Synthesis of α-Sb2O4 nanorods by a facile hydrothermal route

α-Sb₂O₄ nanorods with diameter of 50–150 nm and length of 100–300 nm have been successfully synthesized by a hydrothermal method using SbCl₃ and I₂ as reaction reagents. The obtained sample is characterized by X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, and selected-area electron diffraction. The results confirm that the product is pure single-crystalline α-Sb₂O₄ nanorods with few dislocations and defects. The vibrational property of the nanorods is investigated by Raman spectroscopy. Raman spectrum of α-Sb₂O₄ nanorods has a small red shifts compared with that of the bulk α-Sb₂O₄ powders. A possible growth mechanism of α-Sb₂O₄ nanorods is proposed as three stages: the hydrolyzation of SbCl₃ under strong acid condition, the oxidation of Sb₄O₅Cl₂ and the growth of α-Sb₂O₄ nanorods with the aid of iodine transport.     

The formation of NaMg2Al15O25 in an α-Al2O3 matrix and its effect on the mechanical properties of alumina

An in-situ sintering reaction was designed to produce lath-like \-alumina in an -alumina matrix in order to make alumina ceramics stronger and tougher. The reaction sequence to produce \-alumina (NaMg₂Al₁₅O₂₅) requires the formation of \-alumina (NaAl₁₁O₁₇) and spinel (MgAl₂O₄) at around 1100 dgC followed by a solid-state reaction of these two phases to give \-alumina at elevated temperatures; this reaction is complete at around 1600 °C. The in-situ sintering reaction produces near-theoretically dense alumina ceramics in which lath-like \-aluminas are homogeneously distributed. The bending strength and fracture toughness increase to 620 MPa and 5 MPam^1/2, respectively; these increases are thought to be due to the suppression of grain growth as well as the crack deflection and bridging associated with lath-like \-alumina.     

Changing the surface mechanical properties of silicon and α-Al2O3 by ion implantation

Microhardness indentation testing has been used as a means of introducing controlled localized deformation and fracture in both ion-implanted and unimplanted {1 1 1} silicon and {1 0 ˉ1 2} sapphire single crystal surfaces. The microstructural alterations due to implantation with N ₂ ⁺ and Al⁺ into silicon and Y⁺ into sapphire have been characterized using channelled Rutherford backscattering, transmission electron microscopy and electron channelling in the scanning electron microscope. It was found that sapphire only became amorphous at doses ⪞3×10¹⁶ Y⁺cm⁻² which corresponds to a total energy deposition of ∼3×10²³ keV cm⁻³ (∼44 kJ mm⁻³). The low-load microhardness (<50 gf) was found to be sensitive to the thickness of the amorphous layer produced by implantation into both silicon and sapphire. Compared with the parent crystal, this layer was found both to be softer and to behave in a relatively plastic manner with considerable plastic pile-up occurring around indentations in the higher dose specimens. The indentation fracture behaviour was found to be dominated by the presence of implantation-induced compressive stresses. The resulting effects were: (a) a decrease in the size of the radial crack traces (henceK _IC is apparently increased when evaluated using indentation fracture mechanics), (b) a decrease in the frequency of occurrence of lateral break-out in silicon and subsurface lateral cracking in sapphire, (c) initiation of lateral cracks further below the surface in both silicon and sapphire. Thus in general, it is concluded that hardness and surface plasticity are associated with softer amorphous layers whilst indentation fracture modifications are principally stress related.     

Improvement of structural and mechanical properties of alumina coatings by incorporation of TiO2 and α-Al2O3 nanoadditives

Abstract

Alumina coatings embedded with different nanoadditives were fabricated on aluminium alloy by microarc oxidation (MAO). Incorporation of nanograins into the prepared coatings was accomplished by dispersing nanoadditives into different electrolytes during the MAO process. Our results show that nanograins are successfully embedded in the ceramic coatings, and the embedded coatings are compact and have lower porosity. The mechanical properties of the nanograin embedded coatings such as hardness, adhesion and wear resistance are consequently improved, and the samples prepared in aluminate electrolyte with α-Al₂O₃ nanoadditive have better mechanical properties than those prepared in other electrolytes. Our results also show that the mechanical properties of MAO coatings are closely related to the surface structure. The introduction mechanism of nanograins into the ceramic coatings resulted from the reactions occurring in the microarc discharge channels such as diffusion and electrophoresis, which is believed to improve the structure of the prepared coatings.     

The effect of SnO2 on the improvement of mechanical properties of MgO–MgAl2O4 composites

Incorporation of SnO₂ into MgO–spinel (M–S) increased mechanical properties significantly. Relationships between the parameters improving mechanical properties and microstructural variables were examined. Basic parameters improving the mechanical properties of M–S–SnO₂ composites were identified as follows: (a) when microcracks come across with either Mg₂SnO₄ particles or pores; crack branching and deviation of interlinked microcracks or crack arresting occurred more effectively than those of spinel particles, (b) fracture type was converted to intergranular fracture with incorporation of spinel into MgO, and transgranular fracture with addition of SnO₂ to M–S; additionally with the incorporation of additives, (c) critical defect size, (d) work of fracture values increased, and (e) MgO grain size decreased. R_st thermal shock parameter values of M–S–SnO₂ composites were markedly higher than those of M–S materials, associated with low strength loss, high thermal shock damage resistance and thus longer service life of M–S–SnO₂ composites for high-temperature industrial applications.     

Microstructures and mechanical properties of spark plasma sintered Cu–Cr composites prepared by mechanical milling and alloying

Nanograined Cu–8 at.% Cr composite was produced by a combination of mechanical milling (MM), mechanical alloying (MA) and spark plasma sintering (SPS). Commercial Cu and Cr powders were pre-milled separately by MM. The milled Cu and Cr powders were then mechanically alloyed with as-received Cr and Cu powders respectively. After milling, the powder mixtures were separately subjected to SPS. It was found that pre-milling Cr can efficiently decrease the size of grain and reinforcement, resulting in remarkable strengthening. The grain size of Cu matrix was about 82 nm after SPS. The Vickers hardness, compressive yield strength and compression ratio of the composite were 327 HV, 1049 MPa and 10.4%, respectively. The excellent mechanical properties were primarily attributed to dispersion strengthening of the Cr particles and fine grain strengthening of the Cu matrix. The strong Cu/Cr interface and dissolved Cr atoms can also contribute to strengthening of the composite.     

Thermoelectric properties of β-FeSi2 with B4C and BN dispersion by mechanical alloying

The B₄C- and BN-dispersed -FeSi₂ thermoelectric materials were synthesized by mechanical alloying and subsequent hot pressing. The effects of the B₄C and BN dispersion on the thermoelectric properties, such as Seebeck coefficient, electrical resistivity and thermal conductivity etc., of the -FeSi₂ were investigated. For the sample with B₄C and BN addition, a larger amount of the residual phase was detected in the X-ray diffraction patterns than the sample without addition. In the case of the BN addition, the Seebeck coefficient was enhanced by BN addition above 700 K, and the electrical resistivity also increased with increasing amount of BN. This is considered to result from doping of a small amount of B into the phase due to partial decomposition of the BN phase. The fine dispersion of BN particles in the phase matrix was quite effective for reducing the thermal conductivity as compared to the B₄C addition over the entire temperature range. The figure of merit, Z, of the -FeSi₂ was significantly enhanced by BN addition.     

The α- to β-Si3N4 transformation in the presence of liquid silicon

The compacts consisted of , -Si₃N₄ and free silicon are heat treated in the range 1650° C to 1750° C in an argon atmosphere in order to observe the following behaviours; the to phase transformation and variations of the microstructure during heat treatment in silicon nitride. For the microstructural observation of the heat treated specimens, the same grains in the polished surface were investigated before and after eliminating the retained silicon by etching. The to phase transformation, in this case, occurs via silicon melts irrespective of added -Si₃N₄. Both and phases are soluted and precipitated into molten silicon and their morphology are changed from an equiaxed shape to prismatic one. Although elongated grains are precipitated at low temperature or in the early stage of heat treatment, fine precipitated grains are mainly observed with increasing heat treating temperature.     

High-purity disperse α-Al2O3 nanoparticles synthesized by high-energy ball milling

The preparation of disperse fine equiaxed α-Al₂O₃ nanoparticles with narrow size distribution, high purity, and high yield is essential for producing Al₂O₃ nanocrystalline ceramic of fine grains which may exhibit a good toughness. In this work, micron-sized α-Al₂O₃ particles were directly ball-milled and subsequently washed with hydrochloric acid at room temperature. Fracture of large α-Al₂O₃ particles and cold welding of fine α-Al₂O₃ nanoparticles occur simultaneously during ball milling. It leads to the reduction of particle size with increasing milling duration below 80?h and reaches to a dynamic equilibrium with a minimal average particle size of 6.4?nm for milling durations over 80?h. Using the optimized high-energy ball milling parameters, we prepared high-purity disperse equiaxed α-Al₂O₃ nanoparticles with an average particle size of 8?nm and a purity of 99.96% (mass percent) in a high yield. After fractionated coagulation separations, disperse fine equiaxed α-Al₂O₃ nanoparticles with narrow size distribution were obtained. Finally, Al₂O₃ nanocrystalline ceramic with a relative density of 99.8% and an average grain size of 34?nm was sintered from the disperse fine equiaxed α-Al₂O₃ nanoparticles with an average particle size of 4.8?nm and a size distribution of 2–10?nm by pressureless two-step sintering.     

Effect of mechanical activation on the synthesis of α-Fe2O3-Cr2O3 solid solutions

Continuous -Fe₂O₃-Cr₂O₃ solid solution series have been synthesized by two methods: (i) direct heating of coprecipitated hydroxides, and (ii) mechanical pre-treatment followed by heating. It is shown that mechanical treatment leads to a decrease in the preparation temperature of the solid solutions to 623 K. The formation of a continuous solid solution series by direct heating begins only at 773 K. The formation of the solid solutions was established by X-ray diffraction analysis, infrared and Mössbauer spectroscopy. The decrease in synthesis temperature of the -Fe₂O₃-Cr₂O₃ solid solutions is attributed to activation of the samples during their mechanical treatment. The samples obtained have large specific surface areas (up to 130 m² g^–1).     

Facile solution synthesis of α-FeF3·3H2O nanowires and their conversion to α-Fe2O3 nanowires for photoelectrochemical application

We report for the first time the facile solution growth of α-FeF(3)·3H(2)O nanowires (NWs) in large quantity at a low supersaturation level and their scalable conversion to porous semiconducting α-Fe(2)O(3) (hematite) NWs of high aspect ratio via a simple thermal treatment in air. The structural characterization by transmission electron microscopy shows that thin α-FeF(3)·3H(2)O NWs (typically <100 nm in diameter) are converted to single-crystal α-Fe(2)O(3) NWs with internal pores, while thick ones (typically >100 nm in diameter) become polycrystalline porous α-Fe(2)O(3) NWs. We further demonstrated the photoelectrochemical (PEC) application of the nanostructured photoelectrodes prepared from these converted hematite NWs. The optimized photoelectrode with a ~400 nm thick hematite NW film yielded a photocurrent density of 0.54 mA/cm(2) at 1.23 V vs reversible hydrogen electrode potential after modification with cobalt catalyst under standard conditions (AM 1.5 G, 100 mW/cm(2), pH = 13.6, 1 M NaOH). The low cost, large quantity, and high aspect ratio of the converted hematite NWs, together with the resulting simpler photoelectrode preparation, can be of great benefit for hematite-based PEC water splitting. Furthermore, the ease and scalability of the conversion from hydrated fluoride NWs to oxide NWs suggest a potentially versatile and low-cost strategy to make NWs of other useful iron-based compounds that may enable their large-scale renewable energy applications.     

Effect of mechanical milling on Ni–TiH2 powder alloy filler metal for brazing TiAl intermetallic alloy: The microstructure and joint's properties

A TiH₂–50 wt.% Ni powder alloy was mechanically milled in an argon gas atmosphere using milling times up to 480 min. A TiAl intermetallic alloy was joined by vacuum furnace brazing using the TiH₂–50 wt.% Ni powder alloy as the filler metal. The effect of mechanical milling on the microstructure and shear strength of the brazed joints was investigated. The results showed that the grains of TiH₂–50 wt.% Ni powder alloy were refined and the fusion temperature decreased after milling. A sound brazing seam was obtained when the sample was brazed at 1140 °C for 15 min using filler metal powder milled for 120 min. The interfacial zones of the specimens brazed with the milled filler powder were thinner and the shear strength of the joint was increased compared to specimens brazed with non-milled filler powder. A sample brazed at 1180 °C for 15 min using TiH₂–50 wt.% Ni powder alloy milled for 120 min exhibited the highest shear strength at both room and elevated temperatures.     

Fabrication and mechanical properties of β-TCP pieces by gel-casting method

In the present work, dense β-TCP ceramics were fabricated by gel-casting method. The effects of the solids loading on the rheological behavior of β-TCP slurries were investigated. When the concentration of the slurries was increased from 40 to 60 vol.%, the compressive strength of green pieces was raised from 12.4 ± 1.1 to 41.2 ± 2.3 MPa, and flexural strength from 9.4 ± 0.4 to 16.3 ± 0.9 MPa. The density of the final specimens was 97.4% of the theoretical density after pressureless sintering at 1100 °C. The compressive strength, flexural strength, elasticity modulus and the fracture toughness of the sintered pieces were 291 ± 15 MPa, 93.0 ± 8.7 Mpa, 72.4 ± 7.5 GPa and 0.92 ±0.04 Mpa·m^0.5 respectively. SEM images show a compact and uniform microstructure; XRD and FTIR determined the phase and the radical before and after sintering.     

Effects of chelation reactions between metal alkoxide and acetylacetone on the preparation of MgAl2O4 powders by sol–gel process

Magnesium aluminate spinel (MgAl₂O₄) were synthesized via the sol–gel method, using magnesium aluminum double n-butoxide as precursors. The n-butoxide, prepared by the direct reaction of stoichiometric magnesium, aluminum and adequate n-butanol, was modified by acetylacetone as the chelating agent to control the speed of hydrolysis. Gel samples were identified as the mixture of Mg₂Al(OH)₇ and MgAl₂(OH)₈ phases, which converted to pure MgAl₂O₄ spinel phase at 800 °C. Then dense spinel ceramics with 96% of relative density were prepared by sintering in air at 1600 °C. Besides, the morphology of the powders and sintered ceramics has been investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM).     

Kinetics of α- and β-Si3N4 formation from oxide-free high-purity Si powder

Available kinetic data for the nitridation of high-purity oxide-free Si powder are analysed. The analysis suggests that the - and -phases of Si₃N₄ are formed by separate and parallel reaction paths, and kinetic expressions for their formation are reported. The formation of the -phase follows first-order kinetics, while the -phase is formed by a phase-boundary-controlled rate law. These conclusions are consistent with other kinetic and micrographic analyses reported in the literature.     

The effect of silicon on the structure and mechanical properties of an α+Β titanium alloy

Titanium 4 wt% Al-4 wt% Mo-2 wt% Sn containing 0, 0.25 and 0.5 wt% Si has been solution-treated in the + phase field at 900 C. The microstructures obtained at room temperature after cooling from 900 C at various rates have been determined using transmission electron microscopy and the partitioning of the elements between the phases has been established using X-ray energy dispersive analysis on the thin foils. The degree of partitioning increases with decreasing cooling rate: aluminium partitions to the -phase, molybdenum and silicon to the -phase and tin remains uniformly distributed. Silicon is found to inhibit the partitioning of molybdenum: this has a profound effect on the stability of the -phase and the resultant microstructure. In quenched material containing transformed , substantial age hardening can be obtained in the range 350 to 600 C and is associated with precipitation within the orthorhombic martensite phase, possibly occurring via a spinodal mechanism. Silicon has little effect on the microstructure of air-cooled samples but contributes to high-temperature strength via dynamic strain ageing.     

The effect of Mg add on morphology and mechanical properties of Al–xMg/10Al2O3 nanocomposite produced by mechanical alloying

Effect of Mg content on microstructure and mechanical properties of Al–xMg/10 wt.%Al₂O₃ (x = 0, 5, 10 and 15 wt.%) powder mixtures during milling was investigated. The results show that for the binary Al–Mg matrix, the predominant phase was an Al–Mg solid solution. With the increment of Mg to 15 wt.% the crystallite sizes of 20 h milled powders diminish from 44 to 26 nm and lattice strains increased from 0.22% to 0.32% caused by Mg atomic penetration into the substitution sites of the Al lattice. With up to 15 wt.% Mg (for 20 h milled composites) microhardness increases from 120 to 230 HV caused by the increment of the Mg concentration and dislocation density as well as the decrease of the crystallite size.     

Fabrication and mechanical properties of α-Al2O3/β-Al2O3/Al/Si composites by liquid displacement reaction

Al₂O₃/metal composites were fabricated by heating three kinds of commercial mullite refractories in contact with Al, and their mechanical properties were investigated. Aluminum reacted with the mullite and SiO₂-glass constituting the mullite refractories and changed them into -Al₂O₃ and Si. Simultaneously, -Al₂O₃ was formed by the reactions among -Al₂O₃ and sodium and potassium oxides in the glassy phase. Also, Al penetrated into the -Al₂O₃/-Al₂O₃/Si composite by partly dissolving Si. Finally, the mullite refractories were changed into -Al₂O₃/-Al₂O₃/Al/Si composites. The phase contents, microstructures and mechanical properties of the resulting composites varied with the composition of the refractories. The content of -Al₂O₃ in the composite was lowest at the lowest Na₂O and K₂O contents in the refractories. Silicon in the composite had its highest content at the highest SiO₂ content. The composite fabricated from SiO₂/Al₂O₃ (in mol) (SAR)=1.85 consisted of 2–5 m Al₂O₃ grains embedded in metal, but that from SAR=1.05 showed a complicated microstructure with small and large grains. The bending strength of the composites fabricated from the refractories of SAR=1.85, 1.24, and 1.05 were 327, 405 and 421 MPa, respectively. Also, the corresponding fracture toughness values were 5.2, 6.1, and 5.5 MPam , respectively.     

The mechanical and electrical properties of ZrO2-Naβ″-Al2O3 composites

ZrO₂-Na-Al₂O₃ composites were prepared by a conventional method using two different powder routes and different milling liquids. The retained tetragonal-phase ZrO₂ was 85 to 90% for composites with 2.4 to 15 vol% ZrO₂. The fracture toughness (K _lc) and strength increased with increasing ZrO₂ content. At 20 vol % ZrO₂,K _lc and bend strength were 4.35 M Pa m^1/2 and 390 MPa, respectively. Stress-induced transformation toughening is the predominant toughening mechanism. Dispersion toughening also contributes to the increase ofK _lc. Surface strengthening was found to be an effective strengthening method for low ZrO₂ levels. The critical tetragonal ZrO₂ grain size was found to increase from 0.86 to 1.02gmm as the ZrO₂ content increased from 2.5 to 15 vol %. A detailed study of the ionic conductivity of the 15 vol % ZrO₂ dispersed sample was conducted by an a.c. technique between –124° C and 300° C. The bulk and total conductivities were calculated via complex-plane analysis. The total (grain and grain-boundary) ionic specific resistivity was 9 cm at 300° C. The activation enthalpies of the bulk and total conductivity processes were 0.30 and 0.32 eV, respectively.