In situ synthesis mechanism of 15R–SiAlON reinforced Al2O3 refractories by Fe–Si liquid phase sintering

15R–SiAlON bonded Al₂O₃ refractories were successfully synthesized using ferrosilicon nitride and alumina by liquid phase sintering. The phase composition and morphology were analyzed by means of X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results show that 15R–SiAlON reinforcement can be in situ obtained in the specimens with 5 wt% ferrosilicon nitride at 1500°C to 1700°C in flowing N₂ of 0.1 MPa. The morphology of 15R–SiAlON is strongly dependent on the morphology of intermediate AlON phases formed at different temperatures. Fe–Si alloys from ferrosilicon nitride form liquid phase and accelerate the formation of 15R‐SiAlON, in which process the wettability of Fe–Si alloys is improved by the increase in Si content, carbon coating on particle, solution process and reactions. The 15R–SiAlON reinforced Al₂O₃ refractory materials possess high cold crushing strength of 138‐171 MPa.     

A study of phase evolution of Fe-doped bismuth tantalate pyrochlore

The phase formation process of Bi₂FeTa₂O_9.5 (sp. gr. Fd-3m) synthesized by traditional ceramic technology was studied. Bismuth orthotantalate BiTaO₄ (sp. gr. Pnna) was found to be the precursor of the pyrochlore phase. The high-temperature synthesis of the iron-containing pyrochlore and the formation of a characteristic dendrite-like microstructure took place at temperatures above 900°С. The pure pyrochlore was obtained at a temperature of 1050°С. At the early stages of calcination, at a temperature close to 650 °C, the main part of iron (III) oxide, as a precursor, interacts with bismuth tantalate, and at temperatures above 950 °C, no Fe₂O₃ traces in the sample were found. Complex oxides Bi₂₅FeO₄₀ (sp. gr. I23), Bi₃TaO₇ (sp. gr. Fm-3m) were found as intermediate phases during the synthesis of pyrochlore. Pyrochlore of the given composition Bi₂FeTa₂O_9.5 had a porous microstructure with a grain size of 0.5–1.0 μm. The calcination temperature did not significantly affect the microstructure of the sample. The porosity of synthesized samples with a pyrochlore structure does not exceed 21%.     

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%.     

Effect of NiS addition on nitridation of alumina to aluminum nitride using polymeric carbon nitride as a nitridation reagent

We investigated the effect of adding nickel(II) sulfide (NiS) on nitridation of alumina (Al₂O₃) to aluminum nitride (AlN) using polymeric carbon nitride (PCN), which was synthesized by polymerization of dicyandiamide at 500 °C. The product powders obtained from nitridation of a mixture of δ-Al₂O₃ and NiS powders (mole ratio of 1:0.01) at various reaction temperatures were characterized by powder X-ray diffraction, ²⁷Al magic-angle spinning nuclear magnetic resonance, and Raman spectroscopy. δ-Al₂O₃ began to convert to AlN at 900 °C and completely converted to AlN at 1300 °C. The as-synthesized sample powders contained nitrogen-doped carbon microtubes (N-doped CMTs) with a length of several tens of mm and thickness of ca. 3 µm. The addition of NiS to δ-Al₂O₃ resulted in the enhancement of the amount of N-doped CMTs and nitridation rate, which might be due to the catalytic action of Ni particles on the thermal decomposition of vaporized PCN. The change in Raman spectra with reaction temperatures indicated that the crystallinity of N-doped CMTs was increased by calcining at higher reaction temperatures.     

Self-assembled one-dimensional carbon nitride architectures

Three kinds of assembled one-dimensional carbon nitride architectures were realized in large scale by a simple solvothermal technique. Carbon nitride nanotube bundles were formed involving the reaction of cyanuric chloride (C₃N₃Cl₃) with sodium at 230 °C and 1.8 MPa in a stainless steel autoclave using NiCl₂ as a catalyst precursor. Without any catalyst, aligned nanoribbons were formed at 290 °C and 3 MPa, and microspheres consisting of hundreds of nanoribbons were formed at 260 °C and 3.5–4.5 MPa. The electron energy-loss spectroscopy (EELS) proved all these 1-D carbon nitride nanostructures are consistent with the stoichiometry of CN. The similarity and difference of their microstructures and optical properties were researched by FTIR, Raman, PL measurements and UV–vis absorption spectra. These assembled CN architectures with well-controlled dimensionality and luminescent property may have potential uses as component of optical nanoscale devices. Their formation mechanisms were briefly discussed.     

Zinc oxide obtained by the solvothermal method with high sensitivity and selectivity to nitrogen dioxide

The effect of heat treatment using [Zn(H₂O)(O₂C₅H₇)₂] precursor ethylene glycol solution (synthesis temperature 125–185 °C, holding time 2–6 h) on the characteristics of the obtained nanocrystalline zinc oxide powder was studied. Under all synthesis conditions crystalline ZnO was formed with a wurtzite structure and an average crystallite size of 8–37 nm. The thermal behaviour and microstructure of the nanostructured ZnO powder were studied. The sensory properties of the obtained films in terms of the detection of hydrogen, methane, carbon monoxide and nitrogen dioxide were studied. The high sensitivity and selectivity of a thick ZnO film (synthesis temperature 145°С, holding time 6 h) when detecting NO₂ was established. It was found, that humidity had almost no effect on response value for NO₂ detection.     

Synthesizing carbon nanotubes and carbon nanofibers over supported-nickel oxide catalysts via catalytic decomposition of methane

Supported-NiO catalysts were tested in the synthesis of carbon nanotubes and carbon nanofibers by catalytic decomposition of methane at 550 °C and 700 °C. Catalytic activity was characterized by the conversion levels of methane and the amount of carbons accumulated on the catalysts. Selectivity of carbon nanotubes and carbon nanofiber formation were determined using transmission electron microscopy (TEM). The catalytic performance of the supported-NiO catalysts and the types of filamentous carbons produced were discussed based on the X-ray diffraction (XRD) results and the TEM images of the used catalysts. The experimental results show that the catalytic performance of supported-NiO catalysts decreased in the order of NiO/SiO₂ > NiO/HZSM-5 > NiO/CeO₂ > NiO/Al₂O₃ at both reaction temperatures. The structures of the carbons formed by decomposition of methane were dependent on the types of catalyst supports used and the reaction temperatures conducted. It was found that Al₂O₃ was crucial to the dispersion of smaller NiO crystallites, which gave rise to the formation of multi-walled carbon nanotubes at the reaction temperature of 550 °C and a mixture of multi-walled carbon nanotubes and single-walled carbon nanotubes at 700 °C. Other than NiO/Al₂O₃ catalyst, all the tested supported-NiO catalysts formed carbon nanofibers at 550 °C and multi-walled carbon nanotubes at 700 °C except for NiO/HZSM-5 catalyst, which grew carbon nanofibers at both 550 °C and 700 °C.     

Incongruent melting of gallium nitride at 7.5 GPa

Melting of GaN was studied at 7.5 GPa and 2300–2400 °C, using a split-sphere multi-anvil apparatus (BARS). Diamond synthesis from graphite in CaCO₃–C system and platinum melting were used as reference points for HPHT cell calibration. The initial samples of GaN were placed into boron nitride or graphite ampoules. No quench glass or quench crystals of GaN have been established. The incongruent GaN melting in the part of the samples at 2300 °C and in all samples at 2400 °C was fixed. Metallic gallium and gallium nitride as well as metallic gallium, gallium nitride and gallium oxide were observed in boron nitride and graphite ampoules, respectively. Results of the experiment indicate that the congruent melting of gallium nitride would occur at pressures > 7.5 GPa.     

Synthesis of NiMn2O4 thin films via a simple solid-state reaction route

Herein, NiMn₂O₄ (MNO) spinel oxide thermistor films were synthesized on a SiO₂/Si substrate via annealing the electron beam evaporated Mn–Ni–Mn metal trilayers in air at different temperatures. The X-ray diffraction (XRD) results indicate that polycrystalline spinel-structured MNO thermistor films were formed. The surface particle size of the series MNO films quickly reduced from ~300 to ~120 nm with a temperature increase from 650 to 750 °C, and then, slowly reduced to 80 nm or even smaller with a temperature increase from 750 to 950 °C. Specifically, 750 °C anneal formed the spinel MNO film with largest B value of 5067 and Ea value of 0.4366. The proposed synthesis route for MNO spinel oxide film has been proven to be feasible.     

Formation of HxN-rich graphitic carbon nitride network from guanidine carbonate salt by pyrolysis

Hydrogenated nitrogen-rich graphitic carbon nitride material was successfully synthesized by pyrolysis at 550 °C of an unusual organic precursor molecule, namely guanidine carbonate. The product was characterized in detail using X-ray diffraction, chemical analysis, diverse spectroscopy techniques (Fourier transform infrared, X-ray photoelectron, ultra-violet visible absorption), nitrogen adsorption, and scanning electron microscopy. The results of characterization clearly confirmed the synthesis of a graphitic carbon nitride material (stacking of tri-s-triazine planes) composed of C_3.00N_4.29H_1.59O_0.77. In order to understand the mechanism of material formation, products obtained at different temperatures between 200 and 500 °C were systematically investigated. The guanidine carbonate salt precursor is presented to be a promising alternative source for the elaboration of the g-C₃N₄ material in relation to the classically used cyanamide, melamine or cyanuric-based molecules. Since it is more environmentally friendly and highly soluble in aqueous solvents this new method is definitely strengthening the possibility of process industrialization.     

Thermal stability and surface modifications of detonation diamond nanoparticles studied with X-ray photoelectron spectroscopy

Detonation nanodiamond (ND) particles were dispersed on silicon nitride (SiN_x) coated sc-Si substrates by spin-coating technique. Their surface density was in the 10¹⁰–10¹¹ cm^?2 range. Thermal stability and surface modifications of ND particles were studied by combined use of X-ray Photoelectron Spectroscopy (XPS) and Field Emission Gun Scanning Electron Microscopy (FEG SEM). Different oxygen-containing functional groups could be identified by XPS and their evolution versus UHV annealing temperature (400–1085 °C) could be monitored in situ. The increase of annealing temperature led to a decrease of oxygen bound to carbon. In particular, functional groups where carbon was bound to oxygen via one σ bond (C–OH, C–O–C) started decomposing first. At 970 °C carbon–oxygen components decreased further. However, the sp²/sp³ carbon ratio did not increase, thus confirming that the graphitization of ND requires higher temperatures. XPS analyses also revealed that no interaction of ND particles with the silicon nitride substrate occurred at temperatures up to about 1000 °C. However, at 1050 °C silicon nitride coated substrates started showing patch-like damaged areas attributable to interaction of silicon nitride with the underlying substrate. Nevertheless ND particles were preserved in undamaged areas, with surface densities exceeding 10¹⁰ cm^?2. These nanoparticles acted as sp³-carbon seeds in a subsequent 15 min Chemical Vapour Deposition run that allowed growing a 60–80 nm diamond film. Our previous study on Si(100) showed that detonation ND particles reacted with silicon between 800 and 900 °C and, as a consequence, no diamond film could be grown after Chemical Vapour Deposition (CVD). These findings demonstrated that the use of a thin silicon nitride buffer layer is preferable insofar as the growth of thin diamond films on silicon devices via nanoseeding is concerned.     

Optical reflectivity studies of GaN and AlN chemical beam epitaxy on GaAs(100)

The deposition of gallium nitride and aluminium nitride thin films on GaAs(100) substrates by chemical beam epitaxy is reported. In-situ dynamic optical reflectivity has been used to compare growth rates of the nitride layers as a function of substrate temperature with their arsenide analogues. The relative growth efficiency of gallium nitride/gallium arsenide from triethyl gallium was found to be in the range 75–85%. The growth temperature for gallium nitride extends to higher temperatures, compared with gallium arsenide, probably due to lower evaporation rates of Ga bound to the nitride surface. At the same beam equivalent pressure, the growth rate of aluminium nitride from ethyldimethyl aluminium alane is approximately one-third of that for gallium nitride from triethyl gallium. Atomic force microscopy reveals that the gallium nitride surface formed at 500 °C is facetted, whereas an aluminium nitride surface deposited at 400 °C exhibits a rounded columnar type growth habit. Reflection anisotropy spectra indicate that atomic nitrogen readily reacts with the GaAs(100)-c(4×4) As stabilized surface at temperatures as low as 400 °C but without the gross facetting that has been observed at higher temperatures.     

Long-term oxidation and ablation behavior of Cf/ZrB2–SiC composites at 1500, 2000, and 2200°C

Although C_f/ZrB₂–SiC composites prepared via direct ink writing combined with low-temperature hot-pressing were shown to exhibit high relative density, high preparation efficiency, and excellent flexural strength and fracture toughness in our previous work, their oxidation and ablation resistance at high and ultrahigh temperatures had not been investigated. In this work, the oxidation and ablation resistance of C_f/ZrB₂–SiC composites were evaluated via static oxidation at high temperature (1500°C) and oxyacetylene ablation at ultrahigh temperatures (2080 and 2270°C), respectively. The thickness of the oxide layer of the C_f/ZrB₂–SiC composites is <40 μm after oxidizing at 1500°C for 1 h. The C_f/ZrB₂–SiC composites exhibit non-ablative properties after oxyacetylene ablation at 2080 and 2270°C for >600 s, with mass ablation rates of 3.77 × 10⁻³ and 5.53 × 10⁻³ mg/(cm² s), and linear ablation rates of −4.5 × 10⁻⁴ and −5.8 × 10⁻⁴ mm/s, respectively. Upon an increase in the ablation temperature from 2080 to 2270°C, the thickness of the total oxide layer increases from 360 to 570 μm, and the carbon fibers remain intact in the unaffected region. Moreover, the oxidation and ablation process of C_f/ZrB₂–SiC at various temperatures was analyzed and discussed.     

Nano-crystalline Ceria Catalysts for the Abatement of Polycyclic Aromatic Hydrocarbons

Nano-crystalline cerium oxide catalysts have been prepared by precipitation and evaluated for the total catalytic oxidation of naphthalene, which is a polycyclic aromatic hydrocarbon (PAH). Ceria synthesised by precipitation with urea was the most active catalyst for oxidation of naphthalene to carbon dioxide. The urea precipitated CeO₂ demonstrated over 90% naphthalene conversion to carbon dioxide at 175°C (100 ppm naphthalene, GHSV=25,000 h⁻¹), whilst ceria precipitated via a carbonate only gave 90% conversion at 275°C. Comparison with known high activity total oxidation catalysts, Mn₂O₃ and 0.5% Pt/γ-Al₂O₃, showed that the urea precipitated CeO₂ was a more effective catalyst for naphthalene total oxidation. At temperatures below those required to achieve catalytic activity the adsorption capacity of urea precipitated ceria for naphthalene was considerably greater than any of the other catalysts examined. The high adsorption capacity of the material provides the advantage that it can be used as a combined catalyst and adsorbent to remove PAHs from waste streams.     

Wear damage of Si3N4-graphene nanocomposites at room and elevated temperatures

Mechanical and tribological properties of nanocomposites with silicon nitride matrix with addition of 1 and 3 wt.% of multilayered graphene (MLG) platelets were studied and compared to monolithic Si₃N₄. The wear behavior was observed by means of the ball-on-disk technique with a silicon nitride ball used as the tribological counterpart at temperatures 25 °C, 300 °C, 500 °C, and 700 °C in dry sliding. Addition of such amounts of MLG did not lower the coefficient of friction. Graphene platelets were integrated into the matrix very strongly and they did not participate in lubricating processes. The best performance at room temperature offers material with 3 wt.% graphene, which has the highest wear resistance. At medium temperatures (300 °C and 500 °C) coefficient of friction of monolithic Si₃N₄ and composite with 1%MLG reduced due to oxidation. Wear resistance at high temperatures significantly decreased, at 700 °C differences between the experimental materials disappeared and severe wear regime dominated in all cases.     

Annealing effects for calcination of tin oxide powder prepared via homogeneous precipitation

Tin oxide powders were prepared from a homogeneous precipitation using the aqueous solution of SnCl₄ with urea as a precipitator at 90 °C and followed by a calcination process. The calcination was performed using two different methods; conventional furnace annealing (CFA) and rapid thermal annealing (RTA). The crystallization of the tin oxide finished at 600 °C regardless of the calcination method used. The crystallite size increased with as the calcination temperature increased due to the crystal growth and agglomeration. The tin oxide calcined using RTA has a relative smaller crystallite size than CFA at the same temperature. The tin oxide powders calcined with RTA showed higher specific surface areas than those that used CFA over a wide range of temperatures.     

Synthesis and spark plasma sintering of Si3N4–ZrN self-healing composites

A Si₃N₄–ZrN wear-resistant self-healing composite material was developed. Si₃N₄–ZrN composite ultrafine powders were synthesized at a temperature of 1200 °С via solid-state reactions without milling and densified by spark plasma sintering at 1650 °C to a relative density of 97 ± 0.5%. Balls 13.494 mm in diameter for ball bearings manufactured by spark plasma sintering had a fine-grained structure with a grain size of 200–500 nm, Vickers hardness of 22.5 ± 1.8 GPa, and indentation fracture toughness of 6.2 ± 0.4 MPa. The tribological properties of the composite were investigated under static and dynamic loading. The self-healing capability of the Si₃N₄–ZrN composite was evaluated in the temperature range 500–550 °С. High-temperature three-point bending tests of notched specimens showed a bending strength of 383 ± 21 MPa at room temperature and 413 ± 30 MPa at 500 °С, which confirmed the self-healing of the composite.     

Development and evaluation of TiB2–AlN ceramic composites sintered by spark plasma

Titanium diboride (TiB₂) is a ceramic material with high mechanical resistance, chemical stability, and hardness at high temperatures. Sintering this material requires high temperatures and long sintering times. Non-conventional sintering techniques such as spark plasma sintering (SPS) can densify materials considered difficult to sinter. In this study, TiB₂–AIN (aluminum nitride) composites were sintered by using the SPS technique at different sintering temperatures (1500 °C, 1600 °C, 1700 °C, 1800 °C, and 1900 °C). x-ray diffraction was used to identify the phases in the composites. mechanical properties such as hardness and indentation fracture toughness was obtained using a vickers indenter. Different toughening mechanisms were identified, and good densification results were obtained using shorter times and lower temperatures than those previously reported.     

Ultra-high elevated temperature strength of TiB2-based ceramics consolidated by spark plasma sintering

Spark plasma sintering of TiB₂–boron ceramics using commercially available raw powders is reported. The B₄C phase developed during reaction-driven consolidation at 1900 °C. The newly formed grains were located at the grain junctions and the triple point of TiB₂ grains, forming a covalent and stiff skeleton of B₄C. The flexural strength of the TiB₂–10 wt.% boron ceramic composites reached 910 MPa at room temperature and 1105 MPa at 1600 °С. Which is the highest strength reported for non-oxide ceramics at 1600 °C. This was followed by a rapid decrease at 1800 °C to 480–620 MPa, which was confirmed by increased number of cavitated titanium diboride grains observed after flexural strength tests.     

Effect of CVI SiC content on ablation and mechanism of C/C-SiC-ZrC-Cu composites

C/C-SiC-ZrC-Cu composites were fabricated by chemical vapor infiltration, precursor infiltration-pyrolysis and vacuum-pressure infiltration methods. During Cu infiltration, the Cu_6·69Si and Cu₃Si new phases are generated through reaction between SiC and molten Cu. The formed Cu_6·69Si, Cu₃Si, ZrC and SiC phases can improve the wettability and interface combination between Cu and the doped carbon matrix. The ablation tests demonstrate that the CVI SiC content significantly affects the structure of protective oxide layer, and induces inverse effects in ablation center at 2500 ^°C and 3000 ^°C. The relatively high CVI SiC content enhances the ablation resistance of composites at 2500 ^°C, but increases the linear ablation rate at 3000 ^°C due to the excessive evaporation and mechanical denudation. During ablation, the formed Si-Zr-C-O layer underneath ablation center and the Si-Cu-C-O layer on transition or marginal areas can prevent carbon matrix from serious oxidation. After ablation for 20 s, the C/C-SiC-ZrC-Cu composites with high CVI SiC content display the best anti-ablation property at 2500 ^°C, and the ablation rates are 3.5 ± 0.1 μm/s and 3.4 ± 0.1 mg/s.