Browsing by Author "Zhang, Z"
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- ItemAmorphous to anatase transformation in atomic layer deposited titania thin films induced by hydrothermal treatment at 120°C(Materials Research Society, 2008-09) Zhang, Z; Triani, G; Fan, LJHydrothermal treatment has been applied successfully to convert amorphous titania films to crystalline anatase at 120°C, a temperature compatible with most polymeric substrates. The amorphous films were deposited at 80°C using atomic layer deposition (ALD). The crystallinity of the films was monitored by x-ray absorption near edge structure (XANES), and the film composition was determined by x-ray photoelectron spectroscopy (XPS). The effect of precursor chemistry and substrate material was investigated. It was found that titania films produced from Ti isopropoxide are easier to crystallize than those from Ti tetrachloride as the Ti precursor. The amorphous to crystalline transformation can be achieved more readily with films deposited on Si than polycarbonate substrates. The effect of a “seed” layer on the amorphous to crystalline transformation was also studied. Preformed anatase crystallites between the Si substrate and the amorphous film were shown to accelerate the crystallization process. The possible mechanisms responsible for the phase transformation are discussed. © 2008, Materials Research Society
- ItemAnion Disorder in Lanthanoid Zirconates Gd2-xTbxZr2O7(American Chemical Society., 2013-08-05) Reynolds, EM; Blanchard, PER; Kennedy, BJ; Ling, CD; Liu, S; Avdeev, M; Zhang, Z; Cuello, GJ; Tadich, A; Jang, LYThe pyrochlore?defect fluorite order?disorder transition has been studied for a series of oxides of the type Gd2?xTbxZr2O7 by a combination of diffraction and spectroscopy techniques. Synchrotron X-ray diffraction data suggest an abrupt transition from the coexistence of pyrochlore and defect fluorite phases to a single defect fluorite phase with increasing Tb content. However neutron diffraction data, obtained at ? ≈ 0.497 Å for all Gd-containing samples to minimize absorption, not only provide evidence for independent ordering of the anion and cation sublattices but also suggest that the disorder transition across the pyrochlore?defect fluorite boundary of Ln2Zr2O7 is rather gradual. Such disorder was also evident in X-ray absorption measurements at the Zr L3-edge, which showed a gradual increase in the effective coordination number of the Zr from near 6-coordinate in the pyrochlore rich samples to near 7-coordinate in the Tb rich defect fluorites. These results indicate the presence of ordered domains throughout the defect fluorite region, and demonstrate the gradual nature of the order?disorder transition across the Gd2?xTbxZr2O7 series. © 2013, American Chemical Society.
- ItemAverage and local ordering of Yb2(Ti2-xYbx)O7-x/2 ‘stuffed’ pyrochlores: the development of a robust structural model(Elsevier, 2021-10-01) Mullens, BG; Zhang, Z; Avdeev, M; Brand, HEA; Cowie, BCC; D'Angelo, AM; Múzquiz, MS; Kennedy, BJThe long-range (average) and short-range (local) structures in the Yb2(Ti2-xYbx)O7-x/2 (x = 0.00–0.67) series were studied using a combination of diffraction and spectroscopic techniques. The average structure, established from synchrotron X-ray and neutron powder diffraction data, shows the development of multiphase regions from x = 0.134 and the formation of anti-site disorder from x = 0.335. The local structure, established from X-ray absorption near-edge structure (XANES), shows a gradual evolution of short-range disorder. The crystal field splitting energy of the Ti4+ ions decreases from 2.15 to 1.91 eV with increasing Yb3+ content, reflecting the increase in coordination number from 6 to predominantly 7. Electrochemical impedance spectroscopic studies show an increase in oxygen ionic conductivity by almost a factor of 3 upon doping with small amounts of Yb3+ (x = 0.067). These results imply that the disordering across the anion and cation sublattices are different and inducing small amounts of disorder into the pyrochlore structure may lead to applications in solid-oxide fuel cells. © 2021 Elsevier Inc.
- Item(Ca0.37Sr0.63)TiO3 perovskite - an example of an unusual class of tilted perovskites(Institute of Physics, 2008-04-02) Howard, CJ; Withers, RL; Knight, KS; Zhang, ZThe structure of Ca0.37Sr0.63TiO3 perovskite has been carefully investigated using electron and neutron diffraction. Electron diffraction supports the supercell and Pbcm space group previously proposed, while high resolution neutron powder diffraction data provide the basis for a new structure refinement. The distortions of the resultant structure relative to the ideal parent perovskite structure type have been decomposed into normal modes. It is found that the primary modes of distortion are octahedral tilting modes associated with the R point (k = [1/2, 1/2, 1/2]∗) and the ξ = 1/4 point on the T line of symmetry (k = [1/2, 1/2, ξ]∗). There is a smaller secondary, probably induced, mode of distortion involving antiferroelectric displacements of the Ti cations associated with the ξ = 1/4 point on the T line of symmetry (k = [0, 0, ξ]∗). This compound provides one of the very few currently known examples of a tilted perovskite structure that shows tilting modes associated with the ubiquitous k = (1/2, 1/2, ξ)∗ modulation wavevectors for a value of ξ different from 0 and 1/2. © 2008, Institute of Physics
- ItemChemical, magnetic and orbital order in the substituted double perovskite Sr(1-x)Ca(x)Mn0.5Ru0.5O3(American Physical Society, 2010-03) Ricciardo, RA; Woodward, PM; Cuthbert, HL; Zhou, QD; Kennedy, BJ; Zhang, Z; Avdeev, M; Jang, LYThe structural and magnetic properties of Sr(1x)Ca(x)Mn0.5Ru0.5O3 have been investigated. SrMn0.5Ru0.5O3 exhibits antiferromagnetic ordering, TN ≈200 K. Neutron powder diffraction of this perovskite indicates the presence of orbital ordering of the occupied Mn+3 dz2 orbitals, stabilizing the spin ordering corresponding to the AFM C-type structure. The substitution of smaller Ca+2 for Sr+2 on the A-site induces a change in the octahedral tilt system, (a0a0c−) to (a−b+a−) forcing a loss of this type of orbital ordering for x ≥ 0.2. This is accompanied by a crossover to a ferromagnetic ground state with 200K ≤ TC ≤ 300K even in the absence of long range chemical order. Magnetic data and neutron powder diffraction of CaMn0.5Ru0.5O3 and Sr0.5Ca0.5Mn0.5Ru0.5O3 reveal reduced magnetic moments and imply a complex magnetic behavior as well as a complete loss of orbital ordering for CaMn0.5Ru0.5O3. X-ray absorption studies show increased electron transfer from Mn to Ru. © 2021 American Physical Society
- ItemA colossal barocaloric effect induced by the creation of a high-pressure phase(Royal Society of Chemistry (RSC), 2023-01-13) Zhang, Z; Jiang, X; Hattori, T; Xu, X; Li, M; Yu, CY; Zhang, Z; Yu, D; Mole, RA; Yano, SI; Chen, J; He, LH; Wang, CW; Wang, H; Li, B; Zhang, ZDAs a promising environment-friendly alternative to current vapor-compression refrigeration, solid-state refrigeration based on the barocaloric effect has been attracting worldwide attention. Generally, both phases in which a barocaloric effect occurs are present at ambient pressure. Here, instead, we demonstrate that KPF6 exhibits a colossal barocaloric effect due to the creation of a high-pressure rhombohedral phase. The phase diagram is constructed based on pressure-dependent calorimetric, Raman scattering, and neutron diffraction measurements. The present study is expected to provide an alternative routine to colossal barocaloric effects through the creation of a high-pressure phase. © Royal Society of Chemistry 2024.
- ItemControlling oxygen defect formation and its effect on reversible symmetry lowering and disorder-to-order phase transformations in nonstoichiometric ternary uranium oxides(American Chemical Society, 2019-04-09) Murphy, GL; Wang, CH; Zhang, Z; Kowalski, PM; Beridze, G; Avdeev, M; Muránsky, O; Brand, HEA; Gu, QF; Kennedy, BJIn situ synchrotron powder X-ray diffraction measurements have demonstrated that the isostructural AUO4–x (A = alkaline earth metal cation) oxides CaUO4–x and α-Sr0.4Ca0.6UO4–x undergo a reversible phase transformation under reducing conditions at high temperatures associated with the ordering of in-plane oxygen vacancies resulting in the lowering of symmetry. When rhombohedral (space group R3̅m) CaUO4–x and α-Sr0.4Ca0.6UO4–x are heated to 450 and 400 °C, respectively, in a hydrogen atmosphere, they undergo a first-order phase transformation to a single phase structure which can be refined against a triclinic model in space group P1̅, δ-CaUO4–x and δ-Sr0.4Ca0.6UO4–x, where the oxygen vacancies are disordered initially. Continued heating results in the appearance of superlattice reflections, indicating the ordering of in-plane oxygen vacancies. Cooling ordered δ-CaUO4–x and δ-Sr0.4Ca0.6UO4–x to near room temperature results in the reformation of the disordered rhombohedral phases. Essential to the transformation is the generation of a critical amount of oxygen vacancies. Once these are formed, the transformation can be accessed continuously through thermal cycling, showing that the transformations are purely thermodynamic in origin. Stoichiometric structures of both oxides can be recovered by heating oxygen deficient CaUO4–x and α-Sr0.4Ca0.6UO4–x under pure oxygen to high temperatures. When heated in air, the amount of oxygen vacancy defects that form in CaUO4–x and α-Sr0.4Ca0.6UO4–x are found to correlate with the A site composition. The inclusion of the larger Sr2+ cation on the A site reduces defect–defect interactions, which increases the amount of defects that can form and lowers their formation temperature. The relative difference in the amount of defects that form can be understood on the basis of oxygen vacancy and U5+ disordering as shown by both ab initio calculations and estimated oxygen vacancy formation energies based on thermodynamic considerations. This difference in defect–defect interactions consequently introduces variations in the long-range ordered anionic lattice of the δ phases despite the isostructural relationship of the α structures of CaUO4–x and Sr0.4Ca0.6UO4–x. These results are discussed with respect to the influence the A site cation has upon anion defect formation and ordering and are also compared to δ-SrUO4–x, the only other material known to be able to undergo a reversible symmetry lowering and disorder-to-order transformation with increasing temperature. © 2019 American Chemical Society
- ItemCore level photoemission spectroscopy and chemical bonding in Sr2Ta2O7(Elsevier, 2009-06-12) Atuchin, VV; Grivel, JC; Zhang, ZElectronic parameters of constituent element core levels of strontium pyrotantalate (Sr2Ta2O7) were measured with X-ray photoelectron spectroscopy (XPS). The Sr2Ta2O7 powder sample was synthesized using standard solid state method. The valence electron transfer on the formation of the Sr–O and Ta–O bonds was characterized by the binding energy differences between the O 1s and cation core levels, Δ(O–Sr) = BE(O 1s) − BE(Sr 3d5/2) and Δ(O–Ta) = BE(O 1s) − BE(Ta 4f7/2). The chemical bonding effects were considered on the basis of our XPS results for Sr2Ta2O7 and earlier published structural and XPS data for other Sr- and Ta-containing oxide compounds. The new data point for Sr2Ta2O7 is consistent with the previously derived relationship for a set of Sr-bearing oxides. The binding energy difference Δ(O–Sr) was found to decrease with increasing bond distance L(Sr–O). © 2009, Elsevier Ltd.
- ItemCrystal structure of Ln(1/3)NbO(3) (Ln = Nd, Pr) and phase transition in Nd1/3NbO3(Elsevier, 2007-06) Zhang, Z; Howard, CJ; Kennedy, BJ; Knight, KS; Zhou, QDThe crystal structure of the A-site deficient perovskite Ln1/3NbO3 (Ln=Nd, Pr) at room temperature has been determined, for the first time, as orthorhombic in space group Cmmm using high-resolution neutron powder diffraction. Pertinent features are the alternation of unoccupied layers of A-sites and layers partly occupied by Ln cations, as well as out-of-phase tilting of the NbO6 octahedra around an axis perpendicular to the direction of the cation/vacancy ordering. The phase transition behaviour of Nd1/3NbO3 has also been studied in situ. This compound undergoes a continuous phase transition at around 650°C to a tetragonal structure in space group P4/mmm due to the disappearance of the octahedral tilting. The analysis of spontaneous strains shows that this phase transition is tricritical in nature. © 2007, Elsevier Ltd.
- ItemCrystal structures and phase transition in (Sr0.8Ce0.2)(Mn(1-y)Coy)O-3 (y=0 and 0.2): the influence of Jahn-Teller distortion(Institute of Physics, 2009-03-25) Zhang, Z; Howard, CJ; Kennedy, BJ; Matsuda, M; Miyake, MWe have studied the crystal structures of (Sr0.8Ce0.2)(Mn1-yCoy)O-3 (y = 0 and 0.2) using neutron diffraction. Both (Sr0.8Ce0.2)MnO3 and (Sr0.8Ce0.2)(Mn0.8Co0.2)O-3 have a tetragonal structure in space group I4/mcm at room temperature, and the octahedral tilt angle around the c-axis is nearly the same. The only significant difference is the shape of the Mn(Co)O-6 octahedron: it is elongated in (Sr0.8Ce0.2)MnO3 due to the cooperative Jahn-Teller (JT) effect, but essentially regular in (Sr0.8Ce0.2)(Mn0.8Co0.2) O3 due to the absence of JT-active Mn3+ ions. With increasing temperature, both compounds undergo a continuous phase transition at around 400°C to a cubic structure in Pm (3) over barm, with no indication of a distinct transition in (Sr0.8Ce0.2) MnO3 from the removal of the static JT distortion. In addition, the temperature dependence of the octahedral tilt angle is very similar in the two samples, implying that the JT distortion has minimal effect on the octahedral tilting and the phase transition to cubic. X-ray absorption near-edge structure (XANES) analysis indicates that the Ce oxidation state is predominantly 4+ in both samples. The electrical conductivity is higher in (Sr0.8Ce0.2)MnO3 than in (Sr0.8Ce0.2)(Mn0.8Co0.2)O-3 in the temperature range studied (100-900°C). © 2009, Institute of Physics
- ItemCrystal structures and phase transitions in a-site deficient perovskites Ln1/3TaO3(American Chemical Society, 2008-11-11) Zhou, QD; Saines, PJ; Sharma, N; Ting, J; Kennedy, BJ; Zhang, Z; Withers, RL; Wallwork, KSThe synthesis and structures of the perovskites Ln1/3TaO3 are described. As the size of the Ln cation is reduced, the compounds display a sequence of structure: P4/mmm/La → Cmmm/Ce−Gd → Pmma/Tb, Dy → Pmc21/Ho, Er. Although apparently tetragonal in P4/mmm, electron diffraction patterns of Tm1/3TaO3 reveal this has a complex incommensurate structure. Likewise Gd1/3TaO3 appears metrically tetragonal, but electron diffraction and synchrotron X-ray powder diffraction demonstrate this is actually orthorhombic. The suppression of the spontaneous orthorhombic strain in Gd1/3TaO3 is thought to be due to the proximity to the first-order Cmmm−Pmma transition. Variable temperature studies show both Tb1/3TaO3 and Dy1/3TaO3 undergo a first-order Cmmm−Pmma transition upon heating. © 2008, American Chemical Society
- ItemCrystallization of TiO2 powders and thin films prepared from modified titanium alkoxide precursors(Wiley-Blackwell, 2008-06) Karatchevtseva, I; Cassidy, DJ; Zhang, Z; Triani, G; Finnie, KS; Cram, SL; Barbé, CJThe modification of titanium alkoxides by chemical reactions with ligands yields complexes or molecular clusters that are substantially different from those of the parent alkoxides. In this study, we investigate the structural evolution of powders and thin films prepared from two titanium oxo-alkoxyacylate clusters with different oxo-core structures [Ti6(μ3-O)2(μ2-O)2](CH3COO)8(μ2-OiPr)2(OiPr)6 and [Ti6(μ3-O)6](μ-RCOO)6(OiPr)6 ([6,4] and [6,6], respectively) as a function of annealing temperature. The structural evolution of powders and thin films prepared from the corresponding parent alkoxide Ti(OiPr)4 (TiP) were also investigated for comparison. In all powders, the amorphous-to-anatase transformation occurred upon heating to 400°C. In sharp contrast, the anatase-to-rutile transformation of the powder prepared from the [6,6] cluster was significantly inhibited compared with the conventionally derived powder, with no rutile being detected even after annealing at 800°C for 1 h. This was attributed to the small crystallite size in the [6,6]-derived powder, which is lower than the critical size previously reported for the anatase-to-rutile transformation in similar sol–gel-derived materials. In thin films, the amorphous-to-anatase phase transition also occurred at temperatures as low as 400°C for coatings deposited from conventional TiP precursor and [6,4] cluster solutions. However, in contrast to the corresponding powders no rutile nucleation occurred even at 800°C in either film. © 2008, Wiley-Blackwell.
- ItemDoes local disorder occur in the pyrochlore zirconates?(American Chemical Socity, 2012-12-17) Blanchard, PER; Clements, R; Kennedy, BJ; Ling, CD; Reynolds, EM; Avdeev, M; Stampfl, APJ; Zhang, Z; Jang, LYThe zirconates Ln2Zr2O7 (Ln = lanthanoid) have been studied using a combination of Zr L-edge X-ray absorption near edge structure (XANES) and synchrotron X-ray and neutron powder diffraction methods. These studies demonstrate that as the size of the lanthanoid cation decreases, the local structure evolves smoothly from the ideal pyrochlore toward the defect fluorite rather than undergoing an abrupt transformation. The Zr L-edge spectrum is found to be extremely sensitive to changes in the local coordination environment and demonstrates an increase in local disorder across the pyrochlore oxides. The sensitivity of the XANES measurements enables us to identify the progressive nature of the transition that could not be detected using bulk diffraction techniques. © 2012, American Chemical Society.
- ItemDual-ion intercalation to enable high-capacity VOPO4 cathodes for Na-ion batteries(Elsevier, 2021-01-01) Zhang, Z; Ni, Y; Avdeev, M; Kan, WH; He, GVOPO4 is a potential cathode candidate in sodium-ion batteries for multi-electron reactions to attain high capacity up to 330 mAh g−1. However, most current efforts are focused on the V5+/V4+ redox reaction with a moderate capacity of <150 mAh g−1. Here, we report for the first time the Na intercalation behaviors for both αI- and β-VOPO4 polymorph cathodes through the V5+/V4+/V3+ redox reactions. Electrochemical evaluations suggest further reduction of V4+ ions can be attained for both cathodes, as indicated by the high discharge capacities derived from a large plateau below 1.5 V vs Na/Na+ as well as various structural analyses of electrodes at different discharge states. However, only the layered αI- cathode is preferable for deep sodiation to maintain stable structure over cycling. Theoretical studies by bond-valence sum (BVS) mismatch map calculations reveal similar Na-ion migration pathways in VOPO4•2H2O and αI-NaVOPO4, but H2O molecules in the hydrate cathode have negative effect on the diffusion of Na-ions in betweenVOPO4 layers. © 2020 Elsevier Ltd. All rights reserved.
- ItemEffect of long- and short-range disorder on the oxygen ionic conductivity of Tm2(Ti2–xTmx)O7–x/2 “stuffed” pyrochlores(American Chemical Society, 2021-03-10) Mullens, BG; Zhang, Z; Avdeev, M; Brand, HEA; Cowie, BCC; Múzquiz, MS; Kennedy, BJThe long-range average and short-range local structures in the Tm2(Ti2–xTmx)O7–x/2 (x = 0.00–0.67) series were studied using a combination of diffraction and spectroscopic techniques. The long-range average structure, established from synchrotron X-ray and neutron powder diffraction data, shows the development of multiphase regions from x = 0.134 and the formation of antisite cation disorder from x = 0.402. The crystal field splitting of the Ti4+ ions, as derived from the Ti L3-edge X-ray absorption near-edge structure (XANES) spectroscopy, decreases gradually from 2.17 to 1.92 eV with increasing Tm3+ content (x), reflecting the increase in coordination number from 6 to predominantly 7. This is consistent with a gradual evolution of the short-range local disorder from x = 0.00 to 0.67. These results suggest that local disorder develops gradually throughout the entire composition range, whereas changes in the long-range disorder occur more suddenly. Electrochemical impedance spectroscopic results show an increase in oxygen ionic conductivity at 1000 °C, by a factor of 4 upon doping at x = 0.268. This suggests that inducing small amounts of disorder into the pyrochlore structure, by stuffing, may lead to applications of this material as a solid electrolyte in solid-oxide fuel cells. © 2021 American Chemical Society
- ItemElectronic parameters of Sr2Nb2O7 and chemical bonding.(Elsevier, 2008-06) Atuchin, VV; Grivel, JC; Korotkov, AS; Zhang, ZX-ray photoelectron spectroscopy (XPS) measurements were carried out on a strontium pyroniobate (Sr2Nb2O7) powder sample, which was synthesized using standard solid-state method. The binding energy (BE) differences between the O 1s and cation core levels, Δ(O–Nb)=BE(O 1s)–BE(Nb 3d5/2) and Δ(O–Sr)=BE(O 1s)–BE(Sr 3d5/2), were used to characterize the valence electron transfer on the formation of the Nb–O and Sr–O bonds. The chemical bonding effects were considered on the basis of our XPS results for Sr2Nb2O7 and earlier published structural and XPS data for other Sr- or Nb-containing oxide compounds. The new data point for Sr2Nb2O7 is consistent with the previously derived relationship for a set of Nb5+-niobates that Δ(O–Nb) increases with increasing mean Nb–O bond distance, L(Nb–O). A new empirical relationship between Δ(O–Sr) and L(Sr–O) was also obtained. Interestingly, the correlation between Δ(O–Sr) and L(Sr–O) was found to differ from that between Δ(O–Nb) and L(Nb–O). Possible cause for the difference is discussed. © 2008, Elsevier Ltd.
- ItemEnhanced adhesion of atomic layer deposited titania on polycarbonate substrates(Elsevier, 2007-01-22) Latella, BA; Triani, G; Zhang, Z; Short, KT; Bartlett, JR; Ignat, MInterfacial adhesion of atomic layer deposited titania films on polycarbonate substrates with and without a water-plasma treatment has been studied using in situ observation during microtensile testing. Specific attention is paid to multiple tension-generated transverse cracks in the titania films when subjected to externally applied uniaxial tensile stresses. The strength, fracture toughness and interfacial adhesion of the titania film on polycarbonate were deduced from theoretical models based on experimentally determined parameters. The tensile tests were conducted in a micromechanical tester positioned under an optical microscope allowing in situ viewing of cracking damage. The strain to initiate first cracking and the crack density as a function of strain were obtained. The in situ observations indicated different interfacial behaviour between water-plasma-treated and non-treated samples. It is shown that the water plasma treatment drastically improves the adhesion of the titania film to polycarbonate. Calculations show that the fracture energy required for film debonding in the plasma-treated polycarbonate is 5.9 J/m2 compared to 2.5 J/m2 for the untreated sample. A simple chemical structure model was used to explain the observed differences. © 2007, Elsevier Ltd.
- ItemEvaluation of polymorphism and charge transport in a BaO–CaO–Ta2O5 perovskite phase diagram using TOF-neutron and synchrotron X-ray diffraction, the bond-valence method and impedance spectroscopy(Royal Society of Chemistry (RSC), 2022-10-26) Singh, K; Yang, H; Zhang, Z; Avdeev, M; Huq, Ashfia; Wu, DY; Lee, JJ; Kan, WH; Thangadurai, VAmong the alkaline earth-based perovskite oxides, the Ba-based perovskites have superior chemical stability and tunable electrical/catalytic property via chemical substitution/doping. One of the best-known examples is Ba3Ca1.18Nb1.82O8.73 as a ceramic proton conductor for all-solid-state steam electrolysis and solid oxide fuel cells (SOFCs). Structural ordering variation is often driven by chemical composition, which directly correlates with their chemical/physical properties. In the present work, we develop a comprehensive functional perovskite-type phase diagram for the Ba–Ca–Ta–O quaternary system Ba3Ca1+xTa2−xO9−3x/2 (0 ≤ x ≤ 0.36) with a wide chemical composition between 1000 and 1550 °C, coupled with theoretical calculations to investigate the cation ordering in supercells. Furthermore, the impact of cation clustering on the diffusion pathways of O2− ions was evaluated as a case study. Experimentally, precise cation ordering and other structural features are quantitively determined by TOF-neutron and synchrotron X-ray diffraction analyses. This work provides a comprehensive evaluation of some potential applications of the Ba–Ca–Ta–O quaternary system. The electrochemical impedance data were also systematically studied by impedance spectroscopy genetic programming (ISGP). The electrical conductivity was found to increase from x = 0 to x = 0.27 and then decrease for the end member when x = 0.36 due to a decrease in mobile charge carrier concentration. Interestingly, in dry air, the electrical conductivity was found to increase from x = 0 to x = 0.36. However, only Ba3Ca1.18Ta1.82O8.73 (BCT18) and Ba3Ca1.27Ta1.73O8.595 (BCT27) were found to show an increasing trend in conductivity in humid atmospheres, and this indicates that the clustering effect was pO2 dependent. © Royal Society of Chemistry 2024
- ItemFe speciation in geopolymers with Si/Al molar ratio of ~2.(Elsevier, 2007-05-01) Perera, DS; Cashion, JD; Blackford, MG; Zhang, Z; Vance, ERThe speciation of Fe was studied in metakaolin-based geopolymers to which Fe was added as ferric nitrate solution or freshly precipitated ferric hydroxide. From Mössbauer and near-edge X-ray absorption spectroscopies, coupled with X-ray diffraction and electron microscopy, it was concluded that in as-cured geopolymers the Fe was present in octahedral sites, either as isolated ions in the geopolymer matrix or as oxyhydroxide aggregates which had not reacted with the starting geopolymer components. For material to which iron nitrate was added, heating to 900°C allowed the formation of nepheline and a glass, both of which contained tetrahedrally coordinated, substituted Fe3+. © 2007, Elsevier Ltd.
- ItemGrain size and structure distortion characterization of α-MgAgSb thermoelectric material by powder diffraction(IOP Publishing, 2020-05-21) Li, XY; Zhang, Z; He, L; Avdeev, M; Ren, Y; Zhao, H; Wang, FNanostructuring, structure distortion, and/or disorder are the main manipulation techniques to reduce the lattice thermal conductivity and improve the figure of merit of thermoelectric materials. A single-phase α-MgAgSb sample, MgAg0.97Sb0.99, with high thermoelectric performance in near room temperature region was synthesized through a high-energy ball milling with a hot-pressing method. Here, we report the average grain size of 24–28 nm and the accurate structure distortion, which are characterized by high-resolution neutron diffraction and synchrotron x-ray diffraction with Rietveld refinement data analysis. Both the small grain size and the structure distortion have a contribution to the low lattice thermal conductivity in MgAg0.97Sb0.99. © 2020 Chinese Physical Society and IOP Publishing Ltd