H. Drulis

Hydrogenation equilibria characteristics of LaNi5−xZnx intermetallics

Abstract A series of metal hydride alloys were prepared based on the LaNi 5 formula, by substituting 0, 5, 10, 15 and 20 at.% of Zn for Ni. The obtained pseudobinary LaNi 5− x Zn x alloys have been characterized by X-ray diffraction patterns and hydrogen pressure–composition–temperature ( PCT ) diagrams at temperatures ranging from 293 to 353 K. The P – C isotherms were used to determine thermodynamic functions (enthalpy and entropy) for the hydrogen desorption process. Increasing Zn substitution causes the unit cell volume to increase and the plateau pressures to decrease. The maximum concentration of hydrogen in the tested La(Ni,Zn) 5 alloys slightly decreases with increasing Zn content.

Electrical, thermal, and elastic properties of the MAX-phase Ti2SC

We report on the electronic, thermal, and elastic properties of the layered ternary, Ti2SC. Resistivity, Hall effect, and magnetoresistance were measured as a function of temperature between 2 and 300 K and at fields up to 9 T. The Hall coefficient is negative and roughly temperature independent. The transport results were analyzed within a two-band framework, with electrons as the dominant charge carrier. The room-temperature thermal conductivity (≈60 W/m K) is the highest of any MAX phase measured to date, with a substantial phonon contribution. The specific heat was measured from 2 to 300 K, yielding a Debye temperature of 765 K and in agreement with the Debye temperature of 745 K found from ultrasonic time-of-flight measurements. Young’s, shear, and bulk moduli from the latter measurements were 290, 125, and 145 GPa, respectively. The calculated values of the lattice parameters (a=3.2051 A and c=11.2636 A), and Young’s, shear, and bulk moduli (329, 138, and 179 GPa, respectively), based on the results...

On the heat capacities of M2AlC (M=Ti,V,Cr) ternary carbides

In this paper, we report on the heat capacities cp of bulk polycrystalline samples of Ti2AlC, V2AlC, and Cr2AlC in the 3–260K temperature range. Given the structural and chemical similarities of these compounds it is not surprising that the cp’s and their temperature dependencies were quite similar. Nevertheless, at all temperatures the heat capacity of Cr2AlC was higher than the other two. The density of states at the Fermi level were 3.9, 7.5, and 14.6(eVunitcell)−1 for Ti2AlC, V2AlC, and Cr2AlC, respectively. The results obtained are analyzed using the Debye and Einstein model approximations for cp. Good description of cp is obtained if one assumes that nine phonon modes vibrate according to the Debye model approximation whereas the remaining 3 of 12 modes expected for M2AlC formula unit fulfill an Einstein-like phonon vibration pattern. Debye temperatures θD describing acoustic phonon and Einstein temperature θE describing optical phonon contributions have been estimated for the studied compounds. The...

Magnetic properties and 57Fe Mössbauer effect investigation in U1−xYxFe10Si2 solid solutions

Abstract The solid solutions U1−xYxFe10Si2 have been studied by means of X-ray diffraction, 57Fe Mossbauer effect and magnetic investigations. In the intermediate concentration range, a redistribution of the iron and silicon atoms over non-equivalent positions was observed despite the fact that both ternaries have similar occupancies in all positions. Such a redistribution could be a reason for the slight non-linearity observed in the concentration dependences of lattice parameters and magnetic properties. The results of a hyperfine field study were used to estimate the uranium magnetic moment in UFe0Si2, which is about 0.5μB and coupled ferromagnetically with the iron sublattice magnetic moment.

Hydrogen absorption properties of ZrNi5−xCox alloys

Abstract The structural and hydrogen sorption properties of the ZrNi 5− x Co x ( x =0,1,2 and 3) system have been studied. The alloys were characterized by X-ray diffraction patterns and by pressure-composition-temperature (PCT) diagrams at temperatures ranging from 448 to 523 K. It was found that compositions with 0≤ x ≤2 were solid solutions of AuBe 5 type structure, whereas the ZrNi 2 Co 3 alloy showed a two-phase structure. Substitution of Ni by Co increases the unit cell volume of the monophase alloys by 0.84% per one substituted atom in formula unit. Hydrogen uptake increases the cell volume of the host alloy by up to 2.3%. In the studied conditions of temperature (448 to 523 K) and pressure (0.05 to 2.5 MPa) the PCT diagrams do not exhibit any plateau of hydrogen equilibrium pressure. The hydrogen capacity increases with increasing Co substitution. The largest hydrogen solubility has been found for the ZrNi 2 Co 3 alloy (1.3 hydrogen atoms per formula unit).

57Fe Mössbauer effect in UFexAl12−x system

Abstract 57 Fe Mossbauer effect examinations were carried out on UFe x Al 12− x alloys with x = 4, 5 and 6 in the temperature range 14–370 K. The spectra for x =5 and 6 can be fitted with the Fe atoms distributed over the 8( f ), 8( j ) and 8( i ) positions, whereas for x =4 the Fe atoms are located at the 8( f ) sites only, in agreement with earlier results. The hyperfine fields corresponding to the three sites ( f, j, i ) are 54%, 43% and 3% for x =5 and 58%, 35% and 7% for x =6 respectively. The Curie points determined from H hf ( T ) plots are 162, 265 and 350 K for x =4, 5 and 6 respectively, in fair agreement with the results of magnetic examinations.

Magnetocaloric properties of distilled gadolinium: Effects of structural inhomogeneity and hydrogen impurity

High-purity Gd prepared by distillation is a structurally inhomogeneous system consisting of needle-shaped crystals of cross section 0.5–2.5 μm with near-c-axis orientation embedded in a matrix of nanosized (30–100 nm) grains. By measuring the magnetocaloric effect (MCE) directly, we find that the MCE values differ markedly for the plate-shaped samples cut out of a distillate along and perpendicular to the crystals. The effect of small controlled amounts of impurity (hydrogen) on the properties of distilled Gd is further studied. We observe opposite trends in the MCE response to hydrogen charging with respect to the crystals orientation within the samples and discuss mechanisms interrelating the unique structural morphology with the impurity behavior. As an overall assessment, the Curie temperatures of α-GdHx solid solutions increase from 291 K up to 294 K when increasing hydrogen concentration x from 0 to 0.15. Hydrogenation is found to broaden the ferromagnetic-to-paramagnetic phase transition. Hydroge...

On the heat capacities of Ta2AlC, Ti2SC, and Cr2GeC

Herein we report on the heat capacities cp of bulk predominantly single-phase polycrystalline samples of Ti2SC and Cr2GeC in the 3–1500K temperature range and Ta2AlC in the 3–260K range. At temperatures up to 10K the main contributors to cp for Ta2AlC and Cr2GeC are electronic, with electronic coefficients γ of 7.13 and 26.12mJ∕molK2, respectively. The latter is exceptionally high and is a record for this family of layered ternary carbides and nitrides also known as the MAX phases. In Ti2SC another low-temperature contribution—in addition to a γ of 3.8mJ∕molK2—is manifested by an upturn in cp∕T observed at the lowest temperatures. This feature, appearing as a Schottky-like anomaly, has a local maximum near 4.5K and an intensity of ∼1.9×10−2J∕molK. A defect concentration of ∼3×1021∕mol presumably on the S-sublattice, is proposed as the origin of a two-level energy system responsible for this anomaly. As in previous work on these compounds, the lattice contributions to cp in all compounds are analyzed using...

Magnetic anisotropy and Mössbauer effect studies of YFe11Ti and YFe11TiH

The magnetic properties and 57Fe Mossbauer spectra of the compounds YFe11TiHx (x = 0, 1) were investigated. The magnetocrystalline anisotropy of YFe11Ti and its hydride was studied by analysing the hard and easy magnetization curves of single crystal samples in the temperature range 4.2–300 K. The spontaneous magnetization was determined in wide temperature range 4.2–650 K. It is established that at T = 4.2 K magnetic anisotropy constants K1 and K2 for YFe11TiH1 single crystal reach values of 25.8 K f.u.−1 and 0.24 K f.u.−1, respectively. The Mossbauer effect spectra of YFe11Ti and its hydride were analysed in terms of a model which takes into account the local environment of Fe atoms on three crystallographic sites (8f, 8j and 8i) and an influence of the random distribution of titanium on the 8i site. Upon hydrogenation both the hyperfine fields and the isomer shifts increase. These results are discussed in terms of the hydrogen-induced unit cell expansion and the electron charge transfer from the conduction band onto the H atoms.

Low temperature heat capacity and magnetic susceptibility of Ti3SiC2

In this paper, we report on the heat capacity and magnetic susceptibilities of bulk Ti3SiC2 samples in the 3–250-K temperature range. At 13.6 K, a nonmagnetic sharp anomaly of unknown origin, which disappeared when the sample was heated and quenched in water from 1400 °C, is observed. Curve fitting the cp data up to 100 K clearly shows that only four atoms—most likely one Si and three Ti atoms—of the six in a formula unit contribute to cp at the lowest temperatures. This reduces the Debye temperature θD calculated from over 700 K to ≈575 K. To obtain good agreement between experimental and calculated cp values in the 14 130 K, two other optical modes—the average of two other Raman modes observed at higher wave numbers—are required to enhance the fits to the experimental results. In the absence of the anomaly, θD rise...