Size effects of nano-spaced basal stacking faults on the strength and deformation mechanisms of nanocrystalline pure hcp metals

The size effects of nano-spaced basal stacking faults (SFs) on the tensile strength and deformation mechanisms of nanocrystalline pure cobalt and magnesium have been investigated by a series of large-scale 2D columnar and 3D molecular dynamics simulations. Unlike the strengthening effect of basal SFs on Mg alloys, the nano-spaced basal SFs are observed to have no strengthening effect on the nanocrystalline pure cobalt and magnesium from MD simulations. These observations could be attributed to the following two reasons: (i) Lots of new basal SFs are formed before (for cobalt) or simultaneously with (for magnesium) the other deformation mechanisms (i.e. the formation of twins and the < c + a > edge dislocations) during the tensile deformation; (ii) In hcp alloys, the segregation of alloy elements and impurities at typical interfaces, such as SFs, can stablilise them for enhancing the interactions with dislocation and thus elevating the strength. Without such segregation in pure hcp metals, the < c + a > edge dislocations can cut through the basal SFs although the interactions between the < c + a > dislocations and the pre-existing SFs/newly formed SFs are observed. The nano-spaced basal SFs are also found to have no restriction effect on the formation of deformation twins.     

Martensitic transformations in Ti (Ni,Fe) alloys

In this work we used x-ray structural analysis to investigate the martensitic transformations (MTs)B2R andRB19 in Ti(Ni, Fe) alloys containing 1, 2.5, 4, 5 at. % Fe, and the effect of heat treatment on the MT temperatures and sequence. In the majority of cases, it was found that the MTB2R proceeds through a two-phase region (+R). Heat treatment leads to a narrowing of the temperature interval of the existence of the two-phase region, weakly affects the onset temperatures of the MTsB2R T_R andRB19 M_s, and sharply increases the temperature-of the end of the MT M_e. The existence of the two-phase region is related to the two MT channelsB2R andB2B19 initially intrinsic to the allovs of the given compositions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 19–22, December, 1989.     

Reflection spectra of oxide films on metals (Al,Cu, and Ti)

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 48, No. 3, pp. 405–409, March, 1988.     

Optical properties of ZnTe doped with transition metals (Ti, Cr and Mn)

The electronic and optical properties of $\text{ Zn }_{1-\mathrm{x}}\text{ M }_\mathrm{x}\text{ Te }$ with (M = Cr, Mn, Ti) have been investigated, within generalized gradient approximation (GGA) using the full potential linear augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. This work presents detailed information about optical properties like absorbance, refractive index and reflectivity. The result of this study shows that doped ZnTe material with Ti, Cr and Mn shift the absorption spectrum and reflection to the infrared spectral domain or to the ultra violet region, depending on the nature of the dopant. Dielectric functions for different compositional alloys are calculated for 16-atom cubic supercell structure. The calculated band gaps are fitted with a linear equation: ( $\upalpha \text{ h }\upnu )^{2} = \text{ A }(\text{ h }\upnu -\text{ Eg }$ ). For all types of doping the position of critical points (CP’s) $\text{ E }_{0}, \text{ E }_{1}$ and $\text{ E }_{2}$ show good agreement with the experimental data.     

Stacking faults in ultra-thin films of silver on Al(111) investigated by medium energy ion scattering

Medium energy ion scattering has been used to investigate depositions of 0.2, 1.4, 3.5 and 4.8 ML of silver onto Al(111). Energy profiles indicate alloying to the extent that aluminium is still visible after the deposition of 4.8 ML. From assessments of the visibility, blocking dips and fits using VEGAS simulations it is shown that the first two layers continue the fcc stacking but after that hcp and fcc twin-type stacking faults occur. The 1.4 ML structure is consistent with a mixed structure of 85% fcc and 15% hcp indicating that some silver occupies a third layer. The blocking curve from the structure formed by 3.5 ML equivalent deposition can be simulated by 56% fcc, 32% hcp and 12% fcc twin and that from 4.8 ML by 59% fcc, 23% hcp and 18% fcc twin. This provides direct evidence of the incidence of hcp stacking when silver is deposited onto Al(111) in the range between 2 and 5 ML.     

Lattice vibrations at (111) surfaces and stacking faults in transition metals: Ni

The Falicov and Yndurain transfer matrix formalism is extended to study the lattice vibrations near surfaces and stacking faults. A simple nearest-neighbour central force harmonic potential appropriate to Ni is considered. The bulk density of states obtained is in good agreement with the experimental findings. Modes localised around the (111) surface are found and analysed in detail. Lattice vibrations densities of states around: (i) intrinsic stacking fault, (ii) extrinsic stacking fault and (iii) twin fault, are calculated. Bona flde localised modes are found in the three faults studied.     

Lattice vibronics of bcc phase metals (Ti,Zr and Hf) at higher temperatures

Ashcroft’s analytic bare ion pseudopotential form factor with a modified Hartree dielectric function has been employed to represent the temperature dependent interionic potential. This potential includes both direct ion-ion interaction and indirect ion-electron-ion interaction with and without the effects of ‘d’ bands, in some scantily studied complexbcc metals vizbcc Ti, Zr and Hf. The ab initio radial and tangential force constants extending out to 15th nearest neighbours are computed for the metals. The said potential is used for predicting the binding energy, elastic constants and phonon dispersion of the above mentioned metals and the results are satisfactorily compared with the corresponding measured data.     

Dose response of F center optical absorption in LiF:Mg,Ti (TLD-100)

Optical absorption (OA) of nominally pure single crystal LiF following beta irradiation was measured in order to estimate, the energy and width of the dominant F-band with minimum interference from dopant-related bands. The OA dose response of LiF:Mg,Ti was measured to 30,000 Gy, a level of dose sufficiently high to observe total saturation of the F band, which, we believe, reduces uncertainty in the estimation of the dose filling constant. The dose filling constants for the OA bands associated with the trapping center (4 eV) and competitive center (5.45 eV) responsible for the major dosimetric TL glow peak 5 were also determined. The results of these studies will be used in the framework of a kinetic model which includes the effects of radiation created defects and which will aid in the investigation of the capability of Track Structure Theory to predict OA heavy charged particle (HCP) relative efficiencies.     

Dose reassessment by using PTTL method in MTS-N (LiF:Mg,Ti) thermoluminescent detectors

The thermoluminescent (TLD) method is one of the most commonly used in dose measurements in radiation protection dosimetry. Due to its many advantages this method is widely spread. However, TLD has especially one disadvantage which is very inconvenient: the dose information in already read detectors is erased and in routine standard way the dose can not be reassessed. The positive is that this shortcoming can be eliminated by applying UV radiation. After first readout the same detector can be subjected to UV exposure and then read once again to reassess the dose.This method for reassessment of dose is based on phototransferred thermoluminescence (PTTL). In an irradiated TL detector deep traps are not emptied during the first readout. During exposure to UV, electrons are transferred from deep traps to shallower dosimetric traps. This TL signal emerging during the second readout following UV illumination is called phototransferred thermoluminescence.A method for reassessment of dose in a previously readout TLD is presented in this work. Experiments show that the method works well within region of doses between 5 and 50 mGy, but could be applied for higher doses as well. The efficiency of dose reassessment reaches about 17 percent of the first readout.The method could be a noticeable improvement in TLD dosimetry, giving more opportunities for better control and reliability of measurements.     

Low-temperature phase transformations of PbZr(1-x)Ti(x)O(3) in the morphotropic phase-boundary region

We present anelastic and dielectric spectroscopy measurements of PbZr(1-x)Ti(x)O(3) with 0.455 < or = x < or = 0.53, which provide new information on the low-temperature phase transitions. The tetragonal-to-monoclinic transformation is first order for x < 0.48 and causes a softening of the polycrystal Young's modulus whose amplitude may exceed the one at the cubic-to-tetragonal transformation; this is explainable in terms of linear coupling between shear strain components and tilting angle of polarization in the monoclinic phase. The transition involving rotations of the octahedra below 200 K is visible both in the dielectric and anelastic losses, and it extends within the tetragonal phase, as predicted by recent first-principle calculations.     

Solid-phase reactions between (100) GaAs and thin-film refractory metals (Ti,Zr, V,Nb, Cr,Mo, and W)

Recently, there has been an increased interest in the applications of refractory metals as gate materials for the self aligned gate process in the fabrications of GaAs field effect transistors. In this study, we systematically investigated the thermally induced interface interactions between (100) GaAs substrates and thin films of refractory metals (Ti, Zr, V, Nb, Cr, Mo, and W). Depth profilings of the M/GaAs interfaces were obtained using conventional and heavy ion Rutherford backscattering spectrometry. Phase identifications were achieved by x-ray diffraction.Results on the phase formation sequence, reaction kinetics, the distribution, composition and structure of the reacted phases and the interface reactivity of these contacts will be presented. Correlations between metal properties (electronegativity and metal-metal bond strength) and kinetics of the reactions (activation energy and reactivity of the interfaces) will also be discussed.     

Investigation of the optical absorption characteristics of slow-cooled LiF:Mg,Ti (TLD-100)

The optical absorption (OA) spectrum of LiF:Mg,Ti has been studied as a function of dose at two different cooling rates following the 400 °C pre-irradiation anneal in order to further investigate the role of cooling rate in the thermoluminescence (TL) mechanisms of this material. “Slow-cooling” following the pre-irradiation 400 °C anneal substantially decreases the OA bands at 3.25 eV and 4.0 eV, in agreement with the overall loss in TL peaks 2–5 intensity using slow-cooling routines. Slow-cooling appears to shift the maximum intensity of peak 5 to lower temperatures (a behaviour which has been attributed to an enhanced intensity of peak 5a), however, no difference in the shape of the 4.0 eV OA band is detected following “slow-cooling”. Apparently the OA band related to peak 5a is too close in energy to the peak 5 OA band to be observed due to lack of sufficient resolution and spectral deconvolution process or it is not present at room temperature (RT) and formed during heating of the sample. The intensity of the 4.0 eV OA band does not change if the sample (prior to irradiation to a standard dose of 200 Gy) is irradiated to 4 kGy followed by a 500 °C/1 h post-irradiation anneal. This result demonstrates that the loss of intensity at high levels of dose (so-called radiation damage) of TL glow peak 5 results from alteration of the LCs or to the creation of additional competitive centers and is not correlated with the dose behaviour of the TCs.     

Interaction of bathocuproine with metals (Ca, Mg, Al, Ag, and Au) studied by density functional theory

Electronic structure of the interface between a bathocuproine (BCP) molecule and metal surface was studied by density functional theory. The bond energy of BCP with metal surface increased with decreasing work function of metals, that is, Au, Ag, Al, Mg, and Ca in this order. The charge transfer also increased with decreasing metal work function. It was analyzed that the Fermi level and the lowest unoccupied molecular orbital of BCP are composed of both BCP and metal orbital component, and electron transport properties across the BCP and metal is discussed based on these analyses.     

Dependence of NiTi hydride stability by co-substitution by (Zr,Mg) onto Ti and (Cr,Cu) onto Ni: first-principles study

ABSTRACT

The thermodynamics of hydride formation is one of the most important properties of the metal-hydrogen system, and states its potential for further uptake. For this reason, much research is focused on the use of first principle calculations as a predictive tool in the study of hydride stability. In this paper, First-principles density functional calculations were performed to predict the effect of co-substitution in NiTiH, Ti by Mg and Zr (x?=?0.125, 0.25 and 0.375), as well as Ni by Cu and Cr (y?=?0.125). Structural, thermodynamic stability and electronic properties were investigated. The formation enthalpy when Ti is substituted either by Mg or Zr with respect to their content is calculated and compared to the host NiTiH; it is found that the hydride stability decreases as Mg content increases while it increases when Zr content increases. The substitution of Ni by Cu destabilises the hydride while the stability of the hydride is enhanced when Ni is substituted by Cr. The simultaneous substitution of Ti by Mg (x?=?0.375) and Ni by Cu (y?=?0.125), leads to considerable destabilisation and an increase in cell volume of the hydride. The corresponding Ni_0.875Cu_0.125Ti_0.625Mg_0.375 compound is identified with optimum characteristics among the considered compositions, thereby can be considered as potential material for hydrogen storage.     

Application of phototransferred thermoluminescence (PTTL) for dose re-assessment in routine dosimetry using MTS-N (LiF:Mg,Ti) thermoluminescent detectors

Erasure of the thermoluminescence (TL) signal on detector readout is considered to be a disadvantage of TL dosimetry, as post-readout dose reassessment is then impossible in principle. A method of dose reassessment based on phototransferred thermoluminescence (PTTL) has been developed at the Institute of Nuclear Physics, Polish Academy of Sciences (IFJ PAN) and applied to MTS-N (LiF:Mg,Ti) detectors. We demonstrate the possibility of applying PTTL for dose reassessment in MTS-N TL detectors routinely applied in the dosimetric service at IFJ PAN. Readings of TL detectors exposed to relatively high doses by the customers of our dosimetry service can now be reassessed using our automatic readers. A major obstacle in applying the PTTL method at lower exposures is the presence of residual dose accumulated in LiF:Mg,Ti detectors after many field exposure and readout cycles. Since most of the TL detectors in our service have been already used for a long time (e.g. for over 10 years in the case of some detector batches), we find that our PTTL method of dose reassessment is possible only in detectors which had received doses exceeding 5 mSv.     

Initial stages of oxidation of Mg(0001) and the role of co-adsorbed alkali metals

Soft X-ray Mg 2p photoemission using synchrotron radiation at moderate spectral resolution has been used to investigate the early stages of oxidation of Mg(0001) with and without preadsorbed Na or K. On the alkali-free surface, some evidence is found for a state characterized by a chemical shift intermediate between that of the metallic and oxidic states, but the limited resolution and overlapping spin-orbit split surface and bulk metallic components precludes more thorough characterization. Preadsorbed Na or K enhance the initial rate of oxidation by approximately a factor of two. It is suggested that the mechanism is an enhancement of the rate of oxygen dissociation.     

Non-linearity of the high temperature peak area ratio of 6LiF:Mg,Ti (TLD-600)

Thermoluminescent dosemeters of ⁶LiF:Mg,Ti (TLD-600) were exposed to high-energy heavy ion beams (He, C, Ne, and Ar) and ¹³⁷Cs γ-rays and the glow curves were analyzed. The height of the main peak (peak 5) at about 200°C slightly increased for He from γ-rays and decreased for heavier ions with increasing Linear Energy Transfer (LET). Whereas the high temperature peak (peak 7) area around 260°C simply increased. Non-linearity was seen between the LET and the high temperature peak (peak 7) area ratio (HTR) which was calculated as the TL integrated over 225–275°C from the peak 5 normalized glow curve. Based on these results, average LET and quality factor were estimated for an assumed LET spectra of space radiation by using the HTR method. Considerable differences were found between these estimations and true values.     

Electronic and magnetic properties of basic nanosystems of early 3d transition metals (Sc,Ti, V,Cr, and Mn)

This detailed and systematic theoretical study on the behavior of basic low dimensional (one- and two-dimensional) systems of early 3d transition metals should serve as a guideline to experimentalists as well as to theoreticians. We find that, lowering of dimensionality is favorable for emergence of magnetic ordering in all the systems studied, except Ti monolayers (MLs). For Ti MLs, both nonmagnetic and ferromagnetic states are degenerate within the numerical limits. For such a case, the interactions with substrate would play a decisive role in the magnetic ordering of the atoms in the ML. The total energy calculations show that the nonmagnetic and ferromagnetic states are almost degenerate for Cr and V MLs too; however, anti-ferromagnetic ordering is favored in these. The ferromagnetic ordering in Sc linear chains and anti-ferromagnetic ordering in MLs of Mn and Cr are found to be favored by a relatively larger margin showing good stability. Some low dimensional systems, showing electrons with only one kind of spin available at Fermi energy, may be suitable for spintronics related applications. The linear chains of Cr and Mn, and MLs of Sc are likely to form stable magnetic nanosystems as these exhibit almost saturated magnetic moment per atom around the equilibrium separation. The magnetic moment strengthens considerably as one goes from two- to one-dimension. Our results are supported qualitatively by available experimental results and offer a good insight into these nanosystems.     

Dependence of the thermoluminescent high-temperature ratio (HTR) of LiF:Mg,Ti detectors on proton energy and dose

The high-temperature ratio (HTR) is a parameter quantifying changes of the shape of the high-temperature part of the LiF:Mg,Ti glow-curve after exposure to densely ionizing radiation. It was introduced in order to estimate the ‘effective LET’ of an unknown radiation field and to correct the decreased relative TL efficiency for high-Linear Energy Transfer (LET) radiation.In the present work the dependence of HTR on proton energy (14.5–58 MeV) and dose (0.5–30 Gy) was investigated. All measured HTR values were at the level of 1.2 or higher, therefore significantly different from the respective value for gamma-rays (HTR = 1), but HTR was found to be insensitive to changes of proton energy above 20 MeV. As a result the relationship between HTR and relative TL efficiency is not unequivocal. The HTR was found to be dependent on absorbed dose even for the lowest studied doses.