Fabrication of W-20wt.%Ti alloy by pressureless sintering at low temperature

A powder metallurgical technology of low temperature and pressureless is used to fabricate a W-20wt.%Ti alloy using milled TiH₂ powders and micro-sized W powders. The microstructure of the milled TiH₂ powders and the bulk W–Ti alloy were studied. It is indicated that TiH₂ nanoparticles with the size of 8 to 15 nm were obtained after milling for 48 h and the decomposition temperature decreased from 520.2 °C to 395.5 °C. The W-20wt.%Ti alloy prepared at 1200 °C for 80 min had a relative density of 97.8% which was composed of α-Ti, W and β(W/Ti) solid solution. A preparation mechanism of the W–Ti alloy is also proposed based on the experimental results.     

Preparation of nanostructured high-temperature TZM alloy by mechanical alloying and sintering

Mechanical milling proceeded by sintering was used to synthesize nanostructured temperature-resistant TZM alloy. Milling under Ar for different times (1, 2, 3, 5, 10, 15, 20, 25, and 30 h) and sintering at 1500, 1600 and 1700 °C for 30, 45, 60 and 90 min resulted in increasing of low-energy grain boundaries (LEGBs) and dispersion of TiC and ZrC with a size of ~ 65 nm in the matrix near LEGBs. Morphology and grain size of the products were determined from scanning electron microscope (SEM) images and X-ray diffraction (XRD) patterns, almost precisely. Optimum density of nanostructured TZM alloy ~ 9.95 ± 0.01 g/cm³ was achieved by sintering at 1700 °C for 90 min.     

Pitting behavior of a bulk Ni-18 wt.% Fe nanocrystalline alloy

Three electrodeposited Ni-18 wt.% Fe samples were annealed at 400 °C for 3 h (hrs), 8 h, and 24 h to study the effects of grain size on the electrochemical properties of bulk Ni-18 wt.% Fe in 3.5 wt.% NaCl. The electrochemical results from the annealed samples are compared with those measured for the as-received Ni-18 wt.% Fe material consisting of an average grain size of 23 nanometers (nm). Of the four materials studied, the as-received nanocrystalline alloy less sensitive to localized corrosion.     

Microstructure evolution and mechanical properties of nanocrystalline FeAl obtained by mechanical alloying and cold consolidation

In the present study high energy mechanical milling followed by cold temperature pressing consolidation has been used to obtain bulk nanocrystalline FeAl alloy. Fully dense disks with homogenous microstructure were obtained and bulk material show grain size of 40 nm. Thermal stability of the bulk material is studied by XRD and DSC techniques. Subsequent annealing at a temperature up to 480 °C for 2 h of the consolidated samples enabled supersaturated Fe(Al) solid solution to precipitate out fine metastable Al₅Fe₂, Al₁₃Fe₄ and Fe₃Al intermetallic phases. Low temperature annealing is responsible for the relaxation of the disordered structure by removing defects initially introduced by severe plastic deformation. Microhardness shows an increase with grain size reduction, as expected from Hall-Petch relationship at least down to a grain size of 74 nm, then a decrease at smallest grain sizes. This could be an indication of some softening for finest nanocrystallites. The peak hardening for the bulk nanocrystalline FeAl is detected after isochronal ageing at 480 °C.     

Bulk Mg-3Al-Zn alloy with ultrafine grain size produced by powder metallurgy

Ultrafine-grained Mg-3Al-Zn alloys with an average grain size of 180 nm have been made by powder metallurgy. First, the nanocrystalline powders with mean grain size of 45 nm were produced by ball milling under argon atmosphere, and then through vacuum hot pressing at 633 K for 40 min and warm extrusion at 373 K, bulk solid samples were compacted successfully from the mechanically milled powders, and the relative density of the samples was about 98.87% (1.8003 g/cm³). XRD, SEM and TEM analysis showed that the microstructure of the samples consists of homogeneous equiaxed grains and grain growth has taken place during the consolidation process.     

Vacuum sintering of WC-8 wt.% Co hardmetals from WC powders with different dispersity

The effect of sintering temperature and particle size of tungsten carbide WC on phase composition, density and microstructure of hardmetals WC-8 wt.% Co has been studied using X-ray diffraction, scanning electron microscopy and density measurements. The sintering temperature has been varied in the range from 800 to 1600 °C. The coarse-grained WC powder with an average particle size of 6 μm, submicrocrystalline WC powder with an average particle size of 150 nm and two nanocrystalline WC powders with average sizes of particles 60 and 20 nm produced by a plasma-chemical synthesis and high-energy ball milling, respectively, have been used for synthesis of hardmetals. It is established that ternary Co₆W₆C carbide phase is the first to form as a result of sintering of the starting powder mixture. At sintering temperature of 1100-1300 °C, this phase reacts with carbon to form Co₃W₃C phase. A cubic solid solution of tungsten carbide in cobalt, β-Co(WC), is formed along with ternary carbide phases at sintering temperature above 1000 °C. Dependences of density and microhardness of sintering hardmetals on sintering temperature are found. The use of nanocrystalline WC powders is shown to reduce the optimal sintering temperature of the WC-Co hardmetals by about 100 °C.     

W–15 wt%Cu nano-composite produced by hydrogen-reduction/sintering of WO3–CuO nano-powder

A two-stage hydrogen-reduction/sintering procedure was used to synthesize W–15 wt%Cu nano-composite tablets. Hydrogen-reduction was carried out at 600, 650, 700 and 750 °C for 15–90 min and sintering was performed at 1100, 1150, 1200 and 1250 °C for 60 min. Morphology and grain size of the products were rigorously investigated by scanning electron microscope (SEM), X-ray diffractometer (XRD) and nano-particle Zeta-sizer. Maximum consolidation of the nano-composite product was achieved at 1200 °C. Hydrogen-reduction at 700 °C for 90 min showed an average particle size of ∼72.9 nm. Total reduction was achieved at higher temperatures and longer times. The mixture had a homogeneous structure with 16.1 ± 0.1 g/cm³ density when sintered at 1200 °C for 60 min.     

Electrical and thermal characterization of Sm doped ceria electrolytes synthesized by combustion technique

Nanocrystalline samarium doped ceria electrolyte [Ce_0.9Sm_0.1O_1.95] was synthesized by citrate gel combustion technique involving mixtures of cerium nitrate oxidizer (O) and citric acid fuel (F) taken in the ratio of O/F = 1. The as-combusted precursors were calcined at 700 °C/2 h to obtain fully crystalline ceria nano particles. It was further made into cylindrical pellets by compaction and sintered at 1200 °C with different soaking periods of 2, 4 and 6 h. The sintered ceria was characterized for the microstructures, electrical conductivity, thermal conductivity and thermal diffusivity properties. In addition, the combustion derived ceria powder was also analysed for the crystallinity, BET surface area, particle size and powder morphology. Sintered ceria samples attained nearly 98% of the theoretical density at 1200 °C/6 h. The sintered microstructures exhibit dense ceria grains of size less than 500 nm. The electrical conductivity measurements showed the conductivity value of the order of 10⁻² S cm⁻¹ at 600 °C with activation energy of 0.84 eV between the temperatures 100 and 650 °C for ceria samples sintered at 1200 °C for 6 h. The room temperature thermal diffusivity and thermal conductivity values were determined as 0.5 × 10⁻⁶ m² s⁻¹ and 1.2 W m⁻¹ K⁻¹, respectively.     

Densification and mechanical properties of fine-grained Al2O3-ZrO2 composites consolidated by spark plasma sintering

Alumina matrix composites containing 5 and 10 wt% of ZrO₂ were sintered under 100 MPa pressure by spark plasma sintering process. Alumina powder with an average particle size of 600 nm and yttria-stabilized zirconia with 16 at% of Y₂O₃ and with a particle size of 40 nm were used as starting materials. The influence of ZrO₂ content and sintering temperature on microstructures and mechanical properties of the composites were investigated. All samples could be fully densified at a temperature lower than 1400 °C. The microstructure analysis indicated that the alumina grains had no significant growth (alumina size controlled in submicron level 0.66-0.79 μm), indicating that the zirconia particles provided a hindering effect on the grain growth of alumina. Vickers hardness and fracture toughness of composites increased with increasing ZrO₂ content, and the samples containing 10 wt% of ZrO₂ had the highest Vickers hardness of 18 GPa (5 kg load) and fracture toughness of 5.1 MPa m^1/2.     

Spark plasma sintering consolidation of nanostructured TiC prepared by mechanical alloying

Spark plasma sintering technique was used for the consolidation of nanostructured titanium carbide synthesized by mechanical alloying in order to avoid any important grain growth of the compact materials. The TiC phase was obtained after about 2 h of mechanical alloying. Towards the end of the milling process (20 h), the nanocrystalline powders reached a critical size value of less than 5 nm. Some physical and mechanical properties of the consolidated carbide were reported as a function of the starting grain size powders obtained after different mechanical alloying durations. The crystalline grain size of the bulk samples was found to be increased to a maximum of 120 nm and 91 nm for carbides mechanically alloyed for 2 h and 20 h respectively. The Vickers hardness showed to be improved to about 2700 Hv for a maximum density of 95.1% of the bulk material.     

An aqueous-solution based low-temperature pathway to synthesize giant dielectric CaCu3Ti4O12—Highly porous ceramic matrix and submicron sized powder

A simple, cost-effective and environment-friendly pathway for preparing highly porous matrix of giant dielectric material CaCu₃Ti₄O₁₂ (CCTO) through combustion of a completely aqueous precursor solution is presented. The pathway yields phase-pure and impurity-less CCTO ceramic at an ultra-low temperature (700 °C) and is better than traditional solid-state reaction schemes which fail to produce pure phase at as high temperature as 1000 °C (Li, Schwartz, Phys. Rev. B 75, 012104). The porous ceramic matrix on grinding produced CCTO powder having particle size in submicron order with an average size 300 nm. On sintering at 1050 °C for 5 h the powder shows high dielectric constants (>10⁴ at all frequencies from 100 Hz to 100 kHz) and low loss (with 0.05 as the lowest value) which is suitable for device applications. The reaction pathway is expected to be extended to prepare other multifunctional complex perovskite materials.     

Fabrication and electrical characterization of 15% yttrium-doped barium zirconate—nitrate freeze drying method combined with vacuum heating

We applied a nitrate freeze-drying method to obtain a fine synthesized powder of 15% yttrium-doped barium zirconate. Fine 15% yttrium-doped barium zirconate powder of particle size about 30 nm was obtained by synthesizing at 500 °C in vacuum from a powder mixed by the nitrate freeze-drying method. However, we could not obtain such fine powder by synthesizing in air. Using the powder synthesized in vacuum, large and homogeneous grains of 15% yttrium-doped barium zirconate were easily obtained after sintering. Then, the bulk and grain boundary resistance were evaluated by AC 2-terminal measurement of sample in the form of bar and pellet and DC 4-terminal measurement of bar-shape sample. The grain boundary resistance was not inversely proportional to the grain size as theoretically expected. We concluded that specific grain boundary conductivity varies with samples. Some impurities, evaporation loss of barium oxide and/or other unexpected reasons might affect the grain boundary resistance in 15% yttrium-doped barium zirconate.     

Synthesis and consolidation of TiN/TiB2 ceramic composites via reactive spark plasma sintering

The simultaneous synthesis and densification of TiN/TiB₂ ceramic composites via reactive spark plasma sintering (RSPS) was investigated. Different component ratios (TiH₂/BN (TiN, B)) and heating rates (112.5-300 °C/min) were used to initiate the chemical reaction for TiN/TiB₂ synthesis. The omit RSPS process was revealed to have three stages, which are described separately. The relationships between the RSPS conditions, the microstructure and the properties of sintered ceramic composites were established. A Vickers hardness of 16-25 GPa and a fracture toughness of 4-6.5 MPa m^1/2 were measured for various compositions. Sintered ceramic composites containing 36 wt% TiB₂ with the highest relative density of 97.4 ± 0.4% and an average grain size of 150-550 nm have been obtained.     

Synthesis of (Bi0.5Na0.5)TiO3 (BNT) and Pr doped BNT using the soft combustion technique and its properties

In this work, bismuth sodium titanate (Bi_0.5Na_0.5)TiO₃ (BNT) and praseodymium (Pr)-doped BNT were successfully produced using the soft combustion technique. The effects of Pr doping on stoichiometry, microstructure, density and dielectric properties were studied. Pure Pr-doped BNT was obtained in all samples containing 5, 10 and 20 mol% Pr after calcination at 800 °C for 3 h. The produced powders were then pressed into pellets and sintered at 1100 °C for 3 h. The very similar ionic radii of Pr³⁺ with Bi³⁺ and Na⁺ made it possible to substitute both Bi and Na. The crystallite size and grain size decreased with increasing Pr amount because Pr acted as grain growth inhibitor, both for calcined powders and for sintered pellets. Maximum density was obtained in 5 mol% Pr-doped BNT, beyond which density decreased. The maximum dielectric constant of 756 was obtained in 5 mol% Pr-doped BNT and decreased at higher levels of Pr doping. Pr doped into BNT also caused a decrease in dielectric loss.     

The effect of saccharin addition and bath temperature on the grain size of nanocrystalline nickel coatings

Nanocrystalline nickel coating was synthesized by direct current electrodeposition from a Watts bath at the current density of 100 mA/cm² and pH = 4. The effect of saccharin addition (0-10 g/l) and bath temperature (45-65 °C) on the average grain size of the deposits was investigated by XRD technique. The results showed that the average grain size decreased from 426 nm to 25 nm as the saccharin concentration increased from 0 to 3 g/l, while further increase in saccharin concentration had no significant effect. Theoretical model also indicated a non-linear function for dependence of grain size on saccharin concentration, which was in accordance with experimental results. The experimental results showed that the increases in the bath temperature had no considerable effect on the average grain size of the deposits. A theoretical formula was also established for the temperature dependence of the grain size.     

Fabrication of full density near-nanostructured cemented carbides by combination of VC/Cr3C2 addition and consolidation by SPS and HIP technologies

The aim of present work is to study the effect of VC and/or Cr₃C₂ in densification, microstructural development and mechanical behavior of nanocrystalline WC-12wt.%Co powders when they are sintered by spark plasma sintering (SPS) and hot isostatic pressing (HIP). The results were compared to those corresponding to conventional sintering in vacuum. The density, microstructure, X-ray diffraction, hardness and fracture toughness of the sintered materials were evaluated. Materials prepared by SPS exhibits full densification at lower temperature (1100 °C) and a shorter stay time (5 min), allowing the grain growth control. However, the effect of the inhibitors during SPS process is considerably lower than in conventional sintering. Materials prepared by HIP at 1100 °C and 30 min present full densification and a better control of microstructure in the presence of VC. The added amount of VC allows obtaining homogeneous microstructures with an average grain size of 120 nm. The hardness and fracture toughness values obtained were about 2100 HV₃₀ and close to 10 MPa m^1/2, respectively.     

Crystallization and magnetic behavior of nanosized nickel ferrite prepared by citrate precursor method

NiFe₂O₄ nanoparticles have been synthesized by citrate precursor gel formation with subsequent heat treatment. Differential thermal and thermogravimetric (DTA/TG) analyses show that the metal citrates decomposed around 230 °C followed by crystallization of the ferrite. X-ray diffraction (XRD) patterns reveal the formation of the cubic spinel phase in the samples after sintering the gel at 350 °C, 500 °C and 700 °C. For the samples annealed at 350 °C and 500 °C a small amount of α-Fe₂O₃ was detected whereas single phase was obtained for the sample annealed at 700 °C. The lattice constant a for all the samples is comparable to the value of the bulk material. The mean crystallite size D_XRD of the samples determined from XRD line broadening is 26.2-28.5 nm. Transmission electron microscope (TEM) analysis shows that the single-phase particles form clusters with the particle size in the range of 21-82.5 nm and the most probable value D_TEM of 55.4 nm. Magnetic measurements show that its Curie temperature T_C is close to the bulk value while the spontaneous magnetization M_s at 5 K is lower than that of the bulk. The thermal variation of M_s in the temperature range from 5 to 300 K can be best fitted to a modified Bloch T^α law with the exponent value α ≈ 2. The magnetization data are explained with reference to the disordered surface spins and the finite size effects. In this investigated temperature range, the coercive force H_c decreases linearly with increasing temperature. The coercivity mechanism in the nanoparticle sample with broad particle size distribution is expected to be complex and different factors which affect the H_c value were proposed.     

Oxidation behavior of Al4SiC4 ceramic up to 1700 °C

Isothermal oxidation behavior of Al₄SiC₄ ceramics at the temperature range from 1200 °C to 1700 °C in air for 10-20 h was investigated. The results indicated that this material had an excellent oxidation resistance from 1200 °C to 1600 °C, and the kinetics of oxidation obeyed the parabolic law with an activation energy of 220 ± 20 kJ mol⁻¹. The oxide scales consisted of an outer oxide layer with higher density, a middle oxide layer with a few of large size pores and a reaction layer which is near to the matrix with a number of small size pores over the temperature ranges. A number of pores exist in the middle oxide scale. The oxide surface and cross-sectional morphologies were observed by scanning electron microscope (SEM) technique and the formation mechanism of the oxidation layers was also analyzed.     

Dense La0.9Sr0.1Ga0.8Mg0.2O2.85 electrolyte for IT-SOFC's: Sintering study and electrochemical characterization

This paper presents the sintering behaviour of a La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 coral-like microstructure powder. This is prepared by a successive freeze-drying and self-ignition process followed by calcination at 1200 °C during 1 h. This synthesis method gives great uniformity of the powder and allows shaping into compacts without requiring a grinding step. The grain size distribution (between 0.5 and 4 μm) favours a good sintering behaviour: open porosity disappear at 1400 °C and relative densities over 99% can be achieved after 6 h at 1450 °C. The same powder can also be sintered into a thin disc of ∼100 μm thickness. The characterization of the dense material by impedance spectroscopy shows that the activation energies below and above 600 °C are 1.0 eV and 0.7 eV, respectively. The conductivity at 800 °C is ∼0.11 S cm⁻¹. Special attention is devoted to the temperature range between 200 °C and 400 °C, where the intragrain and intergrain contributions can be distinguished. The analysis of the parameters describing the intragrain constant phase element in the equivalent circuit suggests that, above 325 °C, the system evolves from a distribution of relaxation time to only one relaxation time. The analysis of the data by the complexes permittivity show that ionic oxide conduction mechanism would occur in two steps. In the first, an oxygen vacancy would be released and, in the second, the migration of the ionic oxide would take place in the material.     

Synthesis and characterization of dl-thioctic acid (DLTA)-Fe3O4 nanocomposite

dl-Thioctic acid (DLTA) coated magnetite (Fe₃O₄) NP's have been prepared by the co-precipitation of iron oxide in the presence of DLTA. The product identified as magnetite, which has an average crystallite size of 7 ± 2 nm as estimated from X-ray line profile fitting. Particle size was estimated as 11 ± 1 nm from TEM micrographs. FT-IR analysis showed that the binding of DLTA on the surface of iron oxide is through carboxyl group is bidentate. VSM analysis explained the super-paramagnetic nature of the nanocomposite. TG analysis showed that the 80% of the nanocomposite was DLTA and 10% was Fe₃O₄, respectively. The conductivity measurements displayed the magnetic transition at ∼60 °C for DLTA-Fe₃O₄ NPs. Analysis of the conductivities reveals the fact that the a.c. conductivity shows a frequency-dependent behavior while d.c. electrical conductivity is strongly temperature dependent and is classified into two regions over a limited temperature range of up to 120 °C. Toxicity was tested measured by LDH assay.