Polycrystalline Growth Research Articles

The role of glass composition in the 3D laser fabrication of lithium niobate single crystal in lithium niobosilicate glass

Spatially selective growth of lithium niobate (LiNbO3) crystals deep within lithium niobosilicate (LNS) glass can be achieved via femtosecond laser irradiation, wherein these crystals could serve as optical elements in photonic integrated circuits. In practice, it is challenging to achieve continuous, single-crystal growth of LiNbO3 within LNS glass. This work reports on progress in overcoming this challenge by exploring how the formation of single-crystal LiNbO3 is influenced by the incongruent composition of the glass matrix. An investigation of the parameter space of glass composition ((100 – x) LiNbO3 – x SiO2 for x = 22, 26, 30, and 34), laser scanning speed, and laser power leads to the observation of several growth modes: continuous growth at an optimal speed, segmented growth below it, and polycrystalline growth above it. For certain glass compositions, a balance of long incubation time and high crystal growth rate results in the ability to grow indefinitely long (∼cm) single crystal lines at the optimal scanning speed. Guidance for predicting which glass compositions are most suitable for single crystal growth based on the LNS composition diagram is provided.

Multiscale simulation of powder-bed fusion processing of metallic alloys

We present a computational framework for the simulations of powder-bed fusion of metallic alloys, which combines: (1) CalPhaD calculations of temperature-dependent alloy properties and phase diagrams, (2) macroscale finite element (FE) thermal simulations of the material addition and fusion, and (3) microscopic phase-field (PF) simulations of solidification in the melt pool. The methodology is applied to simulate the selective laser melting (SLM) of an Inconel 718 alloy using realistic processing parameters. We discuss the effect of temperature-dependent properties and the importance of accounting for different properties between the powder bed and the dense material in the macroscale thermal simulations. Using a two-dimensional longitudinal slice of the thermal field calculated via FE simulations, we perform an appropriately-converged PF solidification simulation at the scale of the entire melt pool, resulting in a calculation with over one billion grid points, yet performed on a single cluster node with eight graphics processing units (GPUs). These microscale simulations provide new insight into the grain texture selection via polycrystalline growth competition under realistic SLM conditions, with a level of detail down to individual dendrites.

Regeneration study of MB in recycling runs over nickel vanadium oxide by solvent extraction for photocatalytic performance for wastewater treatments

Recently, researchers are concentrating on the synthesis of composite materials to enhance the efficiency of the materials in various applications. In this work, nickel vanadium oxide (NiV2O6) nanocomposite material is prepared via two methods and the prepared samples have been characterized with basic studies to analyse the effect of preparation method and the reaction time. The XRD studies reveal a polycrystalline growth in both the methods. The broad XRD peaks obtained for samples prepared via hydrothermal method suggests the size reduction and 1D nanostructure formation. The SEM analysis shows the formation of 1D structures in hydrothermal and 3D microsphere structures in solvothermal methods. The possible formation mechanism behind this formation has been discussed in this manuscript. The FTIR peaks in the fingerprint region confirm the formation and vibration of metal-oxygen bonds. The large optical bandgap values obtained from Tauc plot again confirms the formation of nanostructures of the synthesized samples. The photocatalytic activity of nickel vanadium oxide on methylene blue dye under halogen light were performed and, the recyclability of the sample is investigated. It was found from the photocatalytic spectrum that, the samples prepared from both the methods shows a degradation efficiency of more than 80% within 150 min. It was confirmed that the prepared NiV2O6 photocatalyst samples does not lose their degradation ability even after five cycles of repeated usage.

Multi-phase field lattice Boltzmann model of columnar-to-equiaxed transition in entire welding molten pool

Columnar-to-equiaxed transition (CET) often occurs during the solidification of the weld. Moving molten pool and strong fluidity are two notable characteristics in this process. The former causes the deflective epitaxial growth of columnar dendrites, while the latter makes the equiaxed dendrites grow asymmetrically and continuously move, collide and coalesce in the melt. This paper proposed a multi-phase field lattice Boltzmann model to simulate the CET in the entire molten pool. The model can characterize the polycrystalline growth, compute the fluid flow, and simulate the solid motion, collision, and coalescence. It also combined the transient growth condition model and the continuous nucleation model to reproduce the moving molten pool and define heterogeneous nucleation respectively. It further combined the initial grain structure of the base metal to provide a more realistic condition for epitaxial growth. The proposed model was verified by simulating the pure motion of dendrite in lid-driven flow, the free fall of single dendrite in static flow, and the single-dendrite growths in lid-driven and channel flow. The CET in the entire molten pool was simulated under different conditions, including considering or ignoring the fluid flow and gravity, the columnar and equiaxed dendrites growing simultaneously or sequentially. In these simulations, settlement, collision, and coalescence of equiaxed dendrites were reproduced successfully. Clear asymmetric growth, dendrite aggregation effect, and solute vortex were found. Significant differences in the size, symmetry, dendrite spacing, dendrite morphology, and solute distribution were found when the melt flow was considered or ignored. The dense equiaxed dendrite belt was found in the later stage of CET. They gathered in front of columnar dendrites and dominated the following solidification. The typical mixed structure that consists of columnar and equiaxed grains was obtained after complete solidification.

Impact of Substrate Temperature on Structural, Electric and Optical Characteristics of CuO Thin Films Grown by JNS Pyrolysis Technique

The jet nebulizer spray (JNS) pyrolysis method has been successfully employed to grow CuO thin films at various substrate temperatures, ranging from 300 to 600˚C. The XRD analyses revealed the monoclinic phased polycrystalline growth of the samples and exhibited the strong influence of substrate temperature (ST) on the crystallite sizes. Optical transmission and bandgap studies also showed that sample bandgaps clearly rely upon the growth temperatures. The SEM micrographs displayed the agglomerated growth of particles having golf ball-like structures. The occurrence of Cu and O in the samples were confirmed through EDS analyses. The studies on DC electrical conductivities (I-V curves) also show strong dependency on the ST. A p-CuO/n-Si diode was fabricated at the ST of 600˚C and the diode parameters like barrier height (\({\varphi }_{b}\)) and ideality factor (n) were determined under light and dark conditions.

PVD coatings influence (TiCN, BCN, and CrAlN) on the fatigue life behavior of AISI 1045 steel for automotive applications

This work evaluated the influence of titanium carbo-nitride (TiCN), chromium aluminum nitride (CrAlN), and boron carbo-nitride (BCN) coatings deposited on AISI 1045 steel and their behavior in fatigue life. Suitable deposition parameters were established to obtain appropriate deposition times for polycrystalline growth and desired stoichiometry, as well as a stable layer thickness of ~ 3 μm. The physical and chemical properties of the obtained coatings were established by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and nanoindentation. Scanning electron microscopy (SEM) was used for analyzing the fracture surfaces of the samples subjected to fatigue. The analysis of the fatigue behavior of the uncoated and coated substrates was performed under rotary bending conditions applying maximum alternating stresses in the interval of 55 to 70% of the ultimate strength value, i.e., from 479 to 610 MPa, respectively. The test was conducted at room temperature. It was established that the fatigue resistance properties increased for the three types of coated samples, TiCN, BCN, and CrAlN, with values of 9.6%, 4.2%, and 3.9%, respectively, calculated for 1 × 106 cycles. The highest value in fatigue life improvement corresponded to the TiCN coating. This is associated with the increase in the mechanical properties of the coating, as well as the lower presence of tensile type stresses. The mechanical and fatigue results found in these ternary coatings deposited on AISI 1045 steel open up the possibility of future applications in mechanical devices, e.g., automotive applications with high fatigue demands in service conditions.

Strong UV emission in flakes-like ZnS nanoparticles synthesized by cost effective sol-gel method

The objective of the study is to synthesize ZnS nanoparticles (NPs) by simple cost-effective technique and analyze their optoelectronic features. Cubic single phase ZnS NPs were prepared by sol-gel process. XRD data confirms the polycrystalline growth via presence of multiple peaks along various crystallographic planes. The particles are approximately 3 nm in diameter confirming the nano size of the material. FTIR and UV–Vis spectroscopy establishes the presence of all major Zn and S bonding in the sample with slightly increased optical band gap as compared to the bulk ZnS. The larger band gap is related to smaller particle size supported by the XRD data. To understand the light emission behavior of the ZnS NPs, PL spectroscopic analysis was done. Strong UV emission is observed at 380 nm which is attributed to the hole trapping and recombination phenomena with electrons by sulphur or zinc defect states. Other comparatively feeble and broader peak is also seen at 435 which is related to surface dangling bonds. FESEM micrographs reveal the evidence of flake-like outgrowth in the sample. EDX confirms that, Zn and S are the only elements present in the material.

Asymmetric Interfaces in Epitaxial Off-Stoichiometric Fe3+xSi1−x/Ge/Fe3+xSi1−x Hybrid Structures: Effect on Magnetic and Electric Transport Properties

Three-layer iron-rich Fe3+xSi1−x/Ge/Fe3+xSi1−x (0.2 < x < 0.64) heterostructures on a Si(111) surface with Ge thicknesses of 4 nm and 7 nm were grown by molecular beam epitaxy. Systematic studies of the structural and morphological properties of the synthesized samples have shown that an increase in the Ge thickness causes a prolonged atomic diffusion through the interfaces, which significantly increases the lattice misfits in the Ge/Fe3+xSi1−x heterosystem due to the incorporation of Ge atoms into the Fe3+xSi1−x bottom layer. The resultant lowering of the total free energy caused by the development of the surface roughness results in a transition from an epitaxial to a polycrystalline growth of the upper Fe3+xSi1−x. The average lattice distortion and residual stress of the upper Fe3+xSi1−x were determined by electron diffraction and theoretical calculations to be equivalent to 0.2 GPa for the upper epitaxial layer with a volume misfit of −0.63% compared with a undistorted counterpart. The volume misfit follows the resultant interatomic misfit of |0.42|% with the bottom Ge layer, independently determined by atomic force microscopy. The variation in structural order and morphology significantly changes the magnetic properties of the upper Fe3+xSi1−x layer and leads to a subtle effect on the transport properties of the Ge layer. Both hysteresis loops and FMR spectra differ for the structures with 4 nm and 7 nm Ge layers. The FMR spectra exhibit two distinct absorption lines corresponding to two layers of ferromagnetic Fe3+xSi1−x films. At the same time, a third FMR line appears in the sample with the thicker Ge. The angular dependences of the resonance field of the FMR spectra measured in the plane of the film have a pronounced easy-axis type anisotropy, as well as an anisotropy corresponding to the cubic crystal symmetry of Fe3+xSi1−x, which implies the epitaxial orientation relationship of Fe3+xSi1−x (111)[0−11] || Ge(111)[1−10] || Fe3+xSi1−x (111)[0−11] || Si(111)[1−10]. Calculated from ferromagnetic resonance (FMR) data saturation magnetization exceeds 1000 kA/m. The temperature dependence of the electrical resistivity of a Ge layer with thicknesses of 4 nm and 7 nm is of semiconducting type, which is, however, determined by different transport mechanisms.

Non-melt selective enhancement of crystalline structure in molybdenum thin films using femtosecond laser pulses

It is challenging to crystalize a thin film of higher melting temperature when deposited on a substrate with comparatively lower melting point. Trading such disparities in thermal properties between a thin film and its substrate can significantly impede material processing. We report a novel solid-state crystallization process for annealing of high melting point molybdenum thin films. A systematic investigation of laser induced annealing from single pulse to high pulse overlapping is reported upon scanning at fluences lower than the threshold required for the damage/ablation of molybdenum thin films. The approach allows better control of the grain size by changing the applied laser fluence. Atomic force microscopy surface morphology and x-ray diffraction (XRD) analysis reveal significant improvements in the average polycrystalline grain size after laser annealing; the sheet resistance was reduced by 19% of the initial value measured by a Four-point probe system. XRD confirms the enlargement of the single crystal grain size. No melting was evident, although a change in the close packing, shape and size of nanoscale polycrystalline grains is observed. Ultrashort laser induced crystallinity greatly enhances the electrical properties; Hall measurements reinforced that the overall carrier concentration increases after scanning at different laser fluences. The proposed method, based on the aggregation and subsequent growth of polycrystalline and single crystal-grains, leading to enhanced crystallization, has potential to be applicable in thin film processing industry for their wide applications.

Epitaxial growth and polarization reversal characteristics of hybrid improper ferroelectric Ca3Ti2O7 thin films

Layered perovskite oxides A3B2O7 with Ruddlesden–Popper structures have attracted great attention due to their intriguing hybrid improper ferroelectricity. In this work, we prepared high-quality epitaxial Ca3Ti2O7 (CTO) thin films on Nb-SrTiO3 (STO) and Pt–Si substrates by pulsed laser deposition. The epitaxial relationship between the CTO thin film and Nb-STO substrate is [010]CTO//[010]Nb-STO, while the CTO film on Pt–Si is the polycrystalline growth with the preferred orientation of (010). The piezoelectric force microscopy image is attested to the stable existence of ferroelectricity in CTO films at room temperature with different growth patterns. A resistive switching behavior with at least two orders of magnitude is observed in CTO films driven by their ferroelectric polarization reversal. Moreover, a downward self-poling phenomenon is observed in CTO films on different substrates, and the flexoelectric effect induced by the strain gradient is supposed to be the main origin.

Single-Crystal Nanostructure Arrays Forming Epitaxially through Thermomechanical Nanomolding.

For nanostructures in advanced electronic and plasmonic systems, a single-crystal structure with controlled orientation is essential. However, the fabrication of such devices has remained challenging, as current nanofabrication methods often suffer from either polycrystalline growth or the difficulty of integrating single crystals with substrates in desired orientations and locations to create functional devices. Here we report a thermomechanical method for the controlled growth of single-crystal nanowire arrays, which enables the simultaneous synthesis, alignment, and patterning of nanowires. Within such diffusion-based thermomechanical nanomolding (TMNM), the substrate material diffuses into nanosized cavities under an applied pressure gradient at a molding temperature of ∼0.4 times the material's melting temperature. Vertically grown face-centered cubic (fcc) nanowires with the [110] direction in an epitaxial relationship with the (110) substrate are demonstrated. The ability to control the crystal structure through the substrate takes TMNM a major step further, potentially allowing all fcc and body-centered cubic (bcc) materials to be integrated as single crystals into devices.

An enhanced phase field model for micron-scale droplet impact with solidification microstructure formation

In this work, a novel diffuse interface model combining droplet impact with solidification microstructure formation was developed. A number of numerical models simulating droplet impact with solidification have been invented, but few are capable of unveiling the nucleation and growth of polycrystalline crystals on a micro scale. This paper thus aims to propose a diffuse interface model to simulate droplet impact, and moreover, solidification microstructure formation. To implicitly track the evolving liquid-gas interface, the Cahn-Hilliard equation is coupled with the Navier-Stokes equation. A phase field model involving polycrystalline growth is responsible for the capturing of solid-liquid interface and grain-grain boundaries. The current model is discretized explicitly such that it lends itself to shared-memory parallelism like OpenMP. A parallel SOR scheme based on Red/Black ordering is used to solve the pressure velocity coupling. The model was applied to various impact conditions in plasma spraying, including both single and multiple droplet impact, to show its robustness. Besides, with the model being modified slightly, non-equilibrium effect induced by rapid solidification was investigated as well.

Electronic phase derived impedance spectroscopic behavior of La0.5Nd0.2A0.3MnO3 manganites

In the present communication, structural, microstructural and electrical properties of La0.5Nd0.2Ca0.3MnO3 (LNCMO) and La0.5Nd0.2Sr0.3MnO3 (LNSMO) mixed valent manganite films, grown on single crystalline (0001) Al2O3 (ALO) substrates using pulsed laser deposition (PLD) technique, have been studied. After deposition, one set of LNCMO and LNSMO films were annealed in oxygen environment at 500 °C (hereafter referred as LNC–A & LNS–A, respectively) and compared their properties with other set of PLD as–grown LNCMO and LNSMO films (hereafter referred as LNC and LNS, respectively). Single crystalline growth of LNCMO films and polycrystalline growth of LNSMO films have been identified through X–ray diffraction (XRD) measurement. Atomic force microscopy (AFM) measurement for morphological studies indicates that divalent dopant and oxygen annealing process play an important role in governing the grain growth and surface nature of the studied films. A decrease in the value of resistance (R) and impedance (Z) with increase in frequency under the applied magnetic field (H = 1 T) has been observed for all the studied films. For LNC & LNC–A films, reactance is found to decrease monotonically with increase in frequency throughout its range studied while for LNS and LNS–A films, reactance initially increases followed by reduction in its value with increase in frequency under 0 and 1 T magnetic field has been observed. The variation in MR for LNS and LNS–A films has been discussed in the context of charge transport mechanism and Lorentz force. Further, to discuss the role of grain and grain boundaries, nyquist plots and their suitable fits under zero and 1 T external magnetic field have been investigated for all the studied mixed valent manganite films. Important role of phase separation scenario in studied manganite films has been discussed in detail based on magnetization measurement. Double magnetic phase transition and fluctuations in derivative of magnetization have been ascribed to the phase separation scenario of LNCMO / ALO and LNSMO / ALO mixed valent manganite thin films.

Growth of SiGe thin films with uniform and non-uniform Si concentration profiles on insulating substrates by high-speed continuous wave laser annealing

To realize next generation electronics and energy harvesting, crystal growth of silicon-germanium thin film on insulating substrate was investigated by high-speed continuous wave laser annealing. As a result, growth of polycrystalline (poly-) SiGe thin films on insulating substrate became possible. Furthermore, by tuning the laser annealing conditions, we have succeeded in controlling the Si concentration profiles after growth in a wide range from uniform to laterally graded distributions. These results and discussions will facilitate the development of next-generation electronics, and energy harvesting devices.

Thermodynamically consistent phase-field modeling of competitive polycrystalline growth of beta grains during additive manufacturing of Ti6Al4V

In this paper, a thermodynamically consistent multi-phase and multi-component phase-field (PF) model was developed for the prediction of competitive growth of β grains during Ti6Al4V solidification in the directed energy deposition (DED) process. By incorporating the thermodynamic and diffusion mobility databases of the Ti-Al-V system, the concentration evolutions of Al and V solutes were accurately predicted separately for the first time, where the solidified β has a slightly higher Al but lower V than the liquid phase, matching the phase diagrams very well. Influences of cross-diffusion between solutes on concentration distributions were also investigated. The PF modeling predicted elongated β grains of columnar shape during Ti6Al4V solidification, whose growth pattern was highly consistent with the experimental results. A validated DED model was also utilized to acquire the temporal and spatial temperature profiles and heating/cooling rates in the molten pool. By coupling such thermal information, the grain growth and concentration evolutions under the influence of cooling rate were carried out.

Advanced textured monocrystalline silicon substrates with high optical scattering yields and low electrical recombination losses for supporting crack‐free nano‐ to poly‐crystalline film growth

AbstractCrystalline silicon tandem devices with perovskites, CIGS, and nanocrystalline silicon, as well as the TOPCon design, are incompatible with the conventional pyramidal surface texture of silicon. This is a result of crack formation in nano to polycrystalline growth on large sharp surface features. In this work, three texturing approaches are investigated, using alkaline and/or acidic wet chemical etches, that can lead to the crack‐free growth of nano to polycrystalline materials on textured surfaces. In this work, we show that without acidic smoothening, the fraction of &lt;111&gt; pyramidal surface coverage has to remain relatively small to prevent crack formation during crystalline growth on these surfaces. Applying an acidic etch as a function of time continuously smoothens surface features. This shifts the reflection to wider scattering angles and results in higher total reflected intensity with respect to the conventional texture, making it an interesting option for a wide variety of tandem pv applications. Finally, we demonstrate crater‐like features on a &lt;100&gt; monocrystalline silicon surface using an etching process including a sacrificial layer. These craters increase light scattering into wider angles, but to a lesser extent than the former approach. In terms of passivation, we demonstrate the positive effect of a post deposition hydrogen treatment. Initial dilution of the silane plasma improves passivation on a &lt;111&gt; surface, but is detrimental to passivation on a &lt;100&gt; surface, likely because the hydrogen dilution results in epitaxial growth at the c‐Si/a‐Si:H hetero‐interface. A minority carrier lifetime of over 3 ms has been achieved for all texturing approaches, after deposition of a 15 nm a‐Si:H layer on both sides of the wafer, for different a‐Si:H deposition and annealing schemes.

Magnetic Field-Assisted Chemical Vapor Deposition of UO2 Thin Films.

Chemical vapor deposition (CVD) of UO2 thin films from in situ reductive decomposition using a U(VI) precursor ([U(OtBu)6]) was performed under applied magnetic fields (up to 1 T). The molecular mechanism responsible for the formation of U(IV) oxide was determined by nuclear magnetic resonance (NMR) analysis of gaseous byproducts revealed a reductive transformation of uranium hexakis-tert-butoxide into urania. Thin films were grown under zero-field and applied magnetic field conditions that clearly showed the guiding influence of the magnetic field on altering the morphology and crystallographic orientation of grains in UO2 deposits produced under an external magnetic field. Application of magnetic fields was found to reduce the grain size. Whereas films with a ⟨111⟩ preferred orientation were observed under zero-field conditions, the application of magnetic fields (500 mT to 1 T) promoted a polycrystalline growth. X-ray photoelectron spectroscopy confirmed the formation of UO2 films with traces of U(VI) centers present on the surface, which was evidently due to the surface oxidation of coordinatively unsaturated U(IV) centers, which was found to be significantly reduced in the field-assisted process. These findings emphasize the positive effect of magnetic fields on controlling the texture and chemical homogeneity of CVD-grown films. The availability of a magnetic field as an extrinsic parameter for the CVD process adds to the conventional parameters, such as temperature, deposition time, and pressure, and expands the experimental space for thin-film growth.

Selective mask formation and gallium nitride template fabrication on patterned sapphire substrates for light-emitting diodes

Patterned sapphire substrates are composed of multiple planes, leading to the undesirable polycrystalline growth of gallium nitride (GaN) during template deposition. However, patterned sapphire substrates and templates with good crystalline quality would be useful for increasing the light extraction efficiency in light-emitting diodes. In this study, a silicon dioxide (SiO2) mask was selectively introduced onto the lenses of a patterned sapphire substrate to suppress the growth of polycrystalline GaN. The SiO2 mask prevented the negative influence of the patterned sapphire substrate on GaN growth under various conditions. High-resolution x-ray diffraction analysis revealed that the GaN template grew as a single crystal in the presence of the SiO2 mask. Furthermore, the compressive stress generated in the GaN template was relieved due to the suppression of polycrystalline growth.

Influence of Low Axial Temperature Gradient on Ga-Doped Germanium Crystal Growth by the Vertical Bridgman Method

The influence of a low axial temperature gradient on the growth of the Ge:Ga crystal by the vertical Bridgman technique was investigated. The axial temperature gradient of 3.3 K/cm is shown to provide conditions for polycrystalline growth of Ge:Ga crystals. The layers with the distances from 14 to 277 μm are found in the grown crystals with a concave of the solid-liquid interface. The motion of textolite particles in distilled water is visualized by the light cut method at different values of axial and radial temperature gradients. In the absence of the radial temperature gradient, no convection is observed when the axial temperature gradient increases from 0.1 to 6.5 K/cm. In this case, the particle motion velocity increases proportionally to the axial temperature gradient due to the occurrence of thermal diffusion. The convective flow appear in the liquid at the radial temperature gradient of 0.6 K/cm and an axial gradient above 0.1 K/cm.

Flexible and Epitaxial Metal Oxide Thin Film Growth by Photoreaction Processing for Electrical and Optical Applications.

This review summarizes the use of photoreactions that replace conventional heating processes for growing oxide thin films from chemical solutions. In particular, this review outlines key variables in photoreactions that affect epitaxial and polycrystalline thin film growth, including precursor materials, laser wavelength, laser fluence, and carbon. In addition, the features of the photoreaction process that can be controlled at a low temperature by oxygen non-stoichiometry are examined. Likewise, functions that are neither achieved by developing a gradient structure nor controlled by a thermal equilibrium reaction are detailed. Two new concepts are presented, known as photoreaction of nanoparticles (PRNP) and photoreaction of a hybrid solutions (PRHS), in which crystal nuclei are pre-dispersed in a metal-organic compound film. This method has successfully produced flexible phosphor films used as resistor or thermistor electronic components. Finally, thin film growth using different light sources such as flash lamps and femtosecond lasers (fs) is explored.