Thin-film solid-state reactions of solid BaCO3 and BaO vapor with (1 0 0) rutile substrates

Thin-film solid-state reactions in the BaCO₃–TiO₂ and BaO–TiO₂ systems have been experimentally studied in air and in vacuum, respectively, on (1 0 0) TiO₂ substrates. The solid–solid reactions in vacuum and in air showed that different processes occur during BaTiO₃ formation: an intermediate Ba₂TiO₄ compound is always observed in vacuum but not in air. The crystallographic orientations between the reaction products and the TiO₂ substrate were investigated by X-ray diffractometry and transmission electron microscopy. One orientation relationship has been found for BaTiO₃ after a solid–solid reaction, whereas different orientation relationships for BaTiO₃ have been identified after a vapor–solid reaction. At high temperatures, Ti-rich polytitanates were grown in both types of solid-state reactions but with quite different textures. The crystallographic relationships and their origins are analyzed.     

Morphology-dependent crystallization and luminescence behavior of (Y, Eu)2O3 red phosphors

(Y_0.95Eu_0.05)₂O₃ red phosphor particles with three distinctive morphologies of submicron spheres (up to 180 nm), microflowers (up to 10 μm) and microplates (up to 50 × 10 μm) have been converted from their respective precursors autoclaved (100–180 °C, 12 h) from mixed solutions of the component nitrates and hexamethylenetetramine [(CH₂)₆N₄]. The three types of precursors were found to have the approximate compositions M(OH)CO₃·H₂O for the sphere (M = Y and Eu), M₄O(OH)₉NO₃ for the flower and M₂(CO₃)₃·3H₂O for the plate, and their formation domains were defined. Both X-ray diffraction and photoluminescence analysis indicated that a calcination temperature of ?800 °C is needed to attain a homogeneous (Y_0.95Eu_0.05)₂O₃ solid solution and thus improved luminescence. Morphology-confined crystal growth of (Y_0.95Eu_0.05)₂O₃ was observed from the microplates, yielding a significantly higher exposure of the (4 0 0) facets at elevated temperature. The three types of phosphors exhibited a substantial morphology-dependent photoluminescence (PL)/photoluminescence excitation (PLE) behavior, but did not differ much in the positions of the PLE/PL bands or in the asymmetry factor [I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁)] of the luminescence. Upon UV excitation into the charge transfer band at ～240 nm the microplates showed the strongest red emission at ～613 nm (the ⁵D₀ → ⁷F₂ transition of Eu³⁺) at a calcination temperature of 1000 °C, whose intensity was ～2.49 and 1.57 times those of the flowers and spheres, respectively. Fluorescence decay analysis yielded similar lifetimes of ～1.5 ± 0.1 ms for the 613 nm emission of the three morphologies, suggesting that the differing luminescence was largely morphology-dependent, rather than defect-dependent.     

Enhancement of the fracture toughness of bulk L12-based (Al+12.5 at.% M)3Zr (M=Cu,Mn) intermetallics synthesized by mechanical alloying

The microstructural evolution and the mechanical properties of L1₂-type bulk (Al+12.5 at.% M)₃Zr (M=Cu, Mn) intermetallic compounds with a nanocrystalline structure were investigated. The (Al+12.5 at.% M)₃Zr (M=Cu, Mn) powders synthesized by planetary ball milling (PBM) could be successfully consolidated into nearly pore-free bulk compacts at 580 and 620 °C without taking holding time by spark plasma sintering (SPS). Their grain sizes were in the range from 8 to 10 nm. The micro-hardness of the SPS-processed bulks was measured to be 975.8 and 983.9 Hv, respectively. On the other hand, their fracture toughness was barely ∼2 MPam^1/2. It was lower than those (∼4–6 MPam^1/2) of the coarse-grained (∼100 nm) bulk specimens annealed. This result indicates that a grain refinement towards the nanoscale does not have an appreciable effect on improving fracture toughness in brittle intermetallics. Thus, it was found that the fracture toughness could be enhanced by proper annealing and addition of the boron. Furthermore, the effect of grain size on the fracture toughness in nano-sized level was investigated in the bulk specimen prepared by arc melting, using mechanical alloying powders with ball-milling.     

Monoclinic phases and stress-relief conditions in (1 − x)Pb(Mg1/3Nb2/3)TiO3–xPbTiO3 solid solutions

A comparative study on heterophase states in perovskite-type solid solutions of (1 − x)Pb(Mg_1/3Nb_2/3)TiO₃–xPbTiO₃ is carried out for compositions near the morphotropic phase boundary. The conditions for mechanical stress relief at elastic matching of phases are analysed at x = const in a wide temperature range. The heterophase states concerned with the presence of the intermediate monoclinic phase are interpreted using the domain state–interface diagrams calculated for x = 0.28, 0.32 and 0.34. It is shown that optimum volume fraction parameters of the domains in the monoclinic phase of the B type are varied in relatively wide ranges and promote complete stress relief with cubic–monoclinic phase coexistence. Two scenarios of stress relief at x = 0.32 are considered in connection with different heterophase states (either tetragonal–monoclinic of the B type or tetragonal–monoclinic of the C type) in a wide temperature range. Possibilities of elastic matching of two polydomain phases (tetragonal–monoclinic of the B type) with almost equal relative widths of the domains in these phases are shown for x = 0.34. The active role of domains of the monoclinic phases in stress relief and forming the planar unstrained interfaces is discussed.     

Combined experimental and theoretical investigations of the photoluminescent behavior of Ba(Ti, Zr)O3 thin films

The correlation between experimental data and theoretical calculations have been investigated to explain the photoluminescence at room temperature of Ba(Ti_0.75Zr_0.25)O₃ (BTZ) thin films prepared by the polymeric precursor method. The degree of structural order–disorder was investigated by X-ray diffraction, Fourier transform infrared spectroscopy, ultraviolet–visible absorption spectroscopy and photoluminescence (PL) measurements. First-principles quantum mechanical calculations based on density functional theory (B3LYP level) were employed to study the electronic structure of ordered and deformed asymmetric models. The electronic properties are analyzed and the relevance of the present theoretical and experimental results on the PL behavior is discussed. The presence of localized electronic levels and a charge gradient in the band gap due to a break in symmetry, are responsible for the PL in disordered BTZ lattice.     

Microstructure and plastic deformation behavior of Ni3(Ti,X) (X = Nb,Al) single crystals with long-period geometrically closely packed crystal structures

The crystal structure, phase stability and plastic deformation behavior of Ni₃(Ti_1−xNb_x) [x = 0, 0.01, 0.03, 0.05 or 0.10] and Ni₃(Ti_1−yAl_y) [y = 0.16 or 0.28] single crystals were investigated. The substitution of Ti by Nb in Ni₃Ti induced the formation of various long-period stacking ordered (LPSO) structures with 18-fold, 10-fold or 9-fold stacking sequences of closely packed plane (CPP) depending on the Nb content. In compression tests, the yield stress anomaly (YSA) appeared in ternary LPSO crystals as well as in binary Ni₃Ti by slip on the CPP. The change in stacking sequence of CPP in the LPSO phases strongly affects the YSA behavior due to the change in APB energy on the non-closely packed plane.     

(0 0 l)-oriented Bi2Sr2Co2Oy and Ca3Co4O9 films: Self-assembly orientation and growth mechanism by chemical solution deposition

In this study, two typical cobaltate-based thermoelectric films, Bi₂Sr₂Co₂O_y (BSC) and Ca₃Co₄O₉ (CCO), with structures of [Bi₂Sr₂O₄][CoO₂]₂ and [Ca₂CoO₃]^RS[CoO₂], respectively, are prepared by a simple chemical solution deposition on SrTiO₃ (1 0 0), (1 1 0), and (1 1 1) single crystal substrates. X-ray results reveal that all films are c-axis oriented regardless of the orientation of the substrate, suggesting self-assembly orientation. Transmission electron microscopy reveals amorphous/delamination regions for BSC film on SrTiO₃ (1 1 0), and the c-axis stripes of CCO on SrTiO₃ (1 1 1) are inclined at 30° to the interface, whereas the c-axis stripes are parallel to the interfaces for other films. The growth mechanism is established, and the driving force for self-assembly c-axis orientation is attributed to the syneresis stress due to solvent evaporation. The microstructures and properties are also studied and discussed, with the conclusion that self-assembly c-axis oriented layered cobaltates films are good candidates for thermoelectric applications.     

High-temperature thermoelectric properties of Ln(Co,Ni)O3 (Ln = La,Pr, Nd,Sm, Gd and Dy) compounds

The high-temperature thermoelectric properties of polycrystalline LnCo_0.95Ni_0.05O_3±δ (Ln = La, Pr, Nd, Sm, Gd and Dy) compounds have been investigated. The substitution of the lanthanide element produces changes in the crystal structure and in the unit cell lattice parameters. A semiconductor to metal transition takes place for all the compounds at 500 K < T < 800 K. The transition temperature, also monitored by differential scanning calorimetry, shifts to higher temperature with the decrease in the size of the lanthanide element. The Seebeck coefficient has positive values up to T = 1240 K. The thermal conductivity is mainly dominated by the lattice contribution. The dimensionless figure of merit ZT can be enhanced at different temperatures depending on which lanthanide element occupies the A-site in the ABO₃ perovskite-type phases.     

Shifting of the morphotropic phase boundary and superior piezoelectric response in Nb-doped Pb(Zr, Ti)O3 epitaxial thin films

A shift of the morphotropic phase boundary (MPB) and a superior piezoelectric response are observed in Nb-doped Pb(Zr_xTi_1?x)O₃ (PNZT) thin films epitaxially grown on Nb-doped SrTiO₃(1 0 0) (Nb:STO) substrates. X-ray diffraction and Raman spectra characterizations confirm that a phase transition from a tetragonal structure to a rhombohedral structure occurs when the Zr/Ti ratio varies from 20/80 to 80/20. The phenomenological theory and experimental analyses suggest that the MPB of epitaxial PNZT thin films is shifted to the higher Zr/Ti ratio (around 70/30) from the conventional ratio (52/48) due to the misfit compressive stress induced by the substrate. A maximum local effective longitudinal piezoelectric coefficient (d₃₃) up to 307 pm V^?1 is observed at a Zr/Ti ratio of 70/30 in the current compositional range, again confirming the shifting of MPB in epitaxial PNZT thin films. These findings offer a new insight for the fabrication of epitaxial PZT thin films at MPB with a superior piezoelectric response.     

First-principles investigation of electronic,mechanical and thermodynamic properties of L12 ordered Co3(M,W) (M = Al,Ge, Ga) phases

Studies were carried out on the equilibrium structural, temperature-dependent mechanical and thermodynamic properties of the Co₃(M, W) (M = Al, Ge, Ga) phases in terms of first-principles calculations. The results of the ground-state elastic constants revealed that Co₃(M, W) phases are mechanically stable and possess intrinsic ductility. It was found that the elastic heat-resistant properties of Co₃(Ge, W) phase are inferior to those of Co₃(Al, W) and Co₃(Ga, W). Analyzing the charge density difference provides the explanation that the sharp decrease in mechanical properties is mainly due to the weakening of Co–Ge bonding at elevated temperatures for Co₃(Ge, W). The elastic anisotropy as a function of temperature is discussed using a universal index. It is observed that Co₃(M, W) phases show a high degree of elastic anisotropy. The degree of elastic anisotropy could be significantly decreased by an increase in temperature for Co₃(M, W). The lattice vibration is treated with the quasiharmonic phonon approach, considering both the vibrational and thermal electronic contributions. The thermodynamic properties as a function of temperature are computed without any adjustable parameters, including heat capacity, entropy, enthalpy and thermal expansion coefficient.     

DTA/TG study of tungsten oxide and ammonium tungstate reduction by (Mg + C) combined reducers at non-isothermal conditions

In this work the reaction mechanism in the WO₃–Mg/C systems and ammonium paratungstate (APT)–Mg/C systems are studied. As reducer magnesium, carbon or combinations of both are explored. It is shown that in the WO₃–Mg system the reduction undergoes by solid–solid mechanism before melting Mg, where metallic tungsten and MgO are formed. Unlike this system, in the WO₃–C system mainly WO_x (< 880 °C) and WO₂ (> 960 °C) and small amount W is formed. In the WO₃–Mg–C ternary system reduction temperature shifts to higher temperature range and depends on amount of carbon. Similar to WO₃–Mg system, APT–Mg reaction starts and completes in the solid state. Thus, firstly the APT decomposes, then reduction of formed WO₃ takes place at ~ 600 °C yielding W and MgO. Likewise to WO₃–Mg system adding carbon into APT–Mg mixture shifts reduction temperature to even higher temperature zone which can exceed melting point of Mg and further reduction undergoes with molten magnesium. It is shown that the reduction products are MgO and W.     

Combustion synthesis of long-persistent luminescent MAl2O4: Eu2+, R3+ (M = Sr,Ba, Ca,R = Dy,Nd and La) nanoparticles and luminescence mechanism research

Eu²⁺, R³⁺ co-doped alkaline earth aluminates MAl₂O₄: Eu²⁺, R³⁺ (M = Sr, Ba and Ca; R = Dy, Nd and La) nanoparticles with high brightness and long afterglow have been prepared by solution-combustion synthesis at 600 °C without a post-annealing process for the first time. The morphologies and the phase structures of the products have been characterized by transmission electron microscopy and X-ray diffraction. The excitation and emission spectra of the products have been measured by an Edinburgh FLS920 spectrometer at room temperature. The characteristic luminescence of the as-prepared sample has been evaluated, and the reason why the wavelength changed from yellow-green to blue-green and then to blue-purple in visible range when the composition changed from SrAl₂O₄: Eu²⁺, Dy³⁺ to BaAl₂O₄: Eu²⁺, Nd³⁺ and then to CaAl₂O₄: Eu²⁺, La³⁺ has been explained. Furthermore, a new persistent luminescence mechanism was proposed in this article.     

Interfacial tension and contact angle variations of SUS304 melt in contact with solid oxides and CaO-SiO2-Al2O3(CaF2) slags at 1470°C

Study on the interfacial properties of SUS304 melt in contact with pedestal oxides and also liquid slags of varying chemistry was carried out by using a combination of the sessile drop method and a X-ray fluoroscopic technique at 1470°C. Interfacial tension and contact angle were obtained directly from a numerical solution of the Young-laplace equation. Surface tension of SUS304 was found to be 1467 mN.m in Ar-atmosphere which was considerably lower than that of pure iron. This may be due to the existence of surface active elements such as (Cr, Ni and Mn etc. in SUS304. Interfacial tensions of SUS304 in contact with CaO-Sio₂-Al₂O₃(CaF₂) slags were in the range of 925 to 1 148 mN/m. The contact angle between SUS 304 and various pedestal was about 120° regardless of pedestal oxides. A1₂O₃. TiO₂ and SiO₂. Contact angles between SUS304 and the pedestal alumina coexisting with liquid CaO-SiO₂-Al₂O₃CaF₃) were in the range of 125° to 142°.     

Phase stability and electronic structures of YbAl3−xMx (M = Mg, Cu, Zn, In and Sn) studied by first-principles calculations

YbAl₃ is a potential thermoelectric (TE) material to be used in TE power generator utilizing waste heat sources. This work systematically investigated the alloying behavior of M in YbAl_3−xM_x (M = Mg, Cu, Zn, In and Sn) and their electronic structures by first-principles calculations. The solubility of alloying elements M and phase stability of YbAl_3−xM_x were studied by analyzing the elastic constants, formation and cohesive energies. The results show that Mg, In and Sn are the effective alloying elements for preparing YbAl_3−xM_x solid solutions, which are helpful for improving TE properties of YbAl₃.     

Comparison of the corrosion of Cu50Pd50 and Cu50Pd44M6 (M = Y,Mg, or Al) hydrogen separation membrane alloys in simulated syngas containing H2S

The resistance to sulfidation was examined by exposing coupons including a Cu–Pd binary alloy and three ternary alloys with 6 at% Y, Mg, or Al to simulated syngas containing varying amounts of H₂S. The mass change of the samples was determined and the exposed surfaces were characterized by SEM/EDS and XRD. The best corrosion resistance of the ternary alloys was observed in the Cu₅₀Pd₄₄Al₆ alloy. A slight decrease was observed when Mg was added, but both the Al and Mg alloys were roughly on par with the binary composition. The Y addition resulted in a degradation in the corrosion resistance by forming extensive internal Y₂O₃ and Cu_1−xY_xS and external Pd_4−xCu_xS and Pd ₁₃Cu₃S₇.     

Molecular and electronic properties of β-(BEDT-TTF)2PF6 studied by scanning tunneling microscopy

Two different structures based on one-dimensional molecular rows were observed by scanning tunneling microscopy (STM) in the β-(BEDT-TTF)₂PF₆ crystal surface. The temperature dependence of tunneling current-bias voltage curves showed the property of a metal-insulator phase transition. However, metallic electronic states were observed even at 230K between the energy gap, indicating fluctuation of the charge density wave. The metallic property was recovered when the STM tip was softly brought into contact with the sample surface, where the charge density wave is considered to be formd.     

Doping effect of M (M = Nb,Ce, Nd,Dy, Sm,Ag, and/or Na) on the growth of pulsed-laser deposited V2O5 thin films

V₂O₅ thin films were prepared by using a pulsed-laser deposition technique. We investigate the doping effect of elements such as Nb, Ce, Nd, Dy, Sm, Ag, and/or Na, on the growth of V₂O₅ films. X-ray photoelectron spectroscopy results indicate that the oxidation state of vanadium is almost highest. X-ray diffraction study reveals that the doping of Nb urges to crystallize the V₂O₅ films. The Raman measurement shows that the doping of rare earth elements leads to the shift from 146 to 149 cm^?1 about –O–V–O–O–V– bond and from 995 to 998 cm^?1 about VO double bond. The optical measurement study reveals that the doping effect on the VO double bond is classified into two types. The addition of Nb, Ce, Nd, Sm, and/or Dy results in an increase of band gap, while those of Ag, and/or Na decreases a band gap. The electrochromic parameters of V₂O₅ films doped with Nb, and/or Na are superior to that of other elements as an electrochromic device (ECD).     

Crystalline patterns and band structure dimensionality in a series of conducting hybrids associating amide-functionalized EDT-TTF π-donors with the isosteric octahedral anions [FeNO(CN)5]2− and [M(CN)6]3− (M = Co,Fe)

Four new radical cation salts based on the amide functionalized EDT-TTF organic donors EDT-TTF-CONH₂ (D1) and EDT-TTF-(CONH₂)₂ (D2), α′-(D1)₄[FeNO(CN)₅] (1), α′-(D1)₄[Co(CN)₆] (2), β-(D1)₆[Fe(CN)₆] (3) and (D2)₄[FeNO(CN)₅]NB (4), have been synthesized and characterized by X-ray single crystal diffraction experiments, band structure calculations and electrical resistivity measurements. Functionalized organic donors are remarkable for the ability to form cation⋯cation and cation⋯anion type hydrogen bonds which can effectively direct the crystal architecture of molecular conductors. The structural analysis reveals a well-developed hydrogen bond network in the crystals investigated. (D1)₂-dimers or (D2)_n-extended zigzag chains of donors connected through functional groups are found to be stable structural motifs in 1–4. Remarkably, salts 1 and 2 are isostructural despite the presence of anions of different charge (−2 and −3, respectively) and the inherent difference in the degree of charge transfer has a clear effect on the transport properties: σ_RT(2)/σ_RT(1) = 300. Salt 3 differs from 1 and 2 both in the stoichiometry and packing of the conducting organic layer. Crystals of 4 exhibit a superstructure with the incommensurate vector q = ±(0.5, 0.3, 0.2).