Structural transformations of boron trioxide under high pressure and high temperature

A systematic study of boron trioxide under high pressure and high temperature (HPHT) was conducted using a Chinese multi-anvil high-pressure apparatus (CHPA). The HPHT phase diagram was determined using X-ray diffraction measurements. Under high pressure (3.6–5.5?GPa) and low temperature (below 450?°C), the boron trioxide grains were reduced to the nanometer size and the hardness reaches to 13.9?GPa (5.5?GPa and 450?°C). The boroxol rings were produced only in the glass phase that was transformed from the α-B₂O₃ phase under HPHT. And the formation mechanism of boroxol rings was discussed according to Raman spectrum and crystal structure of α-B₂O₃ and β-B₂O₃.     

A Complete Solid Solution with Rutile‐Type Structure in SiO2–GeO2 System at 12 GPa and 1600°C

High pressure and temperature synthesis of compositions made of (Si_1?x,Ge_x)O₂ where x is equal to 0, 0.1, 0.2, 0.5, 0.7, and 1 was performed at 7–12 GPa and 1200–1600°C using a Kawai‐type high‐pressure apparatus. At 12 GPa and 1600°C, all the run products were composed of a single phase with a rutile structure. The lattice constants increase linearly with the germanium content (x), which indicates that the rutile‐type phases in the SiO₂–GeO₂ system form a complete series of solid solutions at these pressure and temperature conditions. Our experimental results show that thermodynamic equilibrium state was achieved in this system at 12 GPa and 1600°C, but not at 1200°C. At lower pressures (7 and 9 GPa) and 1600°C, we observed the decomposition of (Si_0.5,Ge_0.5)O₂ into SiO₂‐rich coesite and GeO₂‐rich rutile phases. The silicon content in the rutile structure increases sharply with pressure in the vicinity of the coesite–stishovite phase transition pressure in SiO₂.     

Crystallization behaviour and properties of BaO-CaO-B2O3-SiO2 glasses and glass-ceramics for LTCC applications

The influence of BaO content (up to 15?mol%) on the crystallization behaviour, structure, thermal properties and microwave dielectric properties of the BaO-CaO-B₂O₃-SiO₂ glasses and glass-ceramics system was investigated. The glasses were produced by melting at 1400?°C and quenching into water, and the glass-ceramics were produced via heat treatment at temperatures between 750 and 800?°C. The results of X-ray diffraction analysis showed that increasing the BaO content raised the resistance of the glass against crystallization and favoured the transformation of β-CaSiO₃ and α-CaSiO₃ phases, which crystallized in the Ba-free and in low BaO content compositions, into SiO₂ and Ba₄Si₆O₁₆, which crystallized in compositions with higher concentrations of BaO. The BaO content had little influence on the glass transition temperature (T_g) and the linear coefficient of thermal expansion (CTE), but strongly reduced the softening point (T_s). Even the addition of BaO as minor additives resulted in a dramatic reduction of the T_s; for example, the T_s decreased from 902?°C for the Ba-free composition to 682?°C for the BaO-containing one (5%). Low values of the dielectric constant (5.9?≤?ε_r ≤?6.63) and dielectric loss (1.12?×?10^?3 ≤?tanδ?≤?3.15?×?10^?3) were measured.     

Diamond formation from graphite in the presence of anhydrous and hydrous magnesium sulfate at high pressures and high temperatures

The diamond forming regions in the anhydrous MgSO₄ and hydrous MgSO₄·H₂O–graphite systems, as well as the morphology of diamond synthesized from both of systems were investigated under the conditions of 6.5–7.7 GPa and 1600–2000 °C. The region was basically the same in both systems, although the melting point of MgSO₄ is at least 600 °C higher than that of MgSO₄·H₂O at 7.7 GPa. In the anhydrous system, the appearance of {111} faces is favored at higher temperatures and that of {100} faces at lower temperatures, while in the hydrous system, {111} faces are predominant at all examined conditions.     

EAFD-loaded vitreous and glass–ceramic materials

SiO₂, Na₂O and CaO were mixed and co-melted with electric arc furnace dust waste. The resulting vitreous materials, produced by quenching at ambient atmosphere, were transformed into glass–ceramics by two-stage heat treatment, under thermal conditions that were determined by differential thermal analysis. X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry and transmission electron microscopy were employed to investigate the physical properties of all products. It was found that whilst wollastonite (CaSiO₃) separates from the parent matrix as the dominant crystalline phase in all glass–ceramic products, the crystallization mode depends on the batch composition. Leaching tests evidenced that vitreous products were chemically durable. Devitrification did not significantly affect leach resistance so glass–ceramic materials retain the leach resistance that was achieved by vitrification.     

In-situ synthesis and densification of boron carbide and boron carbide-graphene nanoplatelet composite by reactive spark plasma sintering

Simultaneous synthesis and densification of boron carbide and boron carbide- graphene nano platelets (GNP) were carried out by reactive spark plasma sintering of amorphous boron and graphene nano platelets at temperature ranging from 1200 to 1600?°C, pressure of 50?MPa and heating rate of 50?°C/min and 100?°C/min. X-ray diffraction and Raman spectroscopy confirmed the formation of required phases. Electron microscopic images revealed the formation of sub-micron and nano sized grains of plate like morphology. Sintered product with high relative density of 96%TD was achieved at a temperature of 1600?°C and heating rate of 50?°C/min for B₄C stoichiometric composition and also exhibited maximum hardness of 21.10?GPa.     

Synthesis of cubic zirconium(IV) nitride,c-Zr3N4, in the 6–8 GPa pressure region

We report on the lowermost pressure and temperature conditions where cubic zirconium(IV) nitride having Th₃P₄-type structure, c-Zr₃N₄, forms in a belt-type apparatus. This novel hard material having exceptional wear resistance by machining of low-carbon steels appears at pressures between 6.5 and 7.7 GPa when heated to 1400–1600 °C. Single-phase samples were obtained at P = 7.7 GPa thus indicating that the industrial scale synthesis of this innovative material is feasible. Chemical reaction of the sample material with the Pt-capsule led to formation of a Pt–Zr alloy, as a minor admixture, which can potentially be used as a binder for cementing of polycrystalline c-Zr₃N₄ tools.     

Improvement of mechanical strength in Y3+/La3+ co-doped silicate glasses for display screen

Y₂O₃ and La₂O₃ co-doped silicate glasses were prepared by conventional melt-quenching method. The effects of Y₂O₃ and La₂O₃ contents on the structural, thermal and mechanical properties of glass samples were investigated by X-ray diffraction (XRD), Differential scanning calorimetry (DSC) and Fourier Transform Infrared Spectroscopy (FTIR). The results showed that the prepared glass samples exhibited excellent mechanical properties and thermal stability. The hardness, elastic modulus and ΔT reaches 860HV, 108?GPa and 299?°C, respectively. Compared with the reported glasses for this type, these values were increased by approximately 19.5%, 28% and 8.7%, respectively. In addition, the transmittance of all glass samples exceeds 90% in visible spectra range. The above results indicate that the prepared glass samples can be applied to the fields of architectural glass, automotive windshield and screen of electronic products.     

Y3Al5O12-α-Al2O3 composites with fine-grained microstructure by hot pressing of Al2O3-Y2O3 glass microspheres

Yttrium aluminate glass microspheres with the eutectic composition 76.8 mol. % Al₂O₃ and 23.2 mol. % Y₂O₃ were prepared by combining the sol-gel Pechini method with flame synthesis. The sol-gel method was applied to achieve the desired composition homogeneity of the prepared glass and hence, improve the microstructure homogeneity and mechanical properties of bulk polycrystalline materials. The latter were prepared by hot pressing, more specifically pressure assisted sintering, at 1050 °C, 1300 °C and 1600 °C using pressures of 30 MPa and 80 MPa and holding times between 0 and 30 min. This also led to the crystallization of the glass. A composite with the Vickers hardness 18.0 ± 0.7 GPa and an indentation fracture toughness 4.9 ± 0.3 MPa.m^1/2 was obtained by sintering at 1600 °C, at the pressure of 80 MPa and with 30 min isothermal heating at the maximum temperature. Improved mechanical properties were observed when increasing the temperature of sintering and the holding time. This can be attributed to the formation of a unique microstructure consisting of α-Al₂O₃ grains in the μm-scale embedded in a YAG (yttrium-aluminium garnet) matrix in the hot-pressed samples.     

Nano-crystal glass-ceramics obtained from high alumina coal fly ash

Glass has been obtained by melting high alumina coal fly ash with fluxing additives. A thermal treatment was employed to convert the obtained glass into nano-crystal glass-ceramics. X-ray diffraction (XRD) patterns show that the main crystalline phases in both the glass-ceramics are anorthite (CaAl₂Si₂O₈) and wollastonite (CaSiO₃). The crystals are homogeneously dispersed within the parent glass. The average crystal size is below 200 nm. Physical and mechanical properties, such as density, thermal expansion coefficient, hardness, and bending strength, of the glass have been examined and the corresponding microstructures are discussed. The results demonstrate that the glass-ceramics have potential for a wide range of construction application.     

In-situ oxide scale investigation of Sm-doped ZrB2/SiC Billets

Samarium-doped zirconium diboride/silicon carbide (Sm-ZBS) ceramics possess an emittance of 0.9 at 1600 °C and develop oxide scales that have excellent ablation performance. This study investigates the oxide scale development of 3 mol% doped Sm-ZBS which contains 80 vol% ZrB₂ and 20 vol% SiC when exposed to temperatures in excess of 1800 °C in an oxidizing atmosphere. Samples were prepared via chemical infiltration of samarium nitrate into spray-dried powders of 80 vol.% ZrB₂/20 vol.% SiC; powders were then pressed into billets and sintered without pressure. Samples cut from these billets were then oxidized for 10, 60, and 300 s, respectively, using an oxyacetylene torch. A Sm-depletion region was observed and believed to form due to glass transport to the surface. X-ray diffraction was used to determine the sequence of oxidation of Sm-ZBS, beginning with the formation of ZrO₂ and Sm₂O₃. The final oxide scale was determined to be c₁-Sm_0.2Zr_0.8O_1.9, with a melting temperature exceeding 2500 °C. SEM and EDS were also used to investigate microstructural formation due to the bursting of convection cells.     

Microstructure and mechanical properties of ZTA composites fabricated by oscillatory pressure sintering

The influence of sintering temperature on the microstructure and mechanical properties of Al₂O₃?20 wt% ZrO₂ composites fabricated by oscillatory pressure sintering (OPS) was investigated by means of X-ray diffraction, scanning electron microscopy, three-point bending test and Vickers indentation. Results were compared to specimens obtained by conventional hot pressing (HP) under a similar sintering schedule. The optimum oscillatory pressure sintering temperature was found to be 1600 °C; almost fully dense materials (99.94% of theoretical density) with homogeneous microstructure could be achieved. The highest flexural strength, fracture toughness and hardness of such composites reached 1145 MPa, 5.74 MPa m^1/2 and 19.08 GPa when sintered at 1600 °C, respectively. Furthermore, the oscillatory pressure sintering temperature could be decreased by more than 50 °C as compared with the HP method, OPS favouring enhanced grain boundary sliding, plastic deformation and diffusion in the sintering process.     

Erbium-doped LAS glass ceramics prepared by spark plasma sintering (SPS)

Dense nanocrystalline glass ceramics of the Li₂O–Al₂O₃–SiO₂ (LAS) system were obtained by spark plasma sintering (SPS) of powders prepared by sol–gel method. The low thermal expansion LAS glass ceramic was chosen as host matrix for erbium ions. ZrO₂ was added both as a nucleating agent and as a possible good environment for the rare earth. The developed crystalline phases were analysed by X-ray diffraction (XRD) and the amorphous phase was quantified. Scanning and transmission electron microscopy (SEM, TEM) was used to investigate the microstructure. A different behaviour during the crystallisation process was observed between the sample prepared through the sintering of powders and the glass produced by the melting technique. A photoluminescence characterisation was also performed.     

Novel single phase (Ti0.2W0.2Ta0.2Mo0.2V0.2)C0.8 high entropy carbide using ball milling followed by reactive spark plasma sintering

Single phase novel (Ti_0.2W_0.2Ta_0.2Mo_0.2V_0.2)C_0.8 high entropy carbide (HEC) compacts were successfully synthesized by reactive spark plasma sintering of ball milled metal-carbon elemental mixture at temperatures of 1400−1800 °C. X-ray diffraction and element distribution maps indicated single phase carbide formation with lattice parameter ranging from 4.307 Å to 4.312 Å with small amount of TiO₂. X-ray energy dispersive spectroscopy (EDS) mapping showed uniform distribution of the transition metals in the carbide phase. The microhardness, elastic modulus, fracture toughness, electrical resistivity and thermal expansion coefficient (25 °C–600 °C) of the compact sintered at 1800 °C were found to be 25.8 ± 2.8 GPa, 461 ± 36 GPa, 3.7 ± 0.4 MPa.m^1/2, 7 × 10⁻⁴ Ω/m² and 7 × 10^-6 K⁻¹ respectively.     

Elevated Temperature Strength Enhancement of ZrB2–30 vol% SiC Ceramics by Postsintering Thermal Annealing

The mechanical properties of dense, hot‐pressed ZrB₂–30 vol% SiC ceramics were characterized from room temperature up to 1600°C in air. Specimens were tested as hot‐pressed or after hot‐pressing followed by heat treatment at 1400°C, 1500°C, 1600°C, or 1800°C for 10 h. Annealing at 1400°C resulted in the largest increases in flexure strengths at the highest test temperatures, with strengths of 470 MPa at 1400°C, 385 MPa at 1500°C, and 425 MPa at 1600°C, corresponding to increases of 7%, 8%, and 12% compared to as hot‐pressed ZrB₂–SiC tested at the same temperatures. Thermal treatment at 1500°C resulted in the largest increase in elastic modulus, with values of 270 GPa at 1400°C, 240 GPa at 1500°C, and 120 GPa at 1600°C, which were increases of 6%, 12%, and 18% compared to as hot‐pressed ZrB₂–SiC. Neither ZrB₂ grain size nor SiC cluster size changed for these heat‐treatment temperatures. Microstructural analysis suggested additional phases may have formed during heat treatment and/or dislocation density may have changed. This study demonstrated that thermal annealing may be a useful method for improving the elevated temperature mechanical properties of ZrB₂‐based ceramics.     

Microwave-specific heating of crystalline species in nuclear waste glass

The microwave heating of a crystal-free and a partially trevorite-crystallized nuclear waste glass simulant was evaluated. Our results show that a 500-mg monolith of partially crystallized waste glass can be heated from room temperature to above 1600°C within 2 minutes using a single-mode, highly focused, 2.45-GHz microwave, operating at 300 W. Using X-ray diffraction measurements, we show that trevorite is no longer detectable after irradiation and thermal quenching. When a crystal-free analog of the same waste glass simulant composition was exposed to the same microwave radiation, it could not be heated above 450°C regardless of the heating time. The reduction in crystalline content achieved by selectively heating spinels in the presence of glass suggests that microwave-specific heating should be further explored as a technique for remediating crystal accumulation in a glass melt.     

Spontaneous nucleation of diamond in the system MgCO3–CaCO3–C at 7.7 GPa

High-pressure and high-temperature experiments were carried out to determine the minimum temperature required for spontaneous nucleation of diamond in the system comprising a carbonate mixture (60 mol% MgCO₃ and 40 mol% CaCO₃) and graphite at 7.7 GPa, for 1 h and longer reaction times up to 12.5 h, and also in the systems MgCO₃–graphite and CaCO₃–graphite for comparison. The above carbonate mixture melted at a temperature between 1600°C and 1700°C. The minimum temperature for spontaneous nucleation of diamond was 1900°C for a reaction time of 1 h; it was lowered to 1700°C for 11 h, but no diamond was formed at 1600°C for 12.5 h. These results indicate that the molten state of the solvent-catalyst and enough reaction time are necessary for diamond nucleation in the systems examined at 7.7 GPa.     

Crystallization and crosslinking of polyamide‐1010 under elevated pressure

The structure and thermal properties of polyamide‐1010 (PA1010), treated at 250°C for 30 min under pressures of 0.7–2.5 GPa, were studied with wide‐angle X‐ray diffraction (WAXD), infrared (IR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Crystals were formed when the pressures were less than 1.0 GPa or greater than 1.2 GPa. With increasing pressure, the intensity of the diffraction peak at approximately 24° was enhanced, whereas the peak at approximately 20° was depressed. The triclinic crystal structure of PA1010 was preserved. The highest melting temperature of the crystals obtained in this work was 208°C for PA1010 treated at 1.5 GPa. Crosslinking occurred under pressures of 1.0–1.2 GPa. Only a broad diffraction peak centered at approximately 20° was observed on WAXD patterns, and no melting and crystallization peaks were found on DSC curves. IR spectra of crosslinked PA1010 showed a remarkable absorption band at 1370 cm^?1. The N? H stretching vibration band at 3305 cm^?1 was weakened. Crystallized PA1010 had a higher thermal stability than crosslinked PA1010, as indicated on TGA curves by a higher onset temperature of decomposition. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2522–2527, 2002     

Effects of UV exposure and thermal history on properties of a preimidized photosensitive polyimide

The structure and properties of a preimidized photosensitive polyimide (Probimide PSPI, a copolyimide of benzophenonetetracarboxylic dianhydride with alkyl groups substituted aromatic diamines) were studied with variations of UV exposure energy and bake temperature by means of wide angle X-ray diffraction, dynamic mechanical thermal analysis, stress-strain analysis, and residual stress analysis. The X-ray diffraction patterns patterns indicate that the PSPI is amorphous in the solid state. The T_g was 378°C ～ 410°C, depending upon the thermal history over the range of 350°C ～ 400°C. At the glass transition region, the dynamic storage modulus E′ was very sensitive to both i-line exposure energy and thermal history. However, the mechanical stress-strain behavior at room temperature was primarily dependent on the thermal history. The mechanical properties were 2.6 GPa ～ 2.9 GPa Young's modulus, 131 MPa ～ 168 MPa tensile strength, 10% ～ 12% yield strain, and 16% ～ 74% elongation at break, depending upon the baking or annealing. These dynamic and static mechanical properties indicate that on the PSPI backbone, crosslinks are formed thermally as well as photochemically. The thermal crosslinks might be formed through thermal liberation of the labile alkyl groups of aromatic diamine moieties and subsequent coupling of the radicals. The thermal degradation was also evidenced in the mechanical properties degraded by baking above 375°C or annealing above 350°C. In addition, during baking and cooling, the residual stress was dynamically measured on Si wafers as a function of temperature. The stress at room temperature was 48 MPa ～ 52 MPa for the PSPI films baked at 350°C or 400°C, regardless of i-line exposure.     

High temperature investigation of SiO2-Al2O3-ZnO-Na2O glass for ceramic-glaze: in‐situ/ex-situ synchrotron diffraction and conventional approaches

The temperature dependent behaviour of a complex aluminosilicate glass (SiO₂-Al₂O₃-ZnO-Na₂O system), which is a reference for ceramic glaze technology, has been determined, by combining techniques that cover a scale ranging from atomistic to macroscopic. The system shows a linear thermal expansion up to about 600 °C. The glass transition temperature is at 620 °C, as observed from Differential Scanning Calorimetry. Ex situ synchrotron diffraction experiments found a further transformation consisting of albite crystallization above 810 °C. This reaction is very slow and induces permanent structural modifications in the material at both intermediate and short ranges, as shown by in situ synchrotron diffraction experiments. These observations explain why ceramic glaze technology still faces challenges for large scale manufacturing and show the critical thermal range where interventions should be focussed. Eventually, melting takes place at 1190 °C, from hot stage microscopy.