Theoretical Bulk Research Articles

Hardness, elasticity, and fracture toughness of polycrystalline spinel germanium nitride and tin nitride

The hardness, elastic moduli, and fracture toughness of the spinel phases, γ–Ge3N4 and γ–Sn3N4, were determined using indentation data and theoretical calculations. Measurements were performed on polycrystalline specimens using the technique of nanoindentation to determine the reduced moduli and hardnesses from the unloading portion of the indent curves. Reduced moduli of γ–Ge3N4 and γ–Sn3N4 were found to be 295 and 167 GPa, respectively. The nanohardnesses of γ–Ge3N4 and γ–Sn3N4 were found to be 31 and 13 GPa, respectively. The shear moduli G0 and Poisson’s ratios ν0 were derived using theoretical bulk moduli B0 obtained from density-functional theory calculations. The calculated values were B0 = 260 GPa, G0 = 146 GPa, ν0 = 0.26 for γ–Ge3N4, and B0 = 186 GPa, G0 = 64 GPa, ν0 = 0.34 for γ–Sn3N4. Fracture toughness was estimated by direct measurement of radial cracks emanating from Vickers microindents. It was determined that for γ–Ge3N4, KIC = 2.3 MPa(m)1/2, while for γ–Sn3N4, KIC = 1.4 MPa(m).1/2

Combustion synthesis of nanocrystalline yttria-doped ceria

Nanocrystalline yttria-doped ceria powder, with the composition Ce0.55Y0.45O1.775, was synthesized by a combustion technique using citric acid as the fuel and the corresponding metal nitrates as oxidants. This process involves mild conditions as the external temperature required to initiate the combustion is only approximately 250 °C. The product was characterized by x-ray diffraction (XRD) to ascertain the phase purity. The crystallite size of these calcined samples, as seen by transmission electron microscopy, was found to be in the range 6 nm to 50 nm. The surface area of the fine powder, as obtained from the Brunauer–Emmett–Teller technique, was about 140 m2/g. The agglomeration behavior as a function of temperature and the lattice thermal expansion studies were carried out using high-temperature XRD. This nanocrystalline powder resulted in nearly theoretical bulk density at a relatively lower temperature, which is attributed to the superior powder properties. The sintered microstructure, as studied by scanning electron microscopy, revealed the presence of fine grains.

Comparison of theory and experiment for the magneto-compositional correlation in a ferromagnetic Fe–V crystal

Abstract Polarised diffuse neutron scattering results are presented for the magneto-compositional response function of ferromagnetic Fe 0.865 V 0.135 . The experimental result agrees well with a calculated result obtained from a first-principles ab initio theory that uses modern spin polarised density functional theory adapted to the KKR-CPA method. Importantly, the experimental results extrapolated to zero scattering vector are found to be close to the expected value determined from both the theoretical calculation and bulk magnetisation measurements.

Minority carrier lifetime and noise in abrupt molecular-beam epitaxy-grown HgCdTe heterostructures

The steady-state lifetime of photogenerated minority carriers has been investigated in heterostructure HgCdTe devices fabricated on molecular-beam epitaxy (MBE) grown material. A wider bandgap capping layer (Hg(1−x)Cd(x)Te, x = 0.44) was grown on a narrower bandgap absorbing layer (Hg(1−x)Cd(x)Te, x = 0.32, λco,80K = 4.57 µm) material in an uninterrupted MBE growth to create an abrupt heterointerface. Steady-state lifetime as a function of temperature over the range 80–300 K was extracted from photoconductive responsivity at an optical wavelength corresponding to the peak responsivity at that temperature. At 80 K, the photoconductors exhibit a specific detectivity of 4.5 × 1011 cm Hz−1/2W−1 (chopping frequency of 1 kHz). For each measurement temperature, the steady-state excess carrier lifetime determined experimentally was compared to the theoretical bulk lifetime for material with x = 0.32 and effective n-type doping density of 3.7 × 1014 cm−3. Theoretical calculations of the Auger-1 lifetime based on expressions developed by Pratt et al. were not able to account for the reduction in lifetime observed at temperatures above 180 K. Two approaches have been attempted to resolve this discrepancy: A semiempirical expression for Auger lifetime attributed to Meyer et al. was used to fit to the data, with the Auger coefficient γ as a fitting parameter. However, the resulting Auger coefficient found in this work is more than an order of magnitude higher than that reported previously. Alternatively, the reduction in effective lifetime above 180 K may be understood as a “loss” of carriers from the narrow bandgap absorbing layer that are promoted across the potential barrier in the conduction band into a low lifetime, wider bandgap capping layer. The reduction in lifetime as a function of inverse temperature for temperatures above 180 K may be fitted by a “cap lifetime” that has an activation energy equal to the change in bandgap across the heterostucture and scaled by a fitting constant.

Electronic Structure and Bulk Properties of β‐SiAlONs

A series of β‐SiAlONs with the composition Si6–zAlzOzN8–z (z= 0.5–4) is prepared by hot isostatic pressing. Evaluated bulk moduli are compared with those calculated using the first‐principles method. Theoretical values are derived from the fit of the energy versus volume dependence. In total‐energy calculations full relaxation of all atomic positions within a supercell is performed. Both experimental and theoretical bulk moduli compare reasonably well and show the decrease in value from ∼240 GPa (z= 0) to ∼170 GPa (z= 4). For the O/N substitutions no preferential occupation of the lattice N sites is observed. Both Al/Si and O/N substitutions cause the local expansion of the structure. In the relaxed structures a sphere of decreased interatomic distances surrounds each substitution site, thus compensating for effect of the local expansion. The increasing Al–O/Si–N substitution rate causes a smooth change of the shape and position of the energy bands. The band gap between occupied and unoccupied states is getting narrower, thus decreasing the isolating properties of the material.

The synthesis and morphology characteristic study of BAO-ODPA polyimide/TiO 2 nano hybrid films

Hybrid nanocomposite films of titanium dioxide (TiO 2) in polyimide (PI) from 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole (BAO) and 4,4′-oxydiphthalic anhydride (ODPA) have been successfully fabricated by an in situ sol–gel process. These nanocomposite films exhibit fair good optical transparency up to 40 wt% of TiO 2 content. X-ray diffraction spectroscopy shows three sharp peaks in pure BAO-ODPA PI. It results from the intermolecular regularity. However, the intermolecular regularity in the hybrid film is disrupted by the introduction of TiO 2 nanoparticles with no sharp peak in XRD spectra. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) results confirm the formation of TiO 2 particles in PI matrix. The surface Ti content is much lower than the theoretical bulk content in all hybrid films. The ratio of the former to the latter increases with the TiO 2 content and levels off at TiO 2 wt%≥20. Transmission electron microscope (TEM) images show that the TiO 2 phase is well dispersed in the polymer matrix. The size of the TiO 2 phase increases from 10 to 40 nm when the TiO 2 content is 5–30 wt%, respectively.

Calculated and experimental low-loss electron energy loss spectra of dislocationsin diamond and GaN

First-principles calculations of electron energy loss (EEL) spectra for bulk GaNand diamond are compared with experimental spectra acquired with a scanningtunnelling electron microscope offering ultra-high-energy resolution in low-lossenergy spectroscopy. The theoretical bulk low-loss EEL spectra, in theEg to10 eV range, are in good agreement with experimental data. Spatially resolvedspectra from dislocated regions in both materials are distinct from bulk spectra.The main effects are, however, confined to energy losses lying above the bandedge. The calculated spectra for low-energy dislocations in diamond are consistentwith the experimental observations, but difficulties remain in understanding thespectra of threading dislocations in GaN.

Calculating the bulk vacancy formation energy (Ev) for a Schottky defect in a perfect Cu(111), Cu(100) and a Cu(110) single crystal

AbstractA study has been made of the bulk vacancy formation energy (Ev) of Cu by calculating the bulk vacancy formation energy for Cu single crystals with different surface orientations ($E_{v}^{(111)} \sim 1.54\ \hbox{eV},\ E_{v}^{(100)} \sim 1.34\ \hbox{eV}$ and $E_{v}^{(110)} \sim 1.07\ \hbox{eV}$). The calculations were performed with the empirical many‐body potentials of Sutton and Chen. The calculations indicate a significant difference in the bulk vacancy formation energy under different surface orientations. The bulk vacancy formation energies were also used to calculate theoretical bulk diffusion coefficients beneath the different surface orientations. These theoretical bulk diffusion coefficients were compared with experimental bulk diffusion coefficients that were calculated from kinetic bulk to surface segregation measurements of Sb that segregates to a Cu(111) and a Cu(110) surface. The calculated bulk diffusion coefficients and the experimental bulk diffusion coefficients showed the same trend regarding the different surface orientations. Copyright © 2002 John Wiley & Sons, Ltd.

Ab initiostudies of structural stability and magnetism inNi3In

To understand the structural stability and magnetism in the intermetallic compound ${\mathrm{Ni}}_{3}\mathrm{In},$ we have performed total energy and electronic structure calculations for ${\mathrm{Ni}}_{3}\mathrm{In}$ as well as ${\mathrm{Ni}}_{3}\mathrm{Al}$ in the cubic ${L1}_{2},$ tetragonal ${D0}_{22},$ and hexagonal ${D0}_{19}$ structures. The highly accurate full-potential linearized augmented-plane-wave method has been used. The calculations are based on first-principles density-functional theory with generalized gradient approximation. The theoretical equilibrium lattice constants and bulk moduli of both the compounds are in good agreement with available experiments. Surprisingly, unlike ${\mathrm{Ni}}_{3}\mathrm{Al}$ and other related intermetallics, ${\mathrm{Ni}}_{3}\mathrm{In}$ in the nonmagnetic state is predicted to energetically favor the ${D0}_{22}$ structure rather than the ${L1}_{2}$ structure. However, the ${L1}_{2}$ and ${D0}_{19}$ structures are found to be magnetically unstable while the ${D0}_{22}$ structure is not. Ferromagnetism would make the ${L1}_{2}$ structure slightly lower in energy than the ${D0}_{22}$ structure by merely 7 meV/f.u. Therefore, the present theoretical work suggests that ${\mathrm{Ni}}_{3}\mathrm{In}$ at low temperatures is a weak ferromagnet and undergoes a structural and magnetic phase transformation as the temperature is raised to the room temperature. This picture would allow one to consistently interpret previous structural experiments and measured x-ray absorption spectra. Moreover, it is also predicted that unlike ${\mathrm{Ni}}_{3}\mathrm{Al}$ and other related intermetallics, there would be a pressure-induced structural phase transition from the ${L1}_{2}$ to ${D0}_{22}$ in ${\mathrm{Ni}}_{3}\mathrm{In}$ at low temperatures. It is hoped that these interesting findings would stimulate further experimental investigations such as temperature-dependent structural, specific-heat, and magnetization experiments on this nearly or weakly magnetic intermetallic compound.

Influence of dislocations on electron energy-loss spectra in gallium nitride

We report on first-principles calculations of electron energy-loss (EEL) spectra for bulk group-III nitrides and of spatially resolved EEL spectra for a variety of edge and screw dislocations in GaN. The theoretical bulk low-loss EEL spectra, in the 0--10 eV range, are in good agreement with recent experimental data. We find gap states associated with undecorated full-core dislocations that lead to absorption below the bulk onset energy in low-loss EEL spectroscopy. It is also found that the electrostatic potential at N atoms in the vicinity of an edge dislocation varies by an order of a volt and casts doubt on any simple interpretation of core-loss spectroscopy.

Theoretical Prediction of Post‐Spinel Phases of Silicon Nitride

New phases of Si3N4 that may be stable at higher pressure than spinel have been searched using a first‐principles plane‐wave pseudopotential method. The CaTi2O4‐type phase is found to be the prime candidate for the post‐spinel phase among six phases selected on the analogy to high‐pressure oxides. The phase transformation from the spinel is predicted to occur at 210 GPa. All silicon atoms of the new phase are coordinated by six anions, similar to the case of the high‐pressure forms of SiO2 and SiC. Because of its high energy at zero pressure, this new phase may be difficult to quench. The bandgap increases with an increase of pressure when compared in the same polymorph. However, the bandgap and the net charge decrease in the order of β, spinel, and CaTi2O4‐type phases at zero pressure. The theoretical bulk modulus of the CaTi2O4‐type phase is comparable with that of spinel.

Roughness characterization of porous soil with acoustic backscatter.

The use of acoustics to determine the pore properties of soils, such as porosity, permeability, and tortuosity, is well established. A theoretical surface impedance and complex bulk wavenumber was developed by K. Attenborough for porous media that incorporated the soil pore properties as parameters [J. Acoust. Soc. Am. 73, 785-799 (1983)]. Acoustic level difference measurements were used as a noninvasive means of finding the soil pore properties. Acoustic reflection measurements showed that the sound field over porous rough surfaces is modified by the surface impedance and by surface roughness. It is not possible to separate the signal modification due to impedance and the signal modification from roughness scattering in a forward scattering measurement. In order to accurately determine the soil pore properties, the roughness effects must be known independently from the surface impedance. A means of measuring roughness apart from impedance would allow the effects of roughness to be taken out of the level difference measurements. The underwater acoustics community has used acoustic backscatter for many years to examine surface roughness. The feasibility of adapting these acoustic backscatter techniques to outdoor porous soil surfaces is examined.

Bond compression bulk modulus and Poisson’s ratio of the polycomponent silicate glasses

Abstract The theoretical bulk modulus and Poisson’s ratio of the 65SiO 2 –15PbO–5CaO–(15− X )K 2 O– X Na 2 O glass system ( X =0–15 mol%) have been calculated using the bond compression model. The atomic ring size of the network has been also obtained for each sample according to the ring deformation model. These parameters have been interpreted quantitatively in terms of the most important structure factors. The composition dependence of these parameters showed that the elasticity and hence the response of this glass system is influenced by the chemical nature of the modifying oxides. Moreover, variation of the estimated atomic ring size with the ratio of bond compression ( K bc ) to experimental ( K e ) bulk modulus was studied to obtain more information about the structure of these glasses. The results showed that the small values of K bc / K e are attributed to the very close (small-ringed) three-dimensional network of these glasses.

A first-principles study of the theoretical strength and bulk modulus of hcp metals

A first-principles method based on the local-density approximation using discrete variational clusters has been used to study the electronic structure of the hcp metals, Be, Mg, Sc, Y, Ti, Zr, Co, Zn and Cd. The binding energy of these metals was calculated in relation to the volume of a unit cell. The variation in the binding energy with the unit cell volume was obtained by means of a polynomial fit. The theoretical tensile strength and bulk modulus of these metals were estimated from the electronic structure and binding energy calculations. The predicted bulk moduli for these metals are in good agreement with experimental findings and other available theoretical data. A linear relationship between the calculated and the experimental strengths is observed.

A first-principles study of the theoretical strength and bulk modulus of hcp metals

A first-principles study of the theoretical strength and bulk modulus of hcp metals

Properties of Nanocrystalline Nickel Particles in Ni-Si0 2 Composites

Nickel-Silica nanocomposites with a nickel content equal to 10, 15, 20 wt% have been prepared by a sol-gel method starting from ethanolic solutions of tetraethoxysilane and nickel nitrate hexahydrate. After selation the samples were reduced in H-> flow at selected temperature (450 °C < T < 600 °C). The morphological, structural and magnetic properties were investigated by transmission electron microscopy (TEM), wide and small angle X-ray scattering (WAXS, SAXS), magnetic susceptibility in zero field cooled and field cooled mode (ZFC and FC), and magnetic hysteresis loop. Nanometric nickel particles are observed in all the investigated samples. TEM, WAXS and SAXS techniques indicate that the average nickel particle size grows slightly but almost regularly with the nickel concentration. TEM results moreover indicate that also the width of the particle size distribution, which can be simulated by log-normal functions, follows this trend. All the sample treated in hydrogen show superparamagnetic behaviour. The magnetisation falls to reach saturation up to highest measuring field of 70 kOe even at 3 K, while the observed coercivity Hc is much higher than the theoretical bulk one. Some uncertainty in the complete interpretation of the sequence of magnetic measurements is attributed to a progressive oxidation of the samples when these are air exposed.

Ab initio calculation of structural and magnetic properties for Fe mono- and bilayers on Mo(1 1 0)

The atomic structures were optimized and magnetic moments calculated for the pseudomorphic Fe overlayers on Mo(1 1 0) substrates employing the all-electron full-potential linearized augmented plane-wave (FP-LAPW) method. Three slabs were considered: (i) 5 monolayer (ML) Mo(1 1 0) substrate, (ii) and (iii) 1 and 2 ML pseudomorphic Fe overlayers on each side of 5 ML Mo(1 1 0) substrate. We found that for the bare Mo substrate, the top Mo–Mo interlayer spacing is contracted by 4.8% with respect to the theoretical bulk Mo(1 1 0) interlayer distance of 2.238 Å. For the 1 ML Fe coverage, the Fe–Mo interlayer spacing has a contraction of 10.3% with respect to the calculated bulk Mo(1 1 0) interlayer spacing, while, for the 2 ML Fe, it is reduced by 9.5%. The Fe–Fe interlayer spacing is also contracted by as much as 11.4% with respect to the theoretical bulk Fe(1 1 0) interlayer distance of 2.004 Å. The inner Mo–Mo interlayer spacings are slightly expanded (<0.5%). The magnetic moment for the 1 ML Fe overlayer on top is enhanced to 2.59 μ B compared to the bulk value of 2.2 μ B. For 2 ML Fe coverage, the magnetic moments are 2.81 μ B and 2.32 μ B for the surface and interface Fe layers, respectively.

Mechanochemical synthesis and sintering behaviour of magnesium aluminate spinel

X-ray amorphous precursor phases for the synthesis of spinel (MgAl2O4) have been prepared by grinding mixtures of gibbsite (Al(OH)3) with brucite (Mg(OH)2) or hydromagnesite (4MgCO3·Mg(OH)2·4H2O). The mechanochemical treatment does not remove any water or carbonate, but converts some of the gibbsite octahedral Al sites into tetrahedral sites and other sites with a 27Al MAS NMR resonance at about 38 ppm. The brucite-derived precursor forms spinel on heating at ≤850°C, by contrast with unground mixtures which show little spinel formation even at 1250°C. The hydromagnesite-derived precursor transforms at about 850°C into a mixture of spinel and hydrotalcite (Mg6Al2(OH)16CO3·4H2O), the latter decomposing to spinel and MgO by 1050°C. Spinel derived from the hydromagnesite-containing precursor shows superior pressureless sintering properties at 1400–1600°C, producing a body of 97% theoretical bulk density at 1600°C. Under the same conditions, the brucite-derived spinel sintered to 72% theoretical density and showed a morphology consisting of widely disparate grain sizes.

Valence-band photoemission from the GaN(0001) surface

A detailed investigation by one-step photoemission calculations of the $\mathrm{GaN}(0001)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1)$ surface in comparison with recent experiments is presented in order to clarify its structural properties and electronic structure. The discussion of normal and off-normal spectra, reveals through the identified surface states, clear fingerprints for the applicability of a surface model proposed by Smith et al. [Phys. Rev. Lett. 79, 3934 (1997)]. Especially the predicted metallic bonds are confirmed. In the context of direct transitions, the calculated spectra allow us to determine the valence-band width and to argue in favor of one of two theoretical bulk band structures. Furthermore, a commonly used experimental method to fix the valence-band maximum is critically tested.

Calculation of theoretical strengths and bulk moduli of bcc metals

A first-principles method based on the local-density approximation using discrete variational clusters has been used to study the electronic structure of bcc metals, Li, Na, K, V, Nb, Ta, Cr, Mo, and W. The binding energy of these metals was calculated, and the relationship between the binding energy and the volume of a unit cell for a given metal was determined. The results show that the contribution of the interaction beyond the third nearest-neighbor atoms to the binding energy is small for transition metals, but large for alkali metals. The theoretical triaxial tensile strength and bulk modulus of these metals were estimated from the electronic structure and binding-energy calculations. The present results of the bulk modulus for Li, Na, K, Nb, Ta, Mo, and W are in good agreement with experiments and other available theoretical data, but the bulk modulus for V and Cr is somewhat overestimated. The theoretical strength obtained in the present work is in good agreement with the results of the linear muffin-tin-orbitals method.