Interstitial Diffusion Research Articles

Oxygen transport properties of Ca-doped Pr2NiO4

Praseodymium nickelate Pr2NiO4+δ with layered Ruddlesden–Popper (R–P) structure is a promising material for the ceramic membranes for oxygen separation and for SOFC cathodes due to its mixed ionic-electronic conducting nature and high oxygen mobility and surface reactivity. However, a low thermal stability and the inadequate electronic conductivity of Pr2NiO4+δ require its doping for a more efficient performance. This work presents results of study of the effect of Ca doping on the structural and transport properties of Pr2NiO4. Pr2−xCaxNiO4 oxides (x = 0–0.6) were synthesized by a co-precipitation method and characterized by XRD, XPS and TEM methods. The oxygen content in these materials and its variation with increasing temperature was evaluated using TGA. The oxygen transport properties of powdered samples were studied by the temperature-programmed oxygen isotope heteroexchange with C18O2 (TPIE). Electrical conductivity of compact samples was measured by a dc four-probe technique. Electrochemical measurements were performed using an impedance spectroscopy method with symmetrically arranged electrodes on the Ce0.8Sm0.2O1.9 electrolyte. It was shown that doping resulted in enhanced electrical conductivity and at a low Ca content the electrochemical activity of electrodes increased while the interstitial oxygen content and oxygen diffusivity gradually decreased. For x > 0.3 increasing the lattice anisotropy resulted in the co-existence of 2–3 channels of oxygen diffusion revealed as separate peaks in TPIE curves. The fast diffusion channel (D⁎ ~ 10−8 cm2/s at 700 °C), with a share in the total diffusion drastically decreasing at big doping levels, corresponds to the fast interstitial oxygen diffusion via the cooperative mechanism while the slow channels (D⁎ < 10−10 cm2/s) are probably related to the oxygen transport in perovskite layers and the complicated transport involving interlayer positions near the dopant cation sites.

Kinetics of the electronic center annealing in Al2O3 crystals

The experimental annealing kinetics of the primary electronic F, F+ centers and dimer F2 centers observed in Al2O3 produced under neutron irradiation were carefully analyzed. The developed theory takes into account the interstitial ion diffusion and recombination with immobile F-type and F2-centers, as well as mutual sequential transformation with temperature of three types of experimentally observed dimer centers which differ by net charges (0, +1, +2) with respect to the host crystalline sites. The relative initial concentrations of three types of F2 electronic defects before annealing are obtained, along with energy barriers between their ground states as well as the relaxation energies.

A first-principles and experimental study of helium diffusion in periclase MgO

The distribution of He isotopes is used to trace heterogeneities in the Earth’s mantle, and is particularly useful for constraining the length scale of heterogeneity due to the generally rapid diffusivity of helium. However, such an analysis is challenging because He diffusivities are largely unknown in lower mantle phases, which can influence the He profiles in regions that cycle through the lower mantle. With this motivation, we have used first-principles simulations based on density functional theory to study He diffusion in MgO, an important lower mantle phase. We first studied the case of interstitial helium diffusion in perfect MgO and found a migration barrier of 0.73 eV at zero pressure. Then we used the kinetic Monte Carlo method to study the case of substitutional He diffusion in MgO, where we assumed that He diffuses on the cation sublattice through cation vacancies. We also performed experiments on He diffusion at atmospheric pressure using ion implantation and nuclear reaction analysis in both as-received and Ga-doped samples. A comparison between the experimental and simulation results are shown. This work provides a foundation for further studies at high-pressure.

Nanostructured Co-Mn containing perovskites for degradation of pollutants: Insight into the activity and stability

The efficient oxidative removal of persistent organic components in wastewater relies on low-cost heterogeneous catalysts that offer high catalytic activity, stability, and recyclability. Here, we designed a series of nanostructured Co-Mn containing perovskite catalysts, LaCo1−xMnxO3+δ (LCM, x = 0, 0.3, 0.5, 0.7, and 1.0), with over-stoichiometric oxygen (δ > 0) to show superior catalytic activity for the degradation of a variety of persistent aqueous organic pollutants by activating peroxymonosulfate (PMS). The nature of LCM for catalysis was comprehensively investigated. A “volcano-shaped” correlation was observed between the catalytic activity and electron filling (eg) of Co in LCM. Among these compounds, LaCo0.5Mn0.5O3+δ (LCM55) exhibited an excellent activity with eg = 1.27. The high interstitial oxygen ion diffusion rate (DO2− = 1.58 ± 0.01 × 10−13 cm2 s−1) of LCM55 also contributes to its catalytic activity. The enhanced stability of LCM55 can be ascribed to its stronger relative acidity (3.22). Moreover, an increased solution pH (pH ≥ 7) generated a faster organic degradation rate and a decrease in metal leaching (0.004 mM) for LCM55 perovskite, justifying it as a potential material for environmental remediation.

Experimental and theoretical comparison of Sb, As, and P diffusion mechanisms and doping in CdTe

Fundamental material doping challenges have limited CdTe electro-optical applications. In this work, the As atomistic diffusion mechanisms in CdTe are examined by spatially resolving dopant incorporation in both single-crystalline and polycrystalline CdTe over a range of experimental conditions. Density-functional theory calculations predict experimental activation energies and indicate that As diffuses slowly through the Te sublattice and quickly along GBs similar to Sb. Because of its atomic size and associated defect chemistry, As does not have a fast interstitial diffusion component similar to P. Experiments to incorporate and activate P, As, and Sb in polycrystalline CdTe are conducted to examine if ex situ Group V doping can overcome historic polycrystalline doping limits. The distinct P, As, and Sb diffusion characteristics create different strategies for increasing hole density. Because fast interstitial diffusion is prominent for P, less aggressive diffusion conditions followed by Cd overpressure to relocate the Group V element to the Te lattice site is effective. For larger atoms, slower diffusion through the Te sublattice requires more aggressive diffusion, however further activation is not always necessary. Based on the new physical understanding, we have obtained greater than 1016 cm−3 hole density in polycrystalline CdTe films by As and P diffusion.

Dissociative Adsorption, Dissolution, and Diffusion of Hydrogen in Liquid Metal Membranes. A Phenomenological Model

There is only limited experimental data and theoretical treatment available in the literature on hydrogen sorption and diffusion in liquid metals, in stark contrast with that in solid metals. This paper utilizes our predictive phenomenological model requiring minimal input, the Pauling Bond Valence-Modified Morse Potential (PBV-MMP) model, for estimating the thermodynamic and kinetic parameters of hydrogen solution and diffusion in liquid metals treated as quasi-crystalline. The PBV-MMP model is a refinement of the Unity Bond Index-Quadratic Exponential Potential (UBI-QEP) model for estimating the energetics of solid metal surface reactions. The sequential kinetic steps of hydrogen dissociative surface adsorption on the feed side, subsurface penetration, and atomic interstitial diffusion in the bulk, followed by these steps in reverse on the permeate side, are thus treated via the PBV-MMP model within Eyring’s transition-state theory framework, while the entropic changes are evaluated via Eyring’s free vo...

On the abnormal fast diffusion of solute atoms in α-Ti: A first-principles investigation

Solute atoms such as Fe, Co, and Ni diffuse abnormally fast in α-Ti, which influences significantly the mechanical properties of the titanium alloys. Various mechanisms (e.g., the interstitial diffusion mechanism and solute-vacancy complex mechanism) have been proposed to account for the fast diffusion of these solutes in α-Ti. To elucidate such diffusion mechanism, a first-principles method is employed to calculate the formation energies, migration energy barriers, and solute-vacancy binding energies of the substitutional and interstitial solute atoms including Al, Si, Sn, V, Mn, Fe, Co, Ni, and Cu in α-Ti. Based on the calculated parameters, the diffusion mechanisms are discussed. Comparing the formation energies of the substitutional and interstitial solutes, we find that all the solute atoms prefer the substitutional configuration to the interstitial one. The interstitial migration energy barriers are quite low for all the solutes. Al and Sn diffuse mainly through normal vacancy mediated mechanism due to the high substitutional to interstitial preferential energy that leads to very low fraction of interstitial solutes (about 10−15∼10−16 at 1000 K) at thermal equilibrium state and high interstitial diffusion activation energy. The 3d metal solute atoms, especially Mn, Fe, and Co, are fast diffusers and the diffusion coefficients are dominated by the interstitial mechanism because of their sizable thermal equilibrium interstitial fractions (several percent at 1000 K). The solute-vacancy complex mechanism is not likely to account for the fast diffusions in α-Ti. We show that the direct chemical interaction between the solute and matrix atoms determines the site-occupancy of the solute atoms in α-Ti besides the atomic size effect that was commonly believed to be responsible for the fast diffusions.

First-principles study for strain effects on oxygen migration in zirconium

Oxygen interstitial atom diffusion in bulk α-Zr is studied using first-principles calculation and nudged elastic band method under various tetragonal strain conditions. Octahedral site is a stable position for an oxygen interstitial in α-Zr. An oxygen interstitial atom may diffuse from one octahedral site to another via a tetrahedral site or a crowdion site. An oxygen atom can travel around zirconium bulk with various combination of these diffusion processes. The diffusion from an octahedral site to a tetrahedral site or a crowdion site is the controlled step. The strain effects on the two diffusion mechanisms are different. In general, the diffusion rate decreases under certain tetragonal compressive strain conditions. As a result, the oxidation rate decreases under certain compressive strain conditions.Since surface coating may be used to protect zirconium metal surface from oxidation, the surface strain generated from the interfacial mismatch can be used for the material design. In our calculations, SiC, TiN, and Al2O3 coatings may slow down the oxidation rate of zirconium because they can induce compressive strain on zirconium near the interfacial region.

Density functional theory calculation of diffusion mechanism of intrinsic defects in rutile TiO2

Diffusion mechanisms of four intrinsic point defects in rutile TiO2, titanium interstitial (TiI), titanium vacancy (Vti), oxygen interstitial (OI) and oxygen vacancy (VO) are studied in the framework of density functional theory with quantum ESPRESSO suite. Diffusion processes are simulated by defect movement between two stable atomic configurations through using the climbing image nudged elastic band (CI-NEB) method.The initial and final atomic structure in the minimum energy path (MEP) are constructed with 3×3×4 perfect supercell matrix of 216 atoms. Considering that oxygen atoms build up TiO6 octahedron and half of the octahedral centers are occupied by Ti atoms in rutile, interstitial defect is constructed by adding one Ti or O atom to the empty oxygen octahedral center, and vacancy defect is constructed by removing one atom from crystal lattice grid. Structural relaxation is performed before performing the NEB calculation with gamma k point sampling in irreducible Brillouin zone with an energy cutoff of 650 eV. As rutile TiO2 has tetragonal symmetry (P42/mnm), the diffusion channel along the[100] direction is equivalent to the[010] direction. Then, the diffusion paths along the direction parallel to c axis ([001] direction) and perpendicular to the c axis ([100] or[110] direction) are chosen to find the minimum diffusion energy path of TiI and OI. As for VTi and VO, diffusion paths are established from the vacancy site to nearest lattice site of the same kind.Calculation results exhibit significant anisotropy of energy barrier and diffusion mechanism. Of all defect species, TiI diffusion along the[001] direction through interstitial mechanism has the lowest activation barrier of 0.5057 eV. In addition, diffusions along the[100] and[110] direction through kick-out mechanism show higher energy barriers of 1.0024 eV and 2.7758 eV, respectively. Compared with TiI, OI shows small barrier discrepancy between different diffusion directions, which is 0.859 eV along[001] and 0.902 eV along[100] direction. For vacancy defects, diffusion can occur only through the vacancy mechanism. The activation barrier energy of symmetrically inequivalent diffusion path of VO is 0.735 eV along the[110] direction, 1.747 eV along the[001] direction, and 1.119 eV from the TiO6 apex site to the equator site. On the other hand, VTi has two inequivalent paths with much larger diffusion energy barriers:2.375 eV along the[111] direction and 3.232 eV along the[001] direction. In summary, the TiI interstitial diffusion along the[001] direction (parallel to the c axis) has the lowest activation barrier in rutile TiO2, which is in excellent agreement with former experimental and theoretical data.

Prediction of interstitial diffusion activation energies of nitrogen, oxygen, boron and carbon in bcc, fcc, and hcp metals using machine learning

Study of diffusion in solids is of fundamental importance in understanding the materials properties such as phase transformations, segregation, and corrosion in processing and operations. Reliable diffusion data is crucial in the successful design of materials and manufacturing processes. In this work, we use gradient boosting with decision tree regression, a robust machine learning method, to develop the model for predicting interstitial diffusion activation energies of the light element X (B, C, O, and N) in 54 metals with bcc, fcc and hcp lattices. With the introduction of elastic strain energy, a dominant concept of interstitial diffusion theory, we are able to train the machine learning model achieving high accuracy of R-squared 0.9 without being over-fitted by using 94 impurity – host binary systems reported in the literature. Furthermore, our study reveals that apart from elastic strain energy, the electronic interaction between the impurity and the host (represented by two fundamental parameters in the well-known Miedema model) also plays an important role. By accurately predicting the diffusion activation energies, we report their trends across 4th, 5th and 6th period and Lanthanides series of the host atoms. More importantly, our predicted data can be used to approximate the transport rates in materials for which little or no diffusion data are available, thus accelerating the design of a wide range of new application-specific materials.

Diffusion and interactions of interstitials in hard-sphere interstitial solid solutions.

Using computer simulations, we study the dynamics and interactions of interstitial particles in hard-sphere interstitial solid solutions. We calculate the free-energy barriers associated with their diffusion for a range of size ratios and densities. By applying classical transition state theory to these free-energy barriers, we predict the diffusion coefficients, which we find to be in good agreement with diffusion coefficients as measured using event-driven molecular dynamics simulations. These results highlight that transition state theory can capture the interstitial dynamics in the hard-sphere model system. Additionally, we quantify the interactions between the interstitials. We find that, apart from excluded volume interactions, the interstitial-interstitial interactions are almost ideal in our system. Lastly, we show that the interstitial diffusivity can be inferred from the large-particle fluctuations alone, thus providing an empirical relationship between the large-particle fluctuations and the interstitial diffusivity.

Photofrin® photodynamic therapy with intratumor photosensitizer injection provides similar tumor response while reducing systemic skin photosensitivity: Pilot murine study.

The goal of this study was to compare tumor response to Photofrin® photodynamic therapy using intravenous and intratumoral injection of photosensitizer. Systemic skin photosensitivity and photosensitizer distribution were also compared between the two delivery methods. SCCVII tumors were initiated in the hind legs of female C3H mice and grown to a volume of ∼1,000 mm3 . Photofrin® was delivered intravenously via the tail vein at a concentration of 2 mg/kg or intratumorally at concentrations ranging from 0.5-2 mg/kg. A 630 nm laser illumination was delivered via interstitial diffuser placement at a fluence rate of 400 mW/cm and fluence of 100 J/cm. Mice were maintained under normal room lighting for 24 hours after treatment, at which point photographs were captured for assessment of skin photosensitivity. Animals were then sacrificed, and their tumors were excised, sectioned, imaged, and stained with hematoxylin and eosin (H&E). H&E slides were imaged to assess necrosis post-PDT, and skin photographs were evaluated by two blinded reviewers for quantification of skin photosensitivity. Whole-body fluorescence imaging was performed before and after photodynamic therapy. Tumor necrosis was not significantly different based on treatment group (P = 0.33), while skin photosensitivity was significantly reduced in animals that received Photofrin® intratumorally (P = 0.0005). Fluorescence imaging revealed similar photosensitizer fluorescence in excised tumors for intratumor and intravenous injection of Photofrin® (P = 0.48), although fluorescence decreased significantly with decreasing intratumor injection concentration (P= 0.01). This pilot study shows that intratumoral administration of Photofrin® has the potential to produce similar tumor outcomes, while reducing systemic skin photosensitivity. Further studies are warranted to characterize and optimize intratumor delivery. Lasers Surg. 50:476-482, 2018. © 2017 Wiley Periodicals, Inc.

Finite size effects: deuterium diffusion in nm thick vanadium layers

We investigate the effect of finite size on the chemical diffusion of deuterium in extremely thin V(001) layers. A five fold increase in the diffusion coefficient is observed at concentrations around 0.2 [D/V], when the thickness of the V is decreased from 28 to 14 atomic layers (≈2.1–4.2 nm). The size dependent deuterium–deuterium interaction energy is argued to be the root of the observed changes as the diffusion rates are similar at low concentrations. The results demonstrate the feasibility of using finite-size effects to enhance the chemical diffusion of light interstitials in solids. We discuss the general applicability of these effects to other systems.

Mechanical Spectroscopy of Ti-15Zr-based Alloys with Mo Addition

In this paper, the effect of substitutional Mo amounts in internal friction and interstitial diffusion mechanisms was analyzed in Ti-15Zr-based alloys. Mechanical spectroscopy was obtained from room temperature up to 730 K with frequencies between 1 Hz and 40 Hz. Internal friction spectra were composed by anelastic relaxation peaks in β-type alloys (metastable and stable), due to stress-induced ordering of oxygen and nitrogen interstitially in octahedral sites of the bcc crystalline structure. Peak decomposition analysis exhibited interactions between matrix-interstitial (Ti-O and Ti-N), substitutional-interstitial (Zr-O, Mo-O and Mo-N), and clusters (Ti-O-O and Zr-O-O). The diffusion results showed that the introduction of Mo facilitates the diffusion of interstitial elements in the metallic matrix.

First-principles investigation on stability and diffusion mechanism of helium impurities in 4H-SiC

The stability, structure, energetics and diffusive properties of He impurities in 4H-SiC have been investigated by first principles calculations in order to understand its irradiation effects. The formation energies indicate a He atom is preferable to occupy the Si pre-existing vacancy at first. In turn, by formation of He defects associating at interstitial and vacancy sites require positive but low formation energies, suggesting its propensity diffusion in the material. By calculating its energy barriers corresponding to both the interstitial diffusion and vacancy assisted diffusion, we suggest that the most probable diffusion pathway involves the exchange of interstitial and vacancy positions with the effective migration energies from 0.1 eV to 1.0 eV. Remarkable, our calculated diffusion barriers are consistent well with previous theoretical data for He diffusion in 3C-SiC.

Interstitial photoacoustic technique and computational simulation for temperature distribution and tissue optical properties in interstitial laser photothermal interaction

Interstitial laser immunotherapy (ILIT) is designed to use photothermal and immunological interactions for treatment of metastatic cancers. The photothermal effect is crucial in inducing anti-tumor immune responses in the host. Tissue temperature and tissue optical properties are important factors in this process. In this study, a device combining interstitial photoacoustic (PA) technique and interstitial laser photothermal interaction is proposed. Together with computational simulation, this device was designed to determine temperature distributions and tissue optical properties during laser treatment. Experiments were performed using ex-vivo porcine liver tissue. Our results demonstrated that interstitial PA signal amplitude was linearly dependent on tissue temperature in the temperature ranges of 20–60[Formula: see text]C, as well as 65–80[Formula: see text]C, with a different slope, due to the change of tissue optical properties. Using the directly measured temperature in the tissue around the interstitial optical fiber diffusion tip for calibration, the theoretical temperature distribution predicted by the bioheat equation was used to extract optical properties of tissue. Finally, the three-dimensional temperature distribution was simulated to guide tumor destruction and immunological stimulation. Thus, this novel device and method could be used for monitoring and controlling ILIT for cancer treatment.

Thermodynamic modelling of fast dopant diffusion in Si

In the present study, nickel and copper fast diffusion in silicon is investigated in the framework of the cBΩ thermodynamic model, which connects point defect parameters with the bulk elastic and expansion properties. All the calculated point defect thermodynamic properties (activation Gibbs free energy, activation enthalpy, activation entropy, and activation volume) exhibit temperature dependence due to the non-linear anharmonic behavior of the isothermal bulk modulus of Si. Calculated activation enthalpies (0.15–0.16 eV for Ni and 0.17–0.19 eV for Cu) are in agreement with the reported experimental results. Small values of calculated activation volumes for both dopants (∼4% of the mean atomic volume) are consistent with the interstitial diffusion of Ni and Cu in Si.

First principles calculations for iodine atom diffusion in SiC with point defects

The formation energies, diffusion barriers, vibration frequencies of complex iodine defects in 3C-SiC are calculated. The effective diffusion rates of these complex defects are evaluated in the temperature range from 300 K to 2000 K. The iodine interstitial diffusion is the dominant diffusion mechanism in bulk SiC since its diffusion energy barriers is the lowest. On the contrary, if the iodine atom substitutes a Si or a C atom, the diffusion rate is the lowest. The diffusion rates of other iodine point defects coupled with vacancies are between the rates of these two cases. The vacancies near the iodine atoms are traps for the iodine and they can slow down the diffusion rate of iodine atoms. The experimental diffusion rate of iodine in SiC is between that of calculated interstitial and substitutional iodine. It may result that the experimental data is from the combination of all the point defect diffusion mechanisms.

Kinetics of dimer F type center annealing in MgF2 crystals

Abstract In this paper, we analyzed experimental annealing kinetics of the primary electronic F centers and dimer F2 centers observed in MgF2 at higher radiation doses and temperatures. The developed phenomenological theory takes into account the interstitial ion diffusion and recombination with the F2-centers, as well as mutual sequential transformation with temperature growth of three types of experimentally observed dimer centers: F2(1), F2(2), F2(3) (which differ tentatively by charges (0, +1, +2) with respect to the host crystalline sites). The results of the electron, neutron and ion irradiation are compared. As the result, the relative initial concentrations of three types of F2 electronic defects before annealing are obtained, along with energy barriers between their ground states as well as the relaxation energies.

An innovative concept for interstitial diffusion in stressed crystals

Existing concepts for diffusion of interstitial atoms in stressed bodies mostly assume that the mechanical driving force stems from the gradient of the hydrostatic stress component. However, both octahedral and tetrahedral interstitial positions in bcc lattice have tetragonal symmetry only. Therefore, deposition of an atom there causes an anisotropic distortion of the lattice. Considering occupation of octahedral positions, three types of them exist differing in misfit eigenstrain tensors and, consequently, in the interaction energies with any stress field. Thus, the state of the system must be described by three independent site fractions (internal variables) corresponding to the three types of octahedral positions. An according new diffusion equation and respective evolution equations for the three site fractions are derived. The concept is demonstrated on a simple example for diffusion of hydrogen in the stress field of a crack in an elastic body loaded by a uniaxial stress. The current new concept is also directly applicable to the occupation of tetrahedral positions.