Non-substituted Sample Research Articles

Site-Selective Substitution by Transition Metal Cations for NaCo2O4 via Ion-Exchange

Incorporation of Ni ions into two different sites of the Na and Co sites of NaCo2O4 was carried out by the processes of conventional elemental substitution for Co ion and solid state ion exchange of mobile Na+ ion. The changes of the lattice parameters of “substituted” and “exchanged” samples were different, suggesting that the Ni ions occupy different sites. The Ni-substituted sample for Co showed a significant decrease of Seebeck coefficient keeping the electrical conductivity almost intact, while Ni-exchanged samples for Na showed a lower electrical conductivity than that of the non-substituted sample with keeping the Seebeck coefficient unchanged. These results suggest that Ni cations introduced to the Co site decouples the Co-Co spin exchange, whereas those at the Na site compensates positive holes by providing conduction electrons.

Effect of Partial Substitutions of Rare-earth Metals for Na-site on the Thermoelectric Properties of Na<SUB><i>x</i></SUB>Co<SUB>2</SUB>O<SUB>4</SUB> Prepared by the Polymerized Complex Method

Polycrystalline (Na 1 - y M y ) x Co 2 O 4 (M=Nd, Sm, Dy and Yb; y = 0.01 ∼ 0.05) samples were synthesized by a polymerized complex method. The effects of partial substitution of rare-earth metals for a Na-site on the thermoelectric properties of Na x Co 2 O 4 were investigated, and their differences with the sample prepared by a conventional solid-state reaction (SSR) method were evaluated. In case of the Nd, Sm and Dy-substituted samples, each rare-earth metal content in the matrix phase was slightly higher than that of the sample prepared by the SSR method, resulting in the slight increase in the thermoelectric power. Dy-substituted samples with y = 0.03 and 0.05 showed the decrease in the electrical resistivity compared to the non-substituted sample because of the enhanced c-axis orientation. The power factors of the Dy-substituted samples were higher than that of the non-substituted sample over the entire temperature range, and the sample with y = 0.05 showed the maximum power factor value of 1.5 x 10 - 3 Wm - 1 K - 2 at 1073 K. On the other hand, Yb-substituted samples with y ≤ 0.03 were almost single Na x Co 2 O 4 phase. The maximum Yb content in the matrix phase corresponded to y . = . 0.03 from the EDX analysis. The value was remarkably higher than that of the sample prepared by the SSR method. For the Yb-substituted samples with y < 0.03, the c-axis orientation was enhanced with increasing the nominal Yb composition in conjunction with the rise in the lattice parameter, c. The Yb substitution caused the increase in the electrical resistivity and thermoelectric power compared to the non-substituted sample over the entire temperature range, which is considered to be due to the reduction in the carrier density resulted from the Yb substitution for Na-site. The power factor of the sample with y = 0.03 was enhanced over the entire temperature range, and the maximum power factor, 1.5 × 10 - 3 Wm - 1 K - 2 was obtained at 1073 K.

Thermoelectric Properties of (Na1‐yMy)xCo2O4 (M: K, Sr, Y, Nd, Sm and Yb; y = 0.01≈0.35).

AbstractFor Abstract see ChemInform Abstract in Full Text.

Effect of strontium substitution on superconductivity in YBa 2 Cu 4 O 8

Superconducting powders with the composition Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 ( x = 0.00-0.50) have been prepared by an aqueous sol-gel method. The effects of strontium substitution on the properties of compounds were studied by resistivity measurements, X-ray powder diffraction, thermogravimetric analysis, electron microscopy and elemental analysis. The data indicate that single-phase Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 superconducting samples were obtained. They also show that doping with strontium has a strong effect on the superconducting properties of the YBa 2 Cu 4 O 8 phase. The critical temperature is enhanced from 78 K (for a non-substituted sample) to 88 K (for Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 ). Elemental analysis data clearly indicate that small amounts of strontium enter the copper sites in the Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 compounds. A point-defect chemistry approach, which explains the enhancement of T c by substituting strontium for barium, is presented.

Mössbauer spectra of α- and γ-polymorphs of FeOOH and Fe2O3 : effects of poor crystallinity and of Al-for-Fe substitution

AbstractThis paper presents various aspects, revealed by Mössbauer spectroscopy, of structural and magnetic properties of Al-substituted small-particle soil-related oxides. For goethite we focus on the relations between the hyperfine fields on the one hand, and crystallinity and Al content on the other. It is argued that these relations only provide a rough estimate of the Al content in natural samples. The ferrimagnetic-like behaviour reflected in the external-field Mössbauer spectra (4.2 K, 60 kOe) of certain Al goethites is presented. The spectra obtained for lepidocrocites are not spectacular, but confirmed that up to ∼10 at.% Al can be incorporated in the structure. Three differently-made series of hematites are considered. The Morin transition and spin structures in hematite are very sensitive to crystallinity and Al content, and probably to the presence of structural OH–. Integral low-energy electron Mössbauer spectroscopy on non-substituted samples indicates that the Morin-transition temperature in the surface layers (2 to 5 nm) is not significantly shifted from the bulk value. Measurements in extremely high magnetic fields (140 kOe) have shown that a spin-flip transition is induced in highly-substituted samples which exhibit no Morin transition in zero field. The use of external fields is crucial for the characterization and precise determination of the hyperfine parameters and site occupancies for maghemites, and for phase analyses of magnetic soils.

Control of Fe valence state and magnetoresistance by means ofT=Taand W substitution inSr2Fe(Mo1−xTx)O6

High-purity ${\mathrm{Sr}}_{2}\mathrm{Fe}({\mathrm{Mo}}_{1\ensuremath{-}x}{T}_{x}){\mathrm{O}}_{6}$ samples with $T=\mathrm{W},$ Ta and $0l~xl~1$ were obtained by means of encapsulation synthesis. For the nonsubstituted samples earlier ${}^{57}\mathrm{Fe}$ M\ossbauer spectroscopy measurements indicate that the Fe ions occupy a fluctuating mixed-valence state of $+2.5.$ [J. Lind\'en et al. Appl. Phys. Lett. 76 (2000) 2925.] ${\mathrm{W}}^{\mathrm{VI}}$ substitution causes increasing amounts of Fe to enter the II state, whereas ${\mathrm{Ta}}^{\mathrm{V}}$ substitution yields increasing amounts of ${\mathrm{Fe}}^{\mathrm{III}}.$ Both substitution schemes lead to a decrease in the intensity of the component assigned to ${\mathrm{Fe}}^{2.5+}.$ Nonsubstituted samples exhibit a characteristic tunneling-type magnetoresistance below ${T}_{\mathrm{C}}.$ Both W and Ta substitution were found to enhance the low-temperature magnetoresistance around the N\'eel temperature of the pure ${\mathrm{Sr}}_{2}{\mathrm{FeWO}}_{6}$ and ${\mathrm{Sr}}_{2}\mathrm{Fe}{\mathrm{TaO}}_{6}$ phases, respectively. The enhancement appears to be related to the colossal magnetoresistance (CMR) effect at the paramagnetic to antiferromagnetic transitions in the areas rich in W or Ta. The transition and consequently the region of non-zero CMR effect are rather broad due to the glass-like behavior of the highly-substituted samples within the low-temperature region. Ta substitution had a stronger influence on the transport properties, magnetization and mixed valency than W substitution had. It is suggested that ${\mathrm{Ta}}^{\mathrm{V}}$ disrupts the double-exchange interaction responsible for the magnetism in the ${\mathrm{Sr}}_{2}\mathrm{Fe}{\mathrm{MoO}}_{6}$ more efficiently than ${\mathrm{W}}^{\mathrm{VI}}.$

Thermoelectric properties of (Na 1− yM y) xCo 2O 4 (M=K, Sr, Y, Nd, Sm and Yb; y=0.01∼0.35)

Polycrystalline samples of (Na 1− y M y ) x Co 2O 4 (M=K, Sr, Y, Nd, Sm and Yb; y=0.01∼0.35) were prepared by a solid state reaction method. In this study, in order to improve the thermoelectric properties of Na x Co 2O 4, the effects of partial substitution of other metals for Na on the thermoelectric properties of Na x Co 2O 4 from room temperature to 1073 K were investigated. For M=Sr, the thermoelectric power and the electrical resistivity increased, and the electronic and lattice contribution to the thermal conductivity decreased compared to the non-substituted sample. These effects suggest that the carrier density was reduced by the substitution of Sr for Na. As a result, the figure of merit of the sample for M=Sr was improved. On the other hand, for other samples in spite of the increase in the electrical resistivity, the thermoelectric power decreased. These results are anomalous effects, which cannot be described merely by a change of the carrier density. For all samples, except for M=Y, the lattice contribution to the thermal conductivity decreased and for all samples, except for M=K, the electronic contribution slightly decreased.

Improved cyclability of Na-birnessite partially substituted by cobalt

We measured the electrode performance of Na-birnessite, which is layered manganese dioxide with sodium ions and water in the interlayer, for use in rechargeable lithium batteries. We focused on the effects of water on the electrode performance. X-ray diffraction (XRD) measurement and thermogravimetric (TG) analysis indicated that the weight loss on heating the material at 100 °C corresponded to the removal of surface water with no structural change, while that at 150 °C was associated with the removal of lattice water, leading to the layer shrinkage. According to this result, we prepared three kinds of electrode samples with different drying conditions, and examined the effect on the electrode performance. Furthermore, to improve the cyclability, we synthesized samples in which cobalt was substituted for some of the manganese. Even with a large substitution ratio of over 30%, we obtained the same initial capacity as the non-substituted sample, and the cyclability was improved. We assume that the substituted cobalt fixed sodium ions in the interlayer in the form of pillars that stabilized the layer structure.

The effect of rare earth substitution on the magnetic properties of Ni0.5Zn0.5MxFe2−xO4 (M: rare earth)

Variation of the complex permeability with frequency of Ni 1− x Zn x Fe 2 O 4 ( x =0.5, 0.6 and 0.7) prepared by a combustion method has been measured over a wide range of frequency, up to 1.8 GHz. Zn content improves permeability but moves the onset of resonance to lower frequencies. The influence on some properties of samples prepared by the insertion of small amounts of Ruthenium, Yttrium and rare-earth cations into the Ni 0.5 Zn 0.5 M x Fe 2− x O 4 ferrite has been also investigated. Relative losses and Curie temperature of all the samples have been measured. Ferrites substituted with Ru and Gd improve microwave behavior compared to non-substituted samples.

Effect of Partial Substitution of 3d Transition Metals for Co on the Thermoelectric Properties of Na&lt;SUB&gt;x&lt;/SUB&gt;Co&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;4&lt;/SUB&gt;

Layered P2 type Na x Co 2 O 4 thermoelectric oxides partially substituted by 3d transition metal (up to 15 at%: Cr, Mn, Fe, Ni, or Zn) for Co were synthesized through calcination and sintering in air. The Cr-substituted samples were mostly decomposed into the NaCrO 2 + Co 2 CrO 4 phases. On the other hand, the X-ray diffraction patterns of the Mn-substituted samples showed a single P2 type γ-Na x Co 2 O 4 phase. The Fe- and Ni-substituted samples were mostly composed of the Na x Co 2 O 4 phase with small amounts of Co 3 O 4 and CoNiO 2 phases. In the case of the Zn-substituted samples, only the Na x (Co 0.95 Zn 0.05 ) 2 O 4 was mostly composed of the Na x Co 2 O 4 phase with a small amount of the NaO phase. The effects of partial substitution of these 3d transition metals for Co except for Cr in the layered Na x Co 2 O 4 on thermoelectric properties were investigated. The electrical resistivity of these partially substituted samples significantly increased over the entire temperature range, as compared to the non-substituted sample, indicating reduction in carrier concentration. Except for Na x (Co 1-y Fe y ) 2 O 4 , the Seebeck coefficient was enhanced by the partial substitution, and in the case of Na x (Co 1-y Fe y ) 2 O 4 , it is suggested that the decrease in the Seebeck coefficient is associated with reduction in the electron-electron correlation due to the substitution. For the Mn-substituted samples, a small grain size and many pores compared to the non-substituted sample were effective for reducing the thermal conductivity. The figure-of-merit Z was enhanced in the case of the Na x (Co 0.95 Zn 0.05 ) 2 O 4 above 550 K and the maximum dimensionless figure-of-merit ZT, 0.68, was obtained at 965 K.

Sol-gel synthesis route for the preparation of Y(Ba1-xSrx)2Cu4O8 superconducting oxides

Superconducting oxide ceramics of composition Y(Ba1−x Sr x )2Cu4O8 (x = 0.00, 0.10, and 0.20) have been prepared by a simple sol-gel method based on the complexation of metal ions in aqueous medium, by chelating acetate and tartarate ligands. Homogeneous sols were obtained by complexing copper ions with tartaric acid, which prevented the flocculation of copper acetate during the gelation process. Single-phase bulk samples were obtained after firing the mixed-metal polymeric acetate-tartarate precursor to 835°C for 70 h in flowing oxygen atmosphere. Thermal decomposition of the gels was studied by thermogravimetry. Effect of strontium substitution on the properties of the compounds was studied by X-ray powder diffraction, electron microscopy, X-ray photoemission spectroscopy and resistivity measurements. These data indicate that nearly monophasic Y(Ba1−x Sr x )2Cu4O8 superconducting samples were obtained for x < 0.30. The Sr-doping in the YBa2Cu4O8 (Y-124) phase shows a pronounced effect on the superconducting properties enhancing the critical temperature from 78 K (for the non-substituted sample) to 88 K (for Y(Ba1−x Sr x )2Cu4O8).

Influence of W/Ta substitution at the Mo site on the Fe valence and magnetoresistive properties of Sr2FeMoO6

High-purity Sr2Fe(Mo1−xTx)O6 samples with T=W, Ta and 0⩽x⩽1 were synthesized using a solid-state reaction scheme. During synthesis a strongly reducing atmosphere was maintained by confining the precursors and an Fe/FeO oxygen getter to fused-silica tubes. Sample quality and homogeneity were confirmed by X-ray diffraction and energy-dispersive X-ray analysis. For the non-substituted samples, 57Fe Mössbauer spectra indicate that the Fe ions occupy a mixed-valence state of +2.5. W6+ forces increasing amounts of Fe to enter the +2 state, whereas Ta5+ substitution yields increasing amounts of Fe3+. Non-substituted samples exhibit a characteristic tunneling-type magnetoresistance below TC. Both W and Ta substitution were found to enhance the low-temperature magnetoresistance around the Néel temperature of the pure Sr2FeWO6 and Sr2FeTaO6 phases, respectively. The enhancement is related to the colossal magnetoresistance effect at the paramagnetic to antiferromagnetic transitions of areas rich in W or Ta.

Change in the melting point of RE 1.8(Ba 1-xSr x) 2.4Cu 3.4O y

The relationship between the substitution rate of Sr for Ba and the melting point of REBaCuO-system superconductor was measured. Various calcined powders of composition RE 1.8(Ba 1- x Sr x ) 2.4Cu 3.4O y , (for x = 0, 0.125, 0.250 and 0.375; RE = Y, Sm and Nd) with 0.5 wt% Pt powder were prepared and the melting points were measured by differential thermal analysis (DTA).The melting points of the YBaCuO-system and the SmBaCuO-system for x = 0.375 were 29 °C and 56 °C higher than that of a non-substituted sample. The melting point of NdBaCuO-system was observed to be almost unaffected by the substitution of Sr for Ba.

High pressure synthesis and magnetic properties for Hg 1− xRe xBa 2Ca 2Cu 3O y

Hg 1− x Re x Ba 2Ca 2Cu 3O y samples with a nominal composition of x=0, 0.1 and 0.2 were synthesized at a high pressure from the powders of raw materials without calcination. We investigated the influence of the nominal Re content ( x) and the nominal oxygen content ( y) on the crystallization, critical temperature and magnetic properties. Almost a single phase of Hg-1223 was obtained in the range 0≤ x≤0.1. The substitution of Re for Hg caused a decrease of T c mag. The lattice parameter ratio ( c/ a) of the Re-substituted samples was smaller than that for the non-substituted samples. The intragrain J c values at B≤5 T for the Re-substituted samples were higher than those for the non-substituted samples at 60–100 K. The irreversibility field ( B irr) at J c=1000 A/cm 2 was improved by Re substitution at 80–100 K. The post oxygenation of our samples was less effective in increasing B irr. This was probably due to Hg depletion during oxygenation.

Oxygen deficiency and electrical conductivity of Nd 1− xA xNiO 3− y (A = alkaline earth) prepared by the low-temperature process

The structure and electrical conductivity of oxygen-deficient Nd 1− x A x NiO 3− y (A = alkaline earth) have been investigated. The samples were prepared from aqueous solutions of metal acetates and EDTA by the low-temperature process. Single-phase NdNiO 3− y and alkaline earth-substituted Nd 0.95A 0.05NiO 3− y (A = Mg, Ca and Cr) were obtained by heating at 700 °C for 20 h in flowing oxygen. All the samples had large numbers of oxygen vacancies with y = 0.25−0.28, resulting in the mixed valence state of Ni 2+ Ni 3+ , as confirmed by iodometric titration and XPS measurements. The lattice constants of the alkaline earth-substituted samples were larger than those of the non-substituted sample. These results were attributed to the difference in ionic radius. The substitution of alkaline earths for Nd decreased the decomposition temperature. All the samples showed resistivity on the order of 10 −1 μ cm at room temperature, and the temperature dependence of the electrical resistance revealed the transition from metallic to semiconducting at lower temperatures. The transition temperature was dependent on the substituted alkaline earths. This behavior is discussed in terms of the changes in lattice constants as well as in the valence of Ni resulting from the oxygen deficiency.

The effect of the substitution of Sm for Ba on the superconductor

Characteristic changes occurring in superconductors on substituting Sm for Ba have been investigated. An appropriate amount of Sm substitution (, in the form of ) promoted grain growth; the grain size became larger than that in the non-substituted sample and had a maximum value at x = 0.07. Single-phase samples were obtained for . The critical temperature increased by about 1.5 K for concentrations up to x = 0.02, while it decreased rapidly beyond this amount. Samples did not show superconductivity above 15 K for . The crystal structure transformed from orthorhombic to tetragonal at x = 0.3. The average valency of the Cu and the oxygen content decreased linearly with increase of x in the ranges of and , while they both increased linearly for the range .

The role of impurity in effect of aging on Tc of YBa 2Cu 3− xM xO y (M = Fe; Ni; Co; X = 0; 0.01) irradiated by fast electrons

The onset transition temperature to superconducting state ( T c on ) and the intergranular weak link coupling temperature ( T m J ) were investigated in electron irradiated (8 MeV; 4 × 10 16el cm −2) YBa 2Cu 3− x M x O y (M = Fe; Ni; Co; X = 0; 0.01) ceramics aged up to 2.6 × 10 6s at 78 K. The T c on and T m J maxima vs aging time ( t a ) were obtained which were expressed clearer in samples with MNi. The positions of maxima on the T c on ( t a ) curve do not depend on M at x = 0.01 while the T m J ( t a ) maxima are shifted to the lower values of t a with respect to the nonsubstituted samples.

Variable-Temperature Mössbauer Spectroscopy of Nano-Sized Maghemite and Al-Substituted Maghemites

AbstractSynthetic aluminum-substituted maghemite samples, γ-(Fe1-xAlx)2O3, have been prepared by thermal decomposition of Al-lepidocrocite (γ-Fe1-xAlxOOH), with × = 0, 0.04, 0.06, 0.14 and 0.18. The particles are needle-shaped and the mean crystallite diameter along the [311] crystallographic direction was found to be between 2.0 and 5.0 nm. Mössbauer spectra were collected at 6 K and from 80 K up to 475 K at steps of 25 K. In a wide range of temperatures the spectra of the non-substituted sample consist of a superposition of a broad sextet and a superparamagnetic doublet, whereas for the Al-maghemites this range is much smaller. From the temperature variation of the fractional doublet area two different parameters were defined: the temperature corresponding to a 50/50 doublet-sextet spectrum (T1/2), and the temperature below which the doublet ceases to exist (T0). These two parameters (T1/2 and T0) decrease from 390 K and 92 K (Al-free sample), to 118 K and 64 K (4 mole % Al) and to 100 K and 48 K (18 mole % Al), respectively. The average hyperfine fields at 6 K undergo a steep drop in going from the Al-free sample (Hhf = 506 kOe) to the sample with 4 mole % Al (Hhf = 498 kOe), but for higher substitutions the effect is much smaller. The A- and B-site quadrupole splittings, obtained from the data between 220 K and 475 K, were found as: ΔEQ,A = 0.86 ± 0.04 mm/s and ΔEQ,B = 0.65 ± 0.04 mm/s for the 4 mole % Al sample. The characteristic Mössbauer temperature, determined from the temperature dependence of the average isomer shift, was found to be in the range of 500–600 K.

Temperature dependence of the hyperfine parameters of maghemite and al-substituted maghemites

Synthetic aluminum-substituted maghemite samples, γ-(AlyFe1-y)2O3 with y=0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by Mossbauer spectroscopy at 8 K and in the range 80 K to 475 K at steps of 25 K. The spectra have been analysed as a superposition of two sextets composed of asymmetrical Lorentzians. The A- and B-site isomer shifts were constrained as: δA=δB-0.12 mm/s. From the temperature dependence of δB it was possible to determine the characteristic Mossbauer temperature and the intrinsic shift. Both quantities clearly increase with increasing Al content, at least up to 10 mole%. The temperature dependence of the A-and B-sites hyperfine fields could be satisfactorily reproduced using the molecular-field theory assuming an antiparallel spin configuration. The exchange integrals were found as: JAB=-25 K; JAA=-18 K and JBB= -3 K. The hyperfine fields show a crossing in the vicinity of 300 K as a result of the relatively strong A-A interaction. The Curie temperature for the non-substituted sample was calculated to be 930 K and decreases to 765 K for the sample with 15 mol% Al. The gradual decrease of the saturation value of the A-site hyperfine field with increasing Al substitution and the constancy of this quantity for the B sites, suggest that the Al cations occupy the B sites.

The Center Shift in Mössbauer Spectra of Maghemite and Aluminum Maghemites

AbstractSynthetic, relatively well-crystallized aluminum-substituted maghemite samples, γ-(Aly·Fe1−y)2O3, with y = 0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by X-ray diffraction and zero-field Mössbauer spectroscopy in the range 8 K to 475 K, and also with an external field of 60 kOe at 4.2 K and 275 K. It was found that there are two different converging models for fitting the zero-field spectra of the maghemites with a superposition of two Lorentzian-shaped sextets, both resulting in inconsistent values for the hyperfine fields (Hhf) and/or the center shifts (δ) of the tetrahedral (A) and octahedral (B) ferric ions. From the applied-field measurements it is concluded that there is a constant difference of 0.12 ± 0.01 mm/s between δB and δA, regardless of the Al content. For the Al-free sample the center shifts are found as: δA = 0.370 mm/s and δB = 0.491 mm/s at 4.2 K and δA = 0.233 mm/s and δB = 0.357 mm/s at 275 K (relative to metallic iron), with an estimated error of 0.005 mm/s. Both δA and δB are observed to decrease with increasing Al concentration. The effective hyperfine fields for the non-substituted maghemite sample are: Heff,A = 575 kOe and Heff,B = 471 kOe at 4.2 K and Heff,A = 562 kOe and Heff,B = 449 kOe at 275 K, with an error of 1 kOe. The B-site hyperfine field remains approximately constant with Al substitution, while for the A site a slight decrease with increasing Al content was observed.