In situ growth of TiCxN1−x whiskers in Al2O3 matrix for ceramic cutting tools

For resolving the dispersing problem of whiskers and fabricating cutting tool materials with excellent properties, an in situ growth technology is used to directly synthesize TiC_xN_1?x whiskers in α-Al₂O₃ matrix by a carbothermal reduction process at a temperature range of 1250–1550 °C. The raw materials are consisted of TiO₂, carbon, nickel, and NaCl. Various molar ratios from 1:3, 1:4, 1:5 to 1:7 of TiO₂:C are experimentally used. For the molar ratio 1:3, a few whiskers can be found only at the synthesis temperature of 1550 °C. For the other molar ratios, large amount of whiskers can be observed at the whole synthesis temperature range. The highest yield of whisker is observed when the synthesis temperature is 1250 °C and the molar ratio of TiO₂:C is 1:4. The compound AlO appears at 1250 °C and AlN instead at 1550 °C. The majority of the synthesized whiskers display an ideal aspect ratio of 10–30 with a diameter of 1–3 μm. No obviously influence on the whiskers growth by the present of α-Al₂O₃ matrix powder can be noted.     

Effect of in situ synthesized TiB whisker on microstructure and mechanical properties of carbon–carbon composite and TiBw/Ti–6Al–4V composite joint

Carbon–carbon composite (C–C composite) and TiB whiskers reinforced Ti–6Al–4V composite (TiB_w/Ti–6Al–4V composite) were brazed by Cu–Ni + TiB₂ composite filler. TiB₂ powders have reacted with Ti which diffused from TiB_w/Ti–6Al–4V composite, leading to formation of TiB whiskers in the brazing layer. The effects of TiB₂ addition, brazing temperature, and holding time on microstructure and shear strength of the brazed joints were investigated. The results indicate that in situ synthesized TiB whiskers uniformly distributed in the joints, which not only provided reinforcing effects, but also lowered residual thermal stress of the joints. As for each brazing temperature or holding time, the joint shear strength brazed with Cu–Ni alloy was lower than that of the joints brazed with Cu–Ni + TiB₂ alloy powder. The maximum shear strengths of the joints brazed with Cu–Ni + TiB₂ alloy powder was 18.5 MPa with the brazing temperature of 1223 K for 10 min, which was 56% higher than that of the joints brazed with Cu–Ni alloy powder.     

Photovoltaic performance of dye sensitized solar cell based on rutile TiO2 scaffold electrode prepared by a 2 step bi-layer process using molten salt matrices

Dye sensitized solar cells were made on TiO₂ scaffold anodes of rutile particles. These TiO₂ scaffold anodes were grown from rutile seeds by using a molten salt synthesis technique. Different thickness coatings of mixed amorphous titanium hydroxide and NaCl–KCl eutectic salt mixture on the rutile seeds were heat treated at different temperatures. The rutile whiskers of different aspect ratios were grown depending on the growth temperature. The best photovoltaic performances were obtained for the device made from the scaffold of 20–50 nm diameter and 0.5–1 μm length nanowhiskers obtained at 700 °C for 5 h of treatment.     

Influence of evolving microstructure on magnetic-hysteresis characteristics in polycrystalline nickel–zinc ferrite,Ni0.3Zn0.7Fe2O4

We report some research findings on the parallel evolutions of microstructural properties and magnetic hysteresis-loop properties; we attempt to elucidate their relationships. The Ni_0.3Zn_0.7Fe₂O₄ toroidal samples were prepared via high-energy ball milling and subsequent moulding; the samples with nanometer/submicron sized compacted powder were sintered from 600 °C to 1400 °C using 100 °C increments. An integrated analysis of phase, microstructural and hysteresis data would point to the existence of three distinct shape-differentiated groups of B–H hysteresis loops which belong to samples with weak, moderate and strong magnetism. The observed grain size with respect to the magnetic-hysteresis behaviour varied from 0.19 μm to 0.23 μm, 0.24 μm to 0.43 μm and 1.07 μm to 4.98 μm for weak, moderate and strong ferromagnetic behaviour respectively. The first occurrence of a strikingly erect and well-defined sigmoid-shape was observable only when sufficient single-phase purity and crystallinity and a sufficiently high volume fraction of multi-domain grains (>0.25 μm) were attained.     

Solvothermal synthesis of Mn2P2O7 and its application in lithium-ion battery

Manganese pyrophosphate, Mn₂P₂O₇ was synthesized by a simple solvothermal method using Mn metal powder and P₂S₅ in ethylene glycol medium at 190–220 °C. Morphology and crystalline structure of the products were characterized by X-ray diffraction and scanning electron microscopy. The flower-like microspheres with diameters of about 2–5 μm are composed of a number of nanoplatelets with thickness of 20–40 nm. The effect of reaction temperature and reaction time on the microstructure of Mn₂P₂O₇ was investigated. The samples were used as active anode materials for lithium-ion battery and their electrochemical properties were examined by constant current charge–discharge cycling. The Mn₂P₂O₇ electrodes exhibited initial reversible capacities of 440–330 mAh g^? 1 depending on the synthetic conditions. From these results, a possible reaction mechanism of Mn₂P₂O₇ with lithium was proposed.     

Martensitic transformation behaviors of rapidly solidified Ti–Ni–Mo powders

For the fabrication of bulk near-net-shape shape memory alloys and porous metallic biomaterials, consolidation of Ti–Ni–Mo alloy powders is more useful than that of elemental powders of Ti, Ni and Mo. Ti₅₀Ni_49.9Mo_0.1 shape memory alloy powders were prepared by gas atomization, and transformation temperatures and microstructures of those powders were investigated as a function of powder size. XRD analysis showed that the B2–R–B19 martensitic transformation occurred in powders smaller than 150 μm. According to DSC analysis of the as-atomized powders, the B2–R transformation temperature (T_R) of the 25–50 μm powders was 18.4 °C. The T_R decreased with increasing powder size, however, the difference in T_R between 25–50 μm powders and 100–150 μm powders is only 1 °C. Evaluation of powder microstructures was based on SEM examination of the surface and the polished and etched powder cross sections and the typical images of the rapidly solidified powders showed cellular morphology. Porous cylindrical foams of 10 mm diameter and 1.5 mm length were fabricated by spark plasma sintering (SPS) at 800 °C and 5 MPa. Finally these porous TiNi alloy samples are heat-treated for 1 h at 850 °C, and then quenched in ice water. The bulk samples have 23% porosity and 4.6 g/cm³ density and their T_R is 17.8 °C.     

Effects of sodium dodecyl sulfate on the oriented growth of nesquehonite whiskers

In this study, nesquehonite MgCO₃·3H₂O whiskers with long length and high aspect ratio were synthesized through reactive crystallization, in which MgCl₂ and Na₂CO₃ precipitated in the presence of sodium dodecyl sulfate (SDS). The effects of SDS concentration, reactive temperature and reactants concentration were investigated. SEM, XRD and FT-IR were used to character morphology and structure of the products. It was found that the presence of SDS in the mixed aqueous solution was an important parameter for the morphology and size of nesquehonite crystals. The morphology of nesquehonite crystals changed from rod-like to bundle-like with increasing SDS concentration. The average length of nesquehonite whiskers was about 87–180 μm while the aspect ratio was 18–45. The longest length reached 250 μm and the biggest aspect ratio was about 100. The reactive temperature and reactant concentration also affect the size even the crystal type. It is concluded that SDS was beneficial for oriented growth of one-dimensional nesquehonite whiskers based on the formation process of the nesquehonite whiskers. The possible mechanism of SDS in nesquehonite whiskers precipitation was ascribed to physical adsorption of SDS on the individual amorphous nanoparticles or crystallites. The critical point of oriented growth occurred when the SDS concentration was about 4.33 mM.     

Hydrothermal synthesis of large-sized hydroxyapatite whiskers regulated by glutamic acid in solutions with low supersaturation of precipitation

Large-sized hydroxyapatite (HA) whiskers with length of 50–100 μm and width of 0.5 μm were synthesized by a facile hydrothermal method. Dilute reaction solutions containing glutamic acid were used in order to achieve low degree of supersaturation with respective to HA precipitation. The supersaturation values at different hydrothermal temperatures were calculated theoretically from ion concentrations with considerations of all association–dissociation balances between various ions. Experimentally, it was found that HA whiskers with smaller sizes were obtained when raising the additive amount of NaOH or glutamic acid. The regulation mechanism of crystal growth along c-axis was proposed according to the degree of supersaturation, the adsorption/desorption behaviors of glutamate occurred on HA’s (1 0 0) and (0 0 1) planes, as well as the concentration effect of glutamate’s selective adsorption.     

Effect of NaF content on the preparation of zirconia whiskers by molten salt method

Zirconia whiskers were successfully fabricated by molten salt method, using NaVO₃/NaF as composite molten salt. The effect of NaF content on the growth of zirconia whiskers were systematically investigated by XRD, FE-SEM, TEM, SAED, HR-TEM, Raman and XPS. When optimized NaF content is 10%, the obtained zirconia whiskers with monoclinic crystal structure show an length of 1–3 μm and diameters of 100–300 nm. The as-prepared whiskers preferentially grow along [0 0 1] direction and exhibit a smooth surface without distinct defects. Fluoride is proved to play an important role in the growth of zirconia whiskers. On the one hand, fluoride can be served as mass transfer agent, promoting the dissolution and precipitation of ZrO₂ through the mass transfer process of fluorozirconate species. On the other hand, fluoride can act as morphology controlling agent, enhancing the anisotropic growth of ZrO₂ crystals by the formation of fluorine-terminated surface.     

Growth of SiC particles in reaction sintered SiC

For reaction sintered SiC (RSSC) prepared at 1600°C by conventional melt infiltration technique, experimentation with two different particle sizes of initial SiC, viz., 0.2 and 23.65 μm, showed that the large SiC particles remained unaltered and the sizes of the fine-grained SiC increased several times yielding well-developed faceted crystals in the final material. To study the process further, compacts of SiC powder of particle sizes varying between 0.20 and 8.99 μm were reacted with pure Si at 1600°C and the resulting SiC–Si boundaries were studied by optical microscopy. A distinct boundary layer with no penetration of Si in the compact of SiC of 0.2 μm was observed and the width of the SiC–Si boundary was found to be increasing linearly with time. Detailed SEM examination establishes the growth of the SiC upto around 4 μm from 0.2 μm starting powder. No such growth was observed in the case of starting SiC powder coarser than 0.2 μm. The growth of SiC is explained in terms of solution-reprecipitation mechanism.     

In situ synthesis of hybrid-reinforced titanium matrix composites

Novel hybrid-reinforced (TiB + La₂O₃)/Ti composites were in situ synthesized utilizing the reaction between Ti, LaB₆ and B₂O₃ through homogeneous melting in a non-consumable vacuum arc remelting furnace. The thermodynamics of in situ synthesis reaction were analyzed. The phases in the composites were identified by X-ray diffraction (XRD) and the microstructures of the composites were examined by optical microscope (OM), backscattered scanning electron microscope (SEM) and field-emission SEM. Three kinds of reinforcements were found in the composites: La₂O₃ particles (diameter: ∼ 2 μm), TiB whiskers (width: ∼ 3 μm) and TiB plates (thickness: ∼ 1.5 μm). The reinforcements' sizes were fine and they were homogeneously distributed in the matrix.     

Preparation of PMN–PT–BT/Ag nanocomposites by surface modification with MgO sol

Nanocomposites of 0.96(0.91Pb(Mg_1/3Nb_2/3)O₃–0.09PbTiO₃)–0.04BaTiO₃ (PBT) with Ag were prepared via synthesis of low-temperature-sinterable PBT/Ag composite powder and its surface modification with a MgO sol. Dielectric properties and modulus of rupture were investigated as a function of the amount of the MgO sol at sintering temperature in the range from 850 to 1000 °C. The MgO sol seemed to suppress the diffusion and migration of Ag during the heat treatment to give rise to the homogeneous microstructure with 300–80 nm of Ag particles in about 3.5–2.5 μm grains. The proper amount of the MgO sol seemed to be 0.5–1.0 wt.% for the PBT powder with 3.0 mol% Ag. The PBT/Ag nanocomposites showed density >99% T.D., ɛ_rt of 18,000–20,000, tan δ of 1–0.5%, specific resistivity of 10¹² Ω cm, and MOR of 125–145 MPa.     

A low temperature synthesis of nanocrystalline spinel NiFe2O4 and its electrochemical performance as anode of lithium-ion batteries

Nanocrystalline spinel NiFe₂O₄ was synthesized by a novel low temperature route. The crystal structure, composition and morphology of the as-prepared powder were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The average diameter of the particles prepared at 700 °C is about 30 nm. The electrochemical reaction mechanism and charge–discharge mechanism of the nanocrystalline NiFe₂O₄ were proposed based on thermogravimetric analysis (TGA) and cyclic voltammogram study. The charge–discharge tests indicated that the sample calcined at 700 °C shows the highest initial discharge capacity (1400 mAh g⁻¹) attributed to the nanometer size and the better crystallinity of the powder. A discharge capacity stabilizes at about 600 mAh g⁻¹ after 10 cycles. The columbic efficiency is improved. The synthesis method is relatively low cost and convenient for large-scale production.     

Consolidation of Al2O3–GdAlO3 eutectic powder prepared from induction-melted solid and strength at high temperatures

A eutectic powder of Al₂O₃–GdAlO₃ was melted using a Mo crucible by induction heating. The melt was slowly solidified, resulting in a eutectic solid with coarse Al₂O₃ and GdAlO₃ phases. The eutectic solid was ground and sieved into 3–44 μm and 64–124 μm particles. The powders were consolidated to produce a eutectic composite by spark plasma sintering. Mechanical properties of the consolidated eutectic composite were measured at room temperature. High temperature strength was obtained at temperatures up to 1673 K. Superplastic deformation of the eutectic composite was not observed on stress–strain curves at 1673 K, but did occur in the case of a conventional composite at 1573 K.     

Fabrication of Al2O3–ZrB2 in situ composite by SHS dynamic compaction: A novel approach

Al₂O₃–ZrB₂ in situ composites of 97% of theoretical density were successfully fabricated by a novel self-propagating high temperature synthesis (SHS) dynamic compaction, using less expensive raw materials zirconium oxide, boron oxide, and aluminium. The process is fast, energy efficient, where no furnace sintering is required. The process inhibits and controls the grain growth and microstructure. The densification behaviour and correlation with microstructure of the SHS dynamic compacts were compared with the furnace sintered composite samples where the composite powder was prepared by SHS process. The furnace sintered samples showed coarser grain growth and maximum density of 94.5% of theoretical density was achieved. The SHS dynamic compacted in situ composite had much finer grains in the range of 0.5–3 μm with density 95.5% of the theoretical value. The average grain size was found to decrease from 10 μm to 1.4 μm for alumina and from 5.4 μm to 1.0 μm for zirconium diboride from furnace sintering to SHS dynamic compaction, respectively. Addition of Al₂O₃ as a diluent during SHS reaction enhanced the density to 97%. During SHS dynamic compaction, the amount of liquid and the time interval at which the sample stays at high temperature are the controlling factor of the final microstructure and the densification of the composite.     

Low temperature growth of SnO2 nanowires by electron beam evaporation and their application in UV light detection

For the first time, high quality tin oxide (SnO₂) nanowires have been synthesized at a low substrate temperature of 450 °C via vapor–liquid–solid mechanism using an electron beam evaporation technique. The grown nanowires have shown length of 2–4 μm and diameter of 20–60 nm. High resolution transmission electron microscope studies on the grown nanowires have shown the single crystalline nature of the SnO₂ nanowires. We investigated the effect of growth temperature and oxygen partial pressure on SnO₂ nanowires growth. Variation of substrate temperature at a constant oxygen partial pressure of 4 × 10⁻⁴ mbar suggested that a temperature equal to or greater than 450 °C was the best condition for phase pure SnO₂ nanowires growth. The SnO₂ nanowires grown on a SiO₂ substrate were subjected to UV photo detection. The responsivity and quantum efficiency of SnO₂ NWs photo detector (at 10V applied bias) was 12 A/W and 45, respectively, for 12 μW/cm² UV lamp (330 nm) intensity on the photo detector..     

Phase change in calcination process for BaTi2O5 and proposal of a new temperature profile

The mixture of BaCO₃ and rutile-type TiO₂ powders mixed with wet ball-milling was calcined in air and the phase change in calcination process was investigated in detail. The slow heating in the temperature range from 800 to 850 °C was effective to change BaTiO₃ to BaTi₂O₅ in the following heating process. The influence of two factors in calcination for the synthesis of BaTi₂O₅, which are temperature profile with slow heating and packing state of the mixture, was investigated. The slow heating in two temperature ranges, 800–850 °C and 1100–1160 °C, was effective for the synthesis of BaTi₂O₅. The production amount of BaTi₂O₅ was increased by using powder compacts rather than powder form. By calcining powder compacts at 1160 °C for 2 h, the powder of BaTi₂O₅ having an α_s value of 0.93, which is semi-quantitatively corresponding to BaTi₂O₅ production ratio, was obtained.     

Enhanced thermal conductivity of Cu matrix composites reinforced with Ag-coated β-Si3N4 whiskers

Cu matrix composites reinforced with 10 vol.% Ag-coated β-Si₃N₄ whiskers (ASCMMCs) were prepared by powder metallurgy method. With the aim of improving the thermal conductivity of the composites, a quite thin Ag layer was deposited on the surface of β-Si₃N₄ whiskers. The results indicated that thermal conductivity of ASCMMCs with 0.30 vol.% Ag (0.30ASCMMCs) reached up to 273 W m⁻¹ K⁻¹ at 25 °C, which was 98 W m⁻¹ K⁻¹ higher than that of Cu matrix composites reinforced with uncoated β-Si₃N₄ whiskers (USCMMCs). The Ag coating could promote the densification of composites, reduce the aggregation of β-Si₃N₄ whiskers and enhance the Cu/Si₃N₄ interfacial bonding, therefore it could efficiently enhance the thermal conductivity of Cu matrix composites reinforced with β-Si₃N₄ whiskers (SCMMCs).     

Thermal decomposition synthesis of 3D urchin-like α-Fe2O3 superstructures

Urchin-like α-Fe₂O₃ superstructures have been deposited on Si substrate using thermal decomposition FeCl₃ solution at 200–600 °C in the oven. The morphologies and structures of the synthesized urchin-like superstructures have been characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results show that urchin-like α-Fe₂O₃ superstructures were a polycrystal with the rhombohedral structure and typical diameters of 16–20 nm and lengths up to 1.0 μm. The as-prepared α-Fe₂O₃ superstructures have a high Brunauer–Emmett–Teller (BET) surface area of about 60.24 m²/g. The photoluminescence spectrum of the urchin-like α-Fe₂O₃ superstructures consists of one weak emission peak at 548 nm (2.26 eV). A possible new mechanism for the formation of the urchin-like superstructures was also preliminarily discussed.     

The effect of processing parameters in the carbothermal synthesis of titanium diboride powder

The mechanism of the carbothermal method for synthesizing titanium diboride (TiB₂) powder has been studied. Mixtures of TiO₂, H₃BO₃ and carbon were heated in an argon atmosphere at 1000–1600 °C. The effect of the molar ratio and holding time on the phase evolution was studied by X-ray diffraction. The products were also characterized by scanning electron microscopy observations and particle size measurements.For a composition with a molar ratio of TiO₂:H₃BO₃:C = 1:2.4:5 heated for 1 h, the simultaneous presence of TiC and TiB₂ phases at 1100 °C and the transformation of TiO₂ to Ti₂O₃ at 1200 °C and higher confirms that TiB₂ synthesis is based on a TiC formation mechanism, in which TiC may be formed from a reaction between TiO₂ or Ti₂O₃ and carbon. Then TiC may react with liquid B₂O₃ and/or gaseous B₂O₂ to form the TiB₂ phase. The reaction is completed at 1500 °C. Also by increasing the molar ratio of boric acid to 3, the impurities decreased considerably and pressing of the material had an obvious effect on decreasing the impurities, due to an increase of the surface contact of particles, which causes an effective inhibition of boron escape from the reaction chamber. Under these experimental conditions, a relatively narrow size distribution of TiB₂ particles was produced. When the reaction time increased to 1.5–2 h, grain growth of particles occurred. Therefore, a wider distribution of particle size was obtained.