Synthesis of osmium borides by mechanochemical method

Transition metal osmium borides were synthesized by mechanochemical method using high‐energy ball‐milling with Os (Osmium) and B (Boron) powders as raw materials. The formation process, reaction mechanism, and thermal stability of the mechochemically synthesized osmium borides were studied. Almost pure Os₂B₃ phase was obtained when the Os‐to‐B molar ratio was 1:2; while ReB₂‐type hexagonal OsB₂ with a small amount of RuB₂‐type orthorhombic OsB₂ was obtained when the Os‐to‐B molar ratio was 1:3. Stoichiometry OsB₂ was obtained from boron rich starting mixture powders due to the B loss during the high‐energy ball‐milling process. It was also found that WC and osmium oxide were present as contaminants after ball milling for 40 hours. Heat treatment results revealed that the as‐synthesized Os₂B₃ powders are thermally stable in flowing Ar up to 800°C, but a transformation from hexagonal to orthorhombic structure partially occurred for the OsB₂ powders as low as 600°C.     

Oxygen Interaction with Hexagonal OsB2 at High Temperature

The stability of ReB₂‐type hexagonal OsB₂ powder at high temperature with oxygen presence has been studied by thermogravimetric analysis, differential scanning calorimetry, SEM, EDS, and high‐temperature scanning transmission electron microscopy and XRD. Results of the study revealed that OsB₂ ceramics interact readily with oxygen present in reducing atmosphere, especially at high temperature and produces boric acid, which decomposes on the surface of the powder resulting in the formation of boron vacancies in the hexagonal OsB₂ lattice as well as changes in the stoichiometry of the compound. It was also found that under low oxygen partial pressure, sintering of OsB₂ powders occurred at a relatively low temperature (900°C). Hexagonal OsB₂ ceramic is prone to oxidation and it is very sensitive to oxygen partial pressures, especially at high temperatures.     

High temperature Ir segregation in Ir–B ceramics: effect of oxygen presence on stability of IrB2 and other Ir–B phases

The formation of IrB₂, IrB_1.35, IrB_1.1 and IrB monoboride phases in the Ir–B ceramic nanopowder was confirmed during mechanochemical reaction between metallic Ir and elemental B powders. The Ir–B phases were analysed after 90 h of high energy ball milling and after annealing of the powder for 72 h at 1050°C in vacuo. The iridium monoboride (IrB) orthorhombic phase was synthesised experimentally for the first time and identified by powder X-ray diffraction. Additionally, the ReB₂ type IrB₂ hexagonal phase was also produced for the first time and identified by high resolution transmission electron microscope. Ir segregation along disordered domains of the boron lattice was found to occur during high temperature annealing. These nanodomains may have useful catalytic properties.     

Effects of Re addition on phase stability and mechanical properties of hexagonal OsB2

ReB₂‐type hexagonal Osmium diboride (OsB₂) has been predicted to exhibit higher hardness than its orthorhombic phase, but hexagonal‐orthorhombic phase transformation occurs at temperature higher than 600°C, resulting in the decrease in its hardness. Therefore, ReB₂‐type hexagonal OsB₂ samples with Re addition were produced by a combination of mechanochemical method and pressureless sintering technique, and the effects of Rhenium (Re) addition on phase composition, thermal stability and mechanical properties of OsB₂ were investigated in this study. X‐ray diffraction (XRD) analysis of the as‐synthesized powders by high‐energy ball milling indicates the formation of hexagonal Os_1‐xRe_xB₂ solid solution with Re concentration of 5 and 10 at.% without forming a second phase. After being sintered at 1700°C, part of the hexagonal phase in OsB₂ transformed to orthorhombic structure, while Os_0.95Re_0.05B₂ and Os_0.9Re_0.1B₂ maintained their hexagonal structure. This suggests that the thermal stability of the hexagonal OsB₂ was significantly improved with the addition of Re. Scanning electron microscopy (SEM) photographs show that all of the as‐sintered samples exhibit a homogeneous microstructure with some pores and cracks formed throughout the samples with the relative density >90%. The measurements of micro‐hardness, nano‐hardness, and Young's modulus of the OsB₂ increased with Re addition, and these properties of the sample with 5 at.% addition of Re is higher than that with 10 at.% Re.     

Thermal stability and hardness of ReB2 type hexagonal OsB2 with the addition of W

This work attempts to understand the effect of W addition on microstructure, thermal stability, and hardness of ReB₂ type hexagonal osmium diboride (h-OsB₂). h-OsB₂ samples with W atomic concentration of (Os+W) from 0% to 30% were synthesized by mechanochemical method combines with pressure-less sintering. The XRD patterns of the as-synthesized powders indicate the formation of Os_1-xW_xB₂ (x?=?0, 0.1, 0.2 and 0.3) solid solution, which has a ReB₂-type hexagonal structure. After being high temperature sintered, part of the h-OsB₂ phase of the pure OsB₂ transformed to orthorhombic (o) phase, while the h-OsB₂ phase was maintained with the addition of W, which suggests that the thermal stability of the sample was remarkably improved. A macroscopically homogeneous structure with some pores can be found from all groups of the as-sintered Os_1-xW_xB₂ (x?=?0, 0.1, 0.2, 0.3) samples, with some B-rich areas distributed in the W doped samples. The lattice parameters of the Os_1-xW_xB₂ (x?=?0, 0.1, 0.2 and 0.3) solid solutions linearly decreased with the increase of the W concentration. The micro-hardness of the OsB₂ sintered samples is 25?±?2?GPa under an applied load of 0.49?N, which increased to 34?±?2?GPa, 38?±?2 and 37?±?2?GPa, respectively when the W concentration increased from 10, 20 and 30?at%. The increased hardness of the h-OsB₂ can be mainly attributed to the improvement of thermal stability with the addition of W.     

Existence de quatre types de sites reactionnels dans l'oxydation du graphite

We have studied the temperature programmed decomposition of graphite-oxygen surface complexes using a gas flow apparatus under atmospheric pressure with infrared CO and CO₂ detection. We have shown that the oxygen is chemisorbed on four types of sites forming surface complexes which on decomposition give mainly CO and some CO₂. The first two types of sites, A and B, are formed by ‘labile’ carbon atoms created during the degassing carried out prior to each experiment. These two types of sites disappear without reconstitution upon desorption of the complexes. The other two, C and D, are formed by edge carbon atoms, normally linked to other atoms in the graphite lattice. The C type sites form by oxidation only at temperatures below about 950°C and give rise to hexagonal pits with sides oriented in the (101?0) crystallographic direction (‘arm-chair’ configuration). The D type sites form at temperatures above about 950°C and give rise to hexagonal pits oriented in the (11?20) crystallographic direction (‘zig-zag’ configuration). Water inhibits the formation of oxygen surface complexes on the C sites and it may be considered that it is essentially on these sites that the water is chemisorbed.     

Fabrication of ZrB2 ceramics by reactive hot pressing of ZrB and B

Zirconium diboride (ZrB₂) ceramics were prepared by reactive hot pressing of ZrB+B powder mixture. Formation of a transient liquid due to eutectic reaction of ZrB₂+Zr→L_eu(ZrB2+Zr) at 1661°C following peritectic decomposition of 2ZrB=ZrB₂+Zr at 1250°C during heating up of the ZrB+B mixture facilitated densification. The liquid phase was subsequently eliminated via reaction of B with Zr in the eutectic liquid L_eu(ZrB2+Zr) to result in a dense ZrB₂ ceramic. Full density was reached after reactive hot pressing at 1900°C under 30 MPa for 1 h. The ZrB₂ ceramic had a refined microstructure consisting of grains of <1.5 μm in size and relatively good Vickers hardness (21 ± 2 GPa) and flexural strength (595 ± 63 MPa).     

Structural characterization and superconducting properties of MgB<Subscript>2</Subscript> prepared by SHS-method

Self-propagating high-temperature synthesis (SHS) of bulk MgB₂ superconductor from Mg-2B powder blend is reported. This reaction proceeds violently at 100 A under a protective atmosphere. Since the heat of reaction of Mg and B was not enough for chain reaction, then (Ti + C) mixed powders were used as the ignition agent to assist the reaction (Mg + 2B). In this case, the combustion front moved without any difficulty. The diffraction lines of the product can be indexed to a hexagonal MgB₂ phase, with lattice constants a = 3.0845 Å, and c = 3.5259 Å. For comparison, the direct synthesis of (Mg + 2B) mixture was carried out at (800°C–1000°C). It can be found, that high-temperature sintering (1000°C) will induce the formation of impurities. The MgB₂ grains are fine, well compacted and more homogenous. The structure of materials was studied using XRD, FESEM and EDX. M-H curvatures were measured under the magnetic fields between ?80 kOe and 80 kOe. J _c was calculated from width of magnetization hysteresis loops based on the extended Bean Model.     

Pressureless Sintering and Reaction Mechanisms of Ti3SiC2 Ceramics

Easy sinterable Ti₃SiC₂ powder was synthesized from a powder mixture with a molar ratio of 1.0 Ti, 0.3 Al, 1.2 Si, and 2.0 TiC by heating at 1200°C in the flowing Ar. Here, the Al powder acts as a deoxidation agent and provides a liquid phase for the reaction. The powder compacts subjected to pressureless sintering at 1300°C in Ar had a relative density up to 99%. The results of chemical analysis and the measured lattice constant suggest that the Al–Si liquid phase was formed at approximately 1200°C and that liquid‐phase sintering was promoted by the 0.1 molar ratio of Al and the 0.2 molar ratio of Si remaining in excess. The three‐point bending strength, fracture toughness, and electrical resistivity of the sintered samples were 380 MPa, 4.1 MPa m^1/2, and 0.34μΩm, respectively.     

Enhancement of Interparticle Exchange Coupling in CoFe2O4/CoFe2 Composite Nanoceramics Via Spark Plasma Sintering Technology

Powders and nanoceramics composed of composites of CoFe₂O₄, CoFe₂, and a small amount of FeO were prepared by heating CoFe₂O₄ powder in reducing atmosphere and by sintering the product of reducing reaction at 350°C via spark plasma sintering technology. In the powders, increase in the molar ratios of CoFe₂:CoFe₂O₄ and a great change in magnetic parameters were observed with the change in heating temperature from 300°C to 400°C, and the dominance of dipole interaction over exchange coupling in the interparticle interactions was confirmed by the steps in magnetic hysteresis loops and the negative Henkel plots. However, in the nanoceramics, significant enhancement in exchange coupling was found when the sintering temperature was raised to 500°C and 650°C, which was confirmed by both the positivity of Henkel plot and the single‐phase style of the magnetic hysteresis loop.     

On the Rapid Synthesis of the Ternary Mo2GaC

The reaction path to form the Mo₂GaC MAX phase starting with Mo, Ga, and C (molar ratio, 2:1.2:1) powders was investigated in the 850°C and 1000°C temperature range. It was found that Mo₂GaC could be synthesized from reactions between Mo₃Ga and C or Mo₂C and Ga. Powders that contained >90 wt% Mo₂GaC were successfully fabricated by heating a 1:1.4 molar ratio of Mo₂C:Ga to 900°C for 24 h under flowing argon, followed by the dissolution of excess Ga by HCl. The a‐ and c‐lattice parameters were measured to be 3.022(1) and 13.179(5) Å.     

Phase equilibrium in binary La2O3-Dy2O3 and ternary CeO2-La2O3-Dy2O3 systems

Cerium oxide doped with oxides of rare earth elements is a multifunctional material, a wide range of uses which is associated with its unique physicochemical properties. Phase diagrams of multicomponent systems are the physicochemical basis for the creation of new materials with improved characteristics.In this work, phase equilibria in ternary CeO₂–La₂O₃–Dy₂O₃ and binary La₂O₃–Dy₂O₃ systems in the whole concentration range were studied. No new phases have been identified in these systems. An isothermal section of the phase diagram of the CeO₂–La₂O₃–Dy₂O₃ system at a temperature of 1500 °С is constructed. No new phases have been detected in the system. It was found that in the studied ternary system solid solutions are formed on the basis of (F) modification of CeO₂ with structure of fluorite type, monoclinic (B), cubic (C) and hexagonal (A) modifications of Ln₂O₃.In the La₂O₃–Dy₂O₃ binary system (1500–1100 °С) three types of solid solutions are formed: based on hexagonal modification A-La₂O₃, monoclinic modification B-Dy₂O₃ and cubic modification C-Dy₂O₃ separated by two-phase fields (A+B) and (B+C), respectively. The boundaries of the regions of homogeneity of solid solutions based on A-La₂O₃ are determined by compositions containing 35–40, 20–25, 15–20 mol% Dy₂O₃ at 1500, 1250, 1100 °C, respectively. From the obtained data it follows that the solubility of Dy₂O₃ in the hexagonal modification of lanthanum oxide is 39 mol% at 1500 °C, 23 mol. % at 1250 °C and 16 mol% at 1100 °C. The limits of existence of solid solutions based on monoclinic B-modification are determined by compositions containing 30–35, 65–60 (1250 °С), 35–40, 55–60 (1100 °С) 40–45, 70–75 (1500 °C) mol% Dy₂O₃.In the studied system, with a decrease in temperature from 1500° to 1100°C, there is a decrease in the solubility of La₂O₃ in the crystal lattice of cubic solid solutions of C-type from 16 to 10 mol%.     

Synthesis of cerium aluminate by the mechanical activation of aluminum and ceria precursors and firing in controlled atmospheres

Single-phase cerium aluminate was synthesized from mixtures of ceria and metallic aluminum by milling and firing under controlled conditions in reducing (10%H₂ + 90%N₂) or inert atmospheres (N₂ or CO₂). Firing in an inert atmosphere (CO₂) did not yield conversion to cerium aluminate, and conversion was also low after firing in reducing conditions (10%H₂ + 90%N₂) and only improved slightly on changing from powder mixtures with coarse Al powder (15 µm) to mixtures with submicron Al (0.77 µm). High-energy milling promoted reactivity by the combined effects of improved homogeneity, decreasing grain size of the Al precursor, increase in lattice strain and decrease in crystallite size down to 40–50 nm. Extensive oxidation of the metallic Al precursor after long-term milling prevented complete conversion to cerium aluminate even after firing under reducing conditions at temperatures up to 1400°C. Thermodynamic modeling of the Al–Ce–O system provided interpretation for differences between firing in reducing and inert atmospheres. Controlled milling time hinders oxidation of Al to the poorly reactive α-Al₂O₃ polymorph. This was supported by thermogravimetry after controlled milling and yielded phase pure CeAlO₃ at T ≥ 1200°C. The high conversion was achieved even by firing at 1100°C under an inert atmosphere.     

Fabrication of translucent AlN ceramics with MgF2 additive by spark plasma sintering

Translucent AlN ceramics with 0‐2 wt.% MgF₂ additive were prepared by spark plasma sintering. AlN powder was heated temporarily up to 2000°C, before holding at 1850°C for 20 minutes in N₂ gas. The sintered ceramics consisted of a single phase of hexagonal AlN, and showed a transgranular fracture mode. The total transmittance was improved remarkably by the additive, to reach 74% at a wavelength of 800 nm for 1 wt.% MgF₂. For 2 wt.% MgF₂, the transmittance was slightly lower than that for 1 wt.% MgF₂, and an absorption band was observed apparently at around 400 nm. The addition of MgF₂ along with the temporary heating at higher temperatures than the sintering temperature contributed to improve the transmittance remarkably.     

Realizing tremendous electrical transport properties of polycrystalline SnSe2 by Cl-doped and anisotropy

SnSe₂ is regarded as an attractive thermoelectric material for its structural and chemical analogy to SnSe that is claimed with the highest ZT in single crystal. In this study, the pure and Cl-doped SnSe₂ polycrystals (3%, 6%, 9% and 12% molar Cl content) were fabricated in four steps that are hydrothermal synthesis, heating purification, diffusion doping, and spark plasma sintering. The phase structure, lamellar morphology and crystallite orientation were studied for the synthesized SnSe₂ powder and the sintered pellets. The structural evolution was traced from the SnSe₂ hexagonal plates of powders to the (001) oriented grains in pellets. The Cl doping into SnSe₂ was verified by phase composition, lattice parameter, element distribution, and chemical valance. The doped Cl increased both the carrier concentration and the mobility. The anisotropic thermoelectric properties of SnSe₂ bulk materials were investigated as functions of temperature from 50?°C to 300?°C and the doping amount, respectively. The Seebeck coefficient was less anisotropic than the electrical and thermal conduction. The grain orientation influenced the anisotropy of the electrical and thermal conductivity at a similar ratio. The power factors were less dependent on temperature with an optimum in-plane 1.06?mW?m^?1 K^?2 and out-of-plane 0.41?mW?m^?1 K^?2. The highest ZTs of 0.3 were attained at 300?°C in both directions.     

A two-step heating strategy for low-temperature fabrication of high sinterability and fine MgAlON powder with MgAl2O4 as Mg source

Based on the reaction sequence during synthesis of MgAlON powder by solid-state reaction, a two-step heating strategy is proposed to low-temperature fabricate fine MgAlON powder of high sinterability by using MgAl₂O₄ as Mg source, respectively together with AlON and Al₂O₃+AlN. By introduction of an additional dwelling at 1550 °C to the first heating step, more α-Al₂O₃ dissolve into the solid solution at this temperature. By this way, overlarge particles of Al₂O₃ by agglomeration could be avoided in the next heating step to enable fast full reaction at a lower temperature. By dwelling 30 min at 1550 °C followed by 60 min at 1700 °C, single phase MgAlON powders were successfully prepared by solid-state reaction of all the two batches. The fine MgAlON powder synthesized by MAS+Al₂O₃+AlN batch exhibited high sinterability as the MgAlON ceramics pressureless sintered by this powder at 1880 °C without dwelling showed a transmittance up to 68.3%. The phase assemblage and morphology evolution of the mixture during solid-state reaction were monitored, which verified the effectiveness of the proposed two-step heating strategy. The low synthesis temperature of the two-step heating scheme benefits to prepare pure MgAlON powder with small particle size.     

A 3D oxalate-bridged [CuIIFeII] coordination polymer as molecular precursor for CuFe2O4 spinel—photocatalytic features

A 3D heterometallic oxalate-bridged coordination polymer [Cu^IIFe^II₂(H₂O)(terpy)(C₂O₄)₃]_n (terpy = 2,2′:6′,2″-terpyridine) ( 1 ) was investigated both as photocatalyst for the organic dye removal and as a single-source precursor for the preparation of the copper ferrite (CuFe₂O₄) nanocrystals by thermal processing. The dual functionality of 1 was supported by the degradation of aqueous solutions of rhodamine B (RhB) and methylene blue (MB) solutions under visible (Vis) and ultraviolet (UV) light irradiation, powder X-ray diffraction data collection at room temperature, and the optical and scanning electron microscopy analyses. A close inspection of the X-ray diffraction patterns unveiled qualitative and quantitative information on the phase composition obtained after the single-source molecular precursor route to spinel oxide. By optimizing the temperature levels and setting the controlled heating rate at 6 h of holding time, the phase composition of thermal processing of 1 was evaluated—thermal treatment of 1 at 950°C for 6 h and a heating/cooling rate of 10°C min⁻¹ resulted in the formation of solely tetragonal spinel phase of CuFe₂O₄, whereas the formation of both tetragonal and cubic CuFe₂O₄ phases was observed at 950°C by the heating rate of 30°C min⁻¹. To obtain the high-temperature cubic CuFe₂O₄ oxide, compound 1 was heated and then quenched at 925°C, which led to the formation of the cubic spinel ferrite as the main crystalline oxide phase. Moreover, the photocatalytic properties of the t-CuFe₂O₄ spinel were investigated under the same conditions as for 1 . The optical bandgap energies were estimated from UV–Vis absorption spectra for both metal oxide and precursor powder.     

Carbon‐Supported CoSe2 Nanoparticles for Oxygen Reduction Reaction in Acid Medium

Carbon‐supported CoSe₂ nanoparticles, as non‐precious metal cathodic catalyst, were prepared via the in situ surfactant‐free method with the conventional heating. Structural and electrochemical properties of the obtained 20 wt.‐% CoSe₂/C nanoparticles were investigated by means of powder X‐ray diffraction (PXRD), differential thermal gravimetric analysis (DTA‐DTG) and rotating disc electrode (RDE) techniques. CoSe₂ nanoparticles have two kinds of crystal structure after heat treatment under nitrogen at different temperature: orthorhombic at 250 and 300 °C; cubic at 400 and 430 °C. The latter structure has higher oxygen reduction activity than the former in 0.5 M H₂SO₄. CoSe₂/C nanoparticles after heat treatment from 250 to 430 °C, have an onset potential from 0.78 to 0.81 V versus the reference hydrogen electrode (RHE) in O₂‐saturated 0.5 M H₂SO₄ at 25 °C. 20 wt.‐% CoSe₂/C nanoparticles, after heat treatment at 300 °C, promote ca. 3.5 electrons, per oxygen molecule, transferred during the oxygen reduction process. They have an oxidation wave centred at 0.96 V versus RHE and display higher methanol tolerance as compared to 20 wt.‐% Pt/C (E‐TEK).     

Thermal expansion and order-disorder polymorphic transformation in the family of borates BaNaMe(BO3)2, Me = Sc, Y

The thermal behavior of BaNaMe(BO₃)₂, Me = Sc, Y (space group R-3) is studied by the methods of high-temperature X-ray powder diffraction, differential scanning calorimetry, and thermogravimetry in the range from room temperature to 1300°C. It is shown that a polymorphic transition with a two-fold reduction of the unit cell volume is observed at 775 ± 20°C (Sc) and 375 ± 30°C (Y). New polymorphic modifications are solid solutions (Ba, Na)Me(BO₃)₂, (Me = Sc, Y) with a disordered distribution of Ba and Na atoms. Eigenvalues of the thermal expansion tensor of the polymorphic modifications are calculated for BaNaSc(BO₃)₂ and BaNaY(BO₃)₂. The solid solutions are stable approximately up to 819°C (Sc) and 770°C (Y); at higher temperatures, the solid-state decomposition of these phases begins.     

The Role of MgAl2O4 Powder Packing on Phase Stability of Hydroxyapatite During Sintering

MgAl₂O₄ (spinel) was utilized as a packing powder in the sintering of hydroxyapatite (HAp) and the composite of HAp/3 mol% Y₂O₃‐stabilized tetragonal zirconia (3Y‐TZP). The influence of spinel on phase stability of HAp was investigated using X‐ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and electron probe microanalysis (EPMA) to reveal the reaction in the vicinity of the interface between HAp and spinel. When covered with spinel powder, decomposition temperatures for both HAp monolith and HAp in the composite were raised from 1360°C to 1470°C and from 930°C to 1280°C, respectively. SEM images supported the role of spinel on retardation of the decomposition, showing a dense cross section of the monolith after sintering for 2 h at 1400°C with the spinel as opposed to a porous feature without the covering. XRD results indicated that the increase in the decomposition temperatures was accompanied by a decrease in the a‐axis dimension of the hexagonal structure of HAp, probably as a result of the substitution of F^? for OH^?. EPMA revealed that negligible reaction occurred between HAp and spinel even at 1500°C, but the Ca²⁺ in HAp diffused about 20 μm into 3Y‐TZP to form a cubic zirconia solid solution at 1275°C, resulting in the decomposition. The involvement of F^? ion in the contraction of a‐axis parameter and the consequent phase stability were manifested by an increase in the Raman band of the symmetric stretching of the P–O bonds at 962.3 cm^?1 and the appearance of a band for fluoroapatite at 3538 cm^?1.