Crystal Growth in the Combustion Synthesis of α‐Si3N4 Using Si with Different Particle Sizes

In this present work, Si₃N₄ powders with high α‐phase contents and distinct crystal morphologies were prepared via a promising approach of combustion synthesis (CS), using Si powders with different particle sizes as reactants. The influence of Si particle size on phase composition and crystal morphologies in the products was systematically investigated. Two unique crystal morphologies, radial‐spheroidal‐cluster and flowerlike, were observed in the Si₃N₄ products. The crystal growth mechanisms of Si₃N₄ granules in the CS system with disparate Si have been proposed based on experiments and thermokinetic calculations. As conclusion, the α‐phase content in the final product was synergetically dominated by the vaporization process of Si particles and the α–β phase transformation of Si₃N₄ during the after‐burn period. Si₃N₄ powders with high α‐phase content can be obtained from Si powders with an appropriate particle size.     

Fabrication and Characterization of (Mo1‐xNix) (Si1‐xAlx)2 (x = 0.025, 0.05, and 0.1) Alloys

(Mo_1‐xNi_x) (Si_1‐xAl_x)₂ (x = 0.025, 0.05, and 0.1) alloys were prepared using self‐propagating combustion synthesis from Mo, Si, Al, and Ni powders and subsequently hot‐pressed to achieve a high density of above 96.2%. The combustion products were composed of C11_b MoSi₂ and C40 Mo(Si,Al)₂. A systematic shift of diffraction peaks of C11_b MoSi₂ toward low angles was observed, and C40 Mo(Si,Al)₂ diffraction peaks became stronger with increase of alloying elements. The solid‐solution softening and toughening phenomena were observed in (Mo_1‐xNi_x) (Si_1‐xAl_x)₂ alloys. On increasing x from 0.025 to 0.1, the hardness of the alloys decreased from 9.87 to 8.31 GPa, and the fracture toughness increased from 3.79 to 6.48 MPa m^1/2.     

Nitridation of Silicon Powders Catalyzed by Cobalt Nanoparticles

The effect of Co nanoparticles (NPs) on the nitridation of silicon (Si) was studied. Co NPs were deposited homogeneously on the surfaces of Si powders using an in situ reduction method using NaBH₄ as a reducing reagent. Si powders impregnated with 0.5–2.0 wt% Co NPs were nitrided in 1200°C–1400°C for 2 h. The resultant silicon nitride powders were characterized by XRD, FE‐SEM, TEM, and EDS. The results showed that: (1) Co NPs significantly decreased the Si nitridation temperature, and the nitridation could be completed at 1300°C upon using 2 wt% Co NPs as catalysts. For comparison, the Si conversion could not be completed even at a temperature as high as 1400°C in the case without using a catalyst; (2) many Si₃N₄ whiskers with 80–320 nm in diameter and tens micrometers in length were generated and uniformly distributed in the final products. They were single‐crystalline α‐Si₃N₄ grown along the [101] direction. The enhanced nitridation in the case of using Co NPs as a catalyst was attributed two following factors, the increased bond length and weakened bond strength in N₂ caused by the electron donation from the Co atoms to the N atoms.     

Theoretical prediction on structure evolution and optimal properties of silicon modified hexagonal boron nitride as interphase in SiCf/SiC composite

To predict the effects of Si doping on hexagonal boron nitride (h-BN) and to achieve a balance between mechanical and oxidation properties for the interphase modification in SiC_f/SiC composites, we herein calculate and analyze the crystal structures and mechanical properties of (BN)₆₄Si_x (x = 4, 8, 16, 32) models by means of density functional theory (DFT) calculations and ab initio molecular dynamics (aiMD) simulations. The possible trends of crack deflection and self-healing ability are discussed. The modeling shows an obvious transition of (BN)₆₄Si_x from the layered crystal structure and anisotropic mechanical property to amorphous structure and isotropic mechanical property as the Si doping content up to 36.1 wt%. Regarding to the application of interphase in SiC_f/SiC composites, (BN)₆₄Si₁₆ model structure possess the highest debonding potential according to Cook and Gordon’s criteria and illustrates the higher self-healing capacity at elevated temperature.     

Preparation and Application of Strong Near‐Infrared Emission Phosphor Sr3SiO5:Ce3+,Al3+,Nd3+

This work investigated the near‐infrared (NIR) emission properties of mCe³⁺, xNd³⁺ codoped Sr_3?m?x(Si_1?m?xAl_m+x)O₅ phosphors. Samples with various doping concentrations were synthesized by the high‐temperature solid‐state reaction. Al³⁺ ions have the ability to promote Ce³⁺ ions to enter into the Sr²⁺ sites and to improve the visible emission of Ce³⁺. Thus the NIR emission of Nd³⁺ is enhanced by the energy‐transfer process, which occurred from Ce³⁺ to Nd³⁺. The device based on these NIR emission phosphors is fabricated and combined with a commercial c‐Si solar cell for performance testing. Short‐circuit current density of the solar cell is increased by 7.7%. Results of this work suggest that the Sr_2.95Si_0.95Al_0.05O₅:0.025Ce³⁺, 0.025Nd³⁺ phosphors can be used as spectral convertors to improve the efficiency of c‐Si solar cell.     

Characterization of Magnesium‐Doped Hydroxyapatite Prepared by Sol‐Gel Process

Pure and doped hydroxyapatite (HA) nanocrystalline powders (Ca_10‐xMg_x(PO₄)₆OH₂) were synthesized using sol‐gel process. For this, calcium nitrate tetrahydrate, magnesium nitrate hexahydrate, and phosphorous pentoxide were used as precursors for Ca, Mg, and P, respectively. Calculated amounts of magnesium ions (Mg⁺²) especially from 0 to 10% (molar ratio) were incorporated as dopant into the calcium sol solution. The structure and morphology of the gels obtained after mixing the phosphorous and (calcium + magnesium) sol solution were different, and their condensations in time depend on the quantities of magnesium added. The several powders resulting from the gels dried and sintered at 500°C for 1 h were characterized by thermogravimetry (TG), Fourier transform infrared spectroscopy (FTIR), X‐ray diffraction (XRD), and inductively coupled plasma (ICP). Additionally, their agglomeration, morphology, and particle size were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The specific surface area of each sample was measured by the Brunauer–Emmett–Teller (BET) gas adsorption technique. The results of XRD, FTIR, and ICP values ranged between 0.45 and 2.11 mg/L indicated that the magnesium added in the calcium solution was incorporated in the lattice structure of HA so prepared, while those obtained by SEM and TEM confirmed the influence of Mg on their morphology (needle and irregular shape) and crystallite size, which is about 30–60 nm. The as‐prepared powders had a specific surface area ranged between 6.37 and 27.60 m²/g.     

Combustion synthesis of SiC/Si3N4-NW composite powders: The influence of catalysts and gases

Composite powders containing silicon carbide (SiC) particles and silicon nitride nanowires (Si₃N₄-NWs) were synthesized by combustion synthesis, using elemental Si, carbon black, PTFE and small amount of metal powders as raw materials. The catalyst types and environmental gases and pressures have been altered to study their influence upon the crystal growth and the nature of the products. The products were characterized by X-ray diffraction, scanning and transmission electron microscopy. Results reveal that the metal/silicon liquid (e.g. Ni₂Si and Fe₃Si) formed during the combustion process is a key factor for the growth of Si₃N₄-NWs in nitrogen. For the process carried out in non-nitrogen gas (Ar, CO₂ or mixed CO₂/O₂), pure SiC particles were obtained. The rise in nitrogen pressure can promote the growth of Si₃N₄-NWs as well as large SiC particles. The growth of Si₃N₄-NWs could be explained by the SLGS mechanism, and the growth of SiC particles was involved in the gas-phase and liquid-phase mechanisms.     

Eu‐Doped β‐SiAlON Phosphors: Template‐Assistant Low Temperature Synthesis,Dual Band Emission,and High‐Thermal Stability

A hard template route has been successfully developed for synthesis of β‐SiAlON:Eu phosphors at low temperatures. The synthesis utilizes mesoporous silica (SBA‐15) skeleton as an active Si source, combined with the carbothermal reduction and nitridation method. It has been shown that the additional driving force from high surface area and porosity of SBA‐15 enables β‐SiAlON:Eu (with compositions of Si_6?zAl_z?xO_z+xN_8?z?x: xEu, x = 0.010–0.200 and z = 1.000) phosphors to be formed as a dominant phase at low temperature of 1400°C. The resultant β‐SiAlON:Eu phosphor powders exhibit a typical rod‐like morphology and a well dispersed state. By tailoring the Eu²⁺ concentration in the phosphors, a continuous change in emission band can be realized, that is a blue emission dominated for low Eu²⁺ concentrations and a green emission dominated for high Eu²⁺ doping concentrations. Furthermore, the resultant phosphors exhibit a small thermal quenching up to high temperature of 250°C. Therefore, the developed method is beneficial to synthesize LED phosphors of oxynitride systems at lower temperatures.     

High strengthening effects and excellent wear resistance of Ti3Al(Si)C2 solid solutions

The Ti₃Al_1.2−xSi_xC₂ (x = 0, 0.2, 0.4) powders were synthesized from Ti, Al, Si, and TiC powders, and nearly pure Ti₃Al_1.2−xSi_xC₂ bulks were fabricated by the means of two-time hot-pressing method. Significant strengthening effect in bulks was found after the addition of 0.2 Si and 0.4 Si to form Ti₃Al(Si)C₂ solid solutions. The flexural strengths of Ti₃AlSi_0.2C₂ and Ti₃Al_0.8Si_0.4C₂ were 485 and 554 MPa, 14% and 30% larger than the strength of Ti₃AlC₂, respectively. The Vickers hardness of these compounds were separately, 6.95 and 7.57 GPa, representing the enhancements of 37% and 49% over those of Ti₃AlC₂. The tribological behavior was studied by dry-sliding method with a S45C steel at the sliding speed of 30 m/s and the normal load of 20-80 N. The results showed that after incorporating different contents of Si, the friction coefficient was between 0.22 and 0.30, correspondingly lower wear rate was 3.19-2.61 × 10⁻⁶ mm³/Nm. These excellent tribological performances were attributed to the presence of continuous self-generated oxidized films during tribological examination. Finally, the phase compositions and microhardness of the oxidized films were analyzed and characterized.     

Effect of Nitrogen on Properties of Na2O–CaO–SrO–ZnO–SiO2 Glasses

Glasses in the Na₂O–CaO–SrO–ZnO–SiO₂ system have previously been investigated for suitability as a reagent in Al‐free glass polyalkenoate cements (GPCs). These materials have many properties that offer potential in orthopedics. However, their applicability has been limited, to date, because of their poor strength. This study was undertaken with the aim of increasing the mechanical properties of a series of these Zn‐based GPC glasses by doping with nitrogen to give overall compositions of: 10Na₂O–10CaO–20SrO–20ZnO–(40?3x)SiO₂–xSi₃N₄ (x is the no. of moles of Si₃N₄). The density, glass‐transition temperature, hardness, and elastic modulus of each glass were found to increase fairly linearly with nitrogen content. Indentation fracture resistance also increases with nitrogen content according to a power law relationship. These increases are consistent with the incorporation of N into the glass structure in threefold coordination with silicon resulting in extra cross‐linking of the glass network. This was confirmed using ²⁹Si MAS‐NMR which showed that an increasing number of Q² units and some Q³ units with extra bridging anions are formed as nitrogen content increases at the expense of Q¹ units. A small proportion of Zn ions are found to be in tetrahedral coordination in the base oxide glass and the proportion of these increases with the presence of nitrogen.     

Controllable wear behaviors of silicon nitride sliding against sintered polycrystalline diamond via altering humidity

The tribological behaviors of silicon nitride (Si₃N₄) sliding against sintered polycrystalline diamond (PCD) were investigated by varying the relative humidity (RH) in the testing atmosphere. The results indicated that higher RH corresponds to higher wear loss of Si₃N₄ and the wear loss of PCD almost fell close to zero. Especially in the case of 85% RH, both a maximum wear loss of Si₃N₄ and a maximum friction coefficient were achieved. In addition, this study revealed insights into the interface chemistry effects on the wear behavior of Si₃N₄ under humidity. When water molecules were introduced into the testing atmosphere, the hydrolysis reaction occurred on the Si₃N₄ surface with the formation of the Si‐O‐Si bond across the sliding interface. And then, the hydration reaction dominated the process, during which Si‐OH was formed through the bond fracture of the Si‐O‐Si. The X‐ray photoelectron spectroscopy results showed that the ratios of Si‐OH/Si‐O and Si‐N/Si‐OH+Si‐O bonds increased as the relative RH levels increased. As a consequence, the wear loss of Si₃N₄ significantly increased. Thus, due to the hydrolysis and hydration reactions, the tribological behaviors of Si₃N₄ against sintered polycrystalline diamond can be essentially controlled via varying RH levels.     

The Production and Characterization of Ytterbium‐Stabilized Zirconia Films for SOFC Applications

(ZrO₂)_1–x(Yb₂O₃)_x binary systems were investigated in the doping range of 0.02 ≤ x ≤ 0.12. Ytterbium‐doped zirconia powders were synthesized using the Pechini method. X‐ray diffraction (XRD) measurements showed that fcc ZrO₂ was stabilized for 8–12 mol% Yb‐doping rate. The produced Yb‐stabilized Zr (YbSZ) films were characterized; their thickness and homogeneity properties depended on the nature of the YbSZ slurry. All coating parameters were optimized and determined with precoating treatments. The samples were characterized by differential thermal analysis/thermal gravimetry (DTA/TG), scanning electron microscopy (SEM) and ac impedance measurements.     

Equiaxed β–Si3N4 ceramics prepared by rapid reaction‐bonding and post‐sintering using TiO2–Y2O3–Al2O3 additives

Sintered reaction‐bonded Si₃N₄ ceramics with equiaxed microstructure were prepared with TiO₂–Y₂O₃–Al₂O₃ additions by rapid nitridation at 1400°C for 2 hours and subsequent post‐sintering at 1850°C for 2 hours under N₂ pressure of 3 MPa. It was found that α–Si₃N₄, β–Si₃N₄, Si₂N₂O, and TiN phases were formed by rapid nitridation of Si powders with single TiO₂ additives. However, the combination of TiO₂ and Y₂O₃–Al₂O₃ additives led to the formation of 100% β–Si₃N₄ phase from the nitridation of Si powders at such low temperature (1400°C), and the removal of Si₂N₂O phase. As a result, dense β–Si₃N₄ ceramics with equiaxed microstructure were obtained after post‐sintering at high temperature.     

SiC formation for a solar cell passivation layer using an RF magnetron co-sputtering system

In this paper, we describe a method of amorphous silicon carbide film formation for a solar cell passivation layer. The film was deposited on p-type silicon (100) and glass substrates by an RF magnetron co-sputtering system using a Si target and a C target at a room-temperature condition. Several different SiC [Si_1-xC_x] film compositions were achieved by controlling the Si target power with a fixed C target power at 150 W. Then, structural, optical, and electrical properties of the Si_1-xC_x films were studied. The structural properties were investigated by transmission electron microscopy and secondary ion mass spectrometry. The optical properties were achieved by UV-visible spectroscopy and ellipsometry. The performance of Si_1-xC_x passivation was explored by carrier lifetime measurement.     

Analysis of the conduction mechanisms of the Ni doped apatite-type lanthanum silicate electrolyte ceramics synthesized by the combustion method

In this study, the nickel doped apatite-type lanthanum silicate La_9.33Si_6-xNi_xO_26-x (x = 0, .5, 1.0, 1.5, 2.0) (LSNO) electrolyte powders were successfully generated by using the urea nitrate combustion method at 600°C and 6–8 min. The optimal sintering temperature was determined to be 1500°C on the basis of the linear shrinkage, relative density as a function of temperature, and microcosmic analysis. It was observed that Ni²⁺ successfully replaced Si⁴⁺ in [SiO₄] to form the [Si(Ni)O₄] tetrahedra. LSNO had a typical p6₃/m apatite structure of high purity. A significant change in the cell volume of the doped samples was observed, and the cell volume increased with the doped nickel content. The conductivity reached the peak value at x = 1.0 (1.21 × 10⁻³ S·cm⁻¹, 700°C). Nickel doping introduced the oxygen vacancies and expanded the channels for interstitial oxygen conduction. Further, it also directly reduced the amount of interstitial oxygen in the LSO structure. This led to an enhancement in the conductivity of La_9.33Si_6-xNi_xO_26-x, followed by a decrease on increasing the doped nickel content. The conductivity enhancement in the Ni-doped LSO resulted from the combination of two mechanisms, namely, the oxygen vacancy defect and lattice volume enhancement mechanism.     

Oxygen vacancy,permeability and stability of Si doping Pr0.6Sr0.4FeO3-δ ceramic membrane for water splitting

The high-valence inorganic metalloid element of silicon was used for doping perovskite type oxides Pr_0.6Sr_0.4Fe_1-xSi_xO_3-δ (PSFSi_x, x = 0?0.1), as the oxygen transport membranes (OTM) for thermochemical water splitting to hydrogen production, which were fabricated by sol-gel method successfully. The effects of Si doped on microstructure, thermal expansion, permeability, and stability were investigated systematically. Although the crystal structure does not change, the metal-oxygen average binging energy (ABE) and oxygen vacancy concentration at room temperature increase after Si⁴⁺ doped. Furthermore, the phase transition process is reduced below the operating temperature by silicon doped. Due to the increase of ABE, the oxygen vacancy concentration of PSF exceeds that of the Si-doping samples with the temperature elevated. This is why the oxygen permeation flux of Si-doping samples is gradually exceeded by PSF. The long-term stability of water splitting to hydrogen is enhanced because the Si-doping increases the chemical stability of samples in reducing atmosphere.     

In-situ formation of BN layers by nitriding boron powders in BN/Si3N4-based composite ceramics

Boron nitride/silicon nitride (BN/Si₃N₄) composite ceramics were fabricated via the in-situ nitridation of boron (B) and silicon (Si) powders in forming gas (95%N₂/5%H₂) at 1390?°C. The effect of the B content on the phase composition, microstructure, density/porosity, machinability as well as mechanical properties of nitridized BN/Si₃N₄ composite ceramics was investigated. The addition of B slightly increased the nitridation degree of the Si and B powders mixture, and improved the ratio of the β-Si₃N₄ phase significantly at low B contents. B powders may have acted as a nucleating agent to promote the formation of β-Si₃N₄ crystals. A core-shell Si₃N₄/BN structure was revealed by the TEM technique, and the number of BN layers increased with the increase of the B content. The in-situ BN formed by the nitridation of B played a similar role with the BN directly added in enhancing the machinability of the BN/Si₃N₄ composite ceramics. The method of the in-situ nitridation of B is also effective to prepare SiC fiber-reforced BN/Si₃N₄ ceramic matrix composites.     

Oxygen Migration in Dense Spark Plasma Sintered Aluminum‐Doped Neodymium Silicate Apatite Electrolytes

Neodymium silicate apatites are promising intermediate temperature (500°C–700°C) electrolytes for solid oxide fuel cells. The introduction of Al promotes isotropic percolation of O^2?, and at low levels (0.83–2.0 wt% Al) enhances bulk conductivity. To better understand the effect of Al‐doping on intrinsic conductivity, and the impact of grain boundaries on the transport, dense Nd_9.33+x/3Al_xSi_6?xO₂₆ (0 ≤ x ≤ 2) pellets were prepared by spark plasma sintering. Phase purity of the products was established by powder X‐ray diffraction and the microstructure examined by scanning electron microscopy. The ionic conductivity measured by AC impedance spectroscopy for the spark plasma sintered ceramics were compared with transport in single crystals of similar composition. Intermediate Al‐doping (0.5 ≤ x ≤ 1.5) delivered superior overall conductivity for both the polycrystalline and single crystal specimens.     

Effect of Fe‐Contained Species on the Preparation of α‐Si3N4 Fibers in Combustion Synthesis

Herein, Si₃N₄ powders of comparatively high α‐phase but with distinct morphologies, especially α‐Si₃N₄ fibers, were successfully prepared by a developed combustion synthesis (CS) strategy. Different proportions of Fe and Fe₂O₃ were innovatively doped in reactants as additives to control the phase constitution and their relative percentage, as well as morphologies of final microstructures. One step further, the effects of Fe‐contained impurities on the CS process were rationally proposed and verified based on a series of meticulous designed experiments. It turns out that two contradictory effects of metal Fe on the formation of α‐Si₃N₄ synergistically play vital roles in the CS reaction. The existence of metal Fe can accelerate the crystallization of the amorphous SiO₂, which act as protection layer outside the Si powders and subsequently promote the generation of gaseous SiO. These gaseous SiO easily reacts with N₂ and eventually form α‐Si₃N₄. On the other hand, the formation of β‐Si₃N₄ will be promoted by the assistance of some liquid phases, and in this case, they mainly come from the reaction between Fe and Si. For this study, when the content of doped Fe is below 2 mol%, the prior effect on promoting α‐phase content is pronounced. Otherwise, the latter dominates the CS process as the content of Fe additive is further increased above 2 mol%. In a different way, Fe₂O₃ mainly encourages the formation of β phase through the large amount of newly generated liquid phases, although the reduced SiO₂ and Fe may still promote the α/β ratio on some extent.     

Structure,mechanical properties and thermal stability of Ti1-xSixN coatings

Ti_1-xSi_xN coating is a promising candidate for wear resistant applications due to their super-hardness and high thermal stability. Here, we explored the structure, mechanical properties and thermal stability of Ti_1-xSi_xN (x?=?0, 0.13, 0.17 and 0.22) coatings deposited by cathodic arc evaporation. Monolithically grown Si-containing Ti_1-xSi_xN coatings, which are Si-solution in TiN for x?=?0.13 and 0.17, reveal a high hardness of 39.4?±?0.67 and 40.6?±?0.72?GPa, respectively. Then Ti_1-xSi_xN transforms into a nanocomposite structure consisting of cubic Ti(Si)N nanocrystallite enveloped by the amorphous SiN_x tissue phase for x?=?0.22, which exhibits a high hardness of 40.0?±?0.6?GPa. However, increasing of Si content leads to a significant increase in compressive stress from ?0.63?GPa for x?=?0 to ?3.78?GPa for x?=?0.13 to ?4.54?GPa for x?=?0.17 to ?5.51?GPa for x?=?0.22. The hardness of Ti_1-xSi_xN coatings can be maintained up to ~ 1000?°C due to the suppressed grain growth, and then decreases for further elevated annealing temperature, whereas the TiN coating exhibits a continuous drop in hardness towards its intrinsic value of ~ 21.3?GPa.