Oriented Polycrystalline Materials Research Articles

Determination of single crystal thermal expansion in Uranium-6wt%Niobium shape memory alloy using in-situ diffraction and modeling of textured polycrystalline samples

In-situ neutron diffraction and ex-situ dilatometry experiments were performed on a chemically banded, quenched uranium-6wt% niobium shape memory alloy to study the impact of deformation-(detwinning-) induced texture on the evolution of thermally induced strains (and associated stresses). Thermal heating and cooling cycles between 5K and 473K were performed in-situ with neutron diffraction, and the lattice strain evolution is reported for the observed monoclinic α″ phase. Comparisons between the measured diffraction strains and macroscopic dilatometry experiments reveal relationships between micro- and macro-level thermal expansion. Softening of the texture during heating suggests that twin boundary motion can accommodate the large internal thermal strains which are approximately 2% greater than that typically observed during the 175K heating interval used to age more randomly oriented polycrystalline material. Assuming weak constraint of neighboring grains in randomly textured polycrystals, the single crystal thermal expansion tensor is extracted from measurements of lattice strains over the range 5K to 473K. Predictions of polycrystalline thermal expansion, based upon this single crystal thermal expansion tensor, are shown to compare favorably with bulk thermal expansion observations of the as quenched microstructure. However, such a lower-bound estimate is insufficient to explain all aspects of the behavior of the textured material, where the matrix is not isotropic. It is hypothesized that relaxation processes which occur within the quenched microstructure during heating are responsible for the distinct thermal expansion behavior observed during the initial heating cycle compared to cooling and subsequent cycling.

Water sensitivity of heteroepitaxial Cu-MOF films: dissolution and re-crystallization of 3D-oriented MOF superstructures.

3D-oriented metal-organic framework (MOF) films and patterns have recently emerged as promising platforms for sensing and photonic applications. These oriented polycrystalline materials are typically prepared by heteroepitaxial growth from aligned inorganic nanostructures and display anisotropic functional properties, such as guest molecule alignment and polarized fluorescence. However, to identify suitable conditions for the integration of these 3D-oriented MOF superstructures into functional devices, the effect of water (gaseous and liquid) on different frameworks should be determined. We note that the hydrolytic stability of these heteroepitaxially grown MOF films is currently unexplored. In this work, we present an in-depth analysis of the structural evolution of aligned 2D and 3D Cu-based MOFs grown from Cu(OH)2 coatings. Specifically, 3D-oriented Cu2L2 and Cu2L2DABCO films (L = 1,4-benzenedicarboxylate, BDC; biphenyl-4,4-dicarboxylate, BPDC; DABCO = 1,4-diazabicyclo[2.2.2]octane) were exposed to 50% relative humidity (RH), 80% RH and liquid water. The combined use of X-ray diffraction, infrared spectroscopy, and scanning electron microscopy shows that the sensitivity towards humid environments critically depends on the presence of the DABCO pillar ligand. While oriented films of 2D MOF layers stay intact upon exposure to all levels of humidity, hydrolysis of Cu2L2DABCO is observed. In addition, we report that in environments with high water content, 3D-oriented Cu2(BDC)2DABCO recrystallizes as 3D-oriented Cu2(BDC)2. The heteroepitaxial MOF-to-MOF transformation mechanism was studied with in situ synchrotron experiments, time-resolved AFM measurements, and electron diffraction. These findings provide valuable information on the stability of oriented MOF films for their application in functional devices and highlight the potential for the fabrication of 3D-oriented superstructures via MOF-to-MOF transformations.

Grain orientation and transport properties of textured Bi2Te3 alloys

The new material developments in the field of thermoelectrics over the past two decades have resulted in new materials with unique transport properties as well as new synthetic approaches. Enhanced thermoelectric properties of these new materials through thermal conductivity reduction and electrical properties enhancement strategies such as nano-structuring and band engineering have also been reported. Nevertheless, the material system upon which all new thermoelectric material developments are compared with continues to be alloys of Bi2Te3. Approaches for thermoelectric property enhancements for Bi2Te3 alloys have also been undertaken, however, oriented polycrystalline materials continue to be of primary interest for thermoelectric device development. To this end we investigate n and p-type Bi2Te3 alloys provided by II-VI Inc. in order to elucidate the texturing in these materials and their transport properties with texturing. These materials are highly textured, as required in order to maximize the thermoelectric properties, with an 80% enhancement of the thermoelectric properties obtained with texturing.

Disordering process of GeSb2Te4 induced by ion irradiation

The disordering process in crystalline GeSb2Te4 films has been studied by means of ion irradiation with 150 keV Ar+ ions. The effect of the interfaces and the role of the crystal microstructure has been investigated. The disordering path observed in a randomly oriented polycrystalline material with trigonal structure involves the transition to the disordered rocksalt structure (at fluence 7 × 1013 cm−2) and then to the amorphous phase (at 1.5 × 1014 cm−2). In GeSb2Te4 epitaxially grown on Si(1 1 1) the formation of the disordered rocksalt phase (DRS) occurs at much higher fluence (3 × 1014 cm−2) and it is preceded by the conversion of the stable phase into the ordered rocksalt structure (at 5 × 1013 cm−2), with the formation of ordered vacancy layers, associated to a local variation of the stoichiometry. Even by increasing the fluence up to 3.5 × 1014 cm−2, the films remains mainly crystalline.The observed behaviour has been attributed to the effect of the interfaces and suggests the possibility to promote switching between the crystalline phases by interface engineering.

Single layer In-O atomic sheets as phonon and electron barriers in ZnO-In2O3 natural superlattices: Implications for thermoelectricity

The phases in the ZnO half of the ZnO-In2O3 binary system are natural superlattices consisting of a periodic stacking of single InO2 sheets separated by indium doped ZnO blocks with a spacing that depends on the composition according to the relationship In2O3(ZnO)k. Characterization by combined, atomic resolution, aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy analysis indicates that the atomic structure of each InO2 layer consists of a single continuous sheet of octahedrally coordinated InO2. The sheets are also crystallographic inversion boundaries. Analysis of the electrical conductivity, thermal conductivity, and Seebeck coefficient data at 800 °C indicates that the InO2 sheets not only decrease thermal conductivity by phonon scattering but also create potential barriers to electron transport. The origin of the potential barriers, the role of piezoelectric effects, and their dependence on superlattice spacing are discussed qualitatively. It is also argued that the crystallographically aligned InO2 sheets within individual grains are also transport barriers in randomly oriented polycrystalline materials.

Controlling the Crystallinity of Electrochemically Deposited CdS Nanowires

This report describes two distinct growth regimes occurring during the templated electrochemical deposition of CdS nanowires. The abundance of Cd2+ ions at the growth front dictates crystal nucleation and growth processes. At high Cd2+ concentration, randomly oriented polycrystalline material is formed, whereas local depletion of Cd2+ results in highly oriented crystals with the [002] crystalline direction aligned with the nanowire axis. Single nanowires exhibiting both regions were produced for direct imaging of the microstructure associated with these regimes. The current dependence of the time needed to locally deplete Cd2+ allowed for the extraction of a diffusion coefficient for Cd2+ of 1.23 × 10–6 cm2/s, which reflects reduced diffusion in nanoscale channels. Knowledge of these growth regimes and the parameters governing them allows for the growth of materials with targeted structural properties. Specifically, single-crystal materials can be preferentially formed by depositing at relatively high growth currents or by reducing the initial concentration of the Cd precursor in solution.

Superhard nanocomposites: Origin of hardness enhancement, properties and applications

The original finding of Veprek et al. that in nc-TiN/a-Si 3N 4 and in nc-TiN/a-Si 3N 4/TiSi 2 nanocomposites, deposited under conditions which allow complete phase segregation by spinodal mechanism, the maximum hardness of ≥ 45 and > 100 GPa, respectively, is achieved when the thickness of the interfacial Si 3N 4 is about 1 monolayer, has been recently confirmed by both experiments and theory. First principle calculations explain why the decohesion and shear strength of a TiN–SiN x –TiN sandwich is higher than that of bulk SiN x . Combined ab initio DFT calculations of shear resistance of the interfaces, their averaging according to Sachs for randomly oriented polycrystalline material to obtain tensile yield strength, Tabor's criterion, Hertzian analysis and pressure-enhanced flow stress explain in a simple way the experimentally achieved high values of hardness of > 100 GPa, in excess of diamond. Friedel oscillations of the valence charge density, originating from negative charge transfer to the strengthened SiN x interface, cause decohesion and ideal shear to occur between Ti–N bonds near that interface. The extraordinary mechanical properties of these and related quasi-binary superhard nanocomposites can be understood in terms of nearly flaw-free strong materials with no need to invoke any new mechanism of strengthening. We shall present selected examples of industrial applications of the superhard nanocomposite coatings.

The 390 cm−1 feature of polycrystalline hematite—An optical crystallite size effect

Abstract The 390 cm −1 spectral feature of polycrystalline hematite is shown to be caused by a crystallite size effect which is caused by the anisotropic nature of single crystalline hematite. This effect occurs if the probing light is able to resolve the heterogeneous nature of a polycrystalline material and if the corresponding single crystalline material is optically anisotropic. Therefore it does not depend on whether the material of interest is a powder or consolidated. As a consequence, the resulting macroscopic reflectance and transmittance of the polycrystalline material is an average of the microscopic reflectance and transmittance of the individual crystallites. Therefore, randomly oriented polycrystalline materials with large crystallites show characteristic changes in their spectral profile compared to the spectral profiles of polycrystalline materials consisting of crystallites that are small in comparison with the wavelength. The extent of the spectral changes depends on the degree of optical anisotropy of the corresponding single crystalline material. The spectral changes also comprise non-zero cross-polarization terms despite of random orientation. Therefore a characterization of a polycrystalline material with a scalar dielectric function is possible in general only if the material consists of randomly oriented crystallites small compared to the wavelength.

Employing spectra of polycrystalline materials for the verification of optical constants obtained from corresponding low-symmetry single crystals

The determination of optical constants from spectra of low-symmetry single crystals or from spectra stemming from unfavorable crystal faces by dispersion analysis is a difficult task. Besides oscillator frequencies, band positions can additionally depend on oscillator strength as well as on the relative orientation of the transition moment. Therefore the results of the analysis are highly prone to errors. We present an easy method to validate the oscillator data resulting from dispersion analysis by the comparison of measured and simulated spectra of randomly oriented polycrystalline materials. Depending on the crystallite size, either average reflectance and transmittance theory (ARTT) or average refractive index theory (ARIT) can be applied to model the spectra of polycrystalline compounds. As an alternative to ARIT, effective medium theory (EMA) can also be employed. However, since principal dielectric functions do not exist in the general triclinic case, which are needed for the application of EMA, we suggest using the eigenvalues of the dielectric tensor function instead. Our method for the verification of optical constants of single crystals is validated using monoclinic CuO as an example.

Ion beam assisted texturing of polycrystalline Y 2O 3 films deposited via electron-beam evaporation

Ion-beam assisted deposition of polycrystalline Y 2O 3 films using e-beam evaporation was investigated. For growth on non-crystalline substrates, low temperature growth yields randomly oriented polycrystalline material. At elevated temperature, surface energy anisotropy yields a (111) uniaxial texture. For film deposition with irradiation from an Ar ion beam, the out-of-plane texture remained (111) in orientation. The incident Ar ion beam induces an in-plane alignment of the Y 2O 3 films that is relatively broad. A six-fold symmetry in the out-of-plane X-ray diffraction phi-scans was observed for the (111) textured Y 2O 3 films, indicating a multi-variant in-plane texture and suggesting anisotropic damage along both the (110) and (100) projections. The lack of a sharp, single variant in-plane texture with ion beam irradiation is consistent with the relatively weak bond strength in Y 2O 3.

Magnetostriction of Tb 0.36Dy 0.64(Fe 1− xCo x) 2 ( x = 0–0.20) 〈1 1 2〉-oriented crystals

〈1 1 2〉-Oriented crystals of the highly magnetostrictive alloys Tb 0.36Dy 0.64(Fe 1− x Co x ) 2 ( x = 0–0.20) and Tb 0.3Dy 0.7Fe 2 have been prepared by zone melting directional solidification method with growth velocities of 240 and 600 mm/h. It is found that 〈1 1 2〉-oriented crystals were formed with dendritic-solidified morphologies. Narrower lamellar spacing is observed in samples obtained with higher growth velocity. Magnetostrictive properties were measured under various compressive pre-stresses at room temperature (20 °C), elevated temperature (100 °C) and cryogenic temperature (−80 °C). Magnetostriction “jump” effect is less obvious in all Tb 0.36Dy 0.64(Fe 1− x Co x ) 2 〈1 1 2〉-oriented crystals at −80 °C than at 20 and 100 °C. The measurement results reveal that large magnetostriction in the 〈1 1 2〉-oriented polycrystalline materials of Tb 0.36Dy 0.64(Fe 1− x Co x ) 2 can be retained in a wide temperature range from −80 to 100 °C.

Modelling IR spectra of single-phase polycrystalline materials with random orientation in the large crystallites limit$mdash$extension to arbitrary crystal symmetry

A formalism based on Yeh's 4 × 4 matrix algebra is developed to model reflectance and transmittance spectra of randomly oriented polycrystalline materials (sintered ceramics or glass ceramics) with arbitrary crystal symmetry provided that the crystallites are larger than the resolution limit of light. It may be applicable to powders, if the particle sizes are large compared to wavelength or show a broad distribution. The formalism is used to model the spectrum of polycrystalline fresnoite (Ba2TiSi2O8, optically uniaxial). The modelled and the measured spectra show a good correspondence, especially regarding peak position and form. Furthermore, approximations are derived for uniaxial crystal symmetry, which are superior to other approximations known from the literature such as the magic-angle arrangement. As a consequence of the formalism, randomly oriented polycrystalline materials consisting of ordered regions larger than the resolution limit should exhibit a non-zero cross polarization, despite being optically isotropic, which is proved experimentally. For an adequate interpretation of spectra of polycrystalline materials with non-cubic crystal symmetry, the particle size effect arising from the anisotropic crystal structure is demonstrated to be non-negligible.

Growth and magnetostriction of oriented polycrystalline Pr/sub 0.15/Tb/sub x/Dy/sub 0.85-x/Fe/sub 2/ (x=0-0.85)

Oriented polycrystalline materials of Pr/sub 0.15/Tb/sub x/Dy/sub 0.85-x/Fe/sub 2/ have been obtained by the Czochralski method with the cold crucible system under a purified-argon atmosphere. Experiments were carried out over a wide range, and the conditions, including growth of 30-10 mm/hr and temperature gradient of 110-140/spl deg/C/cm, have been optimized for oriented polycrystalline materials growth. The magnetostrictive properties of Pr/sub 0.15/Tb/sub x/Dy/sub 0.85-x/Fe/sub 2/ were found to be very sensitive to both the Pr stoichiometry and to the Tb/Dy ratio. The measurement results reveal that the oriented polycrystalline materials of Pr/sub 0.15/Tb/sub 0.2/Dy/sub 0.65/Fe/sub 2/ possess large magnetostriction at room temperature.

EPR study of preferential orientation of crystallites in N-doped high quality CVD diamond

The directions of preferential growth of free-standing optical-quality CVD diamond wafers have been investigated with the help of electron paramagnetic resonance (EPR). EPR signals of the well-known P1 centre (substitutional nitrogen) have been used as a probe. A computer simulation of EPR spectra of preferentially oriented polycrystalline material allows the estimation of the real orientation and spatial distribution of crystallographic axes of crystallites in the samples oriented preferentially in the [110] direction.

Ba 0.5Sr 0.5TiO 3 thin films deposited by PLD on SiO 2/Si RuO 2/Si and Pt/Si electrodes

Stoichiometric barium strontium titanate (Ba 0.5Sr 0.5TiO 3 or BST) 220-nm thick thin films were deposited by pulsed laser ablation on SiO 2/Si, RuO 2/Ta/SiO 2/Si and Pt/Ti/SiO 2/Si substrates at 400°C. The films were weakly crystalline as-deposited. Crystallization was induced by annealing the films in the range of 550–650°C. The BST films deposited on Pt/Ti/SiO 2/c-Si substrates presented wide cracks that were promoted during the annealing process due to the thermal expansion mismatch between the BST films (α BST=4×10 −6 °C −1) and the Pt (α Pt=9×10 −6 °C −1). Smooth films showing slightly cracked areas were obtained on SiO 2/c-Si and RuO 2/Ta/SiO 2/Si substrates. The ruthenium oxide thermal expansion coefficient is α RuO 2 =5.2×10 −6 °C −1. A cross-sectional analysis at the ferroelectric/substrate interface showed that for the lower annealing temperature (550°C) a mixed amorphous/nanocrystalline microstructure is formed. For temperatures above 600°C a randomly oriented polycrystalline material is obtained. However, an amorphous layer of 4–6 nm still remains on the substrate even after heat-treatments up to 650°C. The dielectric constant of the BST films varied in the range of 30–325.

Ferroelectric and microstructure properties of Ba1−xSrxTiO3 films grown on different electrodes

Ferroelectric Ba1−xSrxTiO3 thin films were deposited by pulsed laser ablation on SiO2/Si, RuO2/Ta/SiO2/Si and Pt/Ti/SiO2/Si substrates. The films were weakly crystalline in the as-deposited condition and subsequent crystallization was induced by annealing the films in the range of 550–650°C. The BST films deposited on Pt/Ti/SiO2/c-Si substrates presented wide cracks that were promoted during the annealing process due to the thermal expansion mismatch between the BST films (αBST = 4 × 10−6°C−1) and the Pt (αPt = 9 × 10−6°C−1). Smooth films showing slightly crackeded areas were obtained on SiO2/c-Si and RuO2/Ta/SiO2/Si substrates. The ruthenium oxide thermal expansion coefficient is αRuO2 = 5.2 × 10−6°C−1. A cross-sectional analysis at the ferroelectric/substrate interface showed that for the lower annealing temperature (550°C) a mixed amorphous/nanocrystalline microstructure is formed. For temperatures above 600°C a randomly oriented polycrystalline material is obtained. However, an amorphous layer of 4–6 nm still remains on the substrate even after heat-treatments up to 650°C. The dielectric constant of the BST films varied in the range of 30–325.

Electron diffraction from polycrystalline materials showing stress induced preferred orientation

The Gibbs free energy as generalized by J. F. Nye [Physical Properties of Crystals (Clarendon Press, Oxford, 1957), p. 179] is minimized in thermodynamic systems held at constant temperature and constant stress. This function is orientation dependent in all crystal systems in stress fields which are not purely hydrostatic. There are situations in which preferred orientation arises as a result of the synthesis of materials under impressed stress conditions such as thin film growth under ion bombardment and the pressing of powders into solids. Here, we derive the orientational constraints for cubic crystals which result from growth under a general biaxial stress field. The sign of the expression δ=s11−s12−12s44 determines the behavior of a cubic crystal. Electron diffraction patterns of face-centered-cubic specimens with both positive and negative values of δ are calculated using a program in MATLAB and displayed in a form suitable for direct comparison with experiment. The use of a biaxial stress with unequal principal components for producing highly oriented polycrystalline material is discussed. In the case of δ positive, as occurs in silicon, the preferred orientation is simply an alignment of the 〈100〉 directions along the principal stresses. For δ negative, as occurs in titanium nitride, the preferred orientation depends on the ratio of the principal stresses and low index directions are aligned with the principal stresses only when the principal stresses are either equal or one of them is zero. In the general case, arc-like diffraction patterns are produced. The results of a calculation of a diffraction pattern from a cross-sectional TiN film are compared with diffraction patterns reported by L. Hultman et al. [J. Appl. Phys. 78, 5395 (1995)] and show good agreement.

Orientation of samarium–cobalt compounds by solidification in a magnetic field

The solidification from the liquid state in a magnetic field produces oriented polycrystalline materials. A high degree of orientation is obtained with Sm–Co compounds solidified in several Tesla. The samples are crystallographically oriented with their easy-magnetization axes lying along the direction of the magnetic field applied during solidification. The process can be applied to the production of bulk anisotropic permanent magnets, without using the powder metallurgy. A model, validated by experimental results in the case of Sm–Co alloys, is proposed to explain the orientation mechanism.

Growth of iron-based Laves phase containing Pr and Nd magnetostrictive materials

The synthesis and magnetostriction of R 1− x Ce x Fe 2 (R=Pr, Nd) have been investigated. The pesudobinary compounds of Laves phase with high-Pr and Nd concentration can be obtained due to the strong Ce 4f bonding. Oriented polycrystalline materials can be produced by means of Czochralski method with cold crucible system. The measurement of magnetostriction reveals that the materials possess large magnetostriction at room temperature.

Diffraction Peaks in X-Ray Spectroscopy: Friend or Foe?

AbstractDiffraction peaks can occur as unidentifiable peaks in the energy spectrum of an x-ray spectrometric analysis. Recently, there has been increased interest in oriented thin polycrystalline films and epitaxial films on single crystal substrates for electronic applications. Since these materials diffract x-rays more efficiently than randomly oriented polycrystalline materials, diffraction peaks are being observed more frequently in x-ray fluorescent spectra In addition, micro x-ray spectrometric analysis utilizes a small, intense, collimated x-ray beam that can yield well defined diffraction peaks. In some cases these diffraction peaks can occur at the same position as elemental peaks. These diffraction peaks, although a possible problem in qualitative and quantitative elemental analysis, can give very useful information about the crystallographic structure and orientation of the material being analyzed. The observed diffraction peaks are dependent on the geometry of the x-ray spectrometer, the degree of collimation and the distribution of wavelengths (energies) originating from the x-ray tube and striking the sample.