Oxygen Impurities Research Articles

Balancing thermal conductivity and strength of hot-pressed AlN ceramics via pre-sintering and annealing process

The heat dissipation requirements of the new generation of semiconductors are placing higher demands on the overall performance of aluminum nitride (AlN) ceramics. This has led to an increasing emphasis on AlN ceramics that combine thermal conductivity and strength. AlN ceramics are often strengthened by hot-press sintering, but their thermal conductivity is typically modest. In this study, pre-sintering and annealing processes are introduced to optimize the thermal conductivity of hot-pressed AlN ceramics to avoid the detrimental effects of oxygen impurities in the AlN lattice. The effect of the oxide layer on the surface of commercial AlN particles is investigated, including density, phase composition, microstructure, and fracture behavior. The effect of annealing on the improvement of thermal conductivity and flexural strength is also verified. Electron paramagnetic resonance (EPR) analysis is performed to examine the concentration of intercrystalline defects under various conditions. Finally, AlN ceramics with a thermal conductivity of 204 W m−1 K−1 and a flexural strength of 376 MPa are obtained.

Engineering and controlling the electronic properties of zigzag and armchair boron nitride nanotubes with various concentrations of oxygen impurities

This research underscored the comprehension of the electronic properties of the various diameters for zigzag and armchair single-wall boron nitride (ZSWBNNTs and ASWBNNTs) under various concentrations of oxygen impurities. The electronic properties of these tubes are investigated by using the first density functional theory (DFT), which implemented in the QUANTUM ESPRESSO package. Results show that the electronic properties of all these tubes can be controlled by using different concentrations of oxygen impurities. In the pristine case, the electronic band gap of these tubes is deepened on the diameter. So, the electronic band gap is increased by increasing the diameter of the tube. Nevertheless, these properties are changed and affected by various concentrations of oxygen impurities. The electronic band gap of ZSWBNNTs reduced by using one and two -hexagonal of oxygen impurities, and the behavior of ASWBNNTs changed from insulator to semiconductor properties by using a 2-hexagonal of oxygen impurities. Our investigation revealed for the first time that the behaviors of ZSWBNNTs and ASWBNNTs are changed from semiconductor and insulator to metallic with seven oxygen impurities. Then, there are very fascinating results. Also, the electronic band gap doesn't only depend on the impurity concentration, but also depends on the geometrical pattern of oxygen impurities in the ASWBNNTs.

AlYN Thin Films with High Y Content: Microstructure and Performance

Pseudobinary nitride alloys display enhanced piezoelectric properties compared to their nonalloyed counterparts enabling their wide application in high‐performance transducers and acoustic wave resonators. Their fabrication remains challenging because of their inherently stochastic nature, which requires in‐depth understanding of the film growth dynamics and the interplay of deposition parameters. Herein, thin Al1−xYxN films are produced with varied yttrium content in the range from x = 0.09 to 0.28 on a gradient seed layer on 200‐mm Si substrates and investigated via various X‐ray diffraction methods, high‐resolution scanning transmission electron microscopy, nanoindentation, and atomic force microscopy. Bulk acoustic wave resonators, solidly mounted on a multilayer acoustic isolation, are fabricated to analyze the piezoelectric performance of the films and to extract corresponding material parameters via fitting of the high‐frequency electrical response by 1D Mason's model. The trend of declining coupling is explained by the lattice softening and the increase in electron density, experimentally observed by monitoring reduced elastic modulus and dielectric constant values, respectively. The absence of expected enhancement of the piezoelectric modulus is interpreted by the presence of oxygen impurities, facilitating the inhomogeneous strain of the AlYN lattice, which effectively cancels the energy flattening phenomenon, found in III–V pseudobinary alloys.

Distillation with separate condensation of components as a new way to prepare especially pure GexTe100−x glasses with precisely desired composition

The study is aimed to develop a method for preparing especially pure GexTe100−x glasses that provides low oxygen impurity content and, at the same time, high accuracy of a given, desired composition. It is shown that the conventional distillation of the Ge20Te80 melt can lead to a deviation in the glass composition from 0.5 to 2.1 at.% depending on the number of stages due to the thermal decomposition of GeTe. Such deviations in the composition have a significant effect on the crystallization of glasses upon heating and the values of the characteristic temperatures. A method for purification of GexTe100−x charge that makes it possible to minimize the deviation of the glass composition from the desired value is developed. The method is based on creating a constant tellurium vapor excess over germanium(II) telluride that suppresses its thermal decomposition. This is achieved by separate condensation of tellurium and GeTe and their subsequent successive evaporation. Two options for practical implementation of the method with the same efficiency, differing in the initial stage of the charge synthesis, are proposed. Using the developed method, Ge20Te80 glass samples with an oxygen impurity content of <10 ppb(wt) and a composition deviation of ≤0.2 at.% are prepared.

Local structure of an oxygen impurity and fluorine vacancy complex in LiYF4

Crystal-growth defects and impurities are among the key factors limiting the performance of fluorine-based optical materials. This study reports the local structure of an oxygen impurity centre in LiYF4 single-crystal. Experimental electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy investigations were combined with first principle theoretical modelling to elucidate the defect structure. It was determined that, after irradiation with X-rays, an electron spin S=1/2 paramagnetic centre is generated. It is stable at room temperature and anneals above 150°C. The alignment of the g-tensor and magnitudes of hyperfine interactions suggest that the centre is an oxygen impurity and fluorine vacancy complex. Highly anisotropic hyperfine interaction with three fluorine atoms within 3 Å of the oxygen defect is observed. The results provide novel insights into oxygen ion incorporation in LiYF4 and are crucial to understanding and minimising radiation-induced degradation of the material.

Design and optimization of superconducting CUSP electromagnetic field structure based on a COMSOL-GMDH-MOSO hybrid strategy

As the semiconductor industry gains momentum, there is a significant focus on producing high-quality silicon wafers with larger diameters. However, the preparation of major-diameter, high-quality silicon single-crystal has encountered challenges such as intense convection inside the melt and high oxygen impurity concentration at the interface between the fluid and solid phases. To tackle these problems, the external superconducting CUSP electromagnetic field structure was designed to optimize the crystal growth parameters. A hybrid strategy of the multiphysics field coupling method, Group Method of Data Handling (GMDH) type neural network identification, and the Multiple Objective Snake Optimizer (MOSO) algorithm was proposed to optimize the superconducting CUSP electromagnetic field structure. A two-dimensional axisymmetric model was established for 18-inch silicon single-crystal preparation using the Magnet system Applied Czochralski (MCZ) method. The amount of longitudinal plies of superconducting CUSP electromagnetic field coils, coil spacing, and the thickness of magnetic shields were selected as design variables, and the radial electromagnetic induction intensity at the graphite crucible inner wall and the superconducting magnet volume were used as dual objective functions. The required samples for GMDH training were provided by COMSOL Multiphysics (COMSOL) numerical calculations, and polynomial mathematical equations were established with the radial electromagnetic induction intensity at the graphite crucible inner wall and the superconducting magnet volume as the dual objective functions. Then the Pareto optimal solution for the electromagnetic field structure design variables was obtained using the dual objective function optimized by MOSO. With the hybrid strategy, numerical simulations and theoretical analysis of the crystal preparation process parameters under the effect of the superconducting electromagnetic field with the optimal structure were carried out. The results displayed that the added superconducting CUSP electromagnetic field reduced the crystal radial and axial thermal gradients and greatly suppressed the melt convection. As a result, the heat distribution inside the hot fused silicon was more even, the thermal gradient at the solid–liquid interface was reduced by 68.4%, and the oxygen impurity content was reduced by 82.1%, which effectively improved the crystal quality. The thermal field simulation results verified that the proposed hybrid strategy provided a new numerical calculation method for obtaining the optimal superconducting electromagnetic field structure.

Growth of N-Polar (0001) GaN in Metal–Organic Vapour Phase Epitaxy on Sapphire

We have systematically studied the growth of N-polar GaN on sapphire in metal–organic vapor phase epitaxy (MOVPE) on different misoriented (0001) sapphire substrates. The key parameter was the NH3 flow, which affects the roughness, growth rate, crystal quality, and impurities. Most parameters show a trend reversal at a V/III ratio around 500 and show either a maximum, such as the growth rate, the sizes of hexagonal hillocks on low misorientations, the yellow luminescence and the mobility, or show a minimum such as the FWHM in X-ray diffraction, the carrier concentration, the surface roughness of large misorientations, or the blue (430 nm) luminescence. This suggests that around a V/III ratio of 500, the surface changes from a Ga-terminated Ga-adlayer surface to a N-terminated 3N-H(2×2) surface. Using extremely low V/III ratios, a smooth N-polar GaN was obtained even on the standard 0.2° misorientation. However, good crystalline quality, low oxygen impurities and smooth surfaces together seem too challenging with low misorientation. The strain-dependent band edge shifted by 14 eV for strain along [0001], which is close to the values reported by Ga-polar GaN.

Formation and Luminescence of Single Oxygen Impurities on the Surface of SiC Nanocrystals.

Impurities that hinder luminescence are a common problem in the synthesis of nanocrystals, and controlling the synthesis reaction could provide a way to avoid or use impurities beneficially. Excited state molecular dynamics is used to determine how oxygen impurities appear in the plasma synthesis of silicon carbide nanocrystals (SiC NCs). Formation of impurities is studied by considering the intermediate structures in the simulated photoreaction. The results show the most probable bonding patterns of silicon, carbon, and oxygen. These intermediates are used as a basis for studying the luminescence of expected oxygen impurities in SiC NCs, where luminescence is studied by first-principles modeling and density matrix dissipative dynamics based on on-the-fly non-adiabatic couplings and the Redfield tensor. Modeling the dissipation of energy from electronic to nuclear degrees of freedom reveals multiple impurities with significant photoluminescence quantum yields.

Fabrication of CZTS Films through a Combined Electrodeposition and Solution Process: An Experimental and First‐Principles Study

AbstractSulfurization has a critical role in preparation of CZTS films since the main drawback of CZTS is due to the presence of S‐based impurities. In general, sulfurization could be performed during or following annealing, or the entire CZTS structure could be prepared through sol‐gel. However, toxic gases are used for the former, and the latter takes prolonged times. Alternatively, a combined electrodeposition and solution process was proposed in the present work to prepare CZTS. Briefly, we electrodeposited CZT layers on a metal substrate, and then immersed the as‐prepared coating in a S containing solution for the sulfurization. The effects of electrodeposition time, electrodeposition order, current density, electrolyte compositions, sulfurization time, and annealing pressure were studied. Overall, the results show that S content of the films varied between 27 and 52 % depending on process parameters, thus, suggesting the effectiveness of the proposed approach regarding sulfurization. However, various secondary phases were also detected as well as considerable amount of oxygen impurity. Accordingly, we opted for a computational approach to elucidate the effect of oxygen impurities, and report that the band gap of the films varied between 0.59 and 0.05 eV depending on the presence and which sites oxygen atoms occupy.

Preparation of isotropic submicron spherical boron nitride particles by restricted template method

AbstractHexagonal boron nitride (h‐BN) has received considerable attention, due to its high thermal conductivity and electrical insulation. However, the intrinsic platelike structure with the strong anisotropic property restricts its applications, and it is necessary to synthesize isotropic spherical h‐BN particles (SP‐BNs) with submicron size. Till now, methods to prepare (SP‐BN) still exist problems, such as high oxygen impurities and pollution, generated by the ammonia and pyrolysis of precursors. Here, a relatively green reaction between the restricted template of carbon nanospheres and boron trioxide (B2O3) under elevated temperature is conducted, and the SP‐BNs with an average diameter of 200–300 nm (named Nano‐BN‐s) have been successfully synthesized. Comprehensive scanning electron microscopy, transmission electron microscopy, and X‐ray diffraction characterizations confirm the obtained products are spherical boron nitride. With the analysis of X‐ray photoelectron spectroscopy and Fourier transform infrared, the reaction mechanism is briefly discussed. These results indicate the reaction occurs on the restricted template of carbon nanospheres, and the C atoms are substituted by B and N atoms as the reaction progress, forming the Nano‐BN‐s. What is more, the restricted template method plays a key role in the design and improves h‐BN‐based materials in the future and may also be extended to form other novel materials.

Oxygen uncoupling chemical looping gasification of biomass over heterogeneously doped La-Fe-O perovskite-type oxygen carriers

Biomass chemical looping gasification (BCLG) is a promising technology utilizes lattice oxygen for partial oxidation to produce high-value fuels. But in the field of BCLG, oxygen uncoupling chemical looping gasification (OU-CLG) is a novel concept that introduces molecular oxygen to optimize traditional gas-solid reactions. In this work, based on the most representative oxygen uncoupling elements successfully doped into La-Fe-O oxygen carriers, the results showed that the reaction performance not be positively correlated with the degree of oxygen uncoupling, only Co doping exhibited a better gasification performance than LaFeO3 did. Further, Co-LF differs from conventional LF in that it performs better in ex-situ gasification than in-situ gasification, with a total gas yield and carbon conversion efficiency of 1078.70 ml/g and 75.15% respectively. Further characterizations revealed that Co doping led to changes in the physical structure and reducibility. More importantly, enriched lattice oxygen and oxygen vacancies induced the formation of disordered reactive oxygen species in perovskite lattice, which promote the further oxygen uncoupling and the formation of oxygen uncoupling vacancies in ex-situ gasification. Moreover, doped but failure to promote the CLG might due to oxygen scarcity, impurity and elemental catalytic properties. Finally, a clear reaction mechanism regarding OU-CLG was proposed.

Amorphous silica particles reinforced nickel matrix composites fabricated by in-situ pyrolysis of polycarbosilane

Microstructures and mechanical properties of Ni matrix composites reinforced with amorphous SiO2 particles by in-situ pyrolysis of polycarbosilane (PCS) coated on Ni powders were studied in this paper. The submicron spherical amorphous SiO2 particles instead of traditional SiC uniformly distributing in the composites result from the generated silicon source by PCS pyrolysis combined with oxygen impurity in the preparation. The prepared composites achieve high yield strength, 172 MPa, and ultimate tensile strength, 494 MPa, as well as outstanding ductility, total elongation 28.4 %, which is improved 13.9 %, 50.6 % and 160.6 % compared with those for pure Ni samples, respectively. The effects of solid solution strengthening, grain refinement strengthening, reinforcement particle strengthening and dislocations behavior of at amorphous/crystalline interface on the combination of high strength and ductility of the composites were discussed.

Effect of oxygen on the wettability and interfacial reaction between the DD5 superalloy and ceramic shell

The sessile-drop method in vacuum environment was developed to study the wettability and interfacial reaction between the DD5 superalloy melt and Al2O3-based ceramic shell. The results show that the content of oxygen in superalloy can affect the wettability and interfacial reaction between alloy melt and ceramic shell. For the DD5 alloy/Al2O3-based ceramic shell system, the wetting angle decreases from 135.5° to 123.5° and the thickness of the interfacial reaction layer increases from 2.2 μm to 7.3 μm when the content of oxygen impurity increases from 4 ppmw to 21 ppmw. The interfacial reaction products between the DD5 alloy and Al2O3-based ceramic shell are mainly Al2O3. The Si element is also detected in a diffuse distribution on the surface of the interfacial reaction layer. The proportion of interfacial reaction area gradually increases with increasing temperature and oxygen impurity content. With the increase of oxygen impurity content, the active Al element in the alloy matrix is gradually enriched and adsorbed in the interfacial reaction layer. It leads to the increase of the interfacial reaction rate and has a significant effect on the reduction of the surface tension of the alloy melt.

Fabrication of nanocrystalline and ultra-hard tungsten alloy by dual-particle dispersion strengthening

Alloying with heterogeneous atoms or second phase particles is an effective method for inhibiting grain growth and achieving ultrafine and nanocrystalline materials. In this work, by varying the component and content of Ti and ZrC, the synergistic effects of dual-particle reinforcement on sintering behavior and final bulk W alloys were investigated. The results revealed that adding both Ti and ZrC not only dramatically suppresses grain growth but also effectively promotes relative density. Nanocrystalline W-3Ti-1ZrC bulks were successfully fabricated with an ultra-high hardness of 1347 kgf/mm2. Further characterization showed that the alloying component and content strongly influence the milled-powder state agglomeration and morphology even in same milling condition. Additionally, the advantage of Ti and ZrC addition can be utilized simultaneously, as Ti retards grain growth and ZrC reacts with oxygen impurities to enhance grain boundary cohesion.

Comparison of thermal diffusion and interfacial reactions for bulk and sputtered titanium on 316L stainless steel

As a first step to devise a hybrid process for the production of TiC wear coatings on 316L, consisting of magnetron sputtering followed by titanium carburization, interfacial reactivity between stainless steel and titanium has to be finely understood. Systematic comparisons were carried out on diffusion couples of increasing chemical and geometrical complexity (Fe/Ti, 316L/Ti, and 316L/sputtered Ti), highlighting the formation mechanisms of interfacial structures. Transmission and scanning electron microscopy composition profiles revealed that long-range microstructures in titanium are the result of iron diffusion and oxygen impurities interactions. FeTi and Fe2Ti intermetallics formation is first kinetically driven, then favors thermodynamic stability, leading to compositional changes during thermal cycles. Their growth is shown to be non-diffusion controlled. These compounds act as diffusion barriers for chromium, and traps for carbon, indirectly generating a complex layered structure at the interface. Differences between bulk and sputtered titanium are exclusively linked to the latter smaller scale, including destabilized diffusion fronts, and superficial TiO formation by oxygen rejection after iron diffusion.

Exsolution of oxygen impurity from diamond lattice and formation of pressurized CO2-I precipitates

Diamond single crystals showing Infra-red features of pressurized CO2-I phase were studied using Transmission Electron Microscopy (TEM) and tomography. Numerous O-containing precipitates with sizes up to 45 nm are observed. The absolute majority of these precipitates decorate dislocation loops or are located inside them; individual scattered precipitates are also present. Morphology of the precipitates varies from quasi-isometric octahedra to highly flattened elongated ones. Close association of the precipitates with the dislocation loops implies their formation by exsolution of oxygen impurity from diamond lattice; the size distribution of the precipitates suggests that the equilibrium state is not yet reached. Presumably, the morphology and precise chemical composition depend on P-T-t evolution of the diamond crystal and corresponding changes in oxygen supersaturation in the lattice. The CO2-containing diamonds often contain micron-sized hexagonal lamellar inclusions. TEM investigation of a lamellae reveals that it consists of high quality graphite showing partial epitaxial relation with comprising diamond. The gap between the graphite and diamond may be enriched with oxygen impurity.

Self-propagating high-temperature synthesis and sintering of AlN‒Y&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;‒Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; compositions

The results of studies on the synthesis of aluminum nitride and AlN‒Al2O3‒Y2O3 compositions with a low oxygen impurity content by the SHS method are presented. The dependence of the lattice parameter c of aluminum nitride on the oxygen impurity content is shown. The conditions for the synthesis of AlN‒Y2O3 compositions with a low oxygen content in the structure of aluminum nitride are determined. The dependence of the composition of oxide phases in compositions on the content of yttrium oxide in the reaction mixture is shown. The optimal content of yttrium oxide in the compositions has been determined. The results of sintering AlN‒Al2O3‒Y2O3 composite powders with different oxygen content in the aluminum nitride structure are shown.

Strengthening the grain boundary of Mo alloy via little TiC addition

The little TiC was added into Mo alloys by facile ball milling and subsequent sintering. Interestingly, ultrafine Mo-TiC alloy (6.53 μm) possesses quasi-cleavage transgranular fracture morphology, which is differed from traditional intergranular fracture morphology of oxide-dispersion-strengthened molybdenum (ODS-Mo) alloy. The alloy interface with the lowest strength changes from Mo grain boundaries (GBs) to grain interior, hence the crack propagates along grain interior. For one thing, some C-deficient TiC (Ti8C5) can adsorb adjacent impurity oxygen at sintering stage to generate in-situ TiOx, thereby purifying Mo matrix. For another thing, coherent interfaces between TiC/TiOx particles and Mo further strengthen Mo matrix. Moreover, a lot of TiC/TiOx particles within Mo grains could efficaciously accumulate dislocations and indirectly improve GBs strength. Compared to ball-milled Mo or ODS-Mo alloy reported in literatures, the sintered Mo-TiC alloys in present work possess a high hardness (468 HV0.2), an excellent match of high strength (1287.3 MPa) and large ductility (18.1%).

Convective Flow Redistribution of Oxygen by Laser Melting of a Zr-Based Amorphous Alloy.

Oxygen impurities play a crucial role in the glass-forming ability and crystallisation behaviour of metallic glasses. In the present work, single laser tracks were produced on Zr59.3-xCu28.8 Al10.4Nb1.5Ox substrates (x = 0.3, 1.3) to study the redistribution of oxygen in the melt pool under laser melting, which provides the basis for laser powder bed fusion additive manufacturing. Since such substrates are commercially not available, they were fabricated by arc melting and splat quenching. X-ray diffraction revealed that the substrate with 0.3 at.% oxygen was X-ray amorphous, while the substrate with 1.3 at.% oxygen was partially crystalline. Hence, it is evident that the oxygen content affects the crystallisation kinetics. Subsequently, single laser tracks were produced on the surface of these substrates, and the melt pools attained from the laser processing were characterised by atom probe tomography and transmission electron microscopy. Surface oxidation and subsequent convective flow redistribution of oxygen by laser melting were identified as causes of the presence of CuOx and crystalline ZrO nanoparticles in the melt pool. Bands of ZrO likely originate from surface oxides that were moved deeper into the melt pool by convective flow. The findings presented here highlight the influence of oxygen redistribution from the surface into the melt pool during laser processing.

Fatigue crack growth behavior of titanium with oxygen impurities: Experiments and modeling

The long-term reliability of structural elements in many advanced applications, such as power generation and transportation infrastructures, is paramount, especially under cyclic damage conditions. In this work, fatigue crack growth experiments were performed in commercial purity titanium with different initial oxygen contents and load ratios (R). To model fatigue crack initiation and growth, a fatigue indicator parameter (FIP) along with high-resolution crystal plasticity simulations were carried out. The crack-nucleation along with microstructurally short crack (MSC) growth models are calibrated for R = 0.1 with different oxygen contents. The resulting calibrated parameters along with the average total FIP values for R = 0.2 were used to predict nucleation and MSC growth behavior. Results indicate that this approach can predict the number of cycles required for crack nucleation and MSC behavior.