Effects of annealing on electrical properties of n-type Bi2Te3 fabricated by mechanical alloying and spark plasma sintering

N-type Bi₂Te₃ thermoelectric materials were firstly prepared by spark plasma sintering (SPS) using mechanically alloyed powders and then annealed at 423–593 K for 10–144 h. The samples were characterized by X-ray diffraction, scanning electron and optical microscope, and the Hall coefficient, electrical resistivity and Seebeck coefficient were measured. The effects of annealing conditions on the electrical transport properties were studied on the basis of the effective-medium-theory (EMT) and antisite defect model. The electrical transport properties of Bi₂Te₃ were reduced by annealing at 523 K and 593 K, but were improved by annealing at 423 K from 10 h to 144 h. The good property stability of the SPSed Bi₂Te₃ annealed at 423 K is attributed to the fact that lattice defects are reduced by appropriate annealing treatments.     

Effect of powder mixing on thermoelectric properties in Bi2Te3-based sintered compounds

The effect of powder mixing on thermoelectric properties was studied in n-type Bi₂Te_2.85Se_0.15 compounds and p-type Bi_0.5Sb_1.5Te₃ compounds. The figure-of-merit of the n- and p-type sintered compounds was strongly affected by carrier mobility. The use of coarse powders (200–300 μm) was beneficial to improve the crystallographic orientation of sintered compound. However, voids in the compound decreased the carrier mobility. As the fine powders (below 45 μm) were blended with coarse powders up to about 30%, the carrier mobility was increased due to the reduction of the voids. The addition of fine powders over 30% degraded the carrier mobility due to the decrease of crystallographic orientation and the increase of particle boundary. When the fine powder content was 20%, the n- and p-type compounds exhibited the maximum figure-of-merit of 2.31 × 10⁻³ K⁻¹ and 2.89 × 10⁻³ K⁻¹, respectively.     

Synthesis of (Mo1−x–Crx)Si2 nanostructured powders via mechanical alloying and following heat treatment

MoSi₂–CrSi₂ nanocomposite powder was successfully synthesized by ball milling of Mo, Si and Cr elemental powders. Effects of the Cr content, milling time and annealing temperature were studied. X-ray diffraction (XRD) was used to characterize the milled and annealed powders. The morphological and microstructural evolutions were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). High temperature polymorph (HTP) of MoSi₂ begins to form after 50 h of milling and completes after 70 h of milling. MoSi₂–CrSi₂ composite powder was also prepared with a combination of short milling time (50 h) and low temperature annealing (850 °C). Annealing led to the HTP to low temperature polymorph (LTP) transformation of MoSi₂. MoSi₂–CrSi₂ nanocomposite powder with the mean grain size less than 50 nm was obtained at the end of milling. This composite maintained its nanocrystalline nature after annealing. A spherical morphology was procured for 50 h milled powder with 0.25 mole Cr.     

Preparation and thermoelectric properties of p-type (Ga2Te3)x–(Bi0.5Sb1.5Te3)1−x (x = 0–0.2) alloys prepared by spark plasma sintering

p-Type (Ga₂Te₃)_x–(Bi_0.5Sb_1.5Te₃)_1−x (x = 0–0.2) alloys were prepared by spark plasma sintering technique, and the effect of gallium telluride (Ga₂Te₃) on the thermoelectric properties was experimentally determined. Measurements have shown that the electrical conductivities of (Ga₂Te₃)_x–(Bi_0.5Sb_1.5Te₃)_1−x are improved and thermal conductivities reduced after the introduction of Ga₂Te₃ in the Bi_0.5Sb_1.5Te₃ alloy without noticeable loss of Seebeck coefficient. The maximum thermoelectric figure of merit ZT of 1.0 is obtained with molar fraction x = 0.1 at 335.5 K, being approximately two times that of the Bi_0.5Sb_1.5Te₃ at the corresponding temperature, thus proving that (Ga₂Te₃)_x–(Bi_0.5Sb_1.5Te₃)_1−x (x = 0–0.2) alloys are to be a promising material for application.     

Thermoelectric properties of silver-doped n-type Bi2Te3-based material prepared by mechanical alloying and subsequent hot pressing

Effect of Ag-doping on thermoelectric properties of n-type Bi₂(Te_0.94,Se_0.06)₃ is studied. Bulk Bi₂(Te_0.94,Se_0.06)₃ sample with single solid solution phase and high relative density (over 95%) is obtained by mechanical alloying and subsequent hot press sintering. The as-sintered sample has a microstructure of randomly distributed plate grains and its size is about 1 μm. A V-shaped variation for the electrical properties with Ag doping is found which corresponds to different Ag atomic position in lattice among the doping range studied. A maximum figure of merit of about 1.7 × 10⁻³/K is achieved.     

Effect of nonstoichiometry of NbCy and TaCy powders on their high-energy ball milling

Nanocrystalline powders of nonstoichiometric carbides NbC_y and TaC_y are experimentally obtained by high-energy milling of coarse-grained powders. For the first time, the effect of nonstoichiometry of NbC_y and TaC_y carbides on the particle size of produced nanopowders is studied. It is shown that the effect of nonstoichiometry on milling is manifested by the concentration dependences of the main characteristics (crystal structure parameters, energy of interatomic bonds, elastic properties) of milled nonstoichiometric compound. The experimental results on NbC_y (0.77 ≤ y ≤ 0.96) and TaC_y (0.81 ≤ y ≤ 0.96) milling are compared with the theoretical model dependences of the particle size D of nanopowders on the duration of milling t and the composition y of nonstoichiometric cubic carbides NbC_y and TaC_y. It is established that the carbide powders with the average nanoparticle size of 20–30 nm and specific surface area of 25–30 m² g^− 1 are produced by 15 h milling, all other factors being equal.     

A novel approach to in situ synthesis of WC-Al2O3 composite by high energy reactive milling

In-situ synthesis of WC-Al₂O₃ composite by milling and its subsequent heat treatment were investigated. Mixtures of Al, W, and C with stoichiometric ratio of W₃AlC₂ were ball milled up to 20 h. Then, the 20-hour ball milled powder was heat treated at different temperatures of 900 and 1200 °C. The reaction path was investigated by X-ray diffractometry (XRD). The particle size and microstructure of powders after milling was investigated by field emission scanning electron microscope (FESEM) equipped with energy-dispersive spectroscopy (EDS). Also, in order to analyze the heating behavior of 20 h ball milled powder mixture during heat treatment, simultaneous thermal analysis (STA) was used. The results showed that after milling for 20 h, the reactants reacted together and new phases including W₂C and (W,Al)C_1 − x were formed. After heat treatment, the semi-stable compounds synthesized at the milling stage, were transferred to more stable compounds including WC and Al₂O₃.     

Preparation of WC nanoparticles by twice ball milling

In order to improve the ball milling efficiency of WC powders and thus to fabricate nano-grained WC–Co cemented carbides with high mechanical properties, WC nanoparticles were prepared by twice ball milling in nylon vessels. The best technology to disperse WC powders in alcohol was investigated at first. Based on the dispersion results, 2 wt.% PEG was used with La₂O₃ as additive to improve ball milling efficiency. The particle size, crystal structure, surface morphology and surface properties were tested by a laser particle sizer, XRD, FE-SEM and FT-IR, respectively. During the first ball milling, sample d achieved the best milling performance, including average particle size (168 nm) and grain size (27.2 nm) among samples a (pure WC), b (with PEG), c (with La₂O₃) and d (with PEG and La₂O₃). La₂O₃ could greatly decrease particle size and grain size while PEG could narrow particle size distribution. During the second milling, the particle size and grain size of sample d reached 89 nm and 13.2 nm at 96 h, respectively. The results indicated that twice ball milling can greatly improve particle size and grain size compared with the first ball milling, and further narrow the size distribution. In conclusion, multiple ball milling can reduce the particle size of certain powders with suitable milling technology.     

Preparation of Mo(Si, Al)2–ZrO2 nanocomposite powders by mechanical alloying

Phase transformations and the final formation of Mo(Si, Al)₂–ZrO₂ nanocomposite during high-energy ball milling of a series of Mo–Si–Al–ZrO₂ powders were investigated. Mechanical alloying led to phase transformations from the initial Mo–Si–Al powders mixture to Mo_ss (2 h) → C40 Mo(Si, Al)₂ (4, 8 h) → Mo_ss (12 h) phases. The phase transformations studied by XRD are discussed considering the alloying and second phase effects. Finally, the Mo_ss matrix reinforced with ZrO₂ particles nanocomposite structure was studied by means of TEM. The Mo_ss matrix phase formed was revealed to be strongly inhomogeneous even after 12 h of mechanical alloying and Mo-, Si- and Al-enriched regions were observed. The ZrO₂ nanostructured phase, evenly distributed in the Mo_ss matrix, had grain size of about 5–20 nm.     

Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-CuO powder mixtures

Understanding the microstructure of W–Cu nanocomposite powder is essential for elucidating its sintering mechanism. In this study, the effect of milling time on the structural characteristics and densification behavior of W-Cu composite powders synthesized from WO₃-CuO powder mixtures was investigated. The mixture of WO₃ and CuO powders was ball-milled in a bead mill for 1 h and 10 h followed by reduction by heat-treating the mixture at 800 °C in H₂ atmosphere with a heating rate of 2 °C/min to produce W-Cu composite powder. The microstructure analysis of the reduced powder obtained by milling for 1 h revealed the formation of W–Cu powder consisting of W nanoparticle-attached Cu microparticles. However, Cu-coated W nanocomposite powder consisting of W nanoparticles coated with a Cu layer was formed when the mixture was milled for 10 h. Cu-coated W nanopowder exhibited an excellent sinterability not only in the solid-phase sintering stage (SPS) but also in the liquid-phase sintering stage (LPS). A high relative sintered density of 96.0% was obtained at 1050 °C with a full densification occurring on sintering the sample at 1100 °C. The 1 h-milled W-Cu powder exhibited a high sinterability only in the LPS stage to achieve a nearly full densification at 1200 °C.     

Thermal and electronic properties of Bi1 − xSbx alloys

Polycrystalline Bi_1 ? xSb_x (x = 0.10, 0.12 and 0.15) semiconducting alloys were synthesized by mechanical alloying in order to achieve homogeneous thermoelectric materials with improved mechanical strength. The homogeneity of the powder samples were repeatedly checked by X-ray diffraction and scanning electron microscopy to standardize the milling conditions. The best possible homogenized material was developed with the milling conditions of BPR 30:1, ball diameter 30 mm, 400 rpm and milling time of 15 h. The electrical resistivity, thermoelectric power and thermal conductivity were measured in the temperature range 300–500 K. It was found through these experiments that the composition with x = 0.12 behaved in a normal semiconducting way, whereas the other two compositions (x = 0.10 and 0.15) showed degenerate semiconductor behaviour. These features have been qualitatively explained from the band structure and interband scattering mechanisms.     

Mechanical properties of Mo-Nb-Si-B quaternary alloy fabricated by powder metallurgical method

Mo-Si-B alloys composed of two intermetallic compound phases (Mo₅SiB₂ and Mo₃Si) and a molybdenum solid solution matrix phase have been investigated for use as high-temperature structural materials due to their high melting point and good creep resistance. However, despite these advantages, Mo-Si-B alloys are difficult to use in practical applications because they have insufficient fracture toughness at room temperature. So, in many researches, microstructure control and the addition of other elements in the α-Mo matrix phase are conducted as an effective way to improve the fracture toughness.In this study, niobium (Nb) was added to a Mo-Si-B alloy by a powder metallurgical method to improve the mechanical properties. First, the Mo and Nb powders were pulverized by high-energy ball milling. Then, the synthesized intermetallic compound powders, which were fabricated by continuous heat treatment under a H₂ atmosphere, were mixed with ball-milled Mo and Nb powder. Pressureless sintering was conducted at 1400 °C for 3 h under a H₂ atmosphere. The Vickers hardness and fracture toughness were measured to investigate the mechanical properties of the sintered Mo-Si-B and Mo-Nb-Si-B alloy. The Vickers hardness was about 425 Hv for a Mo-Nb-Si-B alloy, which was lower value of 165 Hv compared to Mo-Si-B alloy (590 Hv). On the other hand, the fracture toughness of the Mo-Nb-Si-B alloy (14.5 MPa·√m) greatly increased compared to that of the Mo-Si-B alloy (12.6 MPa·√m).     

Characterization of mechanically alloyed nanocomposites in TiO2–B2O3–Mg–C quaternary system

The influence of the simultaneous presence of magnesium and graphite on mechanosynthesis of various nanocomposite powders in TiO₂–B₂O₃–Mg–C quaternary system was investigated. A mixture of boron oxide and titanium dioxide powders along with different amounts of magnesium and graphite was milled using a high-energy planetary ball mill to provide necessary conditions for the occurrence of a mechanically induced self-sustaining reaction (MSR). In the absence of C (100 wt.% Mg), TiB₂ nanopowder was formed as a result of combustion reaction after 34 min of milling. In the presence of both Mg and C, the mechanochemical reaction was completed after different milling times depending on the weight fraction of the reducing agents in the powder mixture. In the presence of x wt.% Mg–y wt.% C (x = 85 and 90; y = 100 − x), the mechanosynthesized composites contained TiB₂ and TiC as major compounds as well as MgO and Mg₃B₂O₆ as unwanted phases. With further increasing the graphite content to 30 wt.%, no mechanical activation was observed after 90 min of milling. The nanocomposite powders showed a bimodal particle size distribution characterized by the presence of several coarse particles (≈ 250 nm) along with finer particles with a mean size of about 75 nm. Formation mechanism of nanocomposites was explained through the analysis of the relevant sub-reactions.     

Synthesis of nanocrystalline magnesium diboride (MgB2) metallic superconductor by mechano-chemical reaction and post-annealing

MgB₂ is recently discovered superconducting compound with a record-breaking transition temperature (T_c = 39 K) for a conventional metallic superconductor. Nanocrystallinity can improve its electrical properties by strong pinning. In the present work we report the results of the synthesis of nanocrystalline MgB₂ superconducting compound by mechano-chemical reaction followed by post-annealing. The first stage of synthesis was carried out from elemental crystalline Mg and amorphous B powders by controlled mechanical alloying (CMA) in the magneto-mill Uni-Ball-Mill 5. X-ray diffraction studies reveal that the nucleation of small amount of MgB₂ is initiated after two-step milling for combined 100 h under protective helium gas. Further reaction to form MgB₂ is accelerated by subsequent annealing of the milled powder at various temperatures. X-ray diffraction shows formation of a well-developed nanocrystalline MgB₂ after annealing of the pre-nucleated powder at the 630–650 °C range for a few hours.     

Synthesis of nanocrystalline Cd–Zn ferrite by ball milling and its stability at elevated temperatures

High energy ball milling of stoichiometric (0.5:0.5:1 mole fraction) mixture of CdO, ZnO and α-Fe₂O₃ powders in air at room temperature results in formation of a non-stoichiometric Zn-rich (Zn,Cd)Fe₂O₄ phase with normal spinel structure having tetrahedral vacancies. The ferrite phase is initiated at 1 h of milling and after 25 h milling, 0.96 mole fraction of ferrite is formed and 0.04 mole fraction of CdO phase remained unreacted. The phase stability study of nanocrystalline non-stoichiometric (Zn,Cd)Fe₂O₄ powder annealed at elevated temperatures reveals that the Zn-rich ferrite phase remained stable up to 973 K and then slowly transformed towards Cd-rich (Cd,Zn)Fe₂O₄ phase following the release of divalent cations from ferrite lattice of normal spinel structure. The non-stoichiometric ferrite phase with almost similar composition has also been obtained by conventional ceramic route by sintering the same stoichiometric mixture at 973 K for 1 h. Microstructure characterization in terms of several lattice imperfections, relative phase abundances, cation distribution, and phase stability studies of unmilled, ball-milled and annealed samples is made by employing the Rietveld's structure refinement methodology using X-ray powder diffraction data. The analysis reveals that the particle size of ferrite phase reduces to ～7 nm after 25 h of milling and after annealing at 1273 K for 1 h it grows up to ～700 nm. However, in case of ferrite prepared by ceramic route it grows up to ～250 nm which is quite less than the annealed ball-milled samples.     

Synthesis of Ti3AlC2 ceramic by high-energy ball milling of elemental powders of Ti,Al and C

Synthesis of the ternary carbide Ti₃AlC₂ by high-energy ball milling of elemental Ti, Al and C powders with a stoichiometric composition was tentatively investigated. The results show that high content Ti₃AlC₂ was successfully obtained after ball milling of powder mixture only for 3 h. The milled products consist of powder and a coarse granule with 8 mm in diameter, and both are mainly composed of Ti₃AlC₂ with TiC as impurity based on X-ray diffractometer (XRD) and energy-dispersive spectroscopy (EDS) characterization. It is believed that a mechanically induced self-propagating reaction (MSR) was triggered to form Ti₃AlC₂ and TiC during high-energy ball milling process.     

Synthesis and characterization of MoSi2-Mo5Si3 nanocomposite by mechanical alloying and heat treatment

The nanocomposite of MoSi₂-Mo₅Si₃ powder was synthesized by mechanical alloying from Mo and Si powder mixture at room temperature. The phase evaluation of powder after various milling durations and heat treatments were assessed via X-ray diffraction (XRD) and a differential thermal analysis (DTA). Morphology and microstructure of powder particles were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results revealed that nanocomposite of MoSi₂-Mo₅Si₃ powder was synthesized by combustion reaction of Mo and Si powder using ball milling. In the early stages of ball milling β-MoSi₂ was produced. However with continued milling for 48 h α-MoSi₂ and Mo₅Si₃ phases were formed. DTA results of 24 h and 48 h as milled mechanical alloyed specimens showed a well-defined peak at 852 °C and 920 °C relating to the formation of α-MoSi₂. The activation energy for 24 h and 48 h milled specimens were –128.6 KJ/mol and –121.4 KJ/mol respectively. Annealing the milled specimens at 1000 °C for 2 h revealed the phase transformation of β-MoSi₂ to α-MoSi₂ and the formation of Mo₅Si₃. The crystallite size of α-MoSi₂ and Mo₅Si₃ were about 9 nm and 12 nm after 48 h mechanical alloying. These values increased slightly to 18 nm and 14 nm after annealing at 1000 °C.     

B4C/TiB2 composite powders prepared by carbothermal reduction method

B₄C–(10–20 vol%)TiB₂ composite powders have been synthesized with the temperature of 1650–1800 °C by carbothermal reduction process using boron acid, carbon black and TiO₂ powder as the starting materials. B/C mole ratio of the starting materials is ascertained, thermodynamics temperature of the reactions is calculated and the effect of ball milling on the composite powders is discussed. The experimental results indicate that B/C mole ratio of the starting materials and composite powders are 4.4 and 3.98–4.03, respectively. The purity of the gained powders is more than 99 wt%. Wet ball milling eliminates the size of the B₄C/TiB₂ composite powders from 30–40 to 3–5 μm by decreasing the conglomeration of the composite powders. XRD and EDS results show that the composite powders are composed of B₄C and TiB₂.     

Thermoelectric properties of the new tellurides SrSc2Te4 and BaSc2Te4 in comparison to BaY2Te4

The two new ternary scandium tellurides SrSc₂Te₄ and BaSc₂Te₄ were prepared by heating the elements at temperatures between 800 °C and 900 °C under vacuum. Both compounds crystallize in the CaFe₂O₄ type, space group Pnma, Z = 4, with lattice dimensions of a = 13.1795(17) Å, b = 4.2323(7) Å, c = 15.367(2) Å, V = 857.2(2) Å³ for SrSc₂Te₄, and a = 13.292(2) Å, b = 4.2694(7) Å, c = 15.523(3) Å, V = 880.9(2) Å³ for BaSc₂Te₄. The structures comprise a three-dimensional network of edge- and corner-sharing ScTe₆ octahedra. The Sr and Ba cations are located in linear channels, surrounded by eight Te atoms forming bicapped trigonal prisms. SrSc₂Te₄ and BaSc₂Te₄ are electron-precise materials exhibiting common oxidation states, Sr²⁺, Ba²⁺, Sc³⁺, and Te²⁻. We calculated the band gaps to be 0.1 eV for SrSc₂Te₄ and 0.2 eV for BaSc₂Te₄. At room temperature, both materials exhibit a high Seebeck coefficient and low electrical conductivity.     

Synthesis and characterization of bismuth alkaline titanate powders

In this work, samples of bismuth alkaline titanate, (K_0.5Na_0.5)_(2?x/2)Bi_(x/6)TiO₃, (x = 0.05–0.75) have been prepared by conventional ceramic technique and molten salts. Metal oxides or carbonates powders were used as starting raw materials. The crystalline phase of the synthesized powders was identified by the X-ray diffraction (XRD) and particle morphology was characterized by scanning electron microscopy (SEM). Solid state reaction method was unsuccessful to obtain pellets. From XRD results, a rhombohedral structure was detected and the parameter lattice were estimated to be a = 5.5478 Å and α = 59.48°. These parameters were used to refine the structure by Rietveld analysis. SEM results showed several morphologies. Apparently, bismuth is promoting the grain growth whose sizes vary from 30 nm to 180 nm It is expected that these materials can be utilized in practical applications as substitutes for lead zirconatetitanate (PZT)-based ceramics.