Neutron Thermodiffraction Research Articles

Structural and dielectric properties of ultra-fast microwave-processed La0.3Ca0.7Fe0.7Cr0.3O3−δ ceramics

Perovskite La0.3Ca0.7Fe0.7Cr0.3O3-δ (LCFCr) is a mixed ionic and electronic conductor (MIEC) that can be employed as an electrode material in reversible solid oxide fuel cells (RSOFCs). In this work, an ultra-fast (15 ​min) one-step microwave (MW)-assisted combustion synthesis route has been developed to obtain phase pure and highly crystalline LCFCr powder. The synthesized powders exhibited a sponge-like microstructure with increased electrochemical reaction sites. Neutron thermodiffraction analysis revealed a structural transition above 500 ​°C from the room temperature (RT) orthorhombic Pnma to a rhombohedral R3c perovskite phase. The oxygen vacancy concentration was found to increase from δ ​= ​0.272(7) at RT to δ ​= ​0.333(5) at 900 ​°C. Furthermore, a 3-dimensional G-type antiferromagnetic structure was detected at RT. MW-sintering of pressed green ceramic pellets was carried out at 950 ​°C for 1 ​h, using a MW-transparent quartz fiber crucible or alternatively a SiC crucible acting as a MW-absorber. Impedance spectroscopy data on sintered ceramic pellets revealed electronic inhomogeneity as demonstrated by the occurrence of three dielectric relaxation processes associated with two grain boundary (GB)-like contributions and one bulk. The dielectric inhomogeneity encountered may be restricted to the extrinsic GB areas, which may be rather thin. More homogeneous dielectric properties of the GBs were found in the pellet that was sintered in the SiC crucible.

Magnetic behavior in commercial iron-silicon alloys controlled by the dislocation dynamics at temperatures below 420 K

A decrease of the temperature dependent coercive forces up to around 370 K is discovered in iron silicon alloys, both in quenched samples and in samples which were previously thermally treated to achieve the highest magnetic quality. Alloys of composition Fe-6 wt.% Si and Fe-3 wt.% Si are studied. This reduction in the coercive force is controlled by an increase in the mobility of the domain walls due to the increase in the dislocation’s mobility enhanced by the movement of vacancies. It is worthwhile to mention that this reduction in coercive force is only present at these slightly elevated temperatures which are markedly smaller than the usual annealing temperatures for heat treatment of iron silicon alloys while it disappears again at room temperature. Neutron thermodiffraction, magnetic hysteresis loops tracer and mechanical spectroscopy are used as experimental techniques.

Stabilization of Metastable Thermoelectric Crystalline Phases by Tuning the Glass Composition in the Cu-As-Te System.

Recrystallization of amorphous compounds can lead to the stabilization of metastable crystalline phases, which offers an interesting way to unveil novel binary or ternary compounds and control the transport properties of the obtained glass ceramics. Here, we report on a systematic study of the Cu-As-Te glassy system and show that under specific synthesis conditions using the spark-plasma-sintering technique, the α-As2Te3 and β-As2Te3 binary phases and the previously unreported AsTe3 phase can be selectively crystallized within an amorphous matrix. The microstructures and transport properties of three different glass ceramics, each of them containing one of these phases with roughly the same crystalline fraction (∼30% in volume), were investigated in detail by means of X-ray diffraction, scanning electron microscopy, neutron thermodiffraction, Raman scattering (experimental and lattice-dynamics calculations), and transport-property measurements. The physical properties of the glass ceramics are compared with those of both the parent glasses and the pure crystalline phases that could be successfully synthesized. SEM images coupled with Raman spectroscopy evidence a "coast-to-island" or dendriticlike microstructure with microsized crystallites. The presence of the crystallized phase results in a significant decrease in the electrical resistivity while maintaining the thermal conductivity to low values. This study demonstrates that new compounds with interesting transport properties can be obtained by recrystallization, which in turn provides a tuning parameter for the transport properties of the parent glasses.

Phase Transitions in the Ruddlesden-Popper Phase Li2CaTa2O7: X-ray and Neutron Powder Thermodiffraction, TEM, Raman, and SHG Experiments.

The structure of the Ruddlesden-Popper layered perovskite Li2CaTa2O7, known for its high photocatalytic water activity since its discovery in 2008, is reinvestigated. This oxide has been characterized by powder X-ray and neutron thermodiffraction, TEM, second harmonic generation (SHG), and Raman experiments on powders and single crystals. It is shown that it undergoes two structural phase transitions (i) around 220 °C, mainly characterized by the progressive emergence of SHG signal at low temperatures, and (ii) at 660 °C, mainly characterized by changes of the temperature behavior of lattice parameters and by the emergence of Raman signals that linearly increase on decreasing temperature. It is shown by powder neutron diffraction profile refinements at RT, 400, and 800 °C that the space groups of the successive phases of Li2CaTa2O7 are the acentric Pna21 (RT ≤ T ≤ 220 °C), Pnma (220 °C ≤ T ≤ 660 °C), and Cmcm (T ≥ 660 °C). A soft mode associated with the transition to the highest symmetry for this structural arrangement (I4/mmm) is also found in the Raman spectra. All these transitions appear continuous: the high temperature ones can be attributed to progressive vanishings of the octahedra tiltings (displacives) while the transition in the vicinity of 220 °C from Pna21 to Pnma exhibits order-disorder character.

Investigating the crystallization process in a FeCrB metallic glass by combining magnetic and neutron thermo-diffraction experiments

The structural evolution of a Fe70Cr10B20 metallic glass was followed by means of in situ high-temperature neutron thermo-diffraction and magnetization measurements. Above 723 K the crystallization of bcc-Fe together with a metastable (FeCr)3B phase with tetragonal crystal structure (space group 14̄) is observed. Further heating gives rise to the transformation of the (FeCr)3B phase into another tetragonal (FeCr)2B phase (space group 14 / mcm) + bcc- Fe. On cooling down to room temperature no additional structural transformations occurred. This two-step crystallization process allows understanding quantitatively the intricate variation of the magnetization at high-temperature

Analysis of the polymorphic transformation of nano- and microcrystalline zirconia doped with CaO and MgO during reaction-sintering process by neutron thermodiffraction. A thermodynamic approach

Abstract Polymorphic transformation of zirconia during reaction-sintering process in nano- and microcrystalline mixtures of monoclinic zirconia and dolomite was analyzed using neutron thermodiffraction experiments. This is being the first time that such an approach has been applied in the analysis of these materials. Taking into account the isoplethal section CaO·MgO–ZrO2, the composition, which allows doped CaO/MgO cubic zirconia to be obtained at the lowest temperature, was chosen to carry out the experimental studies. The comparison of the results in both samples, in terms of their thermodynamic and kinetic aspects makes it possible to analyze the effect of the particle-size on the reaction behavior and temperatures of phase transitions.

A comprehensive study of the crystallization of Cu–As–Te glasses: microstructure and thermoelectric properties

We report a thorough experimental study on the microstructure, thermal behavior and thermoelectric properties of the amorphous composition Cu15As30Te55 and the glass–ceramics related-compounds synthesized by using the Spark Plasma Sintering (SPS) technique. Varying the conditions of the SPS process enables the synthesis of composite glassy-crystalline samples with different crystal/glass ratios. Such treatments result in complex microstructures composed of large glassy domains where nanocrystals of the metastable β-As2Te3 phase are embedded. These domains are separated by regions of the dendritic crystalline phase surrounded by a Cu-rich glassy matrix. The presence of β-As2Te3, confirmed by both powder X-ray diffraction and scanning electron microscopy, suggests that pressure and/or internal stresses play an important role in stabilizing this phase. This conclusion is further supported by neutron thermodiffraction experiments revealing a sharp crossover from the β-As2Te3 to the stable α-As2Te3 phase at temperatures below that of the SPS treatment. Transport properties measurements show that the presence of a crystalline fraction significantly lowers the electrical resistivity by four orders of magnitude. However, the probable intrinsic n-type behavior of β-As2Te3 has a detrimental influence on the thermopower values. Even though the partial crystallization of the glassy matrix leads to an increase in the thermal conductivity, the measured values remain on the order of 1 W m−1 K−1 at 300 K. Besides an overall increase in the dimensionless figure of merit ZT, our results demonstrate that the partial crystallization of an amorphous matrix is an efficient tool to tune the electrical resistivity over several orders of magnitude while maintaining low thermal conductivity values.

Neutron diffraction study of stability and phase transitions in Cu-Sn-In alloys as alternative Pb-free solders

In this work, we present an experimental study of structure and phase stability of ternary Cu-Sn-In alloys around 55 at. % Cu in the temperature range 100 °C≤T≤550 °C. We have followed the real-time sequence of phase transformations in succesive heating and cooling ramps, using state-of-the-art neutron powder thermodiffractometry, complemented with calorimetric studies of the phase transitions. Our results give experimental support to the current assessment of the ternary phase diagram in this composition and temperature range, confirming the sequence of transitions η→(η+L)→(ε+L) with transformation temperatures of 210 °C and 445 °C, respectively. The use of neutrons allowed to overcome common difficulties in phase identification with powder XRD due to absorption and preferred orientation issues. Even the transitions to liquid phases could be successfully identified and monitored in situ, turning the neutron thermodiffraction technique into a valuable tool for phase diagram studies of emerging lead-free solder candidates.

Cordierite synthesis. A time-resolved neutron diffraction study

Abstract In order to elucidate the mechanism of reaction during the cordierite synthesis a kinetic study has been carried out by means of X-ray diffraction and neutron thermo-diffraction. It was thus possible to elucidate between the reactions leading to the cordierite synthesis and those that gave rise to the undesired phases generated during this process. Depending on the reactants used these reactions are different. The present paper presents results obtained during the cordierite synthesis starting from talc, kaolin and hydromagnesite as reactants. This synthesis from natural raw materials takes place through a complex mechanism that has been described. In this kind of synthesis, at the end of the entire process the obtained cordierite is always accompanied by a second phase: spinel. This is the first time that the presence of the μ-cordierite phase has been observed when the synthesis is carried out in solid state from non-submicronic natural raw materials.

Thermal expansion of layered cobaltitesY(Ba1 − xSrx)Co2O5 + δ withx = 0, 0.05 and 0.10

The crystal structure and thermal expansion of Sr-doped layered cobaltitesY(Ba1 − xSrx)Co2O5 + δ (x = 0,0.05 and 0.10) were studied by means of in situ neutron thermodiffraction in the temperature range20 K ≤ T ≤ 570 K. The evolution with temperature of lattice parameters for the phases which crystallize inthis system is presented, as well as their dependence on the oxygen non-stoichiometryδ. Each phase’s volume has been fitted using available models based on the Grüneisenapproximation to the zero-pressure equation of state and using the Ruffa modelbased on the Morse potential, both using a Debye model for the internal energy.The coefficient of volumetric thermal expansion, Debye temperature and otherthermodynamic parameters are presented and compared with other perovskite compounds.

Phase transition in the Ruddlesden–Popper layered perovskite Li 2SrTa 2O 7

The crystal structure of the Ruddlesden–Popper layered perovskite Li 2SrTa 2O 7 has been characterized at various temperatures between −185 and 300 °C by several techniques: X-ray and neutron powder diffraction, single crystal diffraction, transmission electron microscopy and Raman spectroscopy. The low temperature structure has been confirmed to be orthorhombic Cmcm with a small octahedra antiphase tilting ( ΦΦ0) ( ΦΦ0) inside the perovskite blocks. With temperature, the tilting progressively vanishes leading around 230 °C to a tetragonal symmetry (S.G. I4/ mmm). This reversible phase transition, followed by X-ray and neutron thermodiffraction and thermal Raman measurements, is considered as of second order. An attribution of the Raman bands based on normal mode analysis is proposed.

Effect of ceramic nanoparticles on the solid-state reaction mechanism of dolomite–zirconium oxide followed by neutron thermodiffraction measurements

Calcium zirconate–magnesium oxide material was obtained by solid-state reaction from mixed dolomite (CaMg(CO3)2) and zirconia (m-ZrO2) nanopowders. The nanopowders were obtained by high-energy milling, which produced an increase of the superficial free energy of the particles. The role of nanoparticles in the reaction process of monoclinic-zirconia and dolomite was analysed for the first time using neutron thermodiffraction and differential thermal analysis-thermogravimetric techniques. The neutron thermodiffraction of this mixture provides a clear description in situ of the different decomposition and reaction processes that occur in the nanopowders mixture. The results make it possible to analyze the effect of the nanoparticles on the reaction behaviour of these materials.

Neutron thermodiffraction study of the crystallization of Ag–Ge–Se glasses: evidence of anew phase

Silver-containing chalcogenide glasses are potential candidates for electrical memorymanufacturing. In order to study the glasses under dynamic conditions, using thetemperature as the main parameter, a neutron thermodiffraction study of threeAgx(Ge0.25Se0.75)100−x glasses (x = 5,15 and 25) was carried out. Two in situ diffraction experiments were performed:a first series including heating ramps from room temperature up to350 °C and a second one comprising the measurement of an isotherm at about300 °C for 5 h.For the three studied glasses two stable crystalline phases were formed during the heating process: thecubic Ag8GeSe6 appeared first, followed by the crystallization of monoclinicGeSe2. The crystallizationprocess for the Ag-rich (x = 15 and 25) glasses was more complex, with the appearance of a new unstable phase at high temperature,i.e. Ag2GeSe3. Such a phase decomposed with time or temperature to produce a new phase,Ag10Ge3Se11, along withthe stable GeSe2. This phase has never been reported to date while the existence ofAg2GeSe3 was still controversial.

Characterization of nanophase precipitation in a metastable β titanium-based alloy by electrical resistivity, dilatometry and neutron diffraction

The metastable beta Ti-6Mo-5Ta-4Fe (wt.%) alloys was synthesized by cold crucible levitation melting and then quenched in water from the beta phase field. In order to investigate the transformation sequence upon heating, thermal analysis methods such as electrical resistivity, dilatometry and neutron thermodiffraction were employed. By these methods, the different temperatures of transition were detected and solute partitioning was oberved to the beta matrix during the omega and alpha nanophase precipitation

Neutron thermodiffractometry study of silver chalcogenide glasses

Silver-containing chalcogenide glasses are of considerable interest for applications in optical and electrical recordings and as solid electrolytes. Therefore, insight in their structural properties is important. A neutron thermodiffraction study of Agx(Ge0.25Se0.75)100−x glasses with x=15 and 25at.% was carried out. The amorphous samples were heated in situ from room temperature up to 350°C, while the neutron diffraction spectra were collected every degree. For both glasses the Ag8GeSe6 phase appeared first, followed by the crystallization of GeSe2. The crystallization process for the Ag-rich glass was more complex with the appearance of a non-identified third phase. Rietveld refinement helped us in studying the evolution of the lattice constants of each phase upon heating.

Low temperature neutron diffraction and magnetization of Fe25Cu75 solid solutions

Metastable Fe25Cu75 solid solution obtained by high-energy ball milling has been studied by means of neutron thermo-diffraction and magnetization measurements. This material, which crystallizes in a face centered cubic (FCC) crystal structure, is ferromagnetic, with a value for the Curie temperature, around 190K. The linear thermal expansion coefficient, αT, has been estimated from the temperature dependence of the lattice parameter in the temperature range 5–300K. An increase in the value of αT following the normal trend observed in other metallic compounds is observed below 150K, however, above this temperature αT remains almost constant indicating a slight magneto-volume effect in this material.

Nanostructured Fe obtained by high-energy ball milling

A detailed microstructural study of nanocrystalline Fe powder samples, obtained by mechanical attrition, was carried out by X-ray diffraction experiments as a function of the milling time. During the process, the exchange energy between the vessels, balls and Fe powders leads to a change of the Fe microstructure: in particular, a significant decrease in the size of the coherent diffraction domains and the creation of a large number of linear defects which induces microstrains. This microstructure seems to be responsible for the distinct magnetic behaviour of the samples. In addition, on heating, the sample produces the relaxation of the nanostructure, recovering the initial magnetic properties; this process has been followed on the sample with larger milling time by neutron thermo-diffraction from room temperature up to 1200 K. The analysis of the diffraction patterns by the Rietveld method allows determining the microstructure at each stage.

Structural evolution and magnetic properties in Fe 70Cr 10B 20 ribbons

The evolution of the magnetic properties of Fe 70Cr 10B 20 alloy, initially in amorphous state, is studied during the whole crystallization process, from room temperature to 800 °C. Neutron thermodiffraction has been used in order to follow all the structural changes that take place in the sample, including the appearance of tetragonal phases and polymorphic transformations. The formation of these tetragonal phases, with high magnetic anisotropy, leads to a degradation of the soft magnetic response. The study of the correlation between structure and magnetic response helps to optimize the composition and heat treatments in these materials for their application in sensor devices.

Structural properties and high-temperature reactions of the metastable Ω phase in Zr–Nb alloys

This article presents an experimental study and a systematic phenomenological analysis of the structural properties and the high-temperature reactions of the Ω phase formed by quenching (‘q’) bcc (β) Zr–Nb alloys. In the first part of the work an extensive database with lattice parameters (LPs) for the Ω q and the untransformed β q phase is developed on the basis of neutron diffraction measurements. Various striking features of the LP vs. composition relations are detected, and a new method of analysis is applied which involves the concept of a ‘reference behavior’ (RB) describing the probable properties of each individual phase if it were unaffected by the other structures in the heterogeneous alloys. In this way, a detailed evaluation is performed of the effects of Ω q/β q coherence strains upon the LPs of these two phases. In the second part of the work, neutron thermodiffraction studies are reported of the structural properties of the Ω q + β q two-phase alloys at 300 K ⩽ T ⩽ 650 K. Extensive LP vs. time results are discussed. On this basis, the structural properties of the Ω a phase formed isothermally upon aging (‘a’) are established, and compared with those of the Ω q phase. In addition, new information is discussed on the evolution of the Ω a + β a system toward thermodynamic equilibrium. Moreover, it is shown how the present method of analysis may also be used to obtain indirect information of both theoretical and practical interest. In particular, considerable evidence is presented indicating that the Nb content of the Ω a phase decreases upon aging whereas that of β a increases. Finally, it is shown how these composition changes might be used to reconcile much of the long-standing conflicting reports concerning the structural properties of the Ω q and Ω a metastable phases.

Structural Properties and Stability of Metastable Phases in the Zr-Nb System

The lattice parameters of the bcc (b) and omega (W) phases occurring metastably in a series of Zr-rich Zr-Nb alloys have been determined at and above room temperature (TR) using neutron diffraction techniques. In the first place, the effect of temperature changes upon the lattice parameters of the b and W phases in alloys with 10 and 18 at.% Nb was monitored using neutron thermodiffraction. A method of analysis is applied to the data which involves a confrontation between the observed structural properties and an idealised -or ``reference'- behaviour (RB) which admits a simple mathematical description. A generalised form of Vegard's law is adopted as RB for the b phase, whereas a specific RB is proposed for the W structure. The experimental data are well accounted for by this interpretation scheme, leading to a picture of the isothermal reactions occurring at high temperature which involves the transfer of Nb from the W to the b phase. Finally, the neutron diffraction data on the W phase are combined with an electron microscopy study for the alloy with 10 at.% Nb aged at 773 K, which provides information on the composition of this phase and its evolution towards thermodynamic equilibrium.