Solution-precipitation Mechanism Research Articles

Synthesis and Characterization of TiC Nanofibers Obtained via a Modified Carbothermal Process

AbstractA simple method for synthesis of titanium carbide (TiC) nanofibers by a modified carbothermal reduction of electrospun titanium‐based fibers is reported. The effect of synthesis steps on the final product is discussed. Continuous TiC fibers synthesized have a smooth surface morphology, with average diameter of 148 nm and length in the centimeter range. X‐ray diffraction and high‐resolution transmission electron microscopy analysis results indicate that the fibers consist of TiC of high purity. Impact of heat treatment on the morphology and chemical and crystalline composition of the product is also investigated. Based on the experimental evidence, it is hypothesized that the carbothermal reduction of calcined electrospun fibers with precoated carbon proceeds through diffusion‐limited shrinking core pathway while that of the as‐spun fibers follows the reaction‐limited solution precipitation mechanism. The TiC nanofibers also show superior sintering properties by increasing theoretical density of hot pressed TiB2 from 94.5% of theoretical density to 97.9% of theoretical density. When sintered with ceria, it improves the conductivity of the ceria by 18.29 times as compared to 2.94 times by TiC nanoparticles at 800 °C. The TiC nanofibers show metallic behavior as well as potential for application as electrochemical double layer capacitor supercapacitors.

Microstructural evolution mechanism of porous reaction bonded silicon nitride ceramics heat-treated in two powder beds

Porous Si3N4 with various grain morphologies was prepared by direct nitriding of silicon powder and subjected to heat-treatment while embedded in a Si3N4 powder bed. The influence of MgO, added either to the starting silicon or to the powder bed, on the microstructural transformation and morphology of pores at temperatures between 1425 °C and 1700 °C are discussed. In the presence of MgO, α-Si3N4 grains with equiaxed morphology resulted in an interconnected microstructure with spherical pores. By contrast, in the absence of MgO, an α-whisker-dominant microstructure resulted in pores of various morphologies. During the post heat-treatment process, the α-whiskers gradually vanished, and grains recrystallized as very fine β-Si3N4 rods. Consequently, pores became spherical, large and whisker-free. In agreement with the results of SEM-EDX along with XRD analysis, the observed morphology transition and full phase transformation occurred by vapor phase transport of MgO from the powder bed and a subsequent solution-precipitation mechanism. The presence of volatile MgO in the powder bed caused a substantial decrease in weight losses, while enhancing β phase formation, grain coarsening and linear shrinkage. The development of coarse β-rods from α-matte grains in a Si–Mg–O–N glassy phase was related to the presence of substantial liquid phase during the growth mechanism. Compared to the granular morphology, whiskers with high aspect ratio gave rise to a high sintering driving force and led to a maximum value of ≈2% linear shrinkage and porosity of ≈30 vol%. Consequently, this ceramic exhibited the highest compressive strength of ≈10 MPa.

Synthesis of Zr substituted B-site complex Bi4(ZrxTi1-x)3O12 platelet microcrystals

Novel plate-like B-site complex Bi4(ZrxTi1-x)3O12 microcrystals consisted with both (Bi2O2)2+ layer and Zr4+substituted [Bi2Ti3O10]2- blocks have been synthesized through molten salt method. With the increase of Zr4+ content, the lattice constant of Bi4(ZrxTi1-x)3O12 increases. From the Raman analysis, the bending vibration was mainly influenced by the Zr4+ content, while both of bending and stretching vibration was influenced at relative high calcination temperature. The reaction conversion process was mainly controlled by the solution-precipitation mechanism and TiO2 provided the basis crystal nucleation in NaCl-KCl molten salt, where the reactant ionic can precipitation, epitaxial growth and finally generated the final bismuth layer-structure products. Finally, fine crystallinity and orientation of plate-like Zr substituted Bi4(Zr0.1Ti0.9)3O12 microcrystals with high aspect ratio can be successful fabricated at 900 °C, which paved a way to design and fabricate the novel plate-like B-site complex perovskite compounds in the future.

Phase-field simulation of liquid phase migration in the WC–Co system during liquid phase sintering

Abstract Liquid phase sintering is a process for forming high performance, multiple-phase components from powders. The process includes very complex interactions between various mass transportation phenomena, among which the liquid phase migration represents an important one in the aspect of forming a gradient structure in cemented carbide. In the present work, phase-field simulation of the liquid phase migration phenomenon during liquid phase sintering is performed in the WC – Co based cemented carbide. The simulation results are analyzed and compared with the experimentally determined key factors of microstructural evolution, such as contiguity and liquid phase migration rate. The diffusion-controlled solution–precipitation mechanism of the liquid phase migration process in the cemented carbide system is confirmed from the current simulation result, which provides deeper understanding of the microstructural evolution during the liquid phase migration process. These simulations can offer guidance in preventing the liquid phase migration process during liquid phase sintering of cellular cemented carbide.

Cost-effective utilization of mineral-based raw materials for preparation of porous mullite ceramic membranes via in-situ reaction method

Porous mullite ceramic membranes were prepared via in-situ synthesis method using abundant natural minerals including coarse bauxite and fine kaolin powders. Thermal sintering behavior of the green sample shows that a unique dimension expansion appears during the sintering process due to the anisotropic growth of mullite crystals. The XRD analysis suggests the ceramic membrane is mainly composed of mullite phase when sintering above 1450°C. The SEM images confirm that with an increase in sintering temperature, mullitization reaction firstly occurs in kaolin agglomerations, and then bauxite-derived alumina reacts with silica-containing phase by a solution-precipitation mechanism to form an interlocked framework of elongated secondary mullite. Both maximum and average pore sizes increase with increasing sintering temperature. The growth of mullite whiskers has a positive impact on the improvement of porosity and mechanical strength of ceramic membranes. Even after sintering at 1500°C, the sample shows the highest open porosity of 31.6% and good biaxial flexural strength of 100.2MPa.

Fabrication and characterization of 3-D Cf/ZrC composites by low-temperature liquid metal infiltration

Three-dimensional braided carbon fiber-reinforced ZrC matrix composite, 3-D Cf/ZrC, were prepared by liquid metal infiltration process at 1200°C using a Zr2Cu intermetallic compound as infiltrator. The microstructure and properties of the composites were investigated. The results indicated that ZrC with a yield of 35.2±1.8vol.% was certified as the major phase of the composites. The formation of ZrC was controlled by a solution-precipitation mechanism. The obtained composites exhibited good mechanical properties, with a flexural strength of 293.0±12.1MPa, a flexural modulus of 82.7±6.4GPa and a fracture toughness of 9.8±0.9MPam1/2. The mass and linear ablation rates of the composites exposed to oxyacetylene torch were 0.0013±0.0005gs−1 and −0.0009±0.0003mms−1, respectively. The formation of a dense ZrO2 protective layer and the evaporation of residual Cu contributed mainly to the excellent ablation resistance.

Mechanochemical Synthesis of (Sm,Pr)$_{2}$(Co,Fe)$_{17}$ Anisotropic Hard Magnetic Powders

Anisotropic Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.85</sub> Pr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.15</sub> (Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> alloy powders with x = 0.2-0.5, consisting mostly of the rhombohedral (Sm,Pr) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (Co,Fe) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sub> compound, were prepared from a mixture of cobalt, ferric oxide and rare earth oxides subjected to high-energy ball-milling in the presence of calcium reducing agent and calcium oxide dispersant. The milling was followed by a short-time annealing and multi-step washing. The resulting powders were polydispersed, but their average particle size could be tuned from 80 to 300 nm by controlling the excess Ca metal and the annealing temperature. A nanometer-thin oxide shell was observed on a surface of the particles, and it may be, at least in part, responsible for their excellent stability to air. The significant effect of the excess Ca on the particle size, anisotropy and chemical/structural homogeneity is discussed based on the supposed solution-precipitation mechanism of particle growth. Intrinsic coercivity H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> up to 14 kOe was obtained; powders with x = 0.4 and an average particle size of 121 nm exhibited a remanence of 106 emu/g and H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 8.3 kOe.

Infiltration mechanism of diamond/SiC composites fabricated by Si-vapor vacuum reactive infiltration process

Abstract Dense diamond/SiC composites were fabricated by Si vapor vacuum reactive infiltration of carbon-containing diamond porous preform at 1600 °C for 1 h. The microstructural evolution of the composites was investigated. The infiltration mechanisms during reactive infiltration were discussed. The composite consists of diamond, β-SiC and a small amount of Si. Epitaxial growth of nano-sized SiC on diamond and graphite surfaces occurred due to the diffusion-reaction mechanism in the initial stage of infiltration. Growth of micron-sized SiC with no preferential orientation was controlled by solution-precipitation mechanism in the final stage. The infiltration process was determined both by molecular diffusion and capillary effects. Explosive evaporation of molten Si, volume expansion of the solids and heat release during the reaction were the key factors contributing to the rapid densification of diamond/SiC composites. High thermal conductivity (580 W m −1 K −1 ) and low density (3.33 g cm −3 ) of the composites were beneficial to thermal management applications.

Rapid crystallization of amorphous silicon nitride powder accelerated by liquid Si

The complete crystallization time of amorphous silicon nitride powder could be reduced from more than 6h to 30min by adding Si powder in N2 atmosphere. The crystallization process may be affected by the synergy of the reaction between Si and N2 and the liquid phase of Si due to the addition of Si. The α-Si3N4 produced by the added Si and nitrogen may be the seed of the crystallization process. The amorphous silicon nitride powder may be dissolved in the liquid phase and then precipitated as α-Si3N4. This article uses Ar instead of N2 to shield the reaction between Si and N2 in order to analyze the role of the two mechanisms. The added Si powder accelerates the crystallization process effectively in Ar. Powder of more than 70% crystalline phase content is produced by adding 5% Si annealing at 1400°C for 15min. The existence of liquid Si is the key factor to promote the crystallization process and the process follows the solution–precipitation mechanism. The final products are mainly α-Si3N4 and Si2N2O. The SEM results show that most of the products are irregular rod-like particles.

Combustion synthesis of Fe3Al/TiC composite powders

A mixture of (FeTi2+C)+10 at-%Al based on stoichiometry of Fe3Al+TiC was used for a combustion front quenching test. The microstructural evolution during combustion synthesis was analysed by scanning electron microscopy and energy dispersive spectrometry. The phase constituents of the product were evaluated by X-ray diffraction. At first, Al powders were melted at 933 K and surrounded FeTi2 particles and carbon powders. When the temperature increased to 1358 K, FeTi2 melted, and Al and C dissolved in Fe and Ti. Then, solid–liquid and liquid–liquid reactions took place, and TiC and Fe3Al formed. At least at 1723 K, the iron aluminide phase surrounded large TiC particles. Because of formation of TiC0·95, a small amount of carbon remained in matrix and reacted with Fe3Al and formed Fe3AlC. Based on these results, the mechanism of the Fe3Al–TiC combustion synthesis could be described with a ternary solution precipitation mechanism.

Synthesis of single phase β-SiAlON ceramics by reaction-bonded sintering using Si and Al 2O 3 as raw materials

Single-phase β-SiAlON compacts with z = 1 have been synthesized by reaction-bonded sintering using Si and Al2O3 as raw materials. The nitrided samples consisted of fine equiaxed particles and continuous fibers. β-SiAlON fibers with a diameter approximately 50 nm were obtained through the vapor–solid mechanism. After post-sintering at 1650–1750 °C for 2 h, only β-SiAlON phase with z = 1 was obtained. α-Si3N4 completely changed into β-SiAlON through the solution-precipitation mechanism. β-SiAlON shows different microstructures at different post-sintering temperature.

Reactive wetting and infiltration of polycrystalline WC by molten Zr 2Cu alloy

Good wettability with equilibrium contact angles of 41–28° was achieved for molten Zr2Cu alloy on dense polycrystalline WC substrates at 1050–1150 °C in an Ar atmosphere, favoring pressureless infiltration of the molten Zr2Cu alloy into a porous WC preform. Interfacial microstructure examination revealed that the Zr2Cu alloy reacted with WC, yielding W and ZrC at the interface through a solution–precipitation mechanism.

Effect of bauxite addition on densification and mullitization behaviour of West Bengal clay

The effect of bauxite addition on the densification and mullitization of reaction sintered bauxite-clay mixture had been studied in the temperature range 1400–1500°C. The maximum bulk density (2·89 g/cc) and minimum apparent porosity (0·58%) was achieved by addition of 50 wt% bauxite. The impurities present in bauxite and clay formed liquid phase which helped in particle diffusion to aid densification. The X-ray diffraction of sample fired at 1500°C showed cristoballite phase gradually disappearing and at the same time mullite and α-Al2O3 phase appearing at a higher level of bauxite addition. The in situ nascent alumina formed was reactive that facilitated the formation of secondary mullite by solution precipitation mechanism. The presence of bauxite also changed the morphology of the mullite particles. Two types of mullite were distinctly observed in the SEM photographs: elongated primary mullite and equiaxed secondary mullite.

A Phenomenological Model (and Experiments) for Liquid Phase Sintering

Two problems in liquid phase sintering (LPhS) are analyzed. In the first case, the densification obtained from particle rearrangement is related to the volume fraction of the liquid phase and the solubility of the solid in the liquid. The model is based upon the assumption that particle rearrangement is related to the dissolution of the solid into the liquid. Sintering experiments carried out with salt particles and water support the results from the model. These experiments provide further insights into the influence of particle size on densification. In the second problem, the kinetics of densification by particle rearrangement is related to the rate of dissolution of the solid into the liquid. These results are compared to the kinetics of densification by the solution-precipitation mechanism, which is shown to be much slower than particle rearrangement. The results from the two problems are then combined to delineate the regimes of particle rearrangement (stage I) and solution-precipitation (stage II) mechanisms. The implication of the contact angle on sintering behavior is discussed.

Integrated liquid-phase sintering of glass-alumina functionally graded materials

The sintering mechanism of glass-alumina functionally graded materials (G-A FGMs) at 710?C prepared by a novel rapid prototyping manufacturing (RP&amp;M) technology was studied. An integrated kinetics equation was established for the initial 20 minutes. Linear shrinkage is due to the integrated working of both viscous flow of the glass particles and gravitation of the effective activated alumina grain boundaries towards the equilibrium state. The driving force for gravitation of the activated alumina grain boundaries originates from the coactions of passive flow of the alumina particles during viscous flow of the glass particles and the compressive stress formed during cooling. In the middle 10 minutes, the sintering process follows the solution-precipitation mechanism. Confirmed by Scanning Electron Microscope (SEM) and the line scanning Energy Dispersive Spectrum (EDS), a layer of Si-Al eutectic of about 1.04?m on the alumina surface is formed. Then the alkali and alkali earth ions in the glass phase diffuse through this layer and precipitate on the surface. Additionally, the dissolved Al ions can easily transport through the eutectic layer and precipitate in the glass matrix due to the effect of Si in the glass matrix.

The development and behaviour of low-angle normal faults during Cenozoic asymmetric extension in the Northern Apennines, Italy

Movements on low-angle normal faults (LANF) are not predicted by traditional Anderson–Byerlee frictional fault mechanics. Our investigations centre on three normal fault systems active at distinct times during the regional extension of the Northern Apennines, with each showing different degrees of crustal exhumation. These are (from E to W): the Altotiberina fault system, Umbria; the Radicofani fault system, Tuscan mainland; and the Zuccale fault system, Isle of Elba. Regional extension has been a continuous process since middle Miocene, migrating progressively from west to east, with deformation accommodated by a set of E to NE-dipping LANF and more steeply SW-dipping antithetic structures. The LANF acted as regional detachments, accommodating a majority of the extension in the Northern Apennines, with individual faults exhibiting several kilometres of displacement. Regionally, the stress field has been characterised by a vertical σ 1 and a NE–SW- (Tuscan mainland and Umbria) to E–W (Elba)-trending σ 3. Where exposed at the surface, the main LANF detachments possess a well-developed fault core of foliated fault rocks in which fluids have played a key role in weakening due to reaction softening and the onset of stress-induced solution–precipitation mechanisms. We speculate that the E-dipping normal faults initiated with relatively low-angles of dip due to differential drag generated by mantle flow following slab retreat and roll-back beneath the Apennine chain.

TiC particulate-reinforced Al–20Si–5Fe composite fabricated by melt in situ reaction spray forming

A novel technique in which TiC particulates are fabricated by an in situ reaction in molten aluminum is introduced for producing TiC/Al–20Si–5Fe composite. In order to reveal the characteristics of the technique, in this paper the formation mechanism of TiC particulates obtained by this method is studied. Both theoretical and experimental results have shown that the TiC particulates are formed by a diffusion mechanism when the molar fraction of aluminum in the preform is greater than 20.02%. Contrarily, the TiC particulates are formed by a solution-precipitation mechanism when the fraction is less than 20.02%. An unwanted needle-like phase is always present in the spray-formed alloys. Experimental results have shown that the amount of the brittle phase decreases in spray-formed Al–20Si–5Fe preforms with increase in the TiC content, and that the brittle phase can be eliminated completely by adding more than 3 wt.%TiC particulates to the alloy. Moreover, another major problem of coarsened silicon particulates, often encountered in hyper-eutectic Al–Si alloy has also been solved by using the technique.

Reactions at the liquid silicon/silica glass interface

Experiments have been carried out to determine the nature and origin of the spots growing on silica glass surfaces in contact with liquid silicon during CZ–Si crystal growth. Silica glass ampoules were filled with silicon and tempered between 5 min and 40 h at a temperature (1693 K) slightly above the melting point of silicon. Cross sections of the ampoules with solidified silicon have been examined by scanning electron microscopy and optical polarization microscopy. In addition cross sections from commercial silica glass crucibles used in the Czochralski process or dipped into the silicon melt were investigated with the same methods. At the silicon/silica glass interface different reaction zone morphologies were detected. A solution-precipitation mechanism is suggested for the fast lateral growth of the reaction zone, which is proposed to consist of small cristobalite crystals embedded in a silica glass matrix.

Electrical and thermal conductivity of liquid phase sintered SiC

Commercial silicon carbide ceramics are semi-conductors. To obtain SiC ceramics with insulating properties, a synthesised SiC powder which was doped with vanadium and boron to get insulating behaviour due to compensating mechanism, and in addition a commercial powder was used. Both powders were pressureless sintered by liquid phase sintering. Different packing powders were used to influence the electrical behaviour of the dense SiC ceramics. The influence of these conditions on the electrical and thermal conductivity are described. Densification of SiC by a solution-precipitation mechanism in the temperature range around 1900°C results in a decrease of resistivity. The resistivity of the commercial powder could be improved by using a vanadium based packing powder.

Phase evolution in mechanically treated mixtures of kaolinite and alumina hydrates (gibbsite and boehmite)

The phase evolution behaviour of kaolinite–gibbsite mixtures ground for 5 min and 4 h has been investigated by X-ray powder diffraction (XRD). Differential scanning calorimetry (DSC) and MAS NMR. The thermal behaviour of mixtures ground for 5 min was similar to that of kaolinite- α-alumina mixtures. After grinding for 4 h, the mixture is transformed into an amorphous state. Grinding caused the acceleration of thermal reactions in these mixtures, with a degree of mechanochemical reaction being found to occur between kaolinite and α-alumina. Grinding kaolinite-boehmite mixtures produced mullite precursors which were more homogeneous than those in which the kaolinite had been dehydroxylated, suggesting the participation of the structural water from the kaolinite. Secondary mullite is formed during thermal reaction of these mixtures by a solution-precipitation mechanism. The mullite initially formed was alumina-rich, but approached the stoichiometric composition with increasing firing temperature.