Showing papers in "Phase Transitions in 2011"

Thermochemical resistive switching: materials, mechanisms, and scaling projections

Abstract: In this article, resistive switching based on the thermochemical mechanism (TCM) is reviewed. This mechanism is observed when thermochemical redox processes dominate over electrochemical processes. As the switching is based on thermal effects, it is inherently unipolar, i.e., the transitions between the resistive states can be induced by the same bias voltage polarity. NiO has emerged as a “model material” for resistive switching based on the TCM effect and the discussion of the resistance states and the switching processes are focused on this material with the appropriate electrodes, mainly Pt. Unipolar switching is unambiguously filamentary. Conductive filaments are formed during the electroforming process needed prior to memory switching. The SET operation is interpreted as a sequence of threshold switching and subsequent Joule heating which triggers local redox reactions in which oxygen deficient NiO and, if the amount of released oxygen exceeds a certain amount, also metallic Ni will form. The RESET ...

SNBL, a dedicated beamline for combined in situ X-ray diffraction, X-ray absorption and Raman scattering experiments

Abstract: X-ray diffraction, X-ray absorption spectroscopy and Raman scattering are commonly used for studies of crystal and electronic structure of materials. All three techniques have their strong points and limitations but by combining them into one experimental set-up it is possible to exploit their complementarities. The biggest advantage of such a multi-technique approach lies in the observation of dynamic processes. This is where (quasi-)simultaneous data acquisition with different techniques ensures a perfect correlation between these measurements. As a result, one obtains information about the materials structure, which goes beyond the sum obtained by individual experimental methods. The success of the multiple technique approach depends strongly on the rigorous optimisation of all related experimental details. In this article, we describe some of these crucial methodological solutions implemented at Swiss-Norwegian beamline.

Synthesis, thermal and optical behaviour of non-symmetric liquid crystal dimers α-(4-benzylidene-substituted-aniline-4'-oxy)-ω-[pentyl-4-(4'-phenyl)benzoateoxy]hexane

Abstract: A series of six non-symmetric liquid crystal dimers α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-[pentyl-4-(4′-phenyl)benzoateoxy]hexane have been synthesized and characterized. These dimers vary from one another in terms of the terminal substituent (H, CH3, C2H5, F, Cl and Br) with different polarizabilities and molecular sizes. Investigation on the thermal behaviour shows that the dimer with a more polarizable terminal substituent of Br atom exhibits a smectic A phase as well as an enantiotropic nematic phase. Influence of the molecular size of the terminal substituent in stabilizing the liquid crystalline properties is also studied.

Dielectric behaviour of the composite system: multiwall carbon nanotubes dispersed in ferroelectric liquid crystal

Abstract: Ferroelectric liquid crystal–multiwall carbon nanotube (FLC-MWCNTs) composite systems have been prepared by dispersing low (0.03 and 0.10 wt.%) concentration of MWCNTs in the liquid crystal matrix. Relaxation behaviours of pure and composite systems have been investigated. Thermo-optic studies infer a small shift (∼5 K) in the clearing temperature of the dispersed systems. Dielectric measurements reveal that the MWCNTs influenced the permittivity. The relaxation time, relaxation frequency and dielectric strength have been evaluated and discussed. The influence of MWCNTs concentration on the conductivity of composite systems has been investigated.

Random local strain effects in the relaxor ferroelectric BaTi1-xZrxO3: experimental and theoretical investigation

Abstract: We report an investigation of the local structure in homovalent-substituted BaTi1−x Zr x O3 relaxors by a combination of experimental and theoretical methods, namely neutron total scattering, X-ray absorption spectroscopy, and supercell ab-initio calculations It is shown that unlike Zr atoms, Ti atoms are largely displaced in their octahedra, and are thus associated with strong local dipole moments Besides, we give evidence that the difference in the size of Ti4+ and Zr4+ cations leads to a significant size mismatch of the Ti–O6 and Zr–O6 octahedra When they link to form the perovskite structure of BaTi1−x Zr x O3, the O6 octahedra undergo slight distortions in order to accommodate their different sizes It is shown that they are compressed in the direction of Zr neighbors, and expanded in the direction of Ti neighbors The polar Ti displacements, which are sensitive to the octahedral distortions, then become constrained in their orientation according to the local Zr/Ti distribution Such constraints i

A brief history of tilts

Abstract: The following article is a personal, and probably self-indulgent, review of the development of the classification of tilted octahedra in perovskite structures and my role in it. This review is not intended to be a thorough account of the topic, which has in any case been covered elsewhere, but is offered as a historical account of how the description of perovskite tilt systems in use today originally came about.

Phase diagram and extreme Gibbs measures of the Ising model on a Cayley tree in the presence of competing binary and ternary interactions

Abstract: One of the main problems of statistical physics is to describe all Gibbs measures corresponding to a given Hamiltonian. It is well known that such measures form a nonempty convex compact subset in the set of all probability measures. The purpose of this article is to investigate phase diagram and extreme Gibbs measures of the Ising model on a Cayley tree in the presence of competing binary and ternary interactions.

High-pressure phase transitions in BiFeO3: hydrostatic versus non-hydrostatic conditions

Abstract: We report high pressure X-ray diffraction experiments on BiFeO3 (BFO) single crystals in diamond-anvil cells up to 14 GPa. Two data sets are compared, one in hydrostatic conditions, with helium used as a pressure-transmitting medium (PTM), and the other in non-hydrostatic conditions, with silicon oil as a PTM. It is shown that the crystal undergoes different phase transitions in the two cases, highlighting the high sensitivity of BFO to non-hydrostatic stress. Consequences for the interpretation of high pressure structural studies are discussed.

Glass transition temperature and thermal stability of ZnS/PMMA nanocomposites

Abstract: In this article, ZnS nanoparticles were prepared by wet chemical precipitation method using zinc sulphate (ZnSO4), sodium sulphide (Na2S) and thio-glycerol. These nanoparticles were characterized through X-ray diffraction (XRD) and transmission electron microscope (TEM) measurements. The solution-based processing was used to prepare Poly methyl methacrylate (PMMA) nanocomposites with different weight percents (0, 2, 4, 6 and 8) of ZnS nanoparticles. The obtained ZnS/PMMA nanocomposites were characterized through XRD, scanning electron microscope and TEM measurements. The dynamic mechanical analyzer was used to obtain the storage modulus and glass transition temperature (T g) of the nanocomposites. The apparent activation energy of the glass transition region was also determined using the Vogel–Fulcher–Tammann equation. The results indicated that the thermal stability of ZnS/PMMA nanocomposites was higher than PMMA and 6 wt. % of ZnS nanoparticles in PMMA matrix showed the maximum activation energy, which ...

On-line phase transition in La1− x Sr x MnO3 (0.28 ≤ x ≤ 0.36) perovskites through ultrasonic studies

Abstract: La1− x Sr x MnO3 perovskite manganite materials with different compositions (x = 0.28, 0.31 and 0.36) have been prepared employing solid-state reaction technique. On-line evaluation of ultrasonic velocities and longitudinal attenuation of the above samples has been done over a wide range of temperatures using the transmission method. The observed anomalies in velocities, attenuation and elastic moduli reveal the occurrence of lattice softening and hardening near Curie temperature. The observed dramatic hardening in sound velocities and softening in attenuation are correlated with the phase transition, i.e. ferromagnetic to paramagnetic. The increase in magnitude of maximum velocity with change in Sr content at T c indicates the existence of linear magnetostriction effect. The elastic moduli study elucidates the observations made from the above-mentioned studies. The variation in the ultrasonic velocities, longitudinal attenuation and its derived parameters help us to understand the competitions between fe...

High-pressure phase transitions in transition metal carbides XC (X = Ti, Zr, Hf, V, Nb, Ta): a first-principle study

Abstract: First-principle density functional approach has been applied to study the B1→B2 structural phase transition in transition metal carbides (TMCs) (XC; X = Ti, Zr, Hf, V, Nb, and Ta) under the application of pressure. The computations have been performed using ground state total energy calculation approach of the system. The study computes the stability of structure as a function of pressure in original rocksalt (B1) and hypothetical CsCl (B2)-type phases using generalized gradient approximation with Perdew–Burke–Ernzerhof-type parametrization and local density approximation with Ceperley–Alder-type parametrization as exchange correlation functional and observed that the vanadium carbide is found to be the most stable amongst all the carbides taken into consideration. Pressure of more than 400 GPa transforms the original B1-type phase of these carbides to a B2-type phase. We have also calculated the ground state properties, such as lattice constant (a), bulk modulus (B 0), and pressure derivative of bulk mod...

Thermal behaviour of natrite Na2CO3 in the 303–1013 K thermal range

Abstract: This study reports a structural investigation, at a fine temperature scale, of natrite Na2CO3 from 303 to 1013 K by laboratory parallel-beam X-ray powder diffraction. Within the investigated thermal range, Na2CO3 undergoes the γ C2/m(α0γ)0s → β C2/m → α P63/mmc phase transitions. Results indicate that all modulation amplitudes tend to decrease to zero, approaching the γ → β transition temperature. However, a high-amplitude anti-phase modulation of the two symmetry-equivalent C–O1 distances starts at ca 520 K and reaches a maximum value of ca 0.7 valence units at the γ → β transition. Therefore, it seems that O1 instability represents the driving force of the γ → β transition. The β-polymorph is characterized by a disordered structure showing multiple site splitting and a strong anisotropic microstrain, starting immediately after the γ → β transition. The structure of α-Na2CO3 was also refined and found to differ from reference data. Similarly, for α-K2CO3 and α-Rb2CO3 a considerable disorder at the O site...

Phase–field modelling of 180° “Bloch walls” in rhombohedral BaTiO3

Abstract: This article describes investigations of 180° walls in rhombohedral BaTiO3 in the framework of generalized Ginzburg–Landau–Devonshire model, using phase-field simulation approach. It is demonstrated that the 180° wall with the normal parallel to [1−10] direction has a Bloch-like character. Its domain profile can be understood as a pair of very close 71° and 109° walls, which can be continuously separated by a strong transverse electric field. The intermediate region can be considered as a few nanometers thick monoclinic domain.

Effect of the chiral chain length on structural and phase properties of ferroelectric liquid crystals

Abstract: Studies of structural and phase properties obtained on several ferroelectric liquid crystalline materials with 2-alkoxypropionate group used as a chiral centre and without any lateral substitution are presented. In dependence on the chiral chain length these compounds exhibit the cholesteric N* phase, the ferroelectric smectic C* and a low-temperature SmX phase. Values of the spontaneous polarization and spontaneous tilt angle have been determined within the whole range of the SmC* phase. A low-temperature SmX phase has been identified as the orthogonal hexatic SmB* phase. The molecular parameters, namely the layer spacing in the SmC* and SmB* phases and the average intermolecular distances (D) between neighbouring parallel molecules in all investigated phases have been determined using the results of the X-ray diffraction obtained on non-oriented samples. The effect of the chiral chain length on mesomorphic, structural and physical properties of the studied ferroelectric liquid crystalline materials is d...

Raman spectroscopy of Pb(Zr1 − x Ti x )O3 graded ceramics around the morphotropic phase boundary

Abstract: Lead zirconate titanate ceramics Pb(Zr1–x Ti x )O3 with spatially graded composition from X = 46 to 49 were studied by Raman scanning microscopy and the results were compared with Raman spectra in PZT ceramics of different compositions around the morphotropic phase boundary (MPB). An E-symmetry mode located near 200 cm−1 was found to be a measure of the tetragonality of PZT in both standard graded ceramics. The singular value decomposition method was applied to decompose the experimental spectra of the graded ceramics. Two main contributions were found: one is related to the intensity of the main transverse modes – especially the E(TO2) mode – and it gives evidence of the change between tetragonal and rhombohedral symmetries in the sample; the second one shows changes just through the spatial MPB and it is related to the intensities of A1(TO2) and E(LO2) modes. All the phonons involved in the main changes correspond to Zr and Ti vibrations.

Physical properties of new multiferroic hexagonal YbFeO 3 thin film

Abstract: Temperature dependences of dielectric constant and second-harmonic generation (SHG) intensity of hexagonal (h-) YbFeO3 thin films deposited on yttrium stabilized zirconia (YSZ) (111) substrates were measured from 300 to 470 K. The results reveal that h-YbFeO3 film is ferroelectric below around 350 K. From the SQUID measurements, it is also concluded that this thin film is ferrimagnetic in the low-temperature range.

Studies on ionic liquid 1-ethyl-3-methyl imidazolium ethylsulphate complexed with PVA

Abstract: Poly(vinyl alcohol) (PVA) and polymer electrolyte (PVA + 15 wt% LiClO4) doped with 20 wt% ionic liquid (IL) (1-ethyl-3-methyl imidazolium ethylsulphate), [EMIM][EtSO4] have been prepared using solution cast technique. These electrolyte films have been characterized using XRD, DSC, TGA, FT-IR and impedance spectroscopic techniques. It has been found that polymer electrolyte film (PVA + 15 wt% LiClO4) + 20 wt% IL shows maximum amorphosity and conductivity.

Raman scattering of perovskite SmScO3 and NdScO3 single crystals

Abstract: We report an investigation of perovskite-type SmScO3 and NdScO3 single crystals by Raman scattering in various scattering configurations and at different wavelengths. The reported Raman spectra, together with the phonon mode assignment, set the basis for the use of Raman scattering for the structural investigation of rare earth (RE) scandates. Further to the phonon signature, a fluorescence signal is observed for both scandates and is particularly intense for NdScO3 when using a 488 or 514 nm excitation line. A comparison of Raman spectra of RE-scandates with literature Raman data on orthorhombic perovskites shows that the frequency of particular modes scales with the orthorhombic distortion in terms of the rotation (or tilt) angle of the ScO6 octahedra.

Disorder in liquid Cu–Pd alloys

Abstract: The disorder in thermodynamic and microscopic structure of liquid Cu–Pd alloy at 1350 K has been studied using regular associated solution model. For this, we have calculated free energy of mixing (GM ), activity (a), concentration fluctuation in long wavelength limit [SCC (0)] and chemical short-range order parameter (α 1) of liquid Cu–Pd alloy at 1350 K. The energetic and structural asymmetry of liquid Cu–Pd alloys has been successfully explained on the basis of regular associated solution model.

Ferroelectric domains in near-stoichiometric LiNbO3 by e-beam polarization reversal

Abstract: Local polarization reversal and periodical domain structure engineering by an electron beam in the Z-cut of near-stoichiometric LiNbO3 crystal (NSLN) was investigated. The NSLN was grown from a congruent composition containing 6%wt. K2O flux by top-seeded solution growth method. The 0.75 mm thick Z-cut samples were irradiated in the SEM with a lithography system. Both −z and +z crystal sides were studied after selective etching using an optical microscope. The sideways and frontal domain wall motions at different densities of introduced e-beam charges were analyzed. Irradiation conditions for long triangular string-like domain reversal in the crystal plates of 750 µm thickness were determined. The domain gratings were fabricated in the same crystal plates by e-beam. In the bulk crystal, domain gratings (Λ = 10 µm) transformed to the 2D structure. The NSLN crystals hold promise for thick periodical domain structure engineering by e-beam technique.

Hydrogen degradation of steels and its related parameters, a review

Abstract: Many steel structures can absorb varying amounts of hydrogen during manufacturing, processing such as electroplating or acid pickling and during service life by corrosion reactions or cathodic protection. Hydrogen is known to have a devastating effect on strength and ductility of iron and steel components. Many studies have been done on the detrimental effects of hydrogen on iron and different steel structures but there is a need for comprehensive research to cover all related parameters which can control the extent of hydrogen interaction and degradation in steels. In this review, at first hydrogen uptake and different damages associated with absorbed hydrogen in steels are discussed. Then, the effective parameters in the extent of hydrogen interaction and degradation, especially hydrogen embrittlement which is more common in steel components are reviewed.

Azoester-based H-shaped symmetrical mesogenic dimers containing – CH3/–OCH3 terminal substituent

Abstract: Two extensive homologous series of H-shaped symmetrical dimers were synthesized and their thermotropic properties studied by differential scanning calorimetry and on a hot-stage of a polarizing microscope. These compounds consist of two mesogenic units of azoester interconnected through flexible spacers (n = 4) resulting in the structure of ‘H-shaped’ dimeric compounds. The difference between the two series is in the structure of terminal substituents (–CH3 for series I and –OCH3 for series II) attached on the azoester mesogens at one terminus. All these compounds were found to be smectogenic. The effect of different terminal substituents on mesomorphism is discussed. The trans-azobenzene groups of the H-shaped dimeric compounds display a high-intensity π–π* transition at about 365 nm and a low-intensity π–π* transition at around 460 nm, therefore, photochromism can be achieved by the introduction of the azo linkage to the H-shaped dimeric compounds.

The study of transition on NiFe2O4 nanoparticles prepared by co-precipitation/calcination

Abstract: In this study, the NiFe2O4 nanoparticles have been prepared by co-precipitation and calcination process. Using a vibrating sample magnetometer (VSM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectrometer of X-ray (EDX), and X-ray photoelectron spectroscopy (XPS), the samples obtained by co-precipitation and then by further calcination have been analyzed. The experimental results show that the precursor synthesized by co-precipitation is the composite of both amorphous FeOOH and Ni(OH)2, but has no amorphous NiFe2O4. The results of both EDX and XPS revealed that the FeOOH species is wrapped up by Ni(OH)2 species. In the calcination process, the amorphous composite is dehydrated and transformed gradually into crystalline NiFe2O4 nanoparticles, with the metal ions diffusing. The reaction is different from the one used to prepare other ferrite (e.g., CoFe2O4, MnFe2O4, Fe3O4, etc.) nanoparticles directly by co-precipitation. With increasing calcination temperature, the...

Magnetic symmetry of low-dimensional multiferroics and ferroelastics

Abstract: Ferroics are materials with two or more “orientation” states with the ability to switch between them via an applied field. Based on spatial inversion and time reversal symmetry, there are four types of primary ferroics: ferroelectrics described by polarization P (a polar vector with broken spatial inversion symmetry), ferromagnets described by magnetization M (an axial vector with broken time reversal symmetry), ferrotoroidics described by torodization T (an axio-polar vector with both spatial inversion and time reversal symmetries broken) and ferroelastics described by strain e (a symmetric second rank polar tensor with neither spatial inversion nor time reversal symmetry broken but with broken rotational symmetry). Crystals exhibiting simultaneous ferroelectricity and magnetism are called magnetoelectrics. We analyze the magnetic symmetry of one- and two-dimensional (1D and 2D) multiferroics and enumerate 1D and 2D magnetoelectric point groups. We study a representative phase transition to such a point ...

Ferroelectricity and strain effects in orthorhombic YMnO3 thin films

Abstract: We report on the dielectric properties of epitaxial [0 0 1]- and [1 0 0]-textured thin films of antiferromagnetic orthorhombic YMnO3 and their variation under compressive epitaxial strain. It is found that weakly strained YMnO3 films are ferroelectric with a polarization along the c-axis switchable by 90° by an external magnetic field. With reducing film thickness and increasing epitaxial strain, the ferroelectric order is progressively suppressed. Analysis of structural, dielectric and magnetic data indicates that suppression of ferroelectricity with reducing thickness is accompanied by an enlarged ratio a/b of the in-plane cell parameters of the orthorhombic structure and the appearance of a net magnetization. All results can be well-described by considering the multiferroic phase diagram of orthorhombic manganites.

Local conductivity of epitaxial Fe-doped SrTiO3 thin films

Abstract: Atomic Force Microscopy (AFM) measurements have been performed for Fe-doped SrTiO3 thin films with an Fe concentration of 1 and 2 at%. Thin films with a thickness of 15–50 nm were grown by pulsed laser deposition on single crystalline SrTi0.99Nb0.01O3 substrates. Low-energy electron diffraction examination showed that the films are single crystalline. The regions treated with the AFM tip (applied dc voltage up to 6 V) showed inhomogeneity of the electrical conductivity. Resistive switching was studied by the use of local conductive atomic force microscopy and was present in all samples. Fe doping was found to influence the character of switching.

Electronic structure of epitaxial Fe-doped SrTiO3 thin films

Abstract: X-ray photoelectron spectroscopy has been applied to study the electronic structure of the epitaxial Fe-doped SrTiO3 thin films with an Fe concentration of 1 and 2 at%. The results were compared to undoped thin films and lightly Fe-doped single crystal of SrTiO3. Fe doping was found to increase the amount of the in-gap states which can be related to the increased ability of Fe-doped thin films to exhibit resistive switching behaviour. Core level analysis indicates the mixed-valence character of Fe atoms in the films, with a divalent contribution situated on the surface.

Gamma prime and TCP phases and mechanical properties of thermally exposed nickel-base superalloy

Abstract: The effect of long-term thermal exposure and casting superheat on microstructure, topologically close-packed (TCP) phases, γ ′ precipitation and mechanical properties of an experimental Ni-base superalloy were studied. The investigated alloys were produced by investment casting process under two levels of superheat. After solution heat treatment, at 1180°C for 2 h followed by air cooling; the two alloys under investigation were isothermally exposed at 845°C for 24, 200, 1000, and 1500 h. The long-term thermal exposure conditions have a significant impact on the precipitation and morphology of TCP and γ ′ phases. The σ phase precipitated as needle and platelet shapes, whereas the μ phase formed in plate and agglomerated shapes. The μ phase has high concentration of Cr, Mo, W, and Co, while the σ phase has high percentages of Ni and Ti. The μ phase was precipitated after thermal exposure of 200 h in the case of high superheat specimen and after 1000 h in low superheat specimen. The η phase found was also a ...

SAXS and UV–Vis combined to Quick-XAFS monitoring of ZnO nanoparticles formation and growth

Abstract: Time-resolved X-ray absorption-fine structure (Quick-XAFS) and UV–Vis absorption spectroscopies were combined for monitoring simultaneously the time evolution of Zn-based species and ZnO quantum dot (Qdot) formation and growth during the sol–gel synthesis from zinc oxy-acetate precursor solution. The time evolution of the nanostructural features of colloidal suspension was independently monitored in situ by small angle X-ray scattering (SAXS). In both cases, the monitoring was initialized just after the addition of NaOH solution (B = [OH]/[Zn] = 0.5) to the precursor solution at 40°C. Combined time-resolved Quick-XAFS and UV–Vis data showed that the formation of ZnO colloids from the zinc oxy-acetate consumption achieves a quasi-steady-state chemical equilibrium in less than 200 s. Afterwards, the comparison of the ZnO Qdots size and Guinier gyration radius evidences a limited aggregation process coupled to the Qdots growth. The analysis of the experimental results demonstrates that the nanocrystal coales...

Synthesis and phase transitions of mesogenic compounds with functional groups in the tail

Abstract: The properties of two pairs of recently synthesised compounds with mesogenic unit were compared. The four compounds consist of a central segment (based on biphenyl or phenylbenzoate) with symmetric carbon chains ended with vinyl or epoxy groups. Typical measurement techniques (including microscopic observations, differential scanning calorimetry and wide-angle X-ray scattering and dielectric studies) were used to examine phase transitions in the studied materials. The phase transition sequences were determined for all materials. A liquid crystalline phase appeared in some cases. It was observed that replacing the functional group in the tails visibly changes the temperatures and sequences of phase transitions.