Titanium Interstitials Research Articles

Achieving metal-like catalysis from semiconductors for on-surface synthesis

Free of posttransfer, on-surface synthesis (OSS) of single-atomic-layer nanostructures directly on semiconductors holds considerable potential for next-generation devices. However, due to the high diffusion barrier and abundant defects on semiconductor surfaces, extended and well-defined OSS on semiconductors has major difficulty. Furthermore, given semiconductors' limited thermal catalytic activity, initiating high-barrier reactions remains a significant challenge. Herein, using TiO2(011) as a prototype, we present an effective strategy for steering the molecule adsorption and reaction processes on semiconductors, delivering lengthy graphene nanoribbons with extendable widths. By introducing interstitial titanium (Tiint) and oxygen vacancies (Ov), we convert TiO2(011) from a passive supporting template into a metal-like catalytic platform. This regulation shifts electron density and surface dipoles, resulting in tunable catalytic activity together with varied molecule adsorption and diffusion. Cyclodehydrogenation, which is inefficient on pristine TiO2(011), is markedly improved on Tiint/Ov-doped TiO2. Even interribbon cyclodehydrogenation is achieved. The final product's dimensions, quality, and coverage are all controllable. Tiint doping outperforms Ov in producing regular and prolonged products, whereas excessive Tiint compromises molecule landing and coupling. This work demonstrates the crucial role of semiconductor substrates in OSS and advances OSS on semiconductors from an empirical trial-and-error methodology to a systematic and controllable paradigm.

Electronic and Surface Properties of Aluminum (111) Surface Modified by Interstitial and Substitutional Titanium Incorporation

This study investigates the influence of interstitial and substitutional titanium atoms on the electronic properties of aluminum surfaces using density functional theory (DFT). The study focuses on three variables: the presence and arrangement of Ti interstitials on the aluminum surface, the behavior of Ti substitutional and interstitial impurities, and the energetic stability and structural properties of these systems. Multiple DFT methods are employed to derive conclusions regarding the impact of these variables on the surface properties of aluminum. The study provides valuable insights into how different states of interstitial and substitutional Ti can alter the physical characteristics and performance behaviors of the aluminum surface. The understanding of these effects could enable engineers to design more efficient materials with enhanced properties suitable for various industries.

Grain refinement in titanium prevents low temperature oxygen embrittlement

Interstitial oxygen embrittles titanium, particularly at cryogenic temperatures, which necessitates a stringent control of oxygen content in fabricating titanium and its alloys. Here, we propose a structural strategy, via grain refinement, to alleviate this problem. Compared to a coarse-grained counterpart that is extremely brittle at 77 K, the uniform elongation of an ultrafine-grained (UFG) microstructure (grain size ~ 2.0 µm) in Ti-0.3wt.%O is successfully increased by an order of magnitude, maintaining an ultrahigh yield strength inherent to the UFG microstructure. This unique strength-ductility synergy in UFG Ti-0.3wt.%O is achieved via the combined effects of diluted grain boundary segregation of oxygen that helps to improve the grain boundary cohesive energy and enhanced <c + a> dislocation activities that contribute to the excellent strain hardening ability. The present strategy will not only boost the potential applications of high strength Ti-O alloys at low temperatures, but can also be applied to other alloy systems, where interstitial solution hardening results into an undesirable loss of ductility.

Pair distribution function analysis driven by atomistic simulations: Application to microwave radiation synthesized TiO[formula omitted] and ZrO[formula omitted

A workflow is presented for performing pair distribution function (PDF) analysis of defected materials using structures generated from atomistic simulations. A large collection of structures, which differ in the types and concentrations of defects present, are obtained through energy minimization with an empirical interatomic potential. Each of the structures is refined against an experimental PDF. The structures with the lowest goodness of fit Rw values are taken as being representative of the experimental structure. The workflow is applied to anatase titanium dioxide (a-TiO2) and tetragonal zirconium dioxide (t-ZrO2) synthesized in the presence of microwave radiation, a low temperature process that generates disorder. The results suggest that titanium vacancies and interstitials are the dominant defects in a-TiO2, while oxygen vacancies dominate in t-ZrO2. Analysis of the atomic displacement parameters extracted from the PDF refinement and mean squared displacements calculated from molecular dynamics simulations indicate that while these two quantities are closely related, it is challenging to make quantitative comparisons between them. The workflow can be applied to other materials systems, including nanoparticles.

Oxidation and electrochemical characterization of oxide formed on Ti-Al-Zr in a neutral steam environment

The study focuses on the oxidation and the electrochemical behavior of the oxide formed on an alpha titanium alloy, Ti-Al-Zr, in a neutral steam environment. The alloy was exposed in a static autoclave at 400 °C, 10 MPa for durations up to 90 days. The oxidation kinetics and the rate laws were established by the discontinuous gravimetric method. The characterization of the oxide was done by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), glow discharge optical emission spectroscopy (GDOES) and Raman spectroscopy. The electrochemical characterization of the high-temperature oxide film was done by a potentiodynamic polarization experiment and ex-situ electrochemical impedance spectroscopy (EIS) measurement. The defect density of the oxide was calculated using the Mott-Schottky (M-S) analysis. A physical model was developed based on the impedance response, within the framework of point defect model (PDM) theory. The oxidation kinetics followed a nearly fourth power rate law followed by a linear rate law. The oxide consisted of an inner barrier layer and an outer layer containing crystallized grains. The transition in the oxidation kinetics was attributed to the recrystallization of the outer layer. The defect density for all the oxidized specimens was of the order of 1021 cm−3. The EIS study, in conjunction with the PDM, established that the oxidation kinetic was controlled by both titanium interstitials and oxygen vacancies. A correlation between the oxidation kinetics and the kinetics of the interfacial reactions was established and the oxide growth mechanism in the steam environment is discussed.

Predicting virtual creep crack growth in a simulated titanium aluminide alloy microstructure containing voids, precipitates, and grain/grain boundary distortions

Over the years numerous of materials based experimental research programmes have focused on understanding the high temperature failure mechanisms and their relation to the microstructure of engineering alloys. These microstructures are complex in nature and are being continuously developed to improve creep/fatigue and oxidation strength of components at elevated temperatures. Furthermore, with the advent of additive manufacturing (AM) even more diverse microstructures which can contain voids, interstitial Titanium Aluminides (Ti-Al) and grain distortions have evolved at the sub-grain level affecting the creep strength of the alloys. Since measurements of detailed sub-grain properties are difficult to obtain and are likely to contain large levels of scatter a modelling procedure for a meso-scale model in multiaxial creep to identify creep damage is needed. The model using a grain (GR)/grain boundary (GB) scale mesh in the cracking region of a Compact Tension geometry simulates crack growth rates under different microstructural anomalies and variabilities such as voids and precipitates and grain shapes. In this model micro-cracks can randomly initiate and grow under a micro-triaxial stress state at positions where accumulated creep damage reaches critical values. A detailed sensitivity analysis of GB/GR/size/distortion/strength and void/precipitate density and distribution simulations is used to identify the likely reasons for cracking rates, crack branching and data scatter in creep test. Available high temperature material and mechanical properties from a representative Ti-Al intermetallic alloy are used to validate the model. The present analysis sets out the modelling approach and points the way to more complex microstructures which could contain a multiplicity of phases and sub-grain particles. The development of such models is likely to be the preferred approach to predict virtual creep crack growth in both welding and new additive manufacturing processes.

Electronic correlations and Fermi liquid behavior of intermediate-band states in titanium-doped silicon

We study the nature of the electronic states in the intermediate band formed by interstitial titanium in silicon. Our single-site description combines effects of electronic correlations, captured by dynamical mean-field theory, and disorder, modeled using the coherent potential approximation and the typical medium mean-field theory. For all studied concentrations an extended metallic state with a strongly depleted density of states at the Fermi level is obtained. The self-energy is characteristic to Fermi-liquids and for certain temperatures reveals the existence of coherent quasi-particles.

Custom-Made Titanium Mesh Tray for Mandibular Reconstruction Using an Electron Beam Melting System.

Mandibular reconstruction using a titanium mesh tray and autologous bone is a common procedure in oral and maxillofacial surgery. However, there can be material problems—such as broken titanium mesh trays—which may undermine long-term functionality. This study was designed to investigate the optimal conditions for a titanium mesh tray with an ideal mandibular shape and sufficient strength, using computer-aided design, computer-aided manufacturing technology, and electron beam additive manufacturing. Specimens were prepared using Ti-6Al-4V extra low interstitial titanium alloy powder and an electron beam melting (EBM) system. The mechanical strength of the plate-shaped specimens was examined for differences in the stretch direction with respect to the stacking direction and the presence or absence of surface treatment. While evaluating the mechanical strength of the tray-shaped specimens, the topology was optimized and specimens with a honeycomb structure were also verified. Excellent mechanical strength was observed under the condition that the specimen was stretched vertically in the stacking direction and the surface was treated. The results of the tray-shaped specimens indicated that the thickness was 1.2 mm, the weight reduction rate was 20%, and the addition of a honeycomb structure could withstand an assumed bite force of 2000 N. This study suggests that the EBM system could be a useful technique for preparing custom-made titanium mesh trays of sufficient strength for mandibular reconstruction by arranging various manufacturing conditions.

Reducible oxide and allotropic transition induced by hydrogen annealing: synthesis routes of TiO2 thin films to tailor optical response

Optical properties of hydrogenated TiO2 thin films deposited by the ion beam sputtering method are reported. By reducing the oxide by hydrogenation in situ (during deposition) and ex situ (by means of posterior thermal annealing) a controlled phase modification allows to manipulate optical properties to improve photoactivity for solar energy conversion and catalytic applications. The hydrogenation procedure prompts remarkable structural changes and optical properties of the TiO2 hierarchical thin films. Photoemission electron spectroscopy (XPS) results show the presence of titanium in Ti4+ into Ti3+ state of oxidation, i.e., compatible with oxygen vacancies and titanium interstitials. Vibrational properties inspected by IR absorption and Raman spectroscopy allowed identifying potentially catalytic sites and vibration modes associated with hydrogen. The vibrational spectra show that both in situ and ex situ hydrogenation routes contribute to prompt (dis)order in the films, involving the presence of anatase crystalline phase embedded in an amorphous matrix or mixed-phase (anatase + rutile) crystalline TiO2. To avoid debate about the TiO2 theoretical (indirect) and expected and measured (direct, normally reported) optical gap (Egap), the absorption coefficient spectra were analyzed by a recently developed method based on the Boltzmann sigmoidal function. From these analyses, conclusions about defects states prompted by the hydrogenation processes are presented.

Unveiling the interplay between induced native defects and room temperature magnetic ordering in titanium deficient disordered-TiO2 nanoparticles

Understanding the origin of magnetic ordering in an undoped semiconductor with native defects is an open question, which is being explored actively in research. In this investigation, the interplay between magnetic ordering and excess induced native defects in undoped anatase TiO2 nanoparticles is explained using an experimental and theoretical approach. It is demonstrated that structurally disordered TiO2 nanoparticles with a high concentration of native defects such as titanium interstitials and oxygen vacancies are synthesized using controlled atmospheric rapid cooling (i.e. quenching) process. The structural disorders in the lattice have been examined using various spectroscopic and microscopic analyses revealed the existence of Ti deficiency in both pristine and quenched TiO2 nanoparticles. A possible origin of magnetic ordering in titanium deficient anatase TiO2 system is elucidated based on first-principle calculations. It was found that the overall magnetic moment of Ti deficient TiO2 system is determined by the distance between Ti interstitials and its neighboring vacancies (i.e. either V Ti or V Os). However, quenched TiO2 nanoparticles possess excess Ti interstitials, Ti and O vacancies and therefore the net magnetic moment of the system is reduced due to anti-ferromagnetically coupled neighboring Tilattice ions.

Chemical diffusion analysis of the oxidation kinetics of oxygen-deficient, nitrogen doped TiO2 thin films

Modification of TiO2 by nitrogen-doping improves its photocatalytic efficiency in visible-light activated photocatalytic processes. Therefore, the defect structure and the point defect transport of N-doped TiO2 represent important fundamental issues. Shi et al. (2012) have studied the re-oxidation kinetics of chemically reduced N-doped TiO2 (anatase) thin films by means of in situ optical spectroscopy. It was found that the time dependence of optical absorbance due to oxidation follows a parabolic rate law. The present paper is devoted to further development and refinement of the kinetic model. The ambipolar chemical diffusion model accounts for three types of mobile charged point defects with essentially different diffusion coefficients (electrons, holes, and titanium interstitials) as well as for a uniformly distributed background of immobile nitrogen ions. The concentration profiles of diffusing species and oxygen activity profiles have been obtained by numerical as well as by approximate analytical solutions of the chemical diffusion equation. The profiles reveal the presence of two separate singularities: an electron-hole recombination reaction front and a step-like electron concentration front whose coordinate obeys a parabolic rate law.

Electromigration-induced leakage current enhancement and its anisotropy in single crystal TiO2

Time-dependent increases in electrical conductivity in dielectrics exposed to a direct-current voltage bias can be detrimental to device reliability and lifetime, while the same phenomenon may be exploited for resistive switching functionality. The applied electric field induces the electromigration of charged point defects, resulting in spatially inhomogeneous stoichiometry and modifications in the electrode electrostatics. Herein, we monitor the time-dependent changes in leakage current in rutile TiO2 with platinum electrodes. Current–voltage measurements before and after the extended electrical biasing demonstrate that changes in the electrostatics of the cathode Schottky barrier are responsible for the time-dependent increases in leakage current. We also find that electromigration is accelerated along the ⟨001⟩ relative to the ⟨010⟩ crystallographic direction in contrast to what would be predicted by high-temperature diffusion kinetics. We find the observed anisotropic behavior to be consistent with a change in the defect charge state of the titanium interstitials and dominant transport path at lower temperatures. A comparison of optical color front migration kinetics to the temporal evolution of leakage current further supports the interpretation that ionic electromigration near the cathode interface is responsible for the increased leakage current in the electrically degraded state.

Extra low interstitial titanium based fully porous morphological bone scaffolds manufactured using selective laser melting

Lattice structure based morphologically matched scaffolds is rapidly growing facilitated by developments in Additive Manufacturing. These porous structures are particularly promising due to their potential in reducing stress shielding and maladapted stress concentration. Accordingly, this study presents Extra Low Interstitial (ELI) Titanium alloy based morphological scaffolds featuring three different porous architecture. All scaffolds were additively manufactured using Selective Laser Melting from Ti6Al4V ELI with porosities of 73.85, 60.53 and 55.26% with the global geometry dictated through X-Ray Computed Tomography. The elastic and plastic performance of both the scaffold prototypes and the bone section being replaced were evaluated through uniaxial compression testing. Comparing the data, the suitability of the Maxwell criterion in evaluating the stiffness behaviour of fully porous morphological scaffolds are carried out. The outcomes show that the best performing scaffolds presented in this study have high strength (169 MPa) and low stiffness (5.09 GPa) suitable to minimise stress shielding. The matching morphology in addition to high porosity allow adequate space for flow circulation and has the potential to reduce maladapted stress concentration. Finally, the Electron Diffraction X-ray analysis revealed a small difference in the composition of aluminium between the particle and the bonding material at the scaffold surface.

Parameters controlling the crystallization kinetics of nanostructured TiO2 – An overview

Abstract In-situ high-temperature synchrotron radiation diffraction was used to determine the effect of air and argon, calcination time, ion implantation, and pressure on phase transformations of nanostructured TiO2. The as-synthesized TiO2 nanomaterials were amorphous initially, as shown by the diffraction results. Crystalline anatase and rutile phases formed at elevated temperatures after thermal annealing. The anatase-to-rutile transformation (as determined from phase analysis in different atmospheres) is more rapid in argon than in air because of oxygen vacancies. The quantity of anatase decreased slightly with an increase in isothermal heating, whereas the quantity of rutile increased by phase transformation. The implantation of TiO2 nanostructures with different dopants affects the phase crystallization temperature. The implantation of V and Al ions reduces the crystallization temperature because smaller ions substituted for Ti in the TiO2 crystal structure by the dopant. A decrease in structural rigidity results in a relaxation of the Ti bonding environment, but Cr and In ions implantation resulted in a phase transformation inhibition. Cr and In ions substitute into the Ti sublattice and inhibit the anatase-to-rutile transformation. An improved structural rigidity is provided by In3+ and Cr3+ ions, which are larger than the Ti4+ ion. This increased rigidity restricts relaxation in the Ti bonding environment. An increased capillary gas pressure resulted with an increase in temperature. The transformation of the amorphous phase to anatase was enhanced by nanopowder heating under pressure and that of anatase-to-rutile transformation was reduced. This behavior is believed to occur in an oxygen-rich environment and interstitial titanium is expected to form when the material is heated under pressure. This suggests that atmospheric oxygen appears to accelerate the amorphous-to-anatase transformation, whereas interstitial titanium inhibits TiO2 structure relaxation, which is required for the anatase-to-rutile transformation.

Characterization of radiation damage in TiO2 using molecular dynamics simulations

Molecular dynamics simulations are carried out to characterize irradiation effects in TiO2 rutile, for wide ranges of temperatures (300–900 K) and primary knock-on atom (PKA) energies (1–10 keV). The number of residual defects decreases with increased temperature and decreased PKA energy, but is independent of PKA type. In the ballistic phase, more oxygen than titanium defects are produced, however, the primary residual defects are titanium vacancies and interstitials. Defect clustering depends on the PKA energy, temperature, and defect production. For some 10 keV PKAs, the largest cluster of vacancies at the peak of the ballistic phase and after annealing has up to ≈1200 and 100 vacancies, respectively. For the 10 keV PKAs at 300 K, the energy storage, primarily in residual Ti vacancies and interstitials, is estimated at 140–310 eV. It decreases with increased temperature to as little as 5–180 eV at 900 K. Selected area electron diffraction patterns and radial distribution functions confirm that although localized amorphous regions form during the ballistic phase, TiO2 regains full crystallinity after annealing.

Mechanical properties and microstructure of Ti5Si3 based composites prepared by combination of MASHS and SPS in Ti-Si-Ni and Ti-Si-Ni-C systems

Abstract Mechanical properties, synthesis behavior, microstructure and oxidation resistance of Ti5Si3-based composites prepared by combination of mechanically activated self-propagating high-temperature synthesis (MASHS) and spark plasma sintering (SPS) in Ti-Si-Ni and Ti-Si-Ni-C systems were investigated. Ti5Si3-Ti2Ni composite was synthesized in Ti-Si-Ni system while Ti5Si3-TiC-Ti2Ni was formed in the Ti-Si-Ni-C. Microstructural analyses showed that Ni and C extensively distributed in Ti5Si3 phase and formed interstitial and substitutional solid solution titanium silicide. The activation energy values of 126 and 135 kJ/mol were calculated for initiation of combustion reaction in Ti-Si-Ni and Ti-Si-Ni-C systems, respectively. The isothermal oxidation behavior of the samples in air atmosphere was studied by TGA analysis and near the same oxidation resistance was detected. The fracture toughness of fabricated composites in Ti-Si-Ni and Ti-Si-Ni-C systems reached to 5 ± 0.1 and 6.1 ± 0.12 MPam1/2, respectively. The higher fracture toughness in Ti-Si-Ni-C system is related to the uniform distribution of TiC in the composite.

Defects, Adsorbates, and Photoactivity of Rutile TiO2 (110): Insight by First-Principles Calculations.

We investigate the effect of adsorbates on the structure and photoabsorption of reduced TiO2 by first-principles calculations of rutile TiO2(110) in the presence of both titanium interstitials (Tiint's) and adsorbed water or methanol. Our results show that while Tiint's prefer to reside in deep inner layers when the surface is clean, they tend to diffuse toward the surface in the presence of water or methanol. This migration is due to the mutual stabilization of the adsorbates and Tiint defects in the near-surface region. We also find that adsorbed water/methanol changes the orbital character and localization sites of the excess electrons associated with the Tiint. These results can explain why the adsorption of water and methanol enhances the photoabsorption of the reduced TiO2(110) surface.

Effects of Superplastic Forming on Modification of Surface Properties of Ti Alloys for Biomedical Applications

In the present work, both the surface chemical contamination and the mechanical alteration of Ti–6Al–4V (Ti64) and Ti–6Al–4V extra low interstitial (Ti64ELI) titanium alloys subjected to superplastic forming (SPF) for the manufacturing of highly customized biomedical prostheses have been investigated. As case study, a cranial implant was considered. The design of the manufacturing process was assisted by a numerical model calibrated on free inflation experimental tests. Glow discharge optical emission spectrometry (GDOES) analyses, nanoindentation tests, and metallographic analyses allowed to relate the mechanical alteration to the oxygen enrichment due to the environmental exposition during processing. While similar diffusion kinetics were found, different oxidation rates were measured in the two investigated alloys. The hardness variation was strictly related to the oxygen content. In order to verify the material biocompatibility, cytotoxicity tests were conducted on the most oxidized part. Results highlighted that the oxygen enrichment due to the manufacturing process did not significantly affect the cells viability.

Resolving phase stability in the Ti-O binary with first-principles statistical mechanics methods

The Ti-O system consists of a multitude of stable and metastable oxides that are used in wide ranging applications. In this work we investigate phase stability in the Ti-O binary from first principles. We perform a systematic search for ground state structures as a function of oxygen concentration by considering oxygen-vacancy and/or titanium-vacancy orderings over four parent crystal structures: (i) hcp Ti, (ii) $\ensuremath{\omega}$-Ti, (iii) rocksalt, and (iv) hcp oxygen containing interstitial titanium. We explore phase stability at finite temperature using cluster expansion Hamiltonians and Monte Carlo simulations. The calculations predict a high oxygen solubility in hcp Ti and the stability of suboxide phases that undergo order-disorder transitions upon heating. Vacancy ordered rocksalt phases are also predicted at low temperature that disorder to form an extended solid solution at high temperatures. Predicted stable and metastable phase diagrams are qualitatively consistent with experimental observations, however, important discrepancies are revealed between first-principles density functional theory predictions of phase stability and the current understanding of phase stability in this system.

Density functional theory study of atomic and electronic properties of defects in reduced anatase TiO2 nanocrystals

Anatase TiO2 nanocrystals have received considerable attention owing to their promising applications in photocatalysis, photovoltaics, and fuel cells. Although experimental evidence has shown that the performance of nanocrystals can be significantly improved through reduction, the mechanistic basis of this enhancement remains unclear. To shed a light on the chemistry of reduced anatase TiO2 nanocrystals, density functional theory were used to investigate the properties of defects and excess electrons. We demonstrated that oxygen vacancies are stable both on the surface and at the sub-surface of the nanocrystal, while titanium interstitials prefer sub-surface sites. Different defect locations possessed different excess electron structures, which contributed to deep and shallow states in the band gap of the nanocrystals. Furthermore, valence band tailing was observed, resulting in band gap narrowing. The theoretical results presented here deepen our understanding, and show the potential of defects to considerably change the macroscopic properties of anatase TiO2 nanocrystals.