Zone Melting Method Research Articles

Microstructure characteristics and mechanical properties of Al2O3/MgAl2O4 directionally solidified eutectic ceramic

ABSTRACT Al2O3/MgAl2O4 directionally solidified eutectic ceramic (DSEC) rods with Φ 10 mm were prepared with induction heating zone melting (IHZM) method. The phase components of the DSEC were composed of MgAl2O4 phase growing along the preferred orientation 〈311〉 and Al2O3 phases growing along the preferred orientation 〈110〉. In the DSEC, the Al2O3 phase is the matrix phase mixed with part of MgAl2O4 phase and the other part of MgAl2O4 phase forms slice structure. Both the DSEC density and hardness decrease with the increase of the growth rate, due to the increase of micropores in DSEC with the higher growth rate. However, its fracture toughness improved due to the fine interface spacing toughening and the dispersion toughening. The highest hardness and fracture toughness of DSEC are 19.3 GPa and 3.74 MPa·m1/2, respectively, the toughening effect could be attributed to the crack deflection and bifurcation hindering the propagation of crack in DSEC.

Energy transfer and thermal stability enhancement in Ce/Cr co-doped Al2O3/YAG eutectic phosphor ceramics for broadband red-emission

The Ce/Cr co-doped Al2O3/YAG eutectic phosphor ceramics for WLEDs with functional and structural integration were successfully solidified by laser floating zone melting (LFZM) method (>104 K/cm). The influences of Cr3+ doping on microstructure, mechanical properties and optical characteristics of the eutectic phosphor ceramics were systematically and efficiently investigated. The results showed that the eutectic phosphor ceramics presented typical “Chinese script” (CS) irregular eutectic structure, consisting of interpenetrated Al2O3:Cr3+ and YAG:Ce3+/Cr3+ phases. Under the excitation of the blue light (460 nm), the eutectic phosphor ceramics exhibited excellent red-light emission with wavelengths of about 687 nm, 693 nm and 707 nm. The emission intensity could be still maintained at a high level of 68% at 127 °C compared to the intensity at room temperature, referring to good thermal stability of the eutectic phosphor ceramics. Especially, the color rendering index (Ra = 80) and the correlative color temperature (Tc = 5606 K) of the present eutectic phosphor were simultaneously improved by the energy transfer between Ce3+ and Cr3+ ions. The present Ce/Cr co-doped Al2O3/YAG eutectic phosphor ceramics can be used as excellent candidates for high performance warm WLEDs.

Thermal transport properties for unveiling the mechanism of BiSbTe alloys in thermoelectric generation: A glance from synchrotron radiation Bi L3-XAFS

For thermal to electric energy conversion, designing a high efficiency thermoelectric material entails simultaneously optimizing multiple properties of the material. Although it may appear simple to increase electrical power while minimizing thermal losses, the complicated link between these parameters makes optimization difficult, necessitating a more sophisticated approach. The one-pot zone melting method was used to fabricate undoped and Sb doped Bi2Te3 (Bi2−xSbxTe3; x = 0.0, 0.2, 0.6, and 1.0) in pellets form. X-ray diffraction (XRD), Raman and selected area electron diffraction (SAED) revealed rhombohedral polycrystalline nature and a unique high intensity of the Eg2 optical mode according to the nanosheet nature of the alloys as observed in the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. Besides, High Resolution TEM (HRTEM) micrographs depicted twin boundary effect with an angle of 120° for Bi1.8Sb0.2Te3 alloys, which may enhance the electronic transport and the thermoelectric properties of the fabricated compounds. To obtain selective and detailed local structural information of Bi2−xSbxTe3 matrix, synchrotron radiation-based X-ray absorption spectroscopy (XAS) measurements were performed at around Bi L3-edge. A significant influence of the Sb/Bi substitution on the local/atomic structure was observed through the gradual elongation of the average in-plane Bi–Sb bond distance. The combination of different off-line structural characterization techniques such as XRD, Raman, SEM, and HRTEM with synchrotron based XAS technique and Laser-Beam Deflection Spectroscopy (BDS) is necessary to fully describe the samples nature and to get average crystal, morphological and local/electronic structural information for unveiling the BiSbTe mechanism in thermoelectric generation.

Femtosecond laser processing and field emission properties of the FEAs on single crystal GdB6 (100) surface

The single crystal GdB6 were prepared by optical zone melting method. Moreover, using the femtosecond laser micromachining technology processed the field-emission tip arrays (FEAs) on the (100) surface of single crystal GdB6. The single crystal quality, field emission of the FEAs, as well as the electronic structure of single crystal GdB6 are systematically characterized. The quality of single crystal is good. The curvature radius of the produced FEAs with uniform morphology and undamaged crystal surface structure are about 0.5 μm. The FEAs showed good field emission performance, which the turn-on electric field of the FEAs is 2.6 V/μm with a field emission current of 0.46 A/cm2 at 7.3 V/μm and the emission current has high stability. The calculation electronic structure of GdB6 show that the 5d electron of Gd determines the electronic state of excellent emission performance. The high electric field will greatly decrease the surface barrier and work function of GdB6, according with the field emission theory.

Growth and Characterization of Large-Size Inse Single Crystal from Non-Stoichiometric Solution Via a Zone Melting Method

Growth and Characterization of Large-Size Inse Single Crystal from Non-Stoichiometric Solution Via a Zone Melting Method

Electronic structures and emission properties of typical binary single crystal REB<sub>6</sub>

Binary rare earth hexaborides (REB<sub>6</sub>) have different rare earth elements with different valence electron distributions, which lead to different strange physical properties and different emission properties. However, in the electron emission properties, whether PrB<sub>6</sub>, NdB<sub>6</sub>, SmB<sub>6</sub> and GdB<sub>6</sub> all have excellent emission properties remains to be further studied, and the physical mechanism affecting their emission properties needs investigating. In this paper, the electronic structures, work functions of typical binary single crystal REB<sub>6</sub> (LaB<sub>6</sub>, CeB<sub>6</sub>, PrB<sub>6</sub>, NdB<sub>6</sub>, SmB<sub>6</sub>, GdB<sub>6</sub>) are studied by first principles calculations. The single crystal REB<sub>6</sub> are prepared by optical zone melting method, and their thermionic electron emission properties are tested experimentally. The theoretical calculation results show that the typical binary REB<sub>6</sub> have large densities of states near the Fermi level. The d-orbitals with broad distributions in conduction bands are beneficial to electron emission. The localized f-orbital electrons in valence bands are not conducive to their electron emission. The theoretical calculations of work functions of typical binary single crystal REB<sub>6</sub> (100) surface are consistent with the analyses of their electronic structures. The theoretical calculation values of work functions are ordered as GdB<sub>6</sub> (2.27 eV) < CeB<sub>6</sub> (2.36 eV) < LaB<sub>6</sub> (2.40 eV) < PrB<sub>6</sub> (2.58 eV) < SmB<sub>6</sub> (2.63 eV) < NdB<sub>6</sub> (2.91 eV). The experimental test results of thermionic electron emission of single crystal show that the experimental thermionic electron properties are consistent with the theoretical ones. The LaB<sub>6</sub> and CeB<sub>6</sub> both have good thermionic and field emission properties, and the GdB<sub>6</sub> has excellent field emission properties.

Enhancing the Thermoelectric and Mechanical Properties of Bi0.5Sb1.5Te3 Modulated by the Texture and Dense Dislocation Networks.

Bi2Te3-based materials are dominating thermoelectrics for almost all of the room-temperature applications. To meet the future demands, both their thermoelectric (TE) and mechanical properties need to be further improved, which are the requisite for efficient TE modules applied in areas such as reliable micro-cooling. The conventional zone melting (ZM) and powder metallurgy (PM) methods fall short in preparing Bi2Te3-based alloys, which have both a highly textured structure for high TE properties and a fine-grained microstructure for high mechanical properties. Herein, a mechanical exfoliation combined with spark plasma sintering (ME-SPS) method is developed to prepare Bi0.5Sb1.5Te3 with highly improved mechanical properties (correlated mainly to the dislocation networks), as well as significantly improved thermoelectric properties (correlated mainly to the texture structure). In the method, both the dislocation density and the orientation factor (F) can be tuned by the sintering pressure. At a sintering pressure of 20 MPa, an exceptional F of up to 0.8 is retained, leading to an excellent power factor of 4.8 mW m-1 K-2 that is much higher than that of the PM polycrystalline. Meanwhile, the method can readily induce high-density dislocations (up to ∼1010 cm-2), improving the mechanical properties and reducing the lattice thermal conductivity as compared to the ZM ingot. In the exfoliated and then sintered (20 MPa) sample, the figure-of-merit ZT = 1.2 (at 350 K), which has increased by about ∼20%, and the compressive strength has also increased by ∼20%, compared to those of the ZM ingot, respectively. These results demonstrate that the ME-SPS method is highly effective in preparing high-performance Bi2Te3-based alloys, which are critical for TE modules in commercial applications at near-room temperature.

Ce Filling Limit and Its Influence on Thermoelectric Performance of Fe3CoSb12-Based Skutterudite Grown by a Temperature Gradient Zone Melting Method.

CoSb3-based skutterudite is a promising mid-temperature thermoelectric material. However, the high lattice thermal conductivity limits its further application. Filling is one of the most effective methods to reduce the lattice thermal conductivity. In this study, we investigate the Ce filling limit and its influence on thermoelectric properties of p-type Fe3CoSb12-based skutterudites grown by a temperature gradient zone melting (TGZM) method. Crystal structure and composition characterization suggests that a maximum filling fraction of Ce reaches 0.73 in a composition of Ce0.73Fe2.73Co1.18Sb12 prepared by the TGZM method. The Ce filling reduces the carrier concentration to 1.03 × 1020 cm−3 in the Ce1.25Fe3CoSb12, leading to an increased Seebeck coefficient. Density functional theory (DFT) calculation indicates that the Ce-filling introduces an impurity level near the Fermi level. Moreover, the rattling effect of the Ce fillers strengthens the short-wavelength phonon scattering and reduces the lattice thermal conductivity to 0.91 W m−1 K−1. These effects induce a maximum Seebeck coefficient of 168 μV K−1 and a lowest κ of 1.52 W m−1 K−1 at 693 K in the Ce1.25Fe3CoSb12, leading to a peak zT value of 0.65, which is 9 times higher than that of the unfilled Fe3CoSb12.

Impurity Removal Leading to High-Performance CoSb3-Based Skutterudites with Synergistic Carrier Concentration Optimization and Thermal Conductivity Reduction.

Thermoelectric properties of CoSb3-based skutterudites are greatly determined by the removal of detrimental impurities, such as (Fe/Co)Sb2, (Fe/Co)Sb, and Sb. In this study, we use a facile temperature gradient zone melting (TGZM) method to synthesize high-performance CoSb3-based skutterudites by impurity removal. After removing metallic or semimetallic impurities (Fe/Co)Sb, (Fe/Co)Sb2, and Sb, the carrier concentration of TGZM-Ce0.75Fe3CoSb12 can be reduced to 1.21 × 1020 cm-3 and the electronic thermal conductivity dramatically reduced to 0.7 W m-1 K-1 at 693 K. Additionally, removing these impurities also effectively reduces the lattice thermal conductivity from 7.2 W m-1 K-1 of cast-Ce0.75Fe3CoSb12 to 1.02 W m-1 K-1 of TGZM-Ce0.75Fe3CoSb12 at 693 K. As a consequence, TGZM-Ce0.75Fe3CoSb12 approaches a high power factor of 11.7 μW cm-1 K-2 and low thermal conductivity of 1.72 W m-1 K-1 at 693 K, leading to a peak zT of 0.48 at 693 K, which is 10 times higher than that of cast-Ce0.75Fe3CoSb12. This study indicates that our facile TGZM method can effectively synthesize high-performance CoSb3-based skutterudites by impurity removal and set up a solid foundation for further development.

THE REFINING OF TITANIUM BY CRUCIBLELESS ZONE MELTING METHOD

The physical substantiation of titanium refining by crucibleless electron beam zone melting (ZM) method in vacuum was presented. Calculations of the equilibrium, limiting and effective distribution coefficients for impurity elements in the base metal were carried out. The refining of titanium was studied experimentally, and samples with a purity of 99.92 wt.% were received. The total content of impurities was reduced by a factor of 1.5 (from 0.12 to 0.08 wt.%). The concentration of interstitial impurities was significantly reduced (oxygen – by 1.3; carbon – by 2; nitrogen – by 2.5 times). The structure and microhardness were investigated.

High temperature tensile strength of large size Al2O3/ZrO2(Y2O3) directionally solidified eutectic ceramics

Al2O3/ZrO2(Y2O3) directionally solidified eutectic ceramics (DSECs) with a diameter of 10 mm were prepared via the induction heating zone melting (IHZM) method, and Φ 5 mm cylindrical tensile specimens were processed. The effects of both Y2O3 content (1–12%) and the pulling rate (5–30 mm/h) on the tensile strength were investigated. With the increase of Y2O3 content and pulling rate, the tensile strength at 1500 °C of DSEC got the maximum of 167.1 MPa with 1% Y2O3 at 10 mm/h, and 94.3 MPa with 3% Y2O3 at 30 mm/h, respectively. Both high m-ZrO2 phase mass fraction and high intercolony area fraction are harmful to the tensile strength, while the grain refinement strengthening and whisker toughening can enhance the tensile strength.

Changes in the microstructure and mechanical properties of Al2O3/YSZ directionally solidified eutectic ceramics during long-time heat treatment

Al2O3/YSZ directionally solidified eutectic ceramics (DSECs) were prepared using the induction heating zone melting (IHZM) method. The microstructures of the DSECs formed with various crucible pulling rates (2.5–300 mm/h) changed from the discontinuous lamellar into the colony structure, finally into the cell structure. The intracolony had better microstructural stability than the intercolony during heat treatment due to two factors: the size effect and the t-m transformation effect. The hardness changes were mainly related to the release and generation of residual stress caused by repeated heating and cooling. The quantity of cracks in DSECs increased with the heat treatment time, leading to a gradual decrease in the fracture toughness, 300 mm/h DSEC had the lowest reduction in fracture toughness after 500 h heat treatment due to fine microstructure, the fracture toughness was 7.15 MPa·m1/2, which was 76 % of that before the heat treatment (9.39 MPa·m1/2).

High thermoelectric performance bismuth telluride prepared by cold pressing and annealing facilitating large scale application

Bi2Te3-based alloys, generally prepared by traditional zone melting or hot-pressing, have been used in commercial applications for decades. However, the zone melting method is time-consuming and usually accompanied by poor thermoelectric performance, while the hot-pressing sintering needs complex and high-cost equipment. In this work, high-performance p-type Bi0.4Sb1.6Te3 alloy is prepared by cold pressing the ball milled Bi0.4Sb1.6Te3 powder followed by vacuum annealing, which is more suitable for large-scale promoting with cost saving. Owing to the low lattice thermal conductivity brought by fine grains, a peak ZT value of ∼1.15 at 50 °C and an average ZT value of ∼0.84 within 30–250 °C are realized, which are comparable with the excellent performance achieved by zone melting and hot-pressing, and are even better than that of the optimized cold-pressed sample. More importantly, the thermoelectric performance here shows good repeatability from batch to batch. Our work makes the widespread application of Bi0.4Sb1.6Te3 alloy in thermoelectric power generation and refrigeration an important step forward.

Investigation on Low-Temperature Thermoelectric Properties of Ag2Se Polycrystal Fabricated by Using Zone-Melting Method.

Silver selenide, Ag2Se, is a promising low-temperature thermoelectric material which can be used to harvest the low-quality waste heat for electrical power generation or cool the microelectronics. Currently, the investigation on Ag2Se and its derivatives has become a hot topic in the thermoelectric community, but the thermoelectric properties of Ag2Se below 300 K have been rarely investigated. In this study, we prepared Ag2Se by using the zone-melting method. The electrical and thermal transport properties of zone-melted Ag2Se from 5 to 380 K were systematically investigated and compared with the previously reported data of Ag2Se and other typical low-temperature thermoelectric materials, such as Mg3Bi2, Bi2Te3, and BiSb. Ag2Se shows intrinsic semiconductor features, ultrahigh carrier mobility, small density-of-state effective mass, and ultralow lattice thermal conductivity. At 300 K, the zT of zone-melted Ag2Se is 0.75. This study will shed light on the further investigation of Ag2Se.

Effects of Solidification Conditions on Grain Refinement Capacity of TiC in Directionally Solidified Ti6Al4V Alloy

In this study, the effects of solidification conditions on the grain refinement capacity of heterogeneous nuclei TiC in directionally solidified Ti6Al4V alloy were investigated using experimental and numerical approaches. Ti6Al4V powder with and without TiC particles in a Ti6Al4V sheath was melted and directionally solidified at various solidification rates via the floating zone melting method. In addition, by using the phase field method, the microstructural evolution of directionally solidified Ti6Al4V was simulated by varying the temperature gradient G and solidification rate V. As the solidification rate increased, the increment of the prior β grain number by TiC addition also increased. There are two reasons for this: first, the amount of residual potent heterogeneous nuclei TiC is larger. Second, the amount of TiC particles that can nucleate becomes larger. This is because increasing the constitutional undercooling ΔTc leads to the activation of a smaller radius of heterogeneous nuclei and a higher nucleation probability from each radius. At a cooling rate R higher than that in the floating zone melting experiment (R = 3 to 1000 K/s), the maximum degree of constitutional undercooling ΔTc,Max has a peak value, which suggests that constitutional undercooling ΔTc has a smaller contribution at higher cooling rates, such as those that occur during electron beam melting (EBM), including laser powder bed fusion (LPBF).

Synergistic effects of B-In codoping in zone-melted Bi0.48Sb1.52Te3-based thermoelectric

Bismuth-telluride-based alloys are the unique commercial thermoelectric materials near room temperature, and the commonly used zone-melting method is efficient for the mass production. Since the ZTs of zone-melted samples has stagnated around 1.2 for decades, it is urgent for the scientific and industrial societies to break through this bottleneck. Herein, we report simultaneously improved power factor and ZT values in p-type Bi0.48Sb1.52Te3 materials by the addition of amorphous boron and metal indium. B and In dopants not only effectively increase the hole concentration and the density of state effective mass, leading to a high power factor of 55 μW cm−1 K−2, but also introduce various phonon scattering centers to obviously suppress the lattice thermal conductivity. The synergistic effects yield a ZTmax of 1.45 at 350 K, as well as a ZTave of 1.20 and σS2ave of 41 μW cm−1 K−2 (300 ~ 500 K) in the Bi0.48Sb1.515In0.005Te3 + 0.6 wt% B sample, suggesting tiny B-In codoping is a promising strategy for the further development of bismuth telluride thermoelectrics.

The Features of Electronic Conduction in InAs

The electrical properties of n-type crystals of InAs compound, grown from stoichiometric melt by the horizontal zone melting method, have been investigated in the temperature range of 4.2 K-300 K before and after fast neutron irradiation up to high integral fluences of 2×1018n?cm-2. At a fixed temperature electrons concentration (n) increases almost by one order during irradiation, and practically does not change with increasing of temperature. n increases only slightly by increasing of temperature near 300 K, both before and after irradiation. When ? 4×1018cm-3 the change of during irradiation is negligible. Comparison of experimental data of mobility with theory shows that the privileged scattering mechanism of electrons at 300 K is scattering on optical phonons in InAs with 1016-1017 cm-3 and scattering on ions of impurity in InAs with n~1018-1019 cm-3. The analysis shows that during irradiation point type scattering centers of donor-type structural defects with shallow levels in the forbidden zone appear. Consequently, the mobility decreases during irradiation. At 300 K in sample with electrons concentration of 3×1016 cm-3 the mobility decreases by 5 times after irradiation, which is equivalent to the formation of 1.5×1019cm-3 charged point scattering centers.

The Features of Electronic Conduction in InAs

The electrical properties of n-type crystals of InAs compound, grown from stoichiometric melt by the horizontal zone melting method, have been investigated in the temperature range of 4.2 K-300 K before and after fast neutron irradiation up to high integral fluences of 2×1018n∙cm-2. At a fixed temperature electrons concentration (n) increases almost by one order during irradiation, and practically does not change with increasing of temperature. n increases only slightly by increasing of temperature near 300 K, both before and after irradiation. When ≥ 4×1018cm-3 the change of during irradiation is negligible. Comparison of experimental data of mobility with theory shows that the privileged scattering mechanism of electrons at 300 K is scattering on optical phonons in InAs with 1016-1017 cm-3 and scattering on ions of impurity in InAs with n~1018-1019 cm-3. The analysis shows that during irradiation point type scattering centers of donor-type structural defects with shallow levels in the forbidden zone appear. Consequently, the mobility decreases during irradiation. At 300 K in sample with electrons concentration of 3×1016 cm-3 the mobility decreases by 5 times after irradiation, which is equivalent to the formation of 1.5×1019cm-3 charged point scattering centers.

Thermal dependent properties of LaB6–MeB2 eutectic composites

The thermal expansion of polycrystalline LaB6–MeB2 ceramic composites (where Me denotes Ti, Zr, Hf) has been studied experimentally within the temperature range from 20 °C to 1500 °C. These ceramic composites were obtained by crucible-free float zone melting method. The first temperature interval with a negative thermal elongation (NTE) (~1000–1100 °C) coincided in the LaB6–MeB2 composites and LaB6 hexaboride and was associated with La–La atomic bond softening. The hierarchy of the interatomic Me–Me–B–B bonds in the LaB6–MeB2 composite compounds, their interface and residual stresses in the LaB6 matrix and the MeB2 reinforcing phase were identified to be the source of such phenomenon. The strong covalent component of interatomic bonds compresses the crystal lattice of the composites after softening ionic and metallic components and removing residual stresses. The composites exhibited several additional NTE intervals at a higher temperature (~1000–1250°С) which were associated with the of interatomic bonds softening in the TiB2, HfB2, ZrB2 reinforcing fibers. NTE intervals determines transition of the deformation mechanism of composites from low-temperature to high-temperature one. Insignificant plasticity and brittle fracture in the area of low-temperature deformation is replaced with an increase in both plasticity and strength with temperature rise.

Microstructural characterization of arc-melted and directionally solidified near-eutectic molybdenum–silicon–boron alloys

Samples of near-eutectic Mo–Si–B alloys, with the nominal composition corresponding to the Moss + Mo3Si + T2 eutectic area, were prepared by arc-melting or by crucible-free zone-melting methods (directional solidification). In the directionally solidified (DSed) alloy, the volume fraction of the Moss + Mo3Si + T2 eutectic cells appeared to increase compared with that of arc-melted (AMed) alloy, and well-aligned and rod-shaped Moss and T2 phases were formed in a Mo3Si matrix of the eutectic cells. In the AMed alloy, only the T2 phase showed a <100] preferential growth orientation along the solidification direction, whereas the Moss phase displayed no obvious preferred orientation. The Mo3Si matrix showed a weak preference toward <001> growth along the solidification direction in the AMed alloy, and this became much stronger in the DSed alloy. Furthermore, the DSed alloy exhibited an apparent preferential growth orientation relationship of <001>Moss//<001>Mo3Si//<100]T2 along the solidification direction. However, no specific atomic structures were observed at the interfaces, indicating that the three phases grew independently from the liquid without the formation of any habit interface during solidification.