Nitridation behavior of silicon powder compacts of various thicknesses with Y2O3 and MgO as sintering additives

The effects of the nitriding temperature (1300 and 1350°C), holding time (0‐4 hours), and thickness of Si powder compacts on the nitridation behavior of silicon were investigated by examining the nitridation rates, analyzing phase compositions, and observing the microstructures of nitrided compacts. Si powder compacts doped with Y₂O₃ and MgO as sintering additives were prepared with thicknesses of 3, 6, and 9 mm. The phases of nitrided compacts were transformed from Si to α‐Si₃N₄ and β‐Si₃N₄ with an increase in the nitriding temperature and holding time. The degree of nitridation increased with the nitriding temperature and holding time. The β/(α+β) ratio increased with the nitriding temperature and holding time, and with a decrease in the thickness of the Si powder compacts. However, all compacts exhibited the same tendency for a higher β/(α+β) ratio at the compact surface than in the bulk of the compact. The variation in the β/(α+β) ratio for each compact decreased with an increase in the nitriding temperature and holding time.     

Catalytic nitridation preparation of high-performance Si3N4(w)-SiC composite using Fe2O3 nano-particle catalyst: Experimental and DFT studies

The complete conversion from Si into Si₃N₄ was achieved after 2 h nitridation at 1400 °C by using in-situ formed Fe₂O₃ nano-particles (NPs) as a catalyst. Such a synthesis condition was remarkably milder than that (>1450 °C for many hours) required by the conventional Si nitridation method. Density functional theory (DFT) calculations suggest that Fe₂O₃ catalyst accelerates the Si nitridation via weakening the bond strength of absorbed N₂ molecule. Furthermore, Si₃N_4(w)-SiC composites prepared by the present catalytic nitridation method showed excellent high-temperature properties including modulus of rupture (MOR of 29.9 MPa at 1400 °C), thermal shock resistance (residual MOR percentage of 50% at ΔT = 1300 °C), as well as good oxidation resistance and cryolite corrosion resistance against molten cryolite. It can be concluded that, Fe₂O₃ NPs not only greatly accelerated the Si nitridation and Si₃N₄ formation, but also facilitated the epitaxial growth of reinforcement phase of Si₃N₄ whisker in the Si₃N_4(w)-SiC composites.     

Microstructure evolution and wear resistance of nitride/aluminide coatings on the surface of Ti-coated 2024 Al alloy during plasma nitriding

A duplex surface treatment consisting in depositing a Ti film followed by plasma nitriding was adopted to improve the wear resistance of 2024 Al alloys. Nano-grained Ti films were firstly deposited on the substrate surface by using magnetron sputtering, then plasma nitrided for 8 h at 400 °C, 430 °C, 460 °C and 490 °C, in a gas mixture of 40% N₂+60% H₂. Duplex coatings composed of three sublayers (i.e. the outmost TiN_0.3 layer, the intermediate Al₃Ti layer and the inside Al₁₈Ti₂Mg₃ layer) were obtained at nitriding temperature higher than 460 °C. The coatings obtained at 400 °C and 430 °C consisted of mainly α-TiN_0.3 with (002) preferred orientation. The surface hardness of the coatings increased at higher nitriding temperature, reaching the maximum of 500 HV at 490 °C, which was about 8 times higher than that of the uncoated alloy. The friction coefficients of 2024 Al alloy decreased in the coatings prepared at higher nitriding temperature, reaching the lowest values of 0.31 at 490 °C. The wear rate of the coated samples decreased by 56% compared with the uncoated ones. The analysis of worn surface indicated that the nitrided samples exhibited severe adhesive wear at 400 °C that changed to predominant abrasive wear at increased nitriding temperature.     

Improved thermal conductivity of sintered reaction-bonded silicon nitride using a BN/graphite powder bed

Sintered reaction-bonded silicon nitride (SRBSN) with improved thermal conductivity was achieved after the green compact of submicron Si powder containing 4.22 wt% impurity oxygen and Y₂O₃-MgO additives was nitrided at 1400 °C for 6 h and then post-sintered at 1900 °C for 12 h using a BN/graphite powder bed. During nitridation, the BN/10 wt% C powder bed altered the chemistry of secondary phase by promoting the removal of SiO₂, which led to the formation of larger, purer and more elongated Si₃N₄ grains in RBSN sample. Moreover, it also enhanced the elimination of SiO₂ and residual Y₂Si₃O₃N₄ secondary phase during post-sintering, and thus induced larger elongated grains, decreased lattice oxygen content and increased Si₃N₄-Si₃N₄ contiguity in final SRBSN product. These characteristics enabled SRBSN to obtain significant increase (∼40.7%) in thermal conductivity from 86 to 121 W ∙ m⁻¹ ∙ K⁻¹ without obvious decrease in electrical resistivity after the use of BN/graphite instead of BN as powder bed.     

Effect of calcination temperature on the fabrication of transparent lutetium titanate by spark plasma sintering

Transparent lutetium titanate (Lu₂Ti₂O₇) bodies were fabricated by spark plasma sintering using Lu₂O₃ and TiO₂ powders calcined from 700 °C to 1200 °C. No solid-state reaction was identified after calcination at 700 °C, whereas single-phase Lu₂Ti₂O₇ powder was prepared at 1100 and 1200 °C. The calcination at 700 °C promoted densification at the early stages of sintering, whereas residual pores at grain boundaries resulted in Lu₂Ti₂O₇ bodies with low transparency. Low-density and opaque Lu₂Ti₂O₇ bodies formed owing to the coarsening of the powder calcined at 1200 °C. The Lu₂Ti₂O₇ body sintered using the powder calcined at the moderate temperature of 1100 °C had a density of 99.5% with the highest transmittances of 41% and 74% at wavelengths of 550 nm and 2000 nm, respectively.     

Influence of nitrogen overpressure on the nitridation,densification and formation of β-SiAlONs produced by silicothermal reduction

β-SiAlON materials (with z = 1) with 2 mol% DyAG have been synthesised by silicothermal reduction under different nitrogen pressures. The possibility of rapidly nitriding silicon and forming the SiAlON phase has been investigated through its reaction sequence of formation under different pressures. β-SiAlONs were fully nitrided (degree of nitridation greater than 93%) and formed after a 1-h hold at 1400 °C, or after a straight ramp to 1500 °C under 0.7 MPa of nitrogen. This was not achievable under static nitrogen at atmospheric pressure where the degree of nitridation was only 43% and β-SiAlON phase represented only about 60% of the crystalline phase assemblage at the same temperature. The formation of β-SiAlON depended on the formation of Si₃N₄ whose reaction rate was enhanced by nitrogen overpressures.     

Synthesis and structural properties of GaN particles from GaO2H powders

Gallium nitride(GaN) powders have been synthesized by nitriding gallium oxyhydroxide (GaO₂H) powders in the flow of NH₃ gas at a nitridation temperature of 950 °C for 35 min. X-ray powder diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectra reveal that simple heat treatment of GaO₂H in the flow of NH₃ leads to the formation of hexagonl GaN with lattice constants a = 3.191 Å, and c = 5.192 Å at 950 °C through intermediate conversion of β-Ga₂O₃. X-ray photo-electron spectroscopy (XPS) confirms the formation of bonding between Ga and N, and yields that the surface stoichiometry of Ga : N approximates 1 : 1. Transmission electron microscopy (TEM) image indicates that GaN particle is a single crystal, and its morphology is ruleless.     

Rapid low temperature synthesis of a titanate pyrochlore by molten salt mediated reaction

Ytterbium titanate pyrochlore, Yb₂Ti₂O₇, was prepared by molten salt mediated synthesis (MSS) from titanium oxide (TiO₂) and ytterbium oxide (Yb₂O₃) reagents. Potassium and sodium chloride mixtures were used as the molten salt medium and the effects of salt to reagent ratio, salt composition, synthesis temperature, reaction time, and TiO₂ particle size were explored. Synthesis temperatures and times required for formation of single phase Yb₂Ti₂O₇ were found to be lower than those required for solid state synthesis (SSS). Whereas MSS synthesis of single phase Yb₂Ti₂O₇ was achieved with micron-sized powders after a single reaction at 1200 °C for 1 h, SSS with micron-sized powders required an extended reaction time of 36 h at 1350 °C. Yb₂Ti₂O₇ micron-sized powder prepared by MSS showed similar particle size and morphology to that of the TiO₂ precursor demonstrating a template growth mechanism. However, the use of TiO₂ nano-sized powder changed the dominant synthesis mechanism from template growth to dissolution–precipitation and facilitated synthesis of near single phase Yb₂Ti₂O₇ at the remarkably low temperature of 700 °C in only 1 h. The potential application for lanthanide and actinide immobilisation from molten salt reprocessing wastes was demonstrated by preparation of Yb₂Ti₂O₇ by molten salt mediated synthesis from TiO₂ and ytterbium chloride (YbCl₃) reagents.     

SPS processing of bismuth-layer structured ferroelectric ceramics yielding highly textured microstructures

The spark plasma sintering (SPS) behaviour of nano-sized Bi₄Ti₃O₁₂ (BIT) and micron-sized CaBi₂Nb₂O₉ (CBNO) powders is described. The densification process of both powders is very rapid, i.e. the densification occurs within a very narrow time interval (2–3 min using a heating rate of 100 °C min⁻¹ and a pressure of 50 MPa). The BIT powder exhibits a lower densification onset temperature (∼650 °C) and higher maximum shrinkage rate (8.9 × 10⁻³ s⁻¹ at 780 °C) than that of the CBNO powder (∼825 °C and 4.5 × 10⁻³ s⁻¹ at 950 °C). Isothermal compaction studies revealed that fully dense nano-sized BIT compacts could be obtained within the temperature region 750 °C < T_iso < 850 °C while for T_iso > 850 °C compacts containing elongated platelet grains are formed. A new preparation route to produce highly textured compacts is described in detail. Appropriate pre-forms are prepared by spark plasma sintering (SPS) and these fully dense compacts are subject to superplastic deformation in the SPS unit to achieve a total compressive strain of ∼60%. This strain was achieved within a period of 1.5 min and with a maximum strain rates of 1.1 × 10⁻² s⁻¹ achieved at ∼840 °C and 1.3 × 10⁻² s⁻¹ at 1020 °C for the BIT and CBNO compacts, respectively. The X-ray studies showed that the Lotgering orientation factors of grains in the deformed BIT and CBNO compacts are 99% and 70%. The formation of highly textured compacts is suggested to be governed by a superplastic deformation-induced directional dynamic ripening mechanism.     

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.     

Effect of temperature and heating rate on the sintering performance of SiC-Al2O3-Dy2O3 and SiC-Al2O3-Yb2O3 systems

Samples of SiC+10 vol%(Al₂O₃+Dy₂O₃) and SiC+10 vol%(Al₂O₃+Yb₂O₃) mixtures were obtained by cold isostatic pressing and sintered for one hour in a dilatometer at 1800 °C and 1900 °C, applying heating rates of 10, 20 and 30 °C/min. The results of the complete sintering cycle indicated that the heating rates do not significantly influence the shrinkage, but that temperature and total sintering time may be relevant factors. The compacts sintered at 1900 °C shrank on average 9% more than those sintered at 1800 °C, and it was found that the sintering time can be reduced by 40–50% at faster heating rates. The maximum shrinkage rates occurred at temperatures lower than those of the sintering thresholds for the two mixtures, two temperatures and three heating rates. It was also found that after formation of the liquid, the mechanisms of particle rearrangement and solution-precipitation were not as fast as reported in the literature, even at high heating rates, for example 30 °C/min, but they are responsible for much of the shrinkage occurring throughout the sintering cycle.     

Effect of Bi2O3 addition on the sintering and microstructural development of gadolinia-doped ceria ceramics

The effect of small amounts (0.2–2.0 wt.%) of bismuth oxide on the sintering behavior and microstructural development of Ce_0.9Gd_0.1O_1.95 (GDC) submicronized powders has been studied using XRD for the lattice parameter measurements, the constant heating rate (CHR) method in air to monitor the shrinkage kinetics of powder compacts, and scanning electron microscopy (SEM) to study the microstructure of the sintered samples. Sintering of GDC compacts was significantly improved by adding small amounts of Bi₂O₃ (≤2.0 wt.%), and samples of doped-GDC sintered at 1200–1400 °C for 2–4 h were dense bodies (98–99.5% of theoretical density). Measurements showed that the addition of Bi₂O₃ could reduce the sintering temperature by about 250–300 °C lower than that for undoped-GDC samples. A liquid phase-assisting mechanism was assumed as the main cause for the enhancement of the densification process. The average grain size of doped-GDC sintered samples grew with the increasing of Bi₂O₃ addition up to 1.0 wt.%, and then decreased indicating a poor wetting properties of the formed liquid phase.     

Influence of Yb3+ doping on phase stability and thermophysical properties of (Y1-xYbx)3Al5O12 under high temperature

In this work, Yb³⁺ was selected to replace the Y³⁺ in yttrium aluminum garnet (YAG) in order to reduce its thermal conductivity under high temperature. A series of (Y_1-xYb_x)₃Al₅O₁₂ (x=0, 0.1, 0.2, 0.3, 0.4) ceramics were prepared by solid-state reaction at 1600 °C for 10 h. The microstructure, thermophysical properties and phase stability under high temperature were investigated. The results showed that all the Yb doped (Y_1-xYb_x)₃Al₅O₁₂ ceramics were comprised of a single garnet-type Y₃Al₅O₁₂ phase. The thermal conductivities of (Y_1-xYb_x)₃Al₅O₁₂ ceramics firstly decreased and subsequently increased with Yb ions concentration rising from room temperature to 1200 °C. (Y_0.7Yb_0.3)₃Al₅O₁₂ had the lowest thermal conductivity among investigated specimens, which was about 1.62 W m⁻¹ K⁻¹ at 1000 °C, around 30% lower than that of pure YAG (2.3 W m⁻¹ K⁻¹, 1000 °C). Yb had almost no effect on the coefficients of thermal expansion (CTEs) of (Y_1-xYb_x)₃Al₅O₁₂ ceramics and the CTE was approximate 10.7×10⁻⁶ K⁻¹ at 1200 °C. In addition, (Y_0.7Yb_0.3)₃Al₅O₁₂ ceramic remained good phase stability when heating from room temperature to 1450 °C.     

Vapor-phase dehydration of 1,5-pentanediol into 4-penten-1-ol

Dehydration of 1,5-pentanediol was investigated over ZrO₂ and Yb₂O₃ catalysts at 300–450 °C. 1,5-Pentanediol was converted into 4-penten-1-ol together with tetrahydropyran over monoclinic ZrO₂ at temperatures <400 °C, and the selectivity to 4-penten-1-ol exceeded 50 mol%. Modification of ZrO₂ with Li ions increased the selectivity to 4-buten-1-ol up to 70 mol%. Yb₂O₃ also effectively worked as a catalyst in the dehydration of 1,5-pentanediol into 4-buten-1-ol at temperatures <425 °C. Especially, Yb₂O₃ with cubic structure showed higher than 75 mol% selectivity to 4-penten-1-ol.     

Highly transparent AlON ceramics sintered from powder synthesized by carbothermal reduction nitridation

Aluminum oxynitride (AlON) powders were synthesized by the carbothermal reduction and nitridation process using commercial γ-Al₂O₃ and carbon black powders as starting materials. And AlON transparent ceramics were fabricated by pressureless sintering under nitrogen atmosphere. The effects of ball milling time on morphology and particle size distribution of the AlON powders, as well as the microstructure and optical property of AlON transparent ceramics were investigated. It is found that single-phase AlON powder was obtained by calcining the γ-Al₂O₃/C mixture at 1550 °C for 1 h and a following heat treatment at 1750 °C for 2 h. The AlON powder ball milled for 24 h showed smaller particles and narrower particle size distribution compared with the 12 h one, which was benefit for the improvement of optical property of AlON transparent ceramics. With the sintering aids of 0.25 wt% MgO and 0.04 wt% Y₂O₃, highly transparent AlON ceramics with in-line transmittance above 80% from visible to infrared range were obtained through pressureless sintering at 1850 °C for 6 h.     

Nitridation of silicon powder studied by XRD, 29Si MAS NMR and surface analysis techniques

The nitridation of elemental silicon powder at 900–1475 °C was studied by X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), XRD, thermal analysis and ²⁹Si MAS NMR. An initial mass gain of about 12% at 1250–1300 °C corresponds to the formation of a product layer about 0·2 μm thick (assuming spherical particles). XPS and XAES show that in this temperature range, the surface atomic ratio of N/Si increases and the ratio O/Si decreases as the surface layer is converted to Si₂N₂O. XRD shows that above 1300 °C the Si is rapidly converted to a mixture of α- and β-Si₃N₄, the latter predominating >1400 °C. In this temperature range there are only slight changes in the composition of the surface material, which at the higher temperatures regains a small amount of an oxidised surface layer. By contrast, in the interval 1400–1475 °C, the ²⁹Si MAS NMR chemical shift of the elemental Si changes progressively from about −80 ppm to −70 ppm, in tandem with the growth of the Si₃N₄ resonance at about −48 ppm. Possible reasons for this previously unreported change in the Si chemical shift are discussed. ©     

Nano-sized BaSnO3 powder via a precursor route: Comparative study of sintering behaviour and mechanism of fine and coarse-grained powders

Preparation of a very fine BaSnO₃ powder by calcination of a barium tin 1,2-ethanediolato complex precursor and its sintering behaviour are described herein. A rate controlled calcination process to 820 °C leads to a nm-sized BaSnO₃ powder with a specific surface area of S = 15.1 m²/g (d_av. = 55 nm). The powder has a slightly larger cell parameter of a = 412.22(7) pm compared to the single crystal value, which decreases with increasing calcination temperature and reaches the reference value above 1000 °C. The sintering behaviour is compared between fine and coarse-grained BaSnO₃ powders. Corresponding powder compacts of the nano-sized BaSnO₃ achieve a relative density of 90% after sintering at 1600 °C for 1 h and at 1500 °C and a soaking time of 30 h, whereas coarse-grained powder compacts reach only 80% of the relative density at 1650 °C (10 h). Furthermore, the shrinkage mechanisms of fine and coarse-grained powder compacts have been investigated and are discussed.     

Low-temperature sintering of coarse alumina powder compact with sufficient mechanical strength

Coarse alumina powder compacts doped with various amounts of titania and copper oxide were pressurelessly sintered from 900 °C to 1600 °C. Their phase assemblages and microstructural evolution, as well as their properties, were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry/thermogravimetric (DSC/TG) analysis, and three-point bending and wetting test. The role of TiO₂ and CuO during the sintering is discussed in detail. The experimental results show that the liquid phase from the copper oxide appeared at approximately 1200 °C, so the solid-state reaction between alumina and titania took place at a lower temperature. Such solid state-reaction sintering had a strong impact on the grain growth and greatly promoted the densification of the alumina compact. In addition, the liquid phase inhibited the abnormal grain growth and microcracking. As a result, the coarse alumina powder compacts doped with 5 wt% TiO₂–CuO were fully densified and exhibited sufficient flexural strength (342±21 MPa) when sintered at a temperature of 1450 °C for 2 h.     

Synthesis and characterization of nanometric gadolinia powders by room temperature solid-state displacement reaction and low temperature calcination

Nanometric-sized gadolinia (Gd₂O₃) powders were obtained by applying solid-state displacement reaction at room temperature and low temperature calcination. The XRD analysis revealed that the room temperature product was gadolinium hydroxide, Gd(OH)₃. In order to induce crystallization of Gd₂O₃, the subsequent calcination at 600 ∼ 1200 °C of the room temperature reaction products was studied. Calculation of average crystallite size (D) as well as separation of the effect of crystallite size and strain of nanocrystals was performed on the basic of Williamson-Hall plots. The morphologies of powders calcined at different temperatures were followed by scanning electron microscopy. The pure cubic Gd₂O₃ phase was made at 600 °C which converted to monoclinic Gd₂O₃ phase between 1400° and 1600 °C. High-density (96% of theoretical density) ceramic pellet free of any additives was obtained after pressureless sintering at 1600 °C for 4 h in air, using calcined powder at 600 °C.     

Diphasic aluminosilicate gels with two stage mullitization in temperature range of 1200–1300 °C

The crystallization of mullite in amorphous diphasic gel aged for 6 months has been studied using non-isothermal differential scanning calorimetry (DSC) and powder X-ray diffraction with Rietveld structure refinement analysis. The diphasic premullite gels undergo structural changes by aging even when they are calcined at 700 °C. These changes imply segregation of the sample to Al₂O₃-rich and SiO₂-rich regions. From the Al₂O₃-rich region crystallizes poorly defined AlSi spinel at 977 °C followed by two-step mullite crystallization in the temperature interval of 1200–1300 °C. Two overlapped exothermic peaks on DSC scan of aged gel were observed; the first at 1233 °C and the second at 1261 °C. The former is attributed to mullite crystallization by transformation of AlSi spinel, by which excess alumina occurs, which in the second step of mullitization reacts with amorphous SiO₂-rich phase. The activation energy for mullite crystallization in the first step was E_a=935±14 kJ mol⁻¹ and the Avrami exponent n=2.5. The values E_a=1119±25 kJ mol⁻¹ and n=1.2 were obtained for mullite formation in the second step. If amorphous SiO₂-rich phase is extracted from the sample, the value E_a=805±26 kJ mol⁻¹ is obtained. Mullite crystallizing from AlSi spinel (when SiO₂-rich phase has been extracted) differentiates compositionally from that formed by both reactions. Smaller unit cell parameters and higher amount of oxygen vacancies are incorporated into tetrahedral positions of mullite structure, as was determined by Rietveld structure refinement method.