Single-crystal and textured polycrystalline Nd2Fe14B flakes with a submicron or nanosize thickness

This paper reports on the fabrication, structure and magnetic property optimization of Nd₂Fe₁₄B single-crystal and [0 0 1] textured poly-nanocrystalline flakes prepared by surfactant-assisted high-energy ball milling (HEBM). Single-crystal Nd₂Fe₁₄B flakes first with micron and then with submicron thicknesses were formed via continuous basal cleavage along the (1 1 0) planes of the irregularly shaped single-crystal microparticles during the early stage of HEBM. With further milling, [0 0 1] textured polycrystalline submicron Nd₂Fe₁₄B flakes were formed. Finally, crystallographically anisotropic polycrystalline Nd₂Fe₁₄B nanoflakes were formed after milling for 5-6 h. Anisotropic magnetic behavior was found in all of the flake samples. Nd₂Fe₁₄B flakes prepared with either oleic acid (OA) or oleylamine (OY) as the surfactant exhibited similar morphology, structure and magnetic properties. Both the addition of some low-melting-point eutectic Nd₇₀Cu₃₀ alloy and an appropriate post-annealing can increase the coercivity of the Nd₂Fe₁₄B flakes. The coercivity of Nd₂Fe₁₄B nanoflakes with an addition of 16.7 wt.% Nd₇₀Cu₃₀ by milling for 5 h in heptane with 20 wt.% OY increased from 3.7 to 6.8 kOe after annealing at 450 °C for 0.5 h. The mechanism for formation and coercivity enhancement of Nd₂Fe₁₄B single-crystal and textured poly-nanocrystalline flakes with a submicron or nanosize thickness was discussed.     

Ball milling of Fe83Zr6B10Cu1 amorphous alloy containing quenched in crystals

A Fe₈₃B₁₀Zr₆Cu₁ melt-spun amorphous alloy has been ball milled in the presence of partially crystalline ribbon pieces, in order to speed up its mechanical nanocrystallization process. Microstructure and magnetic properties of the resulting powder were studied as a function of the milling time. A continuous increase of the crystalline fraction of α-Fe was observed after pulverization of the sample. A linear increase of Cr contamination with the milling time (up to 1.5 at.% after 24 h) was measured. This element seems to be preferentially in the crystalline phase. Saturation magnetization and coercivity were also measured as a function of temperature. The temperature dependence of coercivity is similar to that found in soft magnetic nanocrystalline alloys.     

Synthesis of SnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Nanomaterials Via High Energy Ball Milling of SnO (Stannous) and α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> (Hematite) Solid Precursors

The synthesis of single phase tin-ferrite, SnFe₂O₄, from tin (II) oxide or stannous oxide (SnO), and hematite (α-Fe₂O₃) solid precursors was carried out via high energy ball milling (HEBM) under wet condition involving the addition of controlled amounts of acetone. The stoichiometric amounts of the precursor materials were ball milled continuously for up to 22 h in a Spex-8000D mill using a ball-to-powder ratio of 40:1, with hardened stainless steel balls in WC-lined jars. The time-dependent formation of the SnFe₂O₄ based on combined X-ray diffraction and room temperature Mössbauer spectroscopy (MS) measurements revealed reaction enhancements associated with particles size reduction. The 22 h milled material indicated that synthesized SnFe₂O₄ had a particle size of 10.91 nm, coercivity of 4.44 mT, magnetic saturation/remanent ratio (M _r/M _s) of 0.085, while its superparamagnetic behavior was confirmed based on the combined MS and vibrating sample magnetometer measurements.     

XRD and TEM characterizations of the mechanically alloyed CuIn0.5Ga0.5Se2 powders

The CuIn_0.5Ga_0.5Se₂ (CIGS) nanocrystalline powders were prepared by mechanical alloying method. Effect of various milling times and higher milling speed on the structure of CIGS nanoparticles was investigated by X-ray diffraction measurements. The Rietveld method was used to refine the XRD data using the MAUD program. Refinement process reveals that the main phase of the CIGS powders milled for different milling times is of chalcopyrite structure. Milling time dependence of the unit-cell parameters and crystallite size has also been reported. The TEM observations demonstrated that the size of agglomerated CIGS powder is about 140 nm. The EDAX analysis of various grains of the milled powder shows that the compositions vary from one grain to another. However, the global composition was found slightly copper rich.     

Structure,phase composition and magnetic properties of Fe–Si–C system due to mechanical activation of Fe–Si alloy in organic liquids

Fineness, structure, phase composition, and magnetic properties of the powders produced by mechanical milling of Fe₈₀Si₂₀ in liquid hydrocarbon environment (heptane, heptane with oleic acid additive) in planetary ball mill have been studied. Under milling the alloys are saturated with the products of organic liquid decomposition – C, O, H, and then metastable amorphous and carbide phases form. The particle size decreases down to 0.1–0.2 μm. Isochronous (775 K, 1 h) annealing leads to the formation of the Fe₈Si₂C iron silicon carbide, as well as Fe₃Si and C – for milling in heptane, and Fe₃C and SiO₂ – for milling with oleic acid additive. The magnetic properties of the powders depend both on the milling time and milling environment, and on the annealing temperature. An increase in the coercivity from 13 up to 226 Oe correlates with the amount of the synthesized Fe₈Si₂C. It is ferromagnetic (T_c = 788 K) and temperature-resistant up to 870 K.     

Structure and magnetic properties of strontium ferrite anisotropic powder with nanocrystalline structure

The phase composition, nanocrystallite size, lattice microstrain and particles morphology of a SrFe₁₂O₁₉ powder subjected to milling and subsequent annealing were studied by various methods. The investigations showed that the high-temperature annealing of the preliminarily milled powder resulted in the increase in the coercive force (μ₀Н_сi) of the SrFe₁₂O₁₉ powder up to 0.4 T owing to the formation of nanocrystalline structure (D ∼ 10³ nm) with low lattice microstrains. However, the annealed powder cannot be textured in an applied magnetic field because of random orientations of the crystallites in powder particles. A processing technique, which includes the low-temperature annealing of powder in an applied magnetic field, was suggested. It allowed us to produce the anisotropic powder of the strontium ferrite with the nanocrystalline structure that ensures the high coercive force of the powder (∼0.4 T) and possibility of the powder texturing in the magnetic field. The prepared samples textured in a magnetic field exhibit the higher both remanence (by a factor of 1.4) and energy product (by a factor of 2.1) as compared to those of isotropic SrFe₁₂O₁₉ samples.     

Low-temperature hysteresis in transport properties of UNi0.5Sb2

We report detailed data of the electrical resistivity and thermoelectric power (TEP) measurements carried out at temperatures 0.4–300 K on single crystals of a deficit diantimonide UNi_0.5Sb₂. Although the composition of this compound and the transition temperature to the antiferromagnetic state are almost the same as those previously reported, we found the hysteretic transition occurring in the ordered state at different temperature (T_t = 81 K) from those estimated in earlier works, i.e. at either T_t = 64 K or T_t1 = 40 K and T_t2 = 85 K. This fact observed in both kinds of applied measurements indicates clearly a high sensitivity of these temperatures [connected to some moment reorientation and the following reconstruction of the Fermi surface (FS)] in this compound, to the used technology of growing its single crystals by different groups of authors. Following the results of a similar character of the parent compound USb₂ and taking into account the recent literature reports, we suppose that the observed anomalies in the resistivity and TEP of UNi_0.5Sb₂ are also caused by the complex electronic structure. Thus changes at its FS in the ordered state (e.g. f -electron dualism leading to band renormalization involving magnons) are forcing the reorientation of the uranium magnetic moments in the successive magnetic unit cells.     

Synthesis of Mo-Si-B intermetallic compounds with continuous α-Mo matrix by pulverization of ingot and hydrogen reduction of MoO3 powders

The fabrication of the intermetallic phase T2-Mo₃Si with continuous matrix of α-Mo was attempted with the combination process of high energy ball milling, pulverization of arc-melted ingot, addition of Mo by hydrogen reduction of MoO₃ and spark plasma sintering processes. High energy ball milling or arc melting of Mo-16.7Si-16.7B (at %) powders were performed to obtain to intermetallic phase T2 and Mo₃Si. The Mo phase of 57 vol% distributed intermetallic compound powders were prepared by hydrogen reduction of MoO₃ and further mixing of elemental Mo powders. X-ray diffractometry analysis revealed that the intermetallic phase T2-Mo₃Si can be produced by the pulverization process of arc-melted ingot. Hydrogen reduction of 1 vol% MoO₃ mixed intermetallic powder followed by further addition of Mo powders was a more adequate method enabling the homogeneous distribution of the Mo phase than that of added MoO₃ powders with total amount. The powder mixture was successfully consolidated by spark plasma sintering yielding a sound microstructure comprising the intermetallic phase T2-Mo₃Si uniformly distributed in a continuous matrix of α-Mo.     

A transmission electron microscopy study of the A-site disordered perovskite Na0.5Bi0.5TiO3

A transmission electron microscopy study of Na_0.5Bi_0.5TiO₃ (NBT) crystals shows two types of ferroelectric domains characterized by interface boundaries lying in the {1 1 0}_C and {1 0 0}_C planes. A one-dimensional {1 0 0}_C modulated texture is also observed locally. The electron diffraction study reveals the presence of (0 0 1)_T tetragonal platelets a few cells thick within the R3c matrix. They develop within the three basal {1 0 0}_C planes of the prototype phase and represent relics of the high-temperature tetragonal phase. The loss of symmetry compared to the average structure of NBT can thus clearly be attributed to the existence of these tetragonal platelets. The {1 0 0}_C modulated texture corresponds to a modulation of strain induced by the coexistence of two types of octahedra tilting systems: a⁻a⁻a⁻ for the rhombohedral matrix and a⁰a⁰c⁺ for the tetragonal platelets. These (0 0 1)_T platelets indirectly intervene in the relaxation process classically encountered at about 230 °C, in marked contrast to the behaviour of Pb(Mg_1/3Nb_2/3)O₃.     

FeSiBPNbCu alloys with high glass-forming ability and good soft magnetic properties

Effects of Cu addition on the glass-forming ability (GFA), thermal stability, magnetic properties and crystallization process of (Fe_0.76Si_0.09B_0.1P_0.05)_99−xNb₁Cu_x (x = 0, 0.25, 0.5, 0.75, 1) alloys were investigated. The introduction of Cu effectively stimulates the precipitation of the α-Fe(Si) without obvious deterioration of the GFA, and successfully modifies the simultaneous precipitation of α-Fe(Si), Fe₂B and Fe₃(B,P) phases in (Fe_0.76Si_0.09B_0.1P_0.05)₉₉Nb₁ alloy into separable precipitation of each phase at different temperatures during annealing, leading to the enhancement of soft magnetic properties. The saturation magnetic flux density of the representative (Fe_0.76Si_0.09B_0.1P_0.05)_98.25Nb₁Cu_0.75 alloy could be enhanced from 1.43 to 1.51 T after annealing at 530 °C for 10 min due to the precipitation of α-Fe(Si) nanoparticles with a diameter of about 22 nm dispersing randomly in the amorphous matrix. The integration of high GFA and excellent soft magnetic properties makes the FeSiBPNbCu alloys promising soft magnetic materials for industrial applications.     

Atomistic mechanism of cyclic phase transitions in Nd–Fe–B based intermetallics

Mechanical milling induced atomic disorder and the mechanism of the cyclic crystal → amorphous → crystal phase transitions in a Nd₂Fe₁₄B intermetallic – based Nd₁₀Fe₈₅B₅ alloy were investigated. Least square first shell fitting of Extended X-ray Absorption Fine-Structure (EXAFS) data of mechanically milled samples revealed that phase evolution during mechanical milling of Nd₁₀Fe₈₅B₅ alloy occurred in two stages as a result of two dynamical effects: milling induced forced atomic mixing and enhanced diffusion due to continuous production of point defects. In the first stage of milling, atomic mixing of Nd and Fe atoms dominated, resulting in extensive chemical and structural disorder in the initially ordered intermetallic, followed by amorphization. The second stage of milling was characterized by a remarkable increase in point defect density. Enhanced diffusion resulted in redistribution of alloy atoms and annihilation of these point defects leading to the precipitation of α-Fe nanocrystals in the amorphous matrix. Thus the time evolution of defect structures formed during mechanical milling and their atomic scale interactions led to the cyclic phase transitions in the Nd₁₀Fe₈₅B₅ alloy. These milling induced changes in atomic order were found to strongly influence magnetic properties. Crystallization of mechanically milled alloy and effect of annealing parameters on magnetic properties was also analyzed.     

A comparison of the local structure in ball-milled and hand ground skutterudite samples using EXAFS

Skutterudites are considered to be good thermoelectrics with high figures of merit, ZT. After synthesis, electrodes are created by grinding and hot pressing the resulting powder. Materials such as NdFe₄Sb₁₂ exhibit a significantly greater figure of merit, ZT, (about 43%) when it is ball milled to produce fine powders (inital powder 160 nm; after hot pressing ∼330 nm). This enhancement is typically attributed to the reduced particle size, which in turn decreases the mean free path of phonons, and consequently decreases the thermal conductivity. This work aims to investigate whether there is any damage to the crystal structure in the particles formed by ball milling, which could also affect its thermal conductivity. Using a temperature dependent, Extended X-ray Absorption Fine Structure (EXAFS) analysis of a hand ground and ball milled sample of the skutterudite Nd_yFe₄Sb₁₂, we have determined that ball milling causes no significant damage to the local structure around any site. Consequently further improvements in ZT may be possible with smaller particles.     

x(Mg0.7Zn0.3)0.95Co0.05TiO3-(1−x)(La0.5Na0.5)TiO3 ceramic at microwave frequency with a near zero temperature coefficient of resonant frequency

Dielectric properties of x(Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃-(1?x)(La_0.5Na_0.5)TiO₃ ceramic were investigated at microwave frequencies. A nearly 0 ppm/°C temperature coefficient of resonant frequency was realized at x = 0.9. A two-phase system was confirmed by XRD analysis. A dielectric material applicable to microwave devices with a Q × f of 20,000–87,000 GHz and a dielectric constant of 21.27–26.2 was obtained at 1100 °C after 4 h of sintering. The microwave dielectric material 0.9(Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃-0.1(La_0.5Na_0.5)TiO₃ sintered at 1150 °C for 4 h has a dielectric constant of 24.56, a Q × f of 68,000 GHz, and a τ_f value of 0 ppm/°C. It is proposed as a candidate dielectric for GPS patch antennas.     

Thermal Spray Deposition,Phase Stability and Mechanical Properties of La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript>/LaAlO<Subscript>3</Subscript> Coatings

This paper deals with the deposition of La₂Zr₂O₇ (LZO) and LaAlO₃ (LAO) mixtures by air plasma spray (APS). The raw material for thermal spray, single phase LZO and LAO in a 70:30 mol.% ratio mixture was prepared from commercial metallic oxides by high-energy ball milling (HEBM) and high-temperature solid-state reaction. The HEBM synthesis route, followed by a spray-drying process, successfully produced spherical agglomerates with adequate size distribution and powder-flow properties for feeding an APS system. The as-sprayed coating consisted mainly of a crystalline LZO matrix and partially crystalline LAO, which resulted from the high cooling rate experienced by the molten particles as they impact the substrate. The coatings were annealed at 1100 °C to promote recrystallization of the LAO phase. The reduced elastic modulus and hardness, measured by nanoindentation, increased from 124.1 to 174.7 GPa and from 11.3 to 14.4 GPa, respectively, after the annealing treatment. These values are higher than those reported for YSZ coatings; however, the fracture toughness (K _IC) of the annealed coating was only 1.04 MPa m^0.5.     

Structure and magnetic properties of a multi-principal element Ni–Fe–Cr–Co–Zn–Mn alloy

A nanocrystalline alloy with a nominal composition of Ni₂₀Fe₂₀Cr₂₀Co₂₀Zn₁₅Mn₅ was produced by mechanical alloying and processed using annealing treatments between 450 and 600 °C for lengths from 0.5 to 4 h. Analysis was conducted using x-ray diffraction, transmission electron microscopy, magnetometry, and first-principles calculations. Despite designing the alloy using empirical high-entropy alloy guidelines, it was found to precipitate numerous phases after annealing. These precipitates included a magnetic phase, α-FeCo, which, after the optimal heat treatment conditions of 1 h at 500 °C, resulted in an alloy with reasonably good hard magnetic properties. The effect of annealing temperature and time on the microstructure and magnetic properties are discussed, as well as the likely mechanisms that cause the microstructure development.     

Switching characteristics of MPB compositions of (Bi0.5Na0.5)TiO3–BaTiO3–(Bi0.5K0.5)TiO3 lead-free ferroelectric ceramics

The polarization switching characteristics of lead-free a(Bi_0.5Na_0.5)TiO₃–bBaTiO₃–c(Bi_0.5K_0.5)TiO₃ (abbreviated as BNBK 100a/100b/100c) ferroelectric ceramics are investigated. For the first time, the strain hystereses of BNBK compositions inside and outside the morphotropic phase boundary (MPB) are presented. The total induced electrostrain (ε_33,total) and apparent piezoelectric coefficient (d₃₃) first increase dramatically and then decrease gradually as the BNBK composition moves from the tetragonal phase to the MPB and then to the rhombohedral phase. The measured polarization hystereses indicate that the BNBK compositions situated near the rhombohedral side of the MPB typically possess higher coercive field (E_c) and remanent polarization (P_r), while the compositions situated near the tetragonal side of the MPB possess higher apparent permittivity. BNBK 85.4/2.6/12 a composition well within the MPB, exhibits an ε_33,total of ∼0.14%, an apparent d₃₃ of 295 pCN⁻¹ and a P_r of 22.5 μC cm⁻². These property values suggest a candidate material for lead-free actuator applications.     

Preparation of CuIn1−xGaxS2 (x = 0.5) flowers consisting of nanoflakes via a solvothermal method

CuIn_1−xGa_xS₂ (x = 0.5) flowers consisting of nanoflakes were successfully prepared by a biomolecule-assisted solvothermal route at 220 °C for 10 h, employing copper chloride, gallium chloride, indium chloride and l-cysteine as precursors. The biomolecule l-cysteine acting as sulfur source was found to play a very important role in the formation of the final product. The diameter of the CuIn_0.5Ga_0.5S₂ flowers was 1-2 μm, and the thickness of the flakes was about 15 nm. The obtained products were characterized by X-ray diffraction (XRD), energy dispersion spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction spectroscopy (SAED), and UV-vis absorption spectroscopy. The influences of the reaction temperature, reaction time, sulfur source and the molar ratio of Cu-to-l-cysteine (reactants) on the formation of the target compound were investigated. The formation mechanism of the CuIn_0.5Ga_0.5S₂ flowers consisting of flakes was discussed.     

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.     

Brilliant effect of Ca substitution in the appearance of magnetic memory in Dy0.5(Sr1−xCax)0.5MnO3 (x = 0.3) manganites

The present work undertakes an investigation of magnetic memory effect in Dy_0.5(Sr_1−xCa_x)_0.5MnO₃ (x = 0 and x = 0.3) nanoparticles heated at 700 °C and 1350 °C. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements displayed the existence of magnetic memory effect in only samples with a Ca concentration x = 0.3. However, there is no sign of magnetic memory in samples with x = 0 of Ca amount. This confirms that the effect of substituting the Sr by Ca on the appearance of magnetic memory is of great importance, admitting that magnetic memory is mainly due to the powerful inter-particle interactions.     

Pulverization mechanism of the multiphase Ti–V-based hydrogen storage electrode alloy during charge/discharge cycling

Pulverization is an important key factor for the electrochemical cycle stability of many hydrogen storage alloys. In this paper, the pulverization mechanism of the multiphase Ti–V-based hydrogen storage alloy which mainly consists of a V-based solid solution phase with the BCC structure and a C14 Laves phase is studied based on a sample material of the Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.6Ni_1.25Fe_0.2 alloy. The microstructure of the alloy and the morphology change of the alloy electrode during the charge/discharge process were observed by transmission electron microscope, scanning electron microscope and atomic force microscope, etc. The effect of mechanical properties of the V-based phase and the C14 Laves phase on the pulverization behavior of the Ti–V-based alloy is discussed. The results show that microcracks initially occur at the phase boundary of the V-based phase and the C14 Laves phase and then extend to the C14 Laves phase in the charge/discharge process. The phase boundary is composed of a Ti segregated amorphous layer with a thickness of about 90 nm, mismatching with the crystallized V-base phase and C14 Laves phase. The toughness of the C14 Laves phase is much lower and the hardness is higher than that of the V-based phase. The weak bonding strength of the phase boundary, the lower toughness of the C14 Laves phase and the large volume expansion/contraction of the C14 Laves phase during charge/discharge cycling are the main factors that cause the pulverization of the Ti–V-based alloy.