Probing the Pore Size of Porous Ceramics with Controlled Amount of Magnetic Impurities via Diffusion Effects on the CPMG Technique

In our work, we will explore the possibility of implementing the well-known Carr–Purcell–Meiboom–Gill pulse sequence to determine the pore size of porous ceramics with magnetic impurities. The proposed approach exploits the diffusion dependence of the spin-echo signal in the presence of internal gradients occurring as a result of susceptibility contrast between the porous matrix and the confined liquid. For calibrating the technique, a comparison of the pore size data with those extracted from the so-called DDIF technique (DDIF, decay due to diffusion in the internal fields) is performed. This approach can be applied for nondestructive in situ characterization of soils, concrete, biological tissues or other structures with micrometer pore size.     

Influence of the degree of crystal perfection and deviation from the stoichiometric composition on the diffusion processes in samarium sulfide

The processes of diffusion of samarium and europium in nonstoichiometric samarium sulfide (SmS) at temperatures in the range 950–1600°C have been investigated by the radioactive isotope method and the method of weight loss upon evaporation of excess samarium. It has been found that there is a correlation between the diffusion coefficient D and the size of coherent X-ray scattering regions in SmS samples, as well as between the diffusion coefficient and the degree of deviation from the stoichiometric composition: the diffusion coefficient of impurities decreases as the size of coherent X-ray scattering regions increases and the stoichiometric composition is approached. The calculation of the diffusion coefficient of electrons in samarium sulfide at T = 77–300 K has demonstrated that the value of D increases with increasing temperature and increasing size of coherent scattering regions.     

Interaction of deep impurities with radiation defects in n-Si at γ-irradiation

This work presents the results of research on peculiarities of radiation defect formation in single crystal n-Si, doped by deep level impurities (Cu, Ni, Ir, Rh, Pt and Au), at irradiation by γ-quanta of ⁶⁰Co. A property of γ-irradiation to create only vacancies and self-interstitial atoms is used to understand the nature of deep levels with participation of these impurities and primary elemental radiation defects. The role of covalence radii and diffusion coefficients in efficiency of radiation defect formation is discussed.     

Optical properties of the surface transition-metal structures in fluoride ionic crystals and peculiarities of their formation during thermal diffusion

The possibility of forming surface films with an elevated concentration of an impurity metal during high-temperature diffusion has been analyzed for a wide series of ionic crystals: LiF with Co, Ni, Mg, Ca, Ba, and Sr impurities; NaF with Co, Mn, Mg, Ca, and Sr; MgF₂ with Co and Ni; and CaF₂ with Co. It is established that films are formed only on alkali halide crystals with impurities of transition metals and are not formed on alkaline earth fluorides with transition metals, as well as on alkali halide crystals activated with other divalent cationic impurities. The dynamics of the increase and decrease in the intensity of centers related to impurity-vacancy dipoles during thermal diffusion is shown. The mechanisms of film formation are explained in terms of the features of growth and structure of ionic crystals with cationic impurities and on the basis of isomorphism rules.     

Anomalous surface diffusion of water compared to aprotic liquids in nanopores

1H nuclear magnetic relaxation dispersion experiments show remarkable differences between water and acetone in contact with microporous glass surfaces containing trace paramagnetic impurities. Analyzed with surface relaxation theory on a model porous system, the data obtained for water show that proton surface diffusion limited by chemical exchange with the bulk phase permits long-range effectively one-dimensional exploration along the pores. This magnetic-field dependence coupled with the anomalous temperature dependence of the relaxation rates permits a direct interpretation in terms of the proton translational diffusion coefficient at the surface of the pores. A universal rescaling applied to these data collected for different pore sizes and on a large variety of frequencies and temperatures, supports this interpretation. The analysis demonstrates that acetone diffuses more slowly, which increases the apparent confinement and results in a two-dimensional model for the molecular dynamics close to surface relaxation sinks. Surface-enhanced water proton diffusion, however, permits the proton to explore a greater spatial extent of the pore, which results in an apparent one-dimensional model for the diffusive motions of the water that dominate nuclear spin relaxation.     

Elemental and chemical-state mapping of boron carbide fracture surfaces

Samples of commercial B₄C and of reactive hot-pressed B₉C were fractured in situ in a scanning Auger microscopy (SAM) system. The B₄C fracture surfaces showed small areas of pore structures, as well as larger relatively smooth areas. Impurities (Cu, Si, S, Ca, N, O and Cr) were found in trace amounts in the pore areas. SAM maps were made of most of these impurities. The nitrogen was found by Auger lineshape in point Auger electron spectroscopy scans to be in the compound BN. Carbon was found on the surface in both boron carbide and as graphite. SAM was used to seperately map B in B₄C and B in BN, as well as C in B₄C and C in graphite, in a pore region. In the smooth areas of B₄C fracture surfaces fewer impurities were found. The carbon occured in B₄C and in localized carbon-rich areas which had graphite surface layers. In contrast, only B and C were found on the B₉C fracture surfaces; but the distributions of these elements were not homogenous. Carbon was found both in elemental form as graphite and chemically bound to boron as a carbide. In comparison with the graphite regions on the B₄C surfaces, the graphite areas on the B₉C fracture surfaces were not localized. The total surface area was nearly balanced between graphite and carbide regions. Argon ion bombardment revealed the graphite areas to be less than 100 nm thick.     

NMR Fast Field Cycling Relaxometry of Unsaturated Soils

The bioavailability of water for plant nutrition in natural soils is controlled by the pore system structure and the interaction of water with the pore walls at variable degrees of saturation. For the characterization of these processes T ₁ relaxometry is particularly suitable because it is not influenced by internal gradients and the frequency dependence of T ₁ includes detailed information about the local dynamics at the pore walls. Using Fast Field Cycling Relaxometry, we have determined T ₁ relaxation dispersion curves of unsaturated soil materials which cover a broad range of textures between pure sand and silt-loam. The mean relaxation rates scale inversely with the water content, as expected according to the Brownstein–Tarr model, which means that the effective pore volume is the only water-contributing fraction. By further analysis of the relaxation dispersion curves we find a bi-logarithmic behavior which is describable by a model of two-dimensional diffusion at the liquid–solid interface in the neighborhood of paramagnetic impurities at the surface. The microscopic wettability, as expressed by the ratio of surface residence time and correlation time, is identical for the soil material but decreases by a factor of two for the sand. This relaxation mechanism is unique for all textures and water contents and proves that the water mobility at the surface does not decrease even at the lowest water contents.     

Quantum diffusion in the solid heliums

We discuss the diffusion of³He impurities in solid⁴He resulting from the quantum exchange of particles on neighboring lattice sites. The formation of (³He)₂ molecules bound by the elastic deformation field around the isotopic impurities can occur at low concentrations, and we propose new experiments to show how these molecules may be observed explicitly.     

EPR diagnostics of laser materials based on ZnSe crystals doped with transition elements

The electron paramagnetic resonance (EPR) spectra observed in laser materials based on zinc selenide (ZnSe) crystals doped with transition elements have been analyzed and identified. It has been shown that, in addition to working impurities (Cr²⁺, Co²⁺, or Fe²⁺), the diffusion layer exhibits EPR spectra of accompanying impurities due to the diffusion of transition elements (chromium, cobalt, or iron) used in the preparation of active materials for quantum electronics (lasers, switches) operating in the mid-infrared range. EPR diagnostics of these impurities can be used in the development of appropriate regimes for minimizing concentrations of accompanying impurities that adversely affect the performance characteristics of laser materials. It has been found that, during the diffusion of transition metals, ions of the accompanying impurity Mn²⁺, which is characterized by extremely informative EPR spectra, are embedded in the crystal lattice. It has been proposed to use these ions as ideal markers to control, on the electronic level, the crystal structure of the active diffusion layer.     

Effect of the order in which powdered ZnS is doped with copper chloride and indium on formation of luminescence centers

We consider the effect of the order in which In and CuCl impurities are added during thermal doping on the luminescence characteristics of ZnS, and also their role in formation of emission centers in zinc sulfide. We show that the order in which the impurities (acting as activators or coactivators) are added to the ZnS plays the determining role in formation of the spectral characteristics of the luminophore obtained. We have established that adding indium first during thermal doping of zinc sulfide with In and CuCl prevents diffusion of Cu into the interior of the ZnS. Adding indium after CuCl or adding indium simultaneously with CuCl prevents formation of the Cu2S and CuS phases or promotes degradation of the indicated phases in ZnS. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 5, pp. 601–605, September–October, 2006.     

Model Studies of Rayleigh Instabilities via Microdesigned Interfaces

The energetic and kinetic properties of surfaces play a critical role in defining the microstructural changes that occur during sintering and high-temperature use of ceramics. Characterization of surface diffusion in ceramics is particularly difficult, and significant variations in reported values of surface diffusivities arise even in well-studied systems. Effects of impurities, surface energy anisotropy, and the onset of surface attachment limited kinetics (SALK) are believed to contribute to this variability. An overview of the use of Rayleigh instabilities as a means of characterizing surface diffusivities is presented. The development of models of morphological evolution that account for effects of surface energy anisotropy is reviewed, and the potential interplay between impurities and surface energy anisotropy is addressed. The status of experimental studies of Rayleigh instabilities in sapphire utilizing lithographically introduced pore channels of controlled geometry and crystallography is summarized. Results of model studies indicate that impurities can significantly influence both the spatial and temporal characteristics of Rayleigh instabilities; this is attributed at least in part to impurity effects on the surface energy anisotropy. Related model experiments indicate that the onset of SALK may also contribute significantly to apparent variations in surface diffusion coefficients.     

Ordering of defects in metal systems exposed to high-energy particles

A diffusion model for pore nucleation and growth in metal systems exposed to high-energy particles is proposed. The pore nucleation is seen as coagulation of excess vacancies. Our rough estimates correlate with experiment. A mechanism of radiation-induced pore formation and spatial ordering in metal systems is suggested. Altai State Technical University. Translated from Izvestiya Vysshikh Uchebnykh, Zevedenii, Fizika, No. 4, pp. 82–86, May, 2000.     

Mechanism of light emission in low energy ion implanted silicon

A silicon wafer implanted with a single low energy (42 keV) silicon ion beam results in strong luminescence at room temperature. The implantation results in the formation of various luminescent defect centers within the crystalline and polymorphous regions of the wafer. The resulting luminescence centers (LC) are mapped using fluorescence lifetime imaging microscopy (FLIM). The emission from the ion-implanted wafer shows multiple PL peaks ranging from the UV to the visible; these emissions originate from bound excitonic states in crystal defects and interfacial states between crystalline/amorphous silicon and impurities within the wafer. The LCs are created from defects and impurities within the wafer and not from nanoparticles.     

Quantum diffusion

Basic ideas and results which characterize quantum diffusion of defects in quantum crystals like solid helium as a new phenomenon are presented. Quantum effects in such media lead to a delocalization of point defects (vacancies, impurities etc.) and they turn into quasiparticles of a new type—defectons, which are characterized not by their position in the crystal lattice but by their quasimomentum and dispersion law. Defecton-defecton and defecton-phonon scattering are considered and an interpolation formula for the diffusion coefficient valid in all interesting temperature and concentration regions is presented. A comparison with the experimental data is made. Some alternative points of view are discussed in detail and the inconsistency of the Kisvarsanyi-Sullivan theory is shown.     

Ion doping of gallium arsenide

Conclusion The survey presented shows that despite the intensive study of the physical problems of ion implantation in GaAs, there is still a number of questions to investigate. Among them are: The role of anomalous diffusion and channeling in the formation of concentration profiles, the anomalous diffusion mechanism, the influence of radiation defects and residual impurities as well as the form of the coating, the interaction between impurities and defects under ion implantation, and the identification of radiation defects, A further study of the above-mentioned questions will permit extension of the application of the ion implantation method to obtain GaAs semiconductor instruments and realization of still unused possibilities of this powerful method of modern semiconductor technology.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 76–90, January, 1980.     

The kinetics of barium precipitation at dislocations in NaCl monocrystals

The kinetics of barium precipitation at dislocations in NaCl monocrystals has been studied in thermally and mechanically treated NaCl + 4 ppm BaCl₂ samples by investigating the isothermic variation of ionic conductivity as a function of time. The course of precipitation which takes place at dislocations located at grain boundaries can be divided into three time regions characterized by diffusion of impurities to dislocation cores at grain boundaries, nucleation, formation of new grain boundaries, etc. At higher number of dislocations an interruption of the precipitation appears due to a local free energy of nucleation minimum at radiusr ₀=6·92×10^–8 cm.     

Gettering of metallic impurities in photovoltaic silicon

 This work addresses the issue of structural defect-metallic impurity interactions in photovoltaic silicon and their effect on minority carrier diffusion length values. Aluminium and phosphorus segregation gettering studies were performed on photovoltaic silicon in order to gain insight into these interactions and quantify the effect of gettering on solar cell performance. Integrated circuit grade silicon was also studied for comparative purposes. Additionally, a novel rapid thermal annealing technique, designed to dissolve metallic impurity precipitates, and Deep Level Transient Spectroscopy were utilized to determine the as-grown impurity concentration in both grades of materials. Significant differences in gettering responses between the two grades of silicon are observed. Gettering treatments greatly improve I.C. grade silicon with a specific gettering temperature providing the optimal response. Photovoltaic grade silicon does not respond as well to the gettering treatments and, in some cases, the material degrades at higher gettering temperatures. The degradation is primarily observed in dislocated regions of multicrystalline photovoltaic silicon. Additionally, these dislocated regions were found to possess the highest as-grown metallic impurity concentration of all the materials studied. The dislocation-free photovoltaic silicon has a higher diffusion length relative to dislocated silicon but could not be improved by the gettering methods employed in this study. A model is presented to describe these phenomena where the high concentration of metallic impurities at dislocations produce relatively low minority carrier diffusion lengths as well as the degrading response with higher gettering temperatures while microdefects create an upper limit to the photovoltaic grade material’s diffusion length. Received: 21 June 1996/Accepted: 2 September 1996     

Transition metals in silicon

A review is given on the diffusion, solubility and electrical activity of 3d transition metals in silicon. Transition elements (especially, Cr, Mn, Fe, Co, Ni, and Cu) diffuse interstitially and stay in the interstitial site in thermal equilibrium at the diffusion temperature. The parameters of the liquidus curves are identical for the Si:Ti — Si:Ni melts, indicating comparable silicon-metal interaction for all these elements. Only Cr, Mn, and Fe could be identified in undisturbed interstitial sites after quenching, the others precipitated or formed complexes. The 3d elements can be divided into two groups according to the respective enthalpy of formation of the solid solution. The distinction can arise from different charge states of these impurities at the diffusion temperature. For the interstitial 3d atoms remaining after quenching, reliable energy levels are established from the literature and compared with recent calculations.     

Isolated molecules in metals

Isolated clusters consisting of a heavy radioactive impurity and a small number of light interstitial atoms can be produced in a metallic host and conveniently studied by hfi techniques. The heavy impurities are introduced by ion implantation using the radioactive impurities as probe atoms or in-situ irradiation of the host matrix. Light interstitials can be introduced by diffusion from the gas phase, by electrolytic charging or co-implantation. If a chemical interaction between the two types of atoms exists, these clusters can be regarded as molecules in metals. In reviewing the existing data, the following topics will be discussed: mechanisms of molecule formation, after-effects, stoichiometry, geometry and stability of the complexes formed. It will be shown that nuclear reaction analysis of the light interstitial atoms is a valuable complementary tool in the study of molecules in metals.