Substitution Of Sr For Ca Research Articles

Intrinsically modified thermoelectric performance of alkaline-earth isovalently substituted [Bi2AE2O4][CoO2]y single crystals

Alkaline-earth elements isovalently substituted into a [Bi2AE2O4][CoO2]y (AE2 = Ca2, Sr2, and CaSr) single crystal with a layered structure were grown by the optical floating zone method. Structural characterization by X-ray diffraction and electron microscopy showed that the layers were oriented perpendicular to the c-axis, as well as the growth of direction was parallel to the ab-plane. The thermoelectric properties, including the Seebeck effect, electrical conductivity and thermal conductivity were investigated. The results of the thermoelectric measurements showed that the full substitution of Ca for Sr in [Bi2Sr2−xCaxO4][CoO2]y has the best overall thermoelectric performance. Compared with the other two cases studied, the full Ca substituted crystal [Bi2Ca2O4][CoO2]y exhibits both reduced resistivity and thermal conductivity, but not a reduced Seebeck coefficient. The enhanced thermoelectric property in [Bi2Ca2O4][CoO2]y is mainly due to lower structural symmetry, which is confirmed by electron microscopy characterization. This work demonstrates that even isovalently substitution can play a crucial role in the thermoelectric effect of layered cobalt oxides.

SrO‐Containing Diopside Glass–Ceramic Sealants for Solid Oxide Fuel Cells: Mechanical Reliability and Thermal Shock Resistance

AbstractA series of glasses and glass–ceramics (GCs) aiming at applications as sealants for solid oxide fuel cells (SOFCs) were synthesized by partial substitution of Ca for Sr in the diopside‐Ba disilicate composition. X‐ray diffraction in conjunction with the Rietveld‐RIR technique were employed to quantify the crystalline (diopside and Sr‐diopside) and amorphous phases in the glasses sintered/heat treated at 850 °C in humidified 10%H2–90%N2 gas mixture for 250 h. Weibull modulus varied in the range 11.6–34.4 implying toward good mechanical reliability of synthesized GCs. Thermal shock resistance of model electrochemical cells made of yttria‐stabilized zirconia, gadolinia‐doped ceria, and lanthanum gallate based solid electrolytes, hermetically sealed by one diopside‐based composition, was evaluated employing quenching from 800 °C in air and water. Suitable thermal expansion coefficient, mechanical reliability, and strong adhesion to stabilized zirconia and metallic interconnects, are all suggesting a good suitability of the sealants for SOFC applications.

Competition between spin fluctuations in Ca2−xSrxRuO4aroundx=0.5

We study the static susceptibilities for charge and spin sectors in paramagnetic states for Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ in $0.5\leq x \leq 2$ within random phase approximation on the basis of an effective Ru $t_{2g}$ orbital Hubbard model. We find that several modes of spin fluctuation around $\boldq=(0,0)$ and $\boldq\sim(0.797\pi,0)$ are strongly enhanced for the model of $x=0.5$. This enhancement arises from the increase of the corresponding susceptibilities for the $d_{xy}$ orbital due to the rotation-induced modifications of the electronic structure for this orbital (i.e., the flattening of the bandwidth and the increase of the density of state near the Fermi level). We also find that the ferromagnetic spin fluctuation becomes stronger for a special model than for the model of $x=0.5$, while the competition between the modes of spin fluctuation at $\boldq=(0,0)$ and around $\boldq\sim (\pi,0)$ is weaker for the special model; in this special model, the van Hove singularity (vHs) for the $d_{xy}$ orbital is located on the Fermi level. These results indicate that the location of the vHs for the $d_{xy}$ orbital, which is controlled by substitution of Ca for Sr, is a parameter to control this competition. We propose that the spin fluctuations for the $d_{xy}$ orbital around $\boldq=(0,0)$ and $\boldq\sim (\pi,0)$ play an important role in the electronic states around $x=0.5$ other than the criticality approaching the usual Mott transition where all electrons are localized.

Substitutions of strontium in mesoporous calcium silicate and their physicochemical and biological properties

Calcium silicate (Ca–Si) based bioceramics have been regarded as a potential bioactive materials for bone tissue regeneration. In this study, we have successfully prepared ordered mesoporous strontium (Sr)-substituted CaSiO3 (Sr–CaSiO3) materials by using a triblock copolymer (P123) as a structure-directing agent. The microstructure and porosity of mesoporous Sr–CaSiO3 materials were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and the N2 adsorption–desorption technique. The substitution of Sr for Ca in mesoporous CaSiO3 did not change the mesoporous structure, but the surface area and pore volume decreased with increasing Sr substitution. The effects of the Sr substitution on the physiochemical and biological properties of mesoporous CaSiO3 materials were evaluated by the ion dissolution, apatite-forming ability, proliferation and alkaline phosphatase (ALP) activity of osteoblast-like MC3T3-E1 cells. The results showed that the increasing Sr substitution decreased the dissolution rate of Ca and Si ions from mesoporous CaSiO3 materials and enhanced the ability to stabilize the pH environment. Mesoporous Sr–CaSiO3 materials have a similar apatite-forming ability to mesoporous CaSiO3 material, and stimulated the proliferation and ALP activity of MC3T3-E1 cells. Furthermore, using gentamicin as a model drug, mesoporous Sr–CaSiO3 materials exhibited a sustained drug release property which could be used in local drug delivery therapy. Furthermore, the drug release rate decreased to some extent with increasing Sr substitution in mesoporous CaSiO3 materials. Therefore, mesoporous Sr–CaSiO3 materials have more potential for application in bone tissue regeneration.

Phase, Microstructure, and Microwave Dielectric Properties of NaCa4−x Sr x Nb5O17 (x = 0 to 4) Ceramics

A series of A5B5O17-type NaCa4−x Sr x Nb5O17 (x = 0 to 4) compounds were processed through a solid-state mixed-oxide route. All the compositions formed dense single-phase ceramics within the detection limit of an in-house x-ray diffraction facility when sintered at 1300°C. The substitution of Sr for Ca changed the crystal symmetry from monoclinic (x = 0) to orthorhombic (x = 1 to 4) along with a slight increase in molar cell volume due to the relatively larger ionic radius of Sr. The relative permittivity (e r) and temperature coefficient of resonance frequency (TCF) increased from 46 to 84 and from −117 ppm/°C to +377 ppm/°C, respectively, while the quality factor (Q × f) decreased from 11,063 GHz to 559 GHz with an increase in x from 0 to 4. Optimum properties were achieved for NaCa3SrNb5O17, which exhibited e r = 57, Q × f = 4628 GHz, and TCF = −41 ppm/°C. Compounds in the NaCa4−x Sr x Nb5O17 series exhibited high e r and Q × f with adjustable TCF; however, further work is required for simultaneous optimization of all three properties.

Piezoelectric properties of langasite group based on the ionic size of cation

As langasite A3BC3D2O14 compounds group with piezoelectric properties has no phase transition up to the melting point of 1400–1500°C, they have been applied for the combustion pressure sensor. As they also have a larger electromechanical coupling factor compared to quartz and nearly the same temperature stability as quartz, the surface acoustic wave (SAW) filters are expected based on the digital transformation of wider bandwidth and higher-bit rates. In the case of three-element compounds such as [R3]A[Ga]B[Ga3]C[GaSi]DO14 (R=La, Pr and Nd), the piezoelectric constant increases with the ionic radius R. In the case of four-element compounds such as [A3]A[B]B[Ga3]C[Si2]DO14 (A=Ca or Sr, B=Ta or Nb), |d11| and k12 values as a function of AL/BL ratio showed a linear relationship completely. There are two effects for the substitutions of A- and B-site cations: the substitution of Sr for Ca brings expansion toward [100] and enlargement in |d11| and k12. On the other hand, as the substitution of Ta for Nb brings not much change, the properties are similar. In this study, five-element compounds such as La3−xSrxTayGa5−x+ySi1+x−2yO14 (0≤x≤3, 0≤y≤1) solid solutions analysed by a single crystal X-ray diffraction are compared with the three- and four-element compounds on the coordinates of oxygen ions. As the coordinates positions are located on the extended line of coordinate positions on the three-element compounds as increasing ionic radius of R-ions in A-site, the piezoelectric properties |d11| and k12 are expected become large.

Structural Analysis of Strontium-Substituted Hydroxyapatites

In this work, the effects of pH on the formation of hydroxyapatite (HA) and Sr-substituted HA (Sr-HA) were studied. Calcium nitrate tetrahydrate and strontium acetate were dissolved in ethanol at the Ca:Sr mole ratio of 9.5:0.5. Diammonium hydrogen phosphate was dissolved in deionized water. The two solutions were mixed thoroughly and the ammonium solution was added in order to adjust the pH of the solution. After precipitation, the system was subjected to the heat until the dried powder was obtained and then calcined at 600°C for 2 h. From XRD study, HA was found as the main phase in all calcined powders. The substitution of Sr for Ca in HA structure was confirmed by the shift of XRD peak to lower angles and the shifts of OH to higher wave number and PO4 bands to lower wave numbers in FT-IR spectra.

Relation between Seebeck Coefficient and Lattice Parameters of (Ca2−y Sr y CoO3) x CoO2

The effect of dopant content on the Seebeck coefficient of the Sr2+-doped (Ca2−ySryCoO3)xCoO2 (y = 0 to 0.4) system was investigated. This system can be described as a misfit layered structure of (CoO2) and (Ca2−ySryCoO3) layers, which have different b lattice parameters denoted by b1 and b2, respectively. Due to the solubility limit of Sr in (Ca2CoO3)xCoO2, systematic investigations were available only up to y = 0.2. Nevertheless, the structural uniqueness enabled partially quantitative analysis of the correlation between the crystal structure and Seebeck coefficient of the system. Substitution of Sr for Ca leads to lattice expansion which accompanies an anisotropic change of the lattice parameters. Among the lattice parameters considered, the increase in the lattice parameter b2 of the insulating layer was larger than the change of any other lattice parameter, which induced in-plane stress in the conducting layer. As a result, as the b1/b2 misfit ratio of (Ca2−ySryCoO3)xCoO2 is decreased, the Seebeck coefficient also decreases. Practical guidance for selecting dopants to enhance thermoelectric performance is proposed.

Interfacial pH: A Critical Factor for Osteoporotic Bone Regeneration

Osteoporosis is a disease attributed to an imbalance in communication between osteoblasts and osteoclasts, possibly arising from a locally acidic microenvironment which hinders normal cell function. However, to date, little or no attention has been paid to these cells' milieu in respect of implant materials. Although it has been claimed for a few biomaterials that they stimulate bone formation, seldom has their surface behavior been invoked to explain behavior. With degradation, ion concentrations and pH at the material's surface must vary and thus may affect osteoblast response directly. On degradation of a recently developed biomaterial, Sr-containing CaSiO3, the interfacial pH was found to be appreciably higher than that of the bulk medium and the "standard" physiological value of 7.4. At these high values (pH > 8), both the proliferation and alkaline phosphatase (ALP) activity of osteoblasts was significantly enhanced, with a maximum response at 10% Sr substitution for Ca. This shows that the chemistry of the solid-liquid interface is a critical factor in bone regeneration, although this has generally been overlooked. Thus, the interfacial pH in particular is to be considered, rather than the bulk value, and this may be of importance in many related contexts in bone-tissue engineering.

Phase, microstructural characterization and dielectric properties of Ca-substituted Sr5Nb4TiO17 ceramics

The effect of Ca substitution for Sr on the phase, microstructure and microwave dielectric properties of the Sr5−x Ca x Nb4TiO17 composition series was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), an LCR meter, and vector network analyzer. Below 1450 °C, Sr5−x Ca x Nb4TiO17 (x = 1, 2, 3, or 4) compositions formed single-phase Sr4CaNb4TiO17, Sr3Ca2Nb4TiO17, Sr2Ca3Nb4TiO17, and SrCa4Nb4TiO17 ceramics, respectively. At x = 0 and 5, Sr5Nb4TiO17 and Ca5Nb4TiO17 formed, but along with Sr2Nb2O7 (at x = 0) and CaNbO3 and CaNb2O6 (at x = 5) secondary phases. Above 1450 °C, all the compositions formed two-phase ceramics. At low frequencies, a phase transition was observed in the composition Sr5Nb4TiO17. The substitution of Ca for Sr enabled processing of highly dense Sr2Ca3Nb4TiO17, with er ~ 53.4, τf ~ −6.5 ppm/°C and Q u × f o ~ 1166 GHz. Further investigations are required to improve the quality factor of these ceramics for possible microwave applications.

Spin State Control of the Perovskite Rh/Co Oxides

We show why and how the spin state of transition-metal ions affects the thermoelectric properties of transition-metal oxides by investigating two perovskite-related oxides. In the A-site ordered cobalt oxide SrYCoO, partial substitution of Ca for Sr acts as chemical pressure, which compresses the unit cell volume to drive the spin state crossover, and concomitantly changes the magnetization and thermopower. In the perovskite rhodium oxide LaRhO, partial substitution of Sr for La acts as hole-doping, and the resistivity and thermopower decrease systematically with the Sr concentration. The thermopower remains large values at high temperatures (>150 μV/K at 800 K), which makes a remarkable contrast to LaSrCoO. We associate this with the stability of the low spin state of the Rh ions.

The effect of Sr concentration on bioactivity and biocompatibility of sol–gel derived glasses based on CaO–SrO–SiO 2–P 2O 5 quaternary system

In the present study, sol–gel derived glasses based on CaO–SrO–SiO 2–P 2O 5 system were prepared and the effect of Sr concentration on in vitro bioactivity and cellular properties of the glasses were investigated. SrO was substituted for CaO in the glass formula up to 10 mol% and in vitro bioactivity of the samples was evaluated by soaking them in simulated body fluid followed by structural characterization using XRD, FTIR and SEM techniques. The effects of various glass compositions on proliferation and differentiation of osteoblastic cells were also evaluated. The results showed that the substitution of Sr for Ca in the glass composition retarded formation of apatite layer onto the glass surfaces. Morphologies of the apatite layers were also different in which abundance of the crystals decreased with increasing Sr concentration. The bioactive glasses did not exert cytotoxic effect on the cells, however the proliferation and alkaline phosphatase activity of the cells on the samples containing low doses of Sr were higher than those of control and the samples with high dose of Sr. Glass specimen with 5 mol% of Sr exhibited appropriate bioactivity with optimal cell proliferation and ALP activity.

Chemical and Physical Pressure Effects on the Magnetic and Transport Properties of the A-Site Ordered Perovskite Sr3YCo4O10.5

We have measured and analysed the magnetization, resistivity and thermopower of polycrystalline samples of Ca-subsituted Sr 3 YCo 4 O 10.5 . While the transition temperature and thermopower systematically decrease with increasing Ca content, the resistivity is nearly independent of the Ca-substitution level. We have furhter found that the substitution of Ca for Sr systematically decreases the lattice paremeters while maintaining the oxygen content unchanged, and have explained the Ca-substitution effects in terms of the spin-state transition induced by chemical pressure. We have also measured the magnetization of one of the samples under hydrostatic pressure. The result shows that the physical-pressure effect is semiquantitatively consistent with the chemical-pressure effect.

Long lasting phosphorescence behavior of Pr 3+-doped CaTiO 3-based oxides

Ceramic samples with nominal compositions of (Ca 1− x Sr x ) 0.9Pr 0.001Zn 0.099TiO 3 ( x = 0, 0.05, 0.10, 0.15, 0.20) were prepared by a mixed oxide method. When examined by the X-ray diffraction (XRD) analysis, the samples revealed a main CaTiO 3 phase with small amounts of a second Zn 2TiO 4 phase. All of the ceramic samples emitted red phosphorescent light at about 613 nm when excited by light of a wide wavelength range (200 nm to about 500 nm). The partial substitution of Sr for Ca, up to approximately 10 at%, significantly enhanced the phosphorescence intensity and lifetime. The long lasting phosphorescence could be observed with naked eyes in dark room for more than 10 min in both air and water at room temperature.

Multinuclear NMR studies of mixedCa1−xSrxF2crystals

The local environments of fluorine and calcium ions in ${\text{Ca}}_{1\ensuremath{-}x}{\text{Sr}}_{x}{\text{F}}_{2}$ single crystals have been studied using $^{19}\text{F}$ and $^{43}\text{C}\text{a}$ magic-angle spinning (MAS) NMR spectroscopies. The fluorine speciation is found to depend on both the distribution of cations (Ca and Sr) around each fluorine site in the fluorite lattice, as well as fluctuations in the ionic character of bonding between fluorine and these cations. The natural abundance $^{43}\text{C}\text{a}$ MAS NMR results show a clear relation between $^{43}\text{C}\text{a}$ chemical shielding and bond distances in halides, with increasing shielding as the fluorite lattice expands and Ca-F distances lengthen. When taken together, the results from $^{19}\text{F}$ and $^{43}\text{C}\text{a}$ NMR are consistent with a random substitution of Sr for Ca in the mixed fluoride system.

Cathodoluminescence and Longevity Properties of Potential Sr1-xMxGa2S4:Eu (M = Ba or Ca) Green Phosphors for Field Emission Displays

We report the cathodoluminescence and aging properties of a series of green phosphors of formula Sr1-xMxGa2S4:Eu (x = 0.0-1.0, M = Ba or Ca) that have potential applications in field emission displays (FEDs). The series of phosphors was synthesized via NaBr-aided solid-state reactions in a flowing H2S stream. A low level (~20%) of Ba or Ca substitution for Sr in SrGa2S4:Eu maintains the orthorhombic phase of pure SrGa2S4:Eu phosphors. Further, a low level (~20%) of Ba or Ca substitution for Sr in SrGa2S4:Eu provides various green colors and sufficient brightness for FED applications. Substitution of Ba or Ca for Sr in SrGa2S4:Eu also improved the stability of the phosphor when it was operated under electron-beam irradiation of 5 kV.

Hole Distribution in (Sr,Ca,Y,La)14Cu24O41Compounds Studied by X-ray Absorption and Emission Spectroscopy

The polarization dependence of soft x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) near the O 1 s absorption edge was measured on two-leg ladder single-crystalline samples of (Sr,Ca,Y,La) 14 Cu 24 O 41 (14-24-41). The hole distributions in 14-24-41 compounds are determined by polarization analysis. For samples with less than or equal to 5 holes/chemical formula (c.f.), all holes reside on the edge-shared chain layer. In the case of Sr 14- x Ca x Cu 24 O 41 (6 holes/c.f.), there is approximately one hole on the two-leg ladder layer, with about five holes remaining on the edge-shared chain layer. By Ca substitution for Sr in the Sr 14- x Ca x Cu 24 O 41 samples, 0.3 holes transfer from the edge-shared chain to the two-leg ladder layer. It is possible that some of the holes on the two-leg ladder layer move from the rung sites to the leg sites upon Ca substitution.

The role of Sr2+ on the structure and reactivity of SrO–CaO–ZnO–SiO2 ionomer glasses

The suitability of Glass Polyalkenoate Cements (GPCs) for use in orthopaedics is retarded by the presence in the glass phase of aluminium, a neurotoxin. Unfortunately, the aluminium ion plays an integral role in the setting process of GPCs and its absence is likely to hinder cement formation. However, the authors have previously shown that aluminium free GPCs may be formulated based on calcium zinc silicate glasses and these novel materials exhibit significant potential as hard tissue biomaterials. To further improve their potential, and given that Strontium (Sr) based drugs have had success in the treatment of osteoporosis, the authors have substituted Calcium (Ca) with Sr in the glass phase of a series of aluminium free GPCs. However to date little data exists on the effect SrO has on the structure and reactivity of SrO-CaO-ZnO-SiO(2) glasses. The objective of this work was to characterise the effect of the Ca/Sr substitution on the structure of such glasses, and evaluate the subsequent reactivity of these glasses with an aqueous solution of Polyacrylic acid (PAA). To this end (29)Si MAS-NMR, differential scanning calorimetry (DSC), X-ray diffraction, and network connectivity calculations, were used to characterize the structure of four strontium calcium zinc silicate glasses. Following glass characterization, GPCs were produced from each glass using a 40 wt% solution of PAA (powder:liquid = 2:1.5). The working times and setting times of the GPCs were recorded as per International standard ISO9917. The results acquired as part of this research indicate that the substitution of Ca for Sr in the glasses examined did not appear to significantly affect the structure of the glasses investigated. However it was noted that increasing the amount of Ca substituted for Sr did result in a concomitant increase in setting times, a feature that may be attributable to the higher basicity of SrO over CaO.

80 K superconductivity in Bi-2201 phase

We report 80K superconductivity observed for the crystals of Bi2+xCaySr2−x−y−zLazCuO6+δ with varying Ca-content y (0≦y≦2−x−z) under various values of excess Bi (0≦x≦0.2) and La (0≦z≦0.4) doping. Powder X-ray diffraction patterns and transmission electron microscope images indicate the crystals of Bi-2201 phase with orthorhombic structure. Temperature dependence of magnetic moment exhibits bulk superconductivity with the transition temperature of 77–87K, while the volume of the superconductivity changes with Ca-content. Those results suggest a model of inhomogeneous distribution of 80K superconductivity with Ca-substitution for Sr.

Crystal structure–polarization property relations in (Sr 0.5− xCa x)Bi 2.25Na 1.25Nb 3O 12 solid solutions

The relationship between crystal structure and polarization properties of (Sr 0.5− x Ca x )Bi 2.25Na 1.25Nb 3O 12 solid solutions was investigated in this study. A single phase of (Sr 0.5− x Ca x )Bi 2.25Na 1.25Nb 3O 12 solid solutions was obtained over the whole composition range; the lattice parameters and the unit cell volume of (Sr 0.5− x Ca x )Bi 2.25Na 1.25Nb 3O 12 solid solutions are linearly decreased with the Ca substitution for Sr. The decrease in the atomic distances of oxygen–oxygen bonds, which compose the equatorial plane, led to the decrease in the volume of the Nb(1)O 6 and Nb(2)O 6 octahedra caused by the Ca substitution for Sr. The covalency of the Nb(1)–O(1) bond increased in the composition range of 0–0.3, while that of the Nb(1)–O(1′) bond decreased in this composition range. The remanent polarizations ( P r) of the solid solutions were improved by the Ca substitution for Sr; the highest P r value of 16 μC/cm 2 was obtained at x = 0.3. Thus, the variations in the P r value caused by the Ca substitution for Sr may be attributed to the variations in the covalency of Nb(1)–O bonds in the Nb(1)O 6 octahedron, which is located the center of the pseudo-perovskite block. On the other hand, the coercive field ( E c) values of the solid solutions are in the order of approximately 45 kV/cm in the composition range of 0–0.5. Also, the Curie temperatures ( T c) of the solid solutions increased with increased the composition x; therefore, the increase in T c value was connected with the decrease in the tolerance factor caused by the Ca substitution for Sr.