Phase transitions in LaFeAsO: structural, magnetic, elastic, and transport properties, heat capacity and Mossbauer spectra

We present results from a detailed experimental investigation of LaFeAsO, the parent material in the series of "FeAs" based oxypnictide superconductors. Upon cooling this material undergoes a tetragonal-orthorhombic crystallographic phase transition at ~160 K followed closely by an antiferromagnetic ordering near 145 K. Analysis of these phase transitions using temperature dependent powder X-ray and neutron diffraction measurements is presented. A magnetic moment of ~0.35 Bohr magnetons per iron is derived from Mossbauer spectra in the low temperature phase. Evidence of the structural transition is observed at temperatures well above the structural transition (up to near 200 K) in the diffraction data as well as the polycrystalline elastic moduli probed by resonant ultrasound spectroscopy measurements. The effects of the two phase transitions on the transport properties (resistivity, thermal conductivity, Seebeck coefficient, Hall coefficient), heat capacity, and magnetization of LaFeAsO are also reported, including a dramatic increase in the magnitude of the Hall coefficient below 160 K. The results suggest that the structural distortion leads to a localization of carriers on Fe, producing small local magnetic moments which subsequently order antiferromagnetically upon further cooling. Evidence of strong electron-phonon interactions in the high-temperature tetragonal phase is also observed.Comment: Revised and expanded magnetization and Mossbauer spectroscopy section. Clarified sample preparation description. This paper contains some results from arXiv:0804.0796. 10 figure

A Search For Temperature Induced Time-Dependent Structural Transitions In 10Mol%Sc2O3-1Mol%Ceo2-Zro2 And 8Mol%Y2O3-Zro2 Electrolyte Ceramics

Both Sc2O3-CeO2-stabilized-ZrO2 (SCSZ) and Y2O3-stabilized-ZrO2 (YSZ) show similar Young\u27s modulus damping at 175-400°C and 200-400°C, respectively, by impulse excitation acoustic technique. The phase transition in SCSZ is considered responsible for the damping; however for YSZ, such a phase transition has never been reported. To clarify the relation of damping and structural transition in these two materials, the time-dependent high temperature stabilities are studied by in situ X-ray diffraction, neutron diffraction and Raman scattering during long-term annealing of SCSZ at 350°C and YSZ at 275°C. The cubic-to-rhombohedral transition in SCSZ is detected. However, the existence of t\u27 phase in YSZ raw powders is confirmed without significant changes of the cubic structure during isothermal annealing. It is concluded that the phase transition contributes to the Young\u27s modulus damping in SCSZ, while the structural transition is excluded from the reason for damping in YSZ

Mechanical Behavior Of La0.8Sr0.2Ga0.8Mg0.2O3 Perovskites

This paper examines the important mechanical properties of commercially purchased La0.8Sr0.2Ga0.8Mg0.2O3 at room temperature and 800 °C. Sr and Mg-doped lanthanum gallates (LSGM) are strong candidates for use as solid electrolytes in lower temperature solid oxide fuel cells operating at or below 800 °C. The material was found to be phase pure with a Young\u27s modulus value of ∼175 GPa. The four point bending strength of the LSGM samples remained almost constant from 121 ± 35 MPa at room temperature to 126 ± 20 MPa at 800 °C. The fracture toughness, as measured by the single edge V notch beam (SEVNB) method, was 1.22 ± 0.06 MPa√m at room temperature, 1.04 ± 0.09 MPa√m at 700 °C followed by a small increase 1.31 ± 0.16 MPa√m at 800 °C. We also report, for the first time, the static subcritical (or slow) crack-growth (SCG) behavior of natural cracks in LSGM performed in four point bending tests at room temperature. The exponent of a power-law representation in the SCG tests was found to be n = 15, a rather low value showing LSGM to be highly susceptible to room temperature SCG. © 2008 Elsevier Ltd and Techna Group S.r.l

Novel High Pressure Hexagonal Osb2 By Mechanochemistry

Hexagonal OsB2, a theoretically predicted high-pressure phase, has been synthesized for the first time by a mechanochemical method, i.e., high energy ball milling. X-ray diffraction indicated that formation of hexagonal OsB2 begins after 2.5 h of milling, and the reaction reaches equilibrium after 18 h of milling. Rietveld refinement of the powder data indicated that hexagonal OsB2 crystallizes in the P63/mmc space group (No. 194) with lattice parameters of a=2.916 Å and c=7.376 Å. Transmission electron microscopy confirmed the appearance of the hexagonal OsB2 phase after high energy ball milling. in situ X-ray diffraction experiments showed that the phase is stable from -225 °C to 1050 °C. The hexagonal OsB2 powder was annealed at 1050 °C for 6 days in vacuo to improve crystallinity and remove strain induced during the mechanochemical synthesis. The structure partially converted to the orthorhombic phase (20 wt%) after fast current assisted sintering of hexagonal OsB 2 at 1500 °C for 5 min. Mechanochemical approaches to the synthesis of hard boride materials allow new phases to be produced that cannot be prepared using conventional methods. © 2014 Elsevier Inc

Thermal Stability Of Hexagonal Osb2

The synthesis of novel hexagonal ReB2-type OsB2 ceramic powder was performed by high energy ball milling of elemental Os and B powders. Two different sources of B powder have been used for this mechanochemical synthesis. One B powder consisted of a mixture of amorphous and crystalline phases and a mixture of 10B and 11B isotopes with a fine particle size, while another B powder was a purely crystalline (rhombohedral) material consisting of enriched 11B isotope with coarse particle size. The same Os powder was used for the synthesis in both cases. It was established that, in the first case, the hexagonal OsB2 phase was the main product of synthesis with a small quantity of Os 2B3 phase present after synthesis as an intermediate product. In the second case, where coarse crystalline 11B powder was used as a raw material, only Os2B3 boride was synthesized mechanochemically. The thermal stability of hexagonal OsB2 powder was studied by heating under argon up to 876 °C and cooling in vacuo down to -225 °C. During the heating, the sacrificial reaction 2OsB 2+3O2→2Os+2B2O3 took place due to presence of O2/water vapor molecules in the heating chamber, resulting in the oxidation of B atoms and formation of B2O 3 and precipitation of Os metal out of the OsB2 lattice. As a result of such phase changes during heating, the lattice parameters of hexagonal OsB2 changed significantly. The shrinkage of the a lattice parameter was recorded in 276-426 °C temperature range upon heating, which was attributed to the removal of B atoms from the OsB2 lattice due to oxidation followed by the precipitation of Os atoms and formation of Os metal. While significant structural changes occurred upon heating due to presence of O2, the hexagonal OsB2 ceramic demonstrated good phase stability upon cooling in vacuo with linear shrinkage of the lattice parameters and no phase changes detected during cooling. © 2014 Elsevier Inc

NEUTRON SCATTERING RESIDUAL STRESS MEASUREMENTS ON GRAY CAST IRON BRAKE DISCS

Abstract Neutron diffraction was used to investigate the effects of a heat treatment designed to remove internal residual stresses in brake discs. It is believed that residual stresses may change the rate of deformation of the discs during severe braking conditions when the disc temperature is increased significantly. Neutron diffraction was used to map out residual strain distributions in a production disc before and after a stress-relieving heat treatment. Results from these neutron diffraction experiments show that some residual strains were reduced by as much as 400 microstrain by stress relieving. EXPERIMENTAL, MEASUREMENT of residual stresses is of considerable value to the prediction of in-service stress corrosion and fatigue crack growth. In addition, information as to the magnitude of the residual stress present after casting is also important, since these stresses result in distortion of cast components. It is believed that relaxation of residual stresses after high energy applications may lead to disc distortion. These distortions can produce an unacceptable phenomenon known as "brake judder". Some manufacturers incorporate a costly additional heat treatment step in an effort to relieve stresses in brake discs prior to final machining. However, as prior experiments had indicated that the residual stresses in production brake discs were not very great, it was decided to quantify and compare the residual stresses in brake discs manufactured with and without the additional heat treatment, Residual stress measurement by X-ray diffraction is a well established technique (1,2), but it is practically limited to near-surface stresses, whereas the deeply penetrating neutron radiation permits nondestructive diffraction measurement of lattice strain within the bulk of large specimens (3). The neutron dikction tescnique was therefore utilized to evaluate the residual stresses in brake discs and to assess the possibility for control of disc distortion, which may contribute to judder, by annealing the discs foliowing casting. Experimental Procedure Production samples of brake discs including a production sample given a stress-relieving heat treatment were provided for the first phase of residual stress measurements. In a second phase one of the production samples was given a stress-relief anneal (after having fmt been examined by neutron scattering) and then remeasured at the previously studied locations. This heat treatment consisted of a soak at 58OOC for 5 hours, followed by controlled cooling at 4 O O U h . r to 30O0C, followed by air cooling to room temperature. This report focusses on the changes in the residual strains in the brake disc before and after this heat treatment. The brake disc, shown The measurements reported in this paper were made at the midthickness of the inboard disc, along a line extending from the inner diameter to the outer diameter of the disc

Characterization of Residual Stress as a Function of Friction Stir Welding Parameters in Oxide Dispersion Strengthened (ODS) Steel MA956

The article of record as published may be found at http://dx.doi.org/101016/j.msea.2015.09.020This article characterizes the residual stresses generated by friction stir welding of oxide dispersion strengthened steel MA956 over a series of welding conditions. A plate of MA956 steel was friction stir welded at three conditions: 500 rpm/25 millimeters per minute (mmpm), 400 rpm/50 mmpm and 400 rpm/100 mmpm. The residual stresses across these welds were measured using both x-ray and neutron diffraction techniques. Longitudinal residual stresses up to eighty percent of the yield strength were observed for the 400 rpm/100 mmpm condition. Increasing the traverse rate while holding the rotational speed fixed increased the residual stress levels in the stir zone and at the stir zone-thermomechanically affected zone interface. The stress profiles displayed the characteristic M shape, and the asymmetry between advancing and retreating stress peaks was limited, occurring mainly on the root side of the weld. The large magnitude of the stresses was maintained throughout the thickness of the plates.USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)AC05-00OR22725; AC52-07NA2734