₃BO_3, ZrO₂ and Mg. The thermal analysis and XRD study reveal the reaction mechanism of ZrB₂ formation by SHS process. Synthesis of H₃BO₃-Mg system results in formation of Mg₃(BO₃)₂ and MgB₄ phases, whereas ZrO₂ is partially reduced to Zr₃O and Zr during synthesis of ZrO₂-Mg system. The reaction between elemental Zr and MgB₄ results in ZrB₂ formation. The particle size of ZrB₂ is found to decrease with the addition of SHS diluent.     

Crystal structure,electron-to-atom ratio and hydrogen storage capacity of zirconium based intermetallic laves phases

Laves phases of the type AB₂, where A is zirconium, have potential for use as hydrogen storage materials. These Laves phases have one of two different crystal structures, namely the C15 (cubic MgCu₂-type) or the C14 (hexagonal MgZn₂-type) Strukterbericht types. Data are presented on the lattice parameters for the two phases and relationships are developed between the lattice parameters of the two phases both for the binary (ZrB₂) and the pseudobinary (Zr(B′_xB″_1?x)2) Laves phases. Electron-to-atom ratios are calculated for both the binary and pseudobinary Laves phases and these are related to hydrogen storage capacity (hydrogen-to-metal ratio, H/M). The hydrogen-to-metal ratio is shown to decrease with increasing electron-to-atom ratio.     

Synthesis of Fe based ZrB2 composite coating by gas tungsten arc welding

Abstract

ZrB₂/Fe composite coating was in situ synthesised by gas tungsten arc welding cladding process on AISI 1020 steel. Zr, B₄C and Fe–B alloy powders were used as precursor powders. The phase composition and microstructure were investigated by X-ray diffraction analysis, optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. Microhardness of ZrB₂/Fe composite coating at room temperature was examined. Main phases obtained from Zr and B₄C precursor are ZrB₂ and α-Fe, and those obtained from Zr and Fe–B precursor are ZrB₂ and FeB. In the upper part of these composite coatings, ZrB₂ phase mainly grows along temperature gradient direction. The middle part of these composite coatings has the highest ZrB₂ content and highest microhardness. Gradient dispersions of ZrB₂ reinforcements appeared in the composite coating from the middle to the bottom, leading to gradient dispersions of microhardness. With decreasing dilution rate, ZrB₂ content and microhardeness increase.     

Surface and structural characterisation of coprecipitated CexZr1?xO2 (0?≤?x?≤?1) mixed oxides

Ce _x Zr_1−x O₂-mixed oxides with three different Ce/Zr ratios (Ce_0.8Zr_0.2O₂, Ce_0.5Zr_0.5O₂ and Ce_0.2Zr_0.8O₂) along with pure cerium and zirconium oxides were prepared by coprecipitation of the metal hydroxides in alkali media and subsequent calcinations at 500 °C, using two different cerium precursors (Ce(NO₃)₃·6H₂O or (NH₄)₂Ce(NO₃)₆). These samples were characterised by N₂ adsorption at −196 °C, XRD, Raman spectroscopy, XPS and H₂-temperature programmed reduction. Besides, the two mixed oxides with higher cerium content were calcined at higher temperature (1000 °C) with the additional purpose of studying their thermal stability and phase homogeneity. XRD and Raman spectroscopy confirm a significant improvement in the insertion of zirconium cations into the ceria lattice when the samples Ce_0.8Zr_0.2O₂ and Ce_0.5Zr_0.5O₂ are synthesised with (NH₄)₂Ce(NO₃)₆ instead of Ce(NO₃)₃·6H₂O. This is attributed to a more homogeneous coprecipitation of cerium and zirconium hydroxides, leading to mixed oxides with better bulk oxygen mobility and smaller lattice parameter. Moreover, the mixed oxides prepared with the (NH₄)₂Ce(NO₃)₆ precursor and calcined at 1000 °C exhibit a single phase whereas phase segregation occurs in the counterpart mixed oxides prepared with the Ce(NO₃)₃·6H₂O precursor. XPS analysis reveal correlations among O/(Ce + Zr) surface atomic ratio and total cerium content for both cerium precursors. Among the samples calcined at 1000 °C, Ce_0.8Zr_0.2O₂ synthesised with Ce(NO₃)₃·6H₂O is the only one that preserves the low-temperature surface reduction peak, and also shows a BET surface area slightly higher than those of the rest of samples calcined at high temperature.     

Spark plasma sintering of ZrB<Subscript>2</Subscript>–ZrC powder mixtures synthesized by MA-SHS in air

Mechanical activation-assisted self-propagating high-temperature synthesis (MA-SHS) in air was successfully applied to the synthesis of the powder mixtures of ZrB₂ and ZrC as a precursor of the ZrB₂–ZrC composite. When the powder mixtures of Zr/B/C = 4/2/3–6/10/1 in molar ratio were mechanically activated (MA) by ball milling for 45–60 min and then exposed to air, they self-ignited spontaneously and the self-propagating high-temperature synthesis (SHS) was occurred to form ZrB₂ and ZrC. The ZrB₂–ZrC composites were produced from these MA-SHS powders by spark plasma sintering (SPS) at 1800 °C for 5–10 min and showed the fine and homogeneous microstructure composed of the <5 μm-sized grains. The mechanical properties of the composites evaluated by Vickers indentation method showed the values of Vickers hardness of 13.6–17.8 GPa and fracture toughness of 2.9–5.1 MPa·m^1/2, depending on the molar ratio of ZrB₂/ZrC. Thus, the better microstructure and mechanical properties of the ZrB₂–ZrC composites were obtained from the MA-SHS powder mixtures, compared with those obtained from the MA powder, the mixing powder and the commercial powder mixtures.     

Synthesis and microstructure characterization of tetragonal Zr1–xTixO2 (x = 0–1) solid solutions

Oxide powders of Zr_1–xTi_xO₂ (x = 0–1) solid solutions with micron-sized particles were synthesized via a solution combustion method. The synthesis process and Zr/Ti molar ratio were optimized to produce powders with the tetragonal crystal structure. X-ray diffraction, Raman spectroscopy and transmission electron spectroscopy results confirm that a full crystallization microstructure with the single tetragonal phase is obtained after calcination at 600 °C while maintaining the crystallite size <30 nm. Zr/Ti oxide mixtures with Zr ≥ 67 mol% exhibit a tetragonal crystal structure and the embedding Ti in ZrO₂ improves the structure stability. The nitrogen sorption results indicate that the powders possess mesoporous morphology with medium specific surface areas (∼10–50 m²/g). Chemical stability tests show that these powders are relatively stable with negligible removal of titanium and zirconium after elution by 0.5 mol/L HCl. Density functional theory was used to calculate the most stable structure with low energy for the selected composition.     

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)°.     

Gerichtet erstarrte Mo‐Zr‐B‐Legierungen

Two high temperature alloys, namely Mo‐13Zr‐25.9B and Mo‐17.4Zr‐34.8B (in at. %), which were specified as eutectic compositions according to the literature were produced with a zone melting (ZM) method [1, 2]. Investigations with a scanning electron microscope demonstrated that the microstructures of both alloys are not completely eutectic. The alloy Mo‐13Zr‐25.9B shows well‐aligned arrangements of their microstructural constituents along the crystallization direction. X‐ray diffraction analysis revealed the phases molybdenum solid solution and zirconium monoboride (ZrB) in each alloy and, additionally, in alloy Mo‐13Zr‐25.9B the phases Mo₂Zr and dimolybdenum boride (Mo₂B) and in alloy Mo‐17.4Zr‐34.8B the phase zirconium diboride (ZrB₂). Moreover, the microhardness of the individual phases was measured. The fracture toughness of both materials was determined using the SEVNB method according to DIN EN ISO 23146. Finally, the creep resistance of the alloys was tested at 1100 °C under compressive loading and compared with other molybdenum alloys and a single‐crystalline nickel based superalloy.     

Mechanochemical Synthesis and SHS of Diborides of Titanium and Zirconium

Some properties of titanium diboride (TiB₂) obtained by explosive mechanochemical synthesis and self-propagated high-temperature synthesis (SHS) have been investigated. The properties of zirconium diboride (ZrB₂) obtained by SHS have also been studied. There is a general opinion that explosive mechanochemical synthesis proceeds by a SHS mechanism. For that reason, it is of interest to compare the properties of a product synthesized from the same reagents, by both mechanochemical synthesis and SHS. In order to elucidate the peculiarities of mechanochemical synthesis, the changes in shape and size of the titanium particles occurring during their mechanical treatment up to the moment of synthesis have been examined. Titanium and zirconium powders with particles differing drastically in shape and size have been used for the synthesis of TiB₂ and ZrB₂ by SHS. It has been shown that irrespective of the difference in properties of the reagents, the products obtained have some common properties characteristic of the synthesis method and important with respect to the practical applications of the borides of titanium and zirconium.     

Synthesis of calcium phosphate powder from calcium lactate and ammonium hydrogen phosphate for the fabrication of bioceramics

A calcium phosphate powder has been synthesized from aqueous 0.25, 0.5, and 1.0 M calcium lactate and ammonium hydrogen phosphate solutions atat a Ca/P = 1, without pH adjusting. According to X-ray diffraction data, the as-synthesized powder consisted of brushite (CaHPO₄ · 2H₂O) and octacalcium phosphate (Ca₈(HPO₄)₂(PO₄)₄ · 5H₂O). After heat treatment in the range 500–700°C, the powders were gray in color because of the destruction of the reaction by-product. The powders heat-treated in the range 500–700°C consisted largely of γ-Ca₂P₂O₇. The ceramics prepared from the synthesized powders by firing at 1100°C consisted of β-Ca₂P₂O₇ and β-Ca₃(PO₄)₂.     

Hydrothermal preparation of Ba(Ti,Zr)O3 fine powders

Fine powders consisting of aggregated submicron crystallites of Ba(Ti,Zr)O₃ in the complete range of Ti/Zr ratios are prepared at 85–130°C by hydrothermal method, starting from TiO₂ + ZrO₂ · xH₂O mixed gel and Ba(OH)₂ solution. The products obtained below 110°C incorporate considerable amounts of H₂O and OH⁻ within the lattice. As-prepared BaTiO₃ is cubic and converts to tetragonal phase after the heat treatment at 1200°C, accompanied by the loss of residual hydroxyl ions. TEM investgations of the growth features show a transformation of the gel to the crystallite. Ba²⁺ ions entering the gel produce chemical changes within the gel, followed by dehydration, resulting in a cubic perovskite phase irrespective of Ti/Zr. The sintering properties of these powders to fine-grained, high density ceramics and their dielectric properties are presented.     

Lewis and Brønsted acids in super-acid catalyst SO<Subscript>4</Subscript><Superscript>2−</Superscript>/ZrO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript>

Super-acid catalyst, SO₄ ^2?/ZrO₂–SiO₂, with high zirconium loading was synthesized and the nature of the surface acid was investigated by FT-IR of pyridine adsorption. With the increasing ZrO₂ content, the Lewis and Brønsted acid sites increased and reached the maximum when Zr/Si (molar ratio) = 1.3. The sample with Zr/Si = 1.3 showed the strongest IR adsorption band in the S=O stretching region (1,300–1,400 cm^?1). Pyrosulfate and monosulfate species existed on the surface of the catalysts and the acidic strength could be enhanced by induction effect of their S=O groups. And there were two kinds of Brønsted acid sites on the surface of the catalysts.     

Hydrothermal synthesis of Pb(Zr<Subscript>0·52</Subscript>Ti<Subscript>0·48</Subscript>)O<Subscript>3</Subscript> powders at low temperature and low alkaline concentration

Pb(Zr_0·52Ti_0·48)O₃ (PZT) powders were prepared by hydrothermal method. The effects of experimental parameters, including Pb/(Zr, Ti) ratio, alkaline concentration, reaction temperature and time on the product powders were studied in detail. Pure PZT powders were synthesized at suitable experimental conditions and Raman spectra confirmed the PZT with a perovskite-type structure. The homogeneous PZT powders with cubic-shaped morphology were formed at alkaline concentration of 1·2 M after reacting at 230°C for 2 h. The pure PZT powders obtained at low temperature and low alkaline concentration were attributed to precursors, TiCl₄, with high activity and mineralizer NaOH with small cation radius.