Floating Zone Melting Research Articles

Unveiling crystallographic texture in laser floating zone melted Al2O3/YAG eutectic ceramic by seed-crystal inducing

This study demonstrated the unique crystallographic features in Al2O3/YAG eutectic by seed-crystal inducing in laser floating zone melting (LFZM) process for crystallographic orientation control. The as-fabricated eutectic ceramics induced by different single crystalline seeds were mainly composed of randomly oriented Al2O3 faceted particles and irregular eutectic, as a result of the initial uncoupled growth at the junction of seed and eutectic. Specifically, along the direction of directional solidification, the eutectic ceramics grew stably with a single crystallographic orientation relationship with low planar disregistry (2 %∼6 %). The stability of phase interface with minimum interfacial strain and better ionic charge balance in Al2O3/YAG eutectic has a greater effect on the regulation of the eutectic growth orientation than the constraint of the single crystalline seed.

Preparation, Deformation Behavior and Irradiation Damage of Refractory Metal Single Crystals for Nuclear Applications: A Review.

Refractory metal single crystals have been applied in key high-temperature structural components of advanced nuclear reactor power systems, due to their excellent high-temperature properties and outstanding compatibility with nuclear fuels. Although electron beam floating zone melting and plasma arc melting techniques can prepare large-size oriented refractory metals and their alloy single crystals, both have difficulty producing perfect defect-free single crystals because of the high-temperature gradient. The mechanical properties of refractory metal single crystals under different loads all exhibit strong temperature and crystal orientation dependence. Slip and twinning are the two basic deformation mechanisms of refractory metal single crystals, in which low temperatures or high strain rates are more likely to induce twinning. Recrystallization is always induced by the combined action of deformation and annealing, exhibiting a strong crystal orientation dependence. The irradiation hardening and neutron embrittlement appear after exposure to irradiation damage and degrade the material properties, attributed to vacancies, dislocation loops, precipitates, and other irradiation defects, hindering dislocation motion. This paper reviews the research progress of refractory metal single crystals from three aspects, preparation technology, deformation behavior, and irradiation damage, and highlights key directions for future research. Finally, future research directions are prospected to provide a reference for the design and development of refractory metal single crystals for nuclear applications.

Insight into TaSi2 Nanostructures with Different Morphologies and Their Field Emission Properties

AbstractRecognizing the strong potential of cold cathodes for important commercial applications in fields such as electronics, there is a growing interest in the exploration of novel 1D nanomaterials. Among various cold cathode materials, TaSi2 is of great interest for its outstanding field emission performance. In this work, the Si‐TaSi2 eutectic composite with nano‐sized highly oriented TaSi2 fibers and semi‐coherent phase interfaces is prepared by the laser floating zone melting technique with a very high‐temperature gradient of 6000 K cm−1 at a solidification rate of 200 µm s−1. On the basis of directionally solidified Si‐TaSi2 eutectic composite, well‐aligned TaSi2 nanorod and nanotip arrays are fabricated by inductively coupling plasma (ICP) etching process and HNO3/HF wet etching process, respectively. The field emission measurements show that the field enhancement factor, turn‐on electric field, and effective work function are strongly affected by tip morphologies. The TaSi2 array with regular nanotip structure possesses the best field emission characteristic among all TaSi2 nanostructures, with a relatively low turn‐on field of 4.8 V µm−1 and a high current density of 733 µA cm−2. These findings preliminarily establish a clear relationship between the performance and structure of the array, providing technical guidance for the application of this material in electronic devices.

Compressive Property and Toughening Mechanism of Directionally Solidified Fe(Al, Ta)/Fe2Ta (Al) Eutectic Composites

Herein, Fe(Al, Ta)/Fe2Ta(Al) eutectic composites are prepared using the electron beam floating zone melting directional solidification technique. The compression and fracture toughness properties of Fe(Al, Ta)/Fe2Ta(Al) eutectic composites at different solidification rates are studied with compression and three‐point bending experiments. The toughening mechanism of the material is also analyzed from morphology, composition, and microstructure. The results show that the yield strength and compression properties of the Fe(Al, Ta)/Fe2Ta(Al) eutectic composite are increased with the increase of the solidification rate. The room‐temperature fracture toughness reaches 30.5 MPa m1/2 when the solidification rate is 4 mm min−1. In addition, the material exhibits high strength performance at room temperature. However, it exhibits poor strength and excellent plasticity due to the softening effect of recrystallization at 600 °C. The toughening mechanism of the Fe(Al, Ta)/Fe2Ta(Al) eutectic composites can be summarized as crack blunting, crack branching, crack deflection, and local deformation.

A direct evidence for lattice selective rotation of molybdenum alloys single crystals in tensile deformation

The Mo-Nb-W-Zr quaternary single crystals were prepared using the electron beam floating zone melting technique. Samples fractured under tension were sectioned along the axial direction. The results revealed an absence of the single crystal-to-polycrystal transition during the tensile process. However, there was a noticeable alteration in crystal orientation between regions near and far from the fracture site. Upon analyzing three distinct regions, it was observed that the axial crystal plane index gradually shifted from the original (-1–11) towards the direction of (-110), passing through (-2–21), (-3–31), and (-4–41). The fracture occurred precisely when the crystal plane twisted to this (-110) direction. This implies that {110} represents a lattice-selective rotation for molybdenum-based single crystal torsion when subjected to stretching along the <111> direction.

Insight into faceted-nonfaceted transition of directionally solidified eutectic ceramic composites by laser floating zone melting and infrared imaging

Faceted-nonfaceted transition was found during directional solidification of an Al2O3/Y3Al5O12(YAG)/ZrO2 eutectic ceramic composite at the pulling rate over 260 μm/s by laser floating zone melting. This transition, for the first time, optimized the colony structure of faceted eutectic composite materials, resulting in a refined and homogeneous microstructure consisting of both rod and lamella. Infrared imaging was employed to uncover temperature field evolution of the melt during the transition relying on an interesting measurement calibration strategy. In addition to an in-depth discussion about this evolution, we revealed that the transition was triggered by a critical undercooling at the solid-liquid interface. A 3D model of this novel eutectic structure was established, by which the mechanism of structural optimization achieved by the transition was discussed. We supposed that the growth orientation with the smallest index could firstly occur faceted-nonfaceted transition under a certain undercooling degree, and subsequently predominated the growth of faceted eutectic composite, which contributed to the optimization of faceted eutectic colony structure.

Determination on the work function of NdB6 (1 0 0) crystal surface by both theory and experiment

In this paper, the work function Φ of the NdB6 (100) crystal surface have been determined by both theory and experiment. By the first-principles calculation, the Φ of NdB6 (100), (110), (111) and (210) crystal surfaces obey the relation of Φ(100) < Φ(210) < Φ(110) < Φ(111), and the theoretical value of Φ for (100) crystal surface is calculated to be 2.791 eV. The UPS measurement and the experiments of thermionic emission have been performed on the high-quality [100] NdB6 single crystal, which is prepared by the method of two optical floating zone melting passes. The Φ of the NdB6 (100) crystal surface has been calculated to be 2.806 eV by the UPS measurement, while the average Φeff of the NdB6 (100) crystal surface is calculated to be 2.813 ± 0.109 eV. The good agreement between theoretical and experimental results indicates that the Φ of the NdB6 (100) crystal surface can be determined to be around 2.80 eV. Furthermore, the narrow Φ distribution range and the low FWHM extracted from the Miram curves and PWFD plots suggest the good emission uniformity of the NdB6 (100) crystal surface.

Comprehensive microstructure regularization mechanism and microstructure–property stability at 1773 K of directionally solidified Al2O3/GdAlO3 eutectic ceramic composite

Melt grown Al2O3/GdAlO3(GAP) eutectic ceramic composite is considered as a promising ultra–high temperature structural material in fields as aerospace, power generation and so on. In order to tune the properties of directionally solidified Al2O3/GAP eutectic ceramic composite, its microstructure evolution is investigated over a wide range of compositions and solidification rates by laser floating zone melting method (LFZM). With the increase of solidification rate and the enrichment of Al2O3 phase, the eutectic microstructure undergoes a transformation from “Chinese script” irregular morphology to rod–like regular and complex regular morphology. The synergistic effects of composition and solidification rate on the microstructure regularization have been quantitatively characterized by the proportion of regular eutectic. High solidification rate (>200 μm/s) is beneficial to the formation of isothermal interface, partly inhibiting the anisotropic growth of Al2O3 faceted phase, which further promotes the coupling growth of the eutectic phases at the front of the interface and facilitates the microstructure regularization. The stability of eutectic spacing is verified to coincide with the Ostwald ripening relationship when the eutectic ceramic composite is thermally exposed at 1773 K for long–term (250 h). The microstructure coarsening rate is less than 0.003 μm/h, and the hardness slightly decreases about 4%, and the fracture toughness remains almost unchanged, which indicates excellent thermal stability of microstructure and property for the Al2O3/GAP eutectic ceramic composite at high temperature.

Number of optical floating zone melting passes dependence of the LaB6 single crystals

LaB6 materials, especially single crystals, are a kind of ideal cathode materials, however, the number of optical floating zone melting passes dependence of the LaB6 single crystals are still unknown. In this paper, the effect of the number of optical floating zone melting passes on the crystal quality, purity and properties of LaB6 single crystals has been investigated. It was found that a truly [100] LaB6 single crystal consisting of no sub-grains or twin crystals was successfully grown regardless of how many zone melting passes. The crystal quality and purity have been verified to be gradually improved with the increase of the number of zone melting passes. It was particularly notable that there were very small differences in the content of impurity elements between as-grown crystals after two or three zone melting passes. The excellent performances of J1kV = 42.22 A/cm2, Jo = 28.99 A/cm2 and Φeff = 2.66 eV were belonged to LaB6 single crystal after two zone melting passes, which indicates that the method of two optical floating zone melting passes is the most suitable at present for the growth of high-quality, high-purity and high-performance LaB6 single crystals.

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.

Collaborative enhancement of luminous efficacy and fracture toughness based on interface design of Al2O3/YAG:Ce3+ eutectic phosphor ceramic grown by laser floating zone melting

A solid-state light source of directionally solidified Al2O3/Y3Al5O12:Ce3+ (YAG:Ce3+) eutectic phosphor ceramic with superior luminous efficacy (∼116.48 lm/W) and high fracture toughness (∼4.0 MPa·m1/2) is fabricated by laser floating zone melting (LFZM) for white light emitting diodes (WLEDs). The CeO2-doped Al2O3/YAG eutectic ceramic presents improved fracture toughness and prevents from the pore formation as compared to the undoped eutectic ceramic. Impressively, the doping of Ce element and the diffusion coefficient play dominant roles in accelerating the formation of cellular structure and causing the enlargement of the eutectic lamella spacing, respectively. Moreover, the optimal phase interface design act as scattering center and enhance the light conversion efficiency. The maximum luminous efficacy is achieved under the comprehensive investigation of the eutectic structure, phase interface as well as Ce3+ dopant. Moreover, the eutectic phosphor ceramic also exhibits excellent capability of thermal self-managing. The as-designed Al2O3/Y3Al5O12:Ce3+ eutectic phosphor ceramics have great potential as structural and functional materials in the high-brightness application of WLEDs.

Preparation of Macrocrystalline Tungsten-Based Alloys by Electron-Beam Floating Zone Melting

Preparation of Macrocrystalline Tungsten-Based Alloys by Electron-Beam Floating Zone Melting

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).

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.

Nanodomain structure of single crystalline nickel oxide

In this work we present a comprehensive study of the domain structure of a nickel oxide single crystal grown by floating zone melting and suggest a correlation between point defects and the observed domain structure. The properties and structure of domains dictate the dynamics of resistive switching, water splitting and gas sensing, to name but a few. Investigating the correlation between point defects and domain structure can provide a deeper understanding of their formation and structure, which potentially allows one to tailor domain structure and the dynamics of the aforementioned applications. A range of inhomogeneities are observed by diffraction and microscopy techniques. X-ray and low-energy electron diffraction reveal domains on the submicron- and nanometer-scales, respectively. In turn, these domains are visualised by atomic force and scanning tunneling microscopy (STM), respectively. A comprehensive transmission electron microscopy (TEM) study reveals inhomogeneities ranging from domains of varying size, misorientation of domains, variation of the lattice constant and bending of lattice planes. X-ray photoelectron spectroscopy and electron energy-loss spectroscopy indicate the crystal is Ni deficient. Density functional theory calculations—considering the spatial and electronic disturbance induced by the favourable nickel vacancy—reveal a nanoscale distortion comparable to STM and TEM observations. The different inhomogeneities are understood in terms of the structural relaxation induced by ordering of nickel vacancies, which is predicted to be favourable.

Directionally Solidified Al2O3/Er3Al5O12 and Al2O3/Yb3Al5O12 Eutectic Ceramics Prepared by Optical Floating Zone Melting

Directionally Solidified Al₂O₃/Er₃Al₅O₁₂ and Al₂O₃/Yb₃Al₅O₁₂ Eutectic Ceramics Prepared by Optical Floating Zone Melting

Distribution control and formation mechanism of gas inclusions in directionally solidified Al2O3-Er3Al5O12-ZrO2 ternary eutectic ceramic by laser floating zone melting

Distribution control and formation mechanism of gas inclusions formed in directionally solidified Al2O3-Er3Al5O12-ZrO2 eutectic ceramic rods are explored during laser floating zone melting. In atmospheric environment, highly-dense bubble-free eutectic rods are well fabricated at low solidification rate (<25 μm/s). Gas inclusions form intermittently when the solidification rate is in the range of 25−50 μm/s, but produce continuously at higher solidification rates (100−200 μm/s). The gas inclusions exhibit an elongated finger-like pattern along the growth direction, which of the maximum value of diameter first increases and then decreases with increasing the solidification rate. Meanwhile, the volume fraction of gas inclusions increased gradually with the solidification rate. Based on the effect of surface tension gradient, heterogeneous nucleation of gas bubbles is evaluated to be the primary formation mechanism of gas inclusions.

Spin glass transition of single-crystalline TmFe2O4−δ

TmFe2O4 is one kind of multiferroic material in which equivalent amounts of Fe2+ and Fe3+ occupy a two-dimensional triangular lattice, leading to charge and spin frustrations. The spin frustration is expected to be increased as the fraction of Fe2+ (Fe3+) becomes larger than that of Fe3+ (Fe2+). We have grown single-crystalline TmFe2O4−δ with oxygen vacancies by using floating zone melting method and examined its magnetic properties. On cooling the compound, a long-range magnetic ordering develops around ∼240 K. With further cooling, a maximum of zero-field-cooled (ZFC) magnetization is observed at 186.2 K. The ac magnetic susceptibility obtained by ZFC process also manifests a maximum in its temperature dependence, and the variation of spin-freezing temperature with frequency of ac magnetic field is explainable in terms of the dynamic scaling law with the critical component of 8.68(8). This value suggests that the spin glass transition occurs at 186.2 K. The effect of external dc magnetic field on the irreversible transition temperature is coincident with the de Almeida–Thouless (AT) line. Aging-memory and rejuvenation effect is also observed below the spin-freezing temperature. These facts support the idea that TmFe2O4−δ undergoes spin glass transition below the ferrimagnetic transition temperature. In other words, TmFe2O4−δ can be regarded as a reentrance spin glass. It is thought that the oxygen vacancies bring about unequal number of Fe2+ and Fe3+ ions and thereby strengthen the magnetic frustration among the iron ions coupled with antiferromagnetic interactions, leading to the spin glass behavior.

Evolutions of rod diameter, molten zone and temperature gradient of oxide eutectic ceramics during laser floating zone melting

Laser floating zone melting (LFZM) is an important directional solidification (DS) technique in oxide ceramic fabrication owing to its very high temperature gradient and rapid cooling rate. In this work, variations and evolutions of rod diameter, molten zone, and temperature gradient are explored in fabrication of Al2O3-based eutectic ceramics. At low solidification rate of 4 μm/s, periodical oscillations with almost the same period (~275 s) are produced in melting zone shape/size, as-grown rod diameter, temperature distribution, and temperature gradient. The temperature gradient fluctuates between 3.6 × 103 K/cm and 6.9 × 103 K/cm with an average value of about 5.3 × 103 K/cm. As increasing the solidification rate, the temperature gradient decreases and the periodical oscillation weakens. Based on the evolutions of the temperature field, interface location and molten zone shape, the formation mechanism of periodical oscillation during LFZM processing is evaluated.

Crystal Growth and Temperature Evolution of the Atomic Structure of Pr3Ga5SiO14 from the Langasite Family

Pr3Ga5SiO14 crystals from the langasite family have been grown by the floating zone melting technique. Their X-ray diffraction analysis is performed at 16 different temperatures in range of 94–293 K. An experiment with ultrahigh resolution at 94 K has yielded the most exact structural model: sp. gr. P321, Z = 1, a = 8.07636(4) A, c = 5.06586(2) A, R1/wR2 = 1.192/1.185%, and Δρmin/Δρmax = –0.93/+0.79 e/A3 for 3852 unique reflections and 123 refined parameters. The temperature evolution of atomic displacement parameters is estimated based on multitemperature data using the extended Einstein and Debye models. Static components of atomic displacements are selected and the characteristic Einstein and Debye temperatures for structure atoms are determined. No structural anomalies have been found.