Crystalline Ingots Research Articles

Characteristics of TlBr single crystals grown using the vertical Bridgman-Stockbarger method for semiconductor-based radiation detector applications

AbstractTlBr single crystals grown using the vertical Bridgman-Stockbarger method were characterized for semiconductor based radiation detector applications. It has been shown that the vertical Bridgman-Stockbarger method is effective to grow high-quality single crystalline ingots of TlBr. The TlBr single crystalline sample, which was located 6 cm from the tip of the ingot, exhibited lower impurity concentration, higher crystalline quality, high enough bandgap (>2.7 eV), and higher resistivity (2.5 × 1011Ω·cm) which enables using the fabricated samples from the middle part of the TlBr ingot for fabricating high performance semiconductor radiation detectors.

AgI alloying in SnTe boosts the thermoelectric performance via simultaneous valence band convergence and carrier concentration optimization

SnTe, a Pb-free analogue of PbTe, was earlier assumed to be a poor thermoelectric material due to excess p-type carrier concentration and large energy separation between light and heavy hole valence bands. Here, we report the enhancement of the thermoelectric performance of p-type SnTe by Ag and I co-doping. AgI (1–6mol%) alloying in SnTe modulates its electronic structure by increasing the band gap of SnTe, which results in decrease in the energy separation between its light and heavy hole valence bands, thereby giving rise to valence band convergence. Additionally, iodine doping in the Te sublattice of SnTe decreases the excess p-type carrier concentration. Due to significant decrease in hole concentration and reduction of the energy separation between light and heavy hole valence bands, significant enhancement in Seebeck coefficient was achieved at the temperature range of 600–900K for Sn1−xAgxTe1−xIx samples. A maximum thermoelectric figure of merit, zT, of ~1.05 was achieved at 860K in high quality crystalline ingot of p-type Sn0.95Ag0.05Te0.95I0.05.

A first step toward bridging silicon carbide crystal properties and physical chemistry of crystal growth

The links between the occurrence of a given silicon carbide (SiC) polytype and crystal growth conditions have been mainly empirically described. Although studies are a little more advanced, a similar description can be applied to point defects and especially dopants in the crystals. We propose a method to determine such links, based on a coupled thermodynamic and mass transport model, where SiC is treated as a solid solution. We implemented such an approach to the case of the seeded sublimation growth process, which is currently the industrial bulk growth process for SiC single crystalline ingots. The computation of both Si and C activities in the SiC crystal, between the SiC–C and SiC–Si two-phase equilibria, allowed first the assessment of the SiC crystal chemistry and second the linking of SiC–C and SiC–Si chemistry with the growth process parameters. We demonstrate that depending on the temperature, pressure and a temperature gradient, the activities of Si and C in the crystals can be described and tuned. The case of cubic polytype (3C-SiC) formation is specifically discussed. Its stabilization requires a high Si activity and a low C activity in the SiC crystal, meaning that the synthesis conditions must be close to the limit at the SiC + Si phase boundary.

Amorphization of Ni61Nb39 Alloy by Laser Surface Treatment

The surface of Ni61 Nb39 crystalline ingot was treated by laser surface melting with different processing parameters. A fully amorphous layer with a thickness of approximately 10 μm could be produced on the top surface under optimal parameters. An amorphous-crystalline composite layer with the depth from 10 to 50 μm, consisting of amorphous matrix and intermetallic phases of Ni3Nb and Ni6Nb7, could be formed. The micro-hardness (about 831 HV) of the treated surface was remarkably improved by nearly 100% compared with the value of the crystalline substrate caused by the formation of the fully amorphous structure. A finite volume simulation was adopted to evaluate the temperature distribution in the laser-affected zone of Ni61 Nb39 alloys and to reveal the mechanism of glass formation in the laser-affected zone.

Magneto-Optical Study of Martensitic Transition in Ni<sub>45</sub>Mn<sub>36.7</sub>In<sub>13.3</sub>Co<sub>5 </sub>(at. %) Single Crystals

Transverse Kerr effect (TKE) was used to study magneto-optical (MO) properties of Ni45Mn36.7In13.3Co5 (at.%) single crystals. A single crystalline ingot of such composition was grown by the Czochralski method. One series of samples was quenched into cold water (WQ) and the other series after quenching was heated at 770 K for 20 min and slowly cooled to assure a complete atomic order (SC). Accordingly to differential scanning calorimetry (DSC), magnetic and magneto-optical (MO) measurements, WQ samples exhibit well-defined martensitic transition (MT), but the SC samples do not show MT. It is found that TKE for WQ samples shows the following features (i) MO signal is well pronounced far below the martensitic transition in spite of a weak magnetization of martensitic phase; (ii) the characteristic temperatures of martensitic transition differ from those for the bulk and depend on annealing conditions; (iii) MO spectra profile do not change significantly during the martensitic transition and is similar but not identical with that for Ni50Mn35In15 thin films and Ni43.7Mn43.6In12.7 ribbons measured before; (iv) MO signal is anisotropic.

Mg Alloying in SnTe Facilitates Valence Band Convergence and Optimizes Thermoelectric Properties

SnTe, a lead-free rock-salt analogue of PbTe, having valence band structure similar to PbTe, recently has attracted attention for thermoelectric heat to electricity generation. However, pristine SnTe is a poor thermoelectric material because of very high hole concentration resulting from intrinsic Sn vacancies, which give rise to low Seebeck coefficient and high electrical thermal conductivity. In this report, we show that SnTe can be optimized to be a high performance thermoelectric material for power generation by controlling the hole concentration and significantly improving the Seebeck coefficient. Mg (2–10 mol %) alloying in SnTe modulates its electronic band structure by increasing the band gap of SnTe and results in decrease in the energy separation between its light and heavy hole valence bands. Thus, solid solution alloying with Mg enhances the contribution of the heavy hole valence band, leading to significant improvement in the Seebeck coefficient in Mg alloyed SnTe, which in turn results in remarkable enhancement in power factor. Maximum thermoelectric figure of merit, ZT, of ∼1.2 is achieved at 860 K in the high quality crystalline ingot of p-type Sn0.94Mg0.09Te.

Thermoelectric properties and violation of the Wiedemann-Franz law in Bi2−x Cu x Se3 (x ≤ 0.1)

We investigated the thermoelectric properties of melted crystalline ingots and hot-press sintered compounds of Bi2−xCuxSe3 (x ≤ 0.1). The divalent cation substitution of the Cu-ion at the Bi-site increases the carrier densities because of hole doping, resulting in a decrease in Seebeck coefficient due to electron-hole mixing. The thermal conductivity of Cu-doped compounds is lower than that of the pristine Bi2Se3 compound. When we obtain the lattice thermal conductivity κph from the Wiedemann-Franz law, it shows almost zero or negative lattice thermal conductivity. This indicates that the Wiedemann-Franz law is violated in the highly Cu-doped Bi2−xCuxSe3 (x = 0.05 and 0.1) compounds. The violation of the Wiedemann-Franz law implies underlying exotic physical properties for these compounds.

Lead-free thermoelectrics: promising thermoelectric performance in p-type SnTe1−xSex system

Lead chalcogenides are the best performers for thermoelectric power generation at mid/high temperatures; however, environmental concern about Pb prevents its use in large-scale thermoelectric applications. SnTe, a Pb-free IV–VI narrow band gap semiconductor, has the potential to be a good thermoelectric material due to its crystal structure and valence band characteristics being similar to those of PbTe. Here, we report the promising thermoelectric performance in high quality crystalline ingots of In-doped SnTe1−xSex (x = 0–0.15) synthesized by a simple vacuum sealed tube melting reaction. First, we have optimized the lattice thermal conductivity of SnTe by solid solution alloying with SnSe. Resonance level formation in the valence band through In doping along with the increase in the contribution of the heavy hole valence band through solid solution alloying significantly improved the Seebeck coefficient, resulting in a promising ZT of ∼0.8 at 860 K in the Pb-free p-type 1.5 mol% In doped SnTe0.85Se0.15 sample.

Effect of dopant Cr ions on the dielectric properties of melt-grown ZnSe crystals

The frequency and temperature dependences of the real and imaginary parts of the permittivity of Cr-doped (nCr ≅ 1018 cm−3) melt-grown ZnSe crystals have been measured in the low-frequency region. It has been established that such doping reduces the heterogeneity of the dielectric properties and the level of energy losses of ac electric field in a crystalline ingot. The effect of dopant atoms on the dielectric properties has been explained as being caused by formation of defect associates with participation of these atoms and intrinsic crystal defects.

Key Properties of NiMnGa Based Single Crystals Grown with the SLARE Technique

AbstractMagnetic shape memory alloys (MSMAs), exhibit large strains and hence are materials, which could substitute giant magnetostrictive and piezoelectrical materials in actuating devices. The actuation stress needed to induce the strain is much lower than in other actuator materials. Since the strain can be induced without phase transformation by a magnetic field, the development of actuators with high working frequencies is possible. However, for reasonable applications, large strains have to be induced with small magnetic fields. Up to now repeatable magnetically induced strains of 5–10% in magnetic fields of less than 500 mT have been achieved only in single crystals. The production of NiMnGa based single crystals is difficult and time consuming. The crystal quality is affected by porosity and impurities. With the Bridgeman based method called Slag Remelting and Encapsulation (SLARE) single crystalline ingots of NiMnGa, NiMnGaFe, and NiMnGaCo of high quality were grown and characterized. The results show that MSMA properties depend on the position within the single crystalline rods due to a composition gradient. The influence of surface treatment demonstrates that the decrease of surface roughness leads to a decrease of twinning stress. MSMAs with twinning stresses above 1 MPa show a magnetic field induced strain (MFIS) when tilting is not restricted by constraints. Softer samples can adapt to constraints much better and show large MFIS. Substituting Ni by Fe and Co, shifted the phase transitions successfully to higher temperatures. NiMnGa alloyed with up to 6 at.% Co showed three different martensite structures: a non‐modulated tetragonal structure, a modulated tetragonal structure, showing the same behavior as NiMnGa with identical structures and a non‐modulated orthorhombic structure with a stress–strain‐behavior explainable by the double twinning mechanism.

Enhanced magnetocaloric effect of a partially crystalline Gd 55Al 20Ni 25 bulk metallic glass

In this work, the magnetic properties and microstructure of a Gd 55Al 20Ni 25 bulk metallic glass (BMG) heat-treated in the supercooled liquid region are studied. The as-cast rod annealed at the onset temperature of glass transition exhibits amorphous characteristics, and the magnetic properties similar to the as-cast rod. While the partially crystalline rod prepared by heating the as-cast rod to a temperature slightly lower than the onset of crystallization temperature and then rapidly cooling to room temperature shows enhancement of the peak value of the magnetic entropy change. The enhanced magnetocaloric effect of the partially crystalline rod was anomalous because the fully crystalline ingot exhibits much lower peak values of magnetic entropy change, indicating the possible formation of some metastable intermediate phases during nucleation and growth. The existence of some metastable phases was observed by careful microstructural investigations, and their formation mechanisms and magnetic properties were studied.

Study on continuous casting of bulk metallic glass

Continuous casting method for massive production of steel, aluminum, copper or other crystalline alloy ingot is a very important industrial technology for its low energy consumption and high productivity. In this study, a new continuous casting method was developed for the massive production of bulk metallic glass ingot with centimeter-scale diameter without length limitation. An intermittent withdrawal procedure was practiced for continuous casting of bulk glassy alloy. The new developed continuous casting method can provide a cooling speed as high as that provided by the stationary mold casting method. A Zr-based glassy alloy rod with a diameter of about 10 mm and length of tens of centimeters was prepared by the continuous casting method for the first time.

Thermoelectric properties of rapid hot pressed polycrystalline Ag1−xPb18SbTe20 synthesized from doping PbTe nanocrystals

AbstractPolycrystalline thermoelectric (TE) materials of n‐type and with compositions of Ag1−xPb18SbTe20 (x = 0, 0.1, 0.2) were prepared by rapid hot pressing from alloy powders, which were obtained by ball milling crystalline ingots. The ingots were synthesized from PbTe nanoparticles, made by solvothermal reactions, and Ag, Sb and Te powders via vacuum‐sealed melting in quartz ampoules. Through rapid hot pressing at 748 K for 15 min under 80 MPa, the polycrystalline samples possessed 95% relative densities with the average grain size of 5 µm. The electrical resistivity, Seebeck coefficient and thermal conductivity of the Ag1−xPb18SbTe20 (x = 0, 0.1, 0.2) samples were measured in the range from room temperature (RT) to 673 K. The thermal conductivity was dramatically decreased, caused by scattering of phonons from grain boundaries and interfaces. By optimizing the values of carrier concentration and carrier transference, the Ag1−xPb18SbTe20 (x = 0.2) sample achieved a maximum ZT value of 1.019 at 673 K.

Composition and thermoelectric power factor variation of (Bi 2Te 3) 0.96(Bi 2Se 3) 0.04 crystal in growth direction

(Bi 2Te 3) 0.96(Bi 2Se 3) 0.04 crystal, which is an n-type thermoelectric semiconductor, has many applications in thermoelectric cooling systems. Single crystal of this composition was grown by Traveling Heater Method. A sensible gradient in thermoelectric power factor was observed in the first quarter length of the prepared crystalline ingot. Characterizing the crystallization procedure and ingot composition, the gradient was attributed to the variation of the Bi 2Se 3 concentration of Bi 2Te 3–Bi 2Se 3 quasi-binary solid solution system. The structural properties were characterized by means of XRD analyses. Results of composition variation (Bi 2Se 3 distribution function) were in good correlation with experimental thermoelectric power factor measured along the grown rod.

Characterization of single crystalline ZnTe and ZnSe grown by vapor phase transport

Tubular furnaces were designed and built to obtain single crystalline ZnTe and ZnSe ingots using respectively physical and chemical transport methods. Different temperature profiles and growth rates were analyzed in order to optimize the necessary crystalline quality for device development.Optical and scanning electron micrographs of the corrosion figures produced by chemical etching were used to obtain the dislocation density and the misorientation between adjacent subgrains in ZnTe and ZnSe wafers. Structural quality of the single crystalline material was determined by transmission electronic microscopy. Optical transmittance was measured by infrared transmission spectrometry and the resulting values were compared to commercial samples.

Recent Advances in Bulk Crystal Growth

AbstractThe growth of high-quality single crystals remains a challenging endeavor of materials science. Crystals of suitable size (from fiber crystals with diameters of tens of micrometers up to crystalline ingots or blocks with volumes up to 1 m3) and perfection (free from precipitates, inclusions, and twins with good uniformity and low concentration of dislocations) are required for fundamental research and practical implementation in microelectronic circuits, electro-optic switches and modulators, solid-state lasers, light-emitting diodes, sensors, and many other devices. In this introductory article of this issue of MRS Bulletin, we describe the two main challenges of today's crystal growth, namely (1) the production of well-established crystalline materials with improved structural perfection and larger size at a lower cost and (2) the bulk growth of new categories of materials with extreme thermodynamic characteristics, such as a very high melting point, high melting dissociation pressure, incongruent phase diagram, and anisotropic segregation. The subsequent six articles provide examples of how the crystal growers took up these challenges, which led to new experimental approaches and technological advances.

Enhancement of Thermoelectric Figure-of-Merit by a Nanostructure Approach

AbstractThe dimensionless thermoelectric figure-of-merit (ZT) in bulk materials has remained about 1 for many years. Here we show that a significant ZT improvement can be achieved in nanocrystalline bulk materials. These nanocrystalline bulk materials were made by hot-pressing nanopowders that are ball-milled from either crystalline ingots or elements. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, the nanostructure approach has been demonstrated in a few thermoelectric material systems, proving its generosity. The approach can be easily scaled up to multiple tons. Thermal stability studies have shown that the nanostructures are stable at the application temperature for an extended period of time. It is expected that such enhanced materials will make the existing cooling and power generation systems more efficient.

Enhanced Thermoelectric Figure-of-Merit in p-Type Nanostructured Bismuth Antimony Tellurium Alloys Made from Elemental Chunks

By ball milling alloyed bulk crystalline ingots into nanopowders and hot pressing them, we had demonstrated high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this study, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 in the temperature range of 75 and 100 degrees C has been achieved. This process is more economical and environmentally friendly than starting from alloyed bulk crystalline ingots. The ZT improvement is caused mostly by the lower thermal conductivity, similar as the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the increased grain boundaries of the nanograins, precipitates, nanodots, and defects. Our material also exhibits a ZT of 0.7 at 250 degrees C, similar to the value obtained when ingot was used. This study demonstrates that high ZT values can be achieved in nanostructured bulk materials with ball milling elemental chunks, suggesting that the approach can be applied to other materials that are hard to be made into ingot, in addition to its advantage of lower manufacturing cost.

High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys

The dimensionless thermoelectric figure of merit (ZT) in bismuth antimony telluride (BiSbTe) bulk alloys has remained around 1 for more than 50 years. We show that a peak ZT of 1.4 at 100 degrees C can be achieved in a p-type nanocrystalline BiSbTe bulk alloy. These nanocrystalline bulk materials were made by hot pressing nanopowders that were ball-milled from crystalline ingots under inert conditions. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, ZT is about 1.2 at room temperature and 0.8 at 250 degrees C, which makes these materials useful for cooling and power generation. Cooling devices that use these materials have produced high-temperature differences of 86 degrees , 106 degrees , and 119 degrees C with hot-side temperatures set at 50 degrees, 100 degrees, and 150 degrees C, respectively. This discovery sets the stage for use of a new nanocomposite approach in developing high-performance low-cost bulk thermoelectric materials.

Hot-pressed (SmFe)–(FeAlPCBSiGa) composites: Influence of preparation conditions

Ball-milling of amorphous ribbons and crystalline ingots and subsequent hot pressing of the resulting powders were used to prepare disc-shaped (rare earth–iron)–(soft magnetic material) composites (diameter of 10 mm and thickness of 1 mm). Amorphous FeAlPCBSiGa was used as a soft magnetic material and SmFe 2 as the (rare earths–iron)-phase. We investigated the influence of different ball-milling conditions and different ratios of the high magnetostrictive (SmFe 2) and the soft magnetic components (FeAlPCBSiGa) on the magnetic properties of ball-milled powders as well as the final disc-shaped samples. In the case of compacted samples we found a variation of the magnetostriction ranging from 50 to more than 430 ppm, coercivity changed from 9 to 16 kA m −1 and saturation magnetization from 88 to 130 A m 2 kg −1. A maximum magnetostrictive susceptibility of 930 ppm T −1 at a field of 0.1 T was found.