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.     

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.     

Thermal stability of ReB2-type transition metal diborides

The thermal stability of metastable ReB₂-type transition metal diborides (TMB₂), which are considered as new type of superhard material, is of vital importance to obtain bulk samples. In the present work, thermal stability of four kinds of ReB₂-type TMB₂ powders, ReB₂, OsB₂, Os_1–xRe_xB₂, and Os_1-xW_xB₂, were synthesized with varied transition metal (TM)-to-B molar ratio by mechanochemical methods and the subsequent annealing was compared. The as-synthesized powders were then consolidated using a pressureless sintering technique. The results showed that the B content required to obtain the pure hexagonal ReB₂-type Os_1–x(TM)_xB₂ phase varied, which indicated different thermal stabilities, such as OsB₂ < Os_0.1W_0.1B₂ < Os_0.9Re_0.1B₂ < Os_0.8W_0.2B₂ < ReB₂ < Os_0.6W_0.4B₂ and Os_0.5W_0.5B₂. Among them, Os_0.6W_0.4B₂ and Os_0.5W_0.5B₂ were found to be relatively thermally stable and could be synthesized with a stoichiometric molar ratio of (Os + W):B = 1:2. It was also found that the thermal stability of TMB₂ with a hexagonal ReB₂ structure could be mainly governed by the length of lattice constant c. The results have guiding significance for the design and preparation of new type of TM borides. In addition, the hardness of TMB₂ can be increased by tailoring the B content in the raw materials more precisely.     

Role of MgO on densification and mechanical properties in spark plasma sintered Os0.9Re0.1B2 ceramic

To promote the densification and therefore the mechanical properties of boride-based ceramics, MgO was added as sintering aid into Os_0.9Re_0.1B₂ powders for densification by using spark plasma sintering (SPS). The Os_0.9Re_0.1B₂ powders were synthesized by mechanochemical method from powder mixture of Os, Re and amorphous B. The role of MgO on densification, phase composition, microstructure and mechanical properties (hardness, fracture toughness and wear behavior) were studied by using X-ray diffraction (XRD), scanning electron microscope (SEM) with energy-dispersive spectroscopy (EDS), micro indentation and ball-on-disk tribometer. The results show that, with the introduction of MgO as sintering aid, the relative density of the Os_0.9Re_0.1B₂ ceramic samples increased. When the MgO content reached 9 wt%, the as-sintered sample is almost fully dense. No obvious regularity was found from the samples with the addition of different content of MgO. Vickers hardness values of the samples with 0, 3 wt% and 9 wt% MgO are found to be very close with each other within the experimental error (~30 GPa), while the sample with the addition of 6 wt% MgO exhibits the highest hardness of ~35 GPa. The fracture toughness of the samples is decreased slightly with the addition of MgO. The friction coefficient and wear rate of the sample with the addition of 6 wt% MgO was also found to be the lowest among all samples, which indicate best wear resistance. As a whole, with the addition content of 6 wt% MgO, the Os_0.9Re_0.1B₂ ceramic sample performs relatively excellent mechanical properties among four groups of samples.     

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.     

Formation of rhenium diboride via mechanochemical–annealing processing of Re and B

Rhenium diboride (ReB₂) powder was prepared by mechanochemical processing of Re–B powder mixtures with subsequent annealing at temperatures of 600 °C to 1200 °C. Reactive evolution during the synthesis was investigated; furthermore, the effects of the amount of excess B on the reactions that occurred during the synthesis were assessed. The substantial reaction of Re with B occurred at 700 °C to form Re₇B₃ with a small amount of ReB₂. At 800 °C, Re₇B₃ converted into ReB₂; this conversion was enhanced with increasing temperature and increasing amount of excess B. At 1000 °C or above, single-phase ReB₂ powder without trace quantities of Re₇B₃ was obtained for compositions with 15 wt% or greater excess B. The synthesized ReB₂ powder particles were submicrometer with the vast majority being ∼500 nm. In addition, the resulting ReB₂ powders were consolidated by hot pressing or spark plasma sintering to examine the sinterability of the powders.     

Enhancement of mechanical properties of Os0.9Re0.1B2 ceramics via buried boron powder assisted sintering

As a new type of hard/super-hard materials, the consolidation of transition metal borides is very critical for obtaining bulk ceramics with excellent properties. In the present work, buried boron powder assisted pressures-less sintering was applied for preparation of Os_0.9Re_0.1B₂ ceramics with the aim for mechanical properties improvement. Os_0.9Re_0.1B₂ powders were firstly synthesized via mechanochemical technique with moral ratios of (Os + Re):B = 1:2.5 and 1:2.25, respectively. Bulk samples were then consolidated using buried powder sintering and exposed sintering, respectively, for comparison. The influence of buried boron powder sintering on the phase composition, microstructure, and mechanical properties (micro-hardness, nano-hardness, and Young's modulus) of Os_0.9Re_0.1B₂ ceramic samples were investigated. The results show that by employing buried powder sintering, B powders surrounded the sample during the sintering process, which on the one hand, inhibited decomposition of Os_0.9Re_0.1B₂ to (Os_0.9Re_0.1)₂B₃, while on the other hand, decreased the grain size of the sample. Further, a columnar to equiaxial transition for the grains was found with grain size decreased when (Os + Re):B = 1:2.25. The samples prepared with buried powder sintering have higher mechanical properties as compared with those prepared with exposed sintering. The sample prepared from (Os + Re): B = 1:2.25 by buried powder sintering had the best mechanical properties among the four studied samples, along with the smallest grain size. The mechanical properties of the samples were greatly influenced by the grain size and relative density.     

Hexagonal OsB2 reduction upon heating in H2 containing environment

The stability of hexagonal ReB₂ type OsB₂ powder upon heating under reforming gas was investigated. Pure Os metal particles were detected by powder X-ray diffraction starting at 375°C and complete transformation of OsB₂ to metallic Os was observed at 725°C. The mechanisms of precipitation of metallic Os is proposed and changes in the lattice parameters of OsB₂ upon heating are analysed in terms of the presence of oxygen or water vapour in the heating chamber. Previous studies suggested that Os atoms possess (0) valence, while B atoms possess both (+3) and (?3) valences in the alternating boron/osmium sheet structure of hexagonal (P63/mmc, No. 194) OsB₂; if controllable method for Os removal from the lattice could be found, the opportunity would arise to form two-dimensional (2D) layers consisting of pure B atoms.     

Microwave-assisted preparation and characterization of nanoscale rhenium diboride

A simple and efficient microwave-assisted preparation of ReB₂-based material is reported utilizing ammonium perrhenate (NH₄ReO₄), magnesium boride (MgB₁₂) reactants and carbon as an absorber of microwave irradiation. The investigation of microwave irradiated NH₄ReO₄ +MgB₁₂ +C mixtures, thermal analysis results and electron microscopy examination reveals that NH₄ReO₄ decomposition produces ReO₃ at early stages of the process. The ReO₃ then exothermically reacts with MgB₁₂ forming the nanoscale Re₃B phase, which converts into ReB₂ upon further irradiation. The coupling of microwave energy with exothermic reactions significantly accelerates the formation of ReB₂. The product primarily consists of ReB₂ as well as B₄C and minor carbon phases. Structural characterization reveals that the average particle size of ReB₂ is ~ 50?nm.     

Reactive Spark Plasma Sintering of rhenium diboride

The simultaneous synthesis and densification of rhenium diboride is investigated starting from Re and B as reactants by using the Spark Plasma Sintering (SPS) apparatus. It is shown that SPS represents an effective technique to synthesize ReB₂ bulk samples with high purity and density. In particular, a dense product with traces of secondary phases (Re₇B₃) is obtained in 35 min of total processing time by applying a maximum temperature of 1600 °C and a mechanical pressure of 20 MPa.     

Production and characterization of spark plasma sintered (Ti,Nb)B2 solid solutions with graphene nanoplatelets and hexagonal boron nitride

For this study, (Ti,Nb)B₂ solid solutions were consolidated by spark plasma sintering. In addition, (Ti,Nb)B₂ with graphene nanoplatelets (GNPs) and hexagonal boron nitride (h-BN) were produced to evaluate the potential of the new structural materials. The phase formation, microstructure, mechanical properties, oxidation resistance and room temperature reflectance, and absorbance features of (Ti,Nb)B₂ were investigated. X-ray diffraction and Transmission electron microscopy observations showed that a complete solid solution phase was formed when the samples were sintered at 1850 °C for 5 min under 50 MPa. Ti_0.75Nb_0.25B₂ exhibited a relative density of ～98.6%, a hardness of ～20.5 GPa, and an indentation fracture toughness of ～3.4 MPa·m^1/2. It was found that the presence of 1 vol% h-BN as an additive enhanced the hardness (～10%) and fracture toughness (～30%) of Ti_0.75Nb_0.25B₂ by activating toughening mechanisms. The GNP added Ti_0.75Nb_0.25B₂ proved to have better oxidation resistance and optical absorbance than the other materials used in the study.     

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.     

Liquid‐Solid Mass Transfer Behavior of a Vertical Array of Closely Spaced Horizontal Tubes in Relation to Catalytic and Electrochemical Reactor Design

The rates of mass transfer at a vertical array of closely spaced horizontal tubes were measured by the limiting‐current technique under single‐phase flow, gas sparging and two‐phase flow. The single‐phase flow data were correlated by the equation: Sh = 0.75 Sc^0.33 Re^0.59. The gas sparging data with no net solution flow were correlated by the equation: J = 0.31(Re_g.Fr)^–0.22. For two‐phase flow, the gas flow was found to enhance the rate of array mass transfer by a factor ranging from 1.25 to 5.25, depending on Re_g and Re. The enhancement ratio increases with decreasing Re and increasing Re_g. For Re ≥ 2500, the rate of mass transfer approaches the value of single‐phase flow, regardless of the value of Re_g, which ranged from 7 to 41. The importance of the present geometry in building electrochemical and catalytic reactors, where exothermic liquid‐solid diffusion‐controlled reactions take place, is highlighted. The present geometry offers the advantage that the outer surface acts as a turbulence promoter while the inner surface acts as a heat exchanger.     

The effects of in-situ formed phases on the microstructure,mechanical properties and electrical conductivity of spark plasma sintered B4C containing Y2O3

B₄Cs without additive and 5, 10 and 15 wt % Y₂O₃ containing B₄Cs were produced by using spark plasma sintering (SPS) technique at different temperatures such as 1820, 1930 and 2030 °C and the effects of in-situ formed phases on the mechanical properties and electrical conductivity of B₄C were investigated. Microstructural investigations showed that the YB₄ phase was formed at 1820 °C and the YB₆ phase at 1930 °C. The hardness values of B₄C-YB₄ composites were higher than the value of B₄C sintered at 1820 °C while lower than that of sintered at 2030 °C. The fracture toughness steadily increased with increasing Y₂O₃ content. The electrical conductivity of B₄C sintered at 2030 °C increased by ～ 40 % with the contribution of in-situ formed YB₄ phase. Compared to B₄C-YB₄, B₄C-YB₆’s hardness was higher, while its fracture toughness and electrical conductivity were lower.     

ZrB2: Adjusting the phase structure to improve the brittle fracture and electronic properties

Although ZrB₂ is a promising ultrahigh-temperature ceramic, the intrinsic brittleness and low fracture toughness are the main bottlenecks. To solve these key problems, by means of first-principles calculations, we predict ZrB₂ new phases, and investigate the influence of new phase on the mechanical properties and electronic properties of ZrB₂. The calculated results show that two new ZrB₂ phases: RuB₂-type with orthorhombic structure (Pmmn, No.59) and ReB₂-type with hexagonal structure (P63/mmc, No.194) are dynamical stability at the ground state. Although RuB₂-type and ReB₂-type structures weaken the volume deformation resistance and shear deformation resistance of ZrB₂, it results in brittle-to-ductile transition due to the formation of weak Zr-B bond along the shear direction. Importantly, ReB₂-type structure improves the electronic properties of ZrB₂ because of the strong charge overlap between conduction band and the valence band near Fermi level (E_F). Therefore, our work can open up a new clue to improve the ductility and electronic properties of ZrB₂.     

FIR-spectroscopy in the high pressure phase of polyethylene

Summary Despite the experimental problems of the high pressure and the high temperature necessary to reach the hexagonal phase of polyethylene a new absorption band was discovered and ascribed to a lattice vibration of this phase. This implies a well ordered lattice over considerable distances. From the frequency which lies between the frequencies of the B_1u– and _B2u– lattice vibrations of the orthorhombic crystal it is deduced that the hexagonal phase consists mainly of helical sequences and that the lattice vibrations do contribute little to the entropy difference orthorhombic hexagonal.     

Low‐temperature sintering and thermal stability of Li2GeO3‐based microwave dielectric ceramics with low permittivity

A low‐permittivity dielectric ceramic Li₂GeO₃ was prepared by the solid‐state reaction route. Single‐phase Li₂GeO₃ crystallized in an orthorhombic structure. Dense ceramics with high relative density and homogeneous microstructure were obtained as sintered at 1000‐1100°C. The optimum microwave dielectric properties were achieved in the sample sintered at 1080°C with a high relative density ~ 96%, a relative permittivity ε_r ~ 6.36, a quality factor Q × f ~ 29 000 GHz (at 14.5 GHz), and a temperature coefficient of resonance frequency τ_f ~ ?72 ppm/^°C. The sintering temperature of Li₂GeO₃ was successfully lowered via the appropriate addition of B₂O₃. Only 2 wt.% B₂O₃ addition contributed to a 21.2% decrease in sintering temperature to 850°C without deteriorating the dielectric properties. The temperature dependence of the resonance frequency was successfully suppressed by the addition of TiO₂ to form Li₂TiO₃ with a positive τ_f value. These results demonstrate potential applications of Li₂GeO₃ in low‐temperature cofiring ceramics technology.     

Microstructure and mechanical properties of high-entropy borides derived from boro/carbothermal reduction

Starting from metal oxides, B₄C and graphite, a suite of high-entropy boride ceramics, formulated (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)B₂, (Hf_0.2Zr_0.2Mo_0.2Nb_0.2Ti_0.2)B₂ and (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ derived from boro/carbothermal reduction at 1600 °C were fabricated by spark plasma sintering at 2000 °C. It was found that the synthetic high-entropy boride crystalized in hexagonal structure and the yield of the targeting phase was calculated to be over 93.0 wt% in the sintered ceramics. Benefitting from the nearly full densification (96.3% ˜ 98.5% in relative density) and the refined microstructure, the products exhibited the relatively high Vickers hardness. The indentation fracture toughness was determined to be comparable with the single transition metal-diboride ceramics. It should be noted that the formation of high-entropy boride ceramics were featured with the relatively high hardness at no expense of the fracture toughness.     

Relationship Between the Orthorhombic and Hexagonal Phases in Dy2TiO5

The thermal expansion of Dy₂TiO₅ in the hexagonal phase was evaluated and compared with the orthorhombic phase using in situ high‐temperature X‐ray diffraction. The crystal structure, volume changes before and after the transformation process, as well as the mechanism behind the thermal expansion behavior was determined and proposed. It was found that in the hexagonal phase, the thermal expansion was caused by the oxygen anions in the axial positions of the trigonal bipyramidal structure moving toward the central Ti atom. While expanding, the movement of these oxygen anions slows the expansion along the c‐axis resulting in a decrease in α₃₃ with temperature. Furthermore, a structural relationship between the orthorhombic and the hexagonal phases was proposed.     

Characterization and analysis of high-entropy boride ceramics sintered at low temperature

Multicomponent transition metal boride composite–sintered bodies were prepared by spark plasma sintering, and the composite sintered bodies prepared at different sintering temperatures (1500–1900°C) were characterized. The experimental results showed that several other compounds diffused into the TiB_x phase at lower sintering temperatures under the combined effect of temperature and pressure due to the nonstoichiometric ratio of TiB_1.5 vacancies. When the temperature reached 1900°C, only the hexagonal phase remained. With the continuous increase of sintering temperature, the Vickers hardness and fracture toughness of the sintered bodies had a trend of increasing first and then decreasing, due to the continuous reduction of the porosity of the cross section of the sintered bodies and the growth of the grain size. The Vickers hardness and fracture toughness of sintered body obtained at 1800°C are the best, which are 24.4 ± 1.8 GPa and 5.9 ± 0.2 MPa m^1/2. At 1900°C, the sintered body was a single-phase hexagonal high-entropy diboride. Its Vickers hardness and fracture toughness were 21.9 ± 1.5 GPa and 5.4 ± 0.2 MPa m^1/2, respectively; it showed a clear downward trend.