Microwave dielectric properties of rutile (Zn1/3Nb2/3)0.40(Ti1−xSnx)0.60O2 (0.15 ≤ x ≤ 0.30) ceramics

Microwave dielectric properties of (Zn_1/3Nb_2/3)_0.40(Ti_1−xSn_x)_0.60O₂ ceramics were investigated as a function of SnO₂ content (0.15 ≤ x ≤ 0.30). A single phase with tetragonal rutile structure was obtained through the entire composition. The unit-cell volume of the specimens was increased with SnO₂ content, due to the larger ionic radius of Sn⁴⁺ (0.69 Å) than that of Ti⁴⁺ (0.605 Å) for octahedral site. Dielectric constant (K) of the sintered specimens was affected by the dielectric polarizability. Quality factor (Qf) was dependent on the degree of reduction of Ti⁴⁺ ion. With an increase of SnO₂ content, the temperature coefficient of resonant frequency (TCF) of the specimens decreased due to the decrease of the octahedral distortion of rutile structure.     

Fabrication and thermoelectric properties of heavily rare-earth metal-doped SrO(SrTiO3)n (n = 1, 2) ceramics

To clarify the effect of substitutional electron doping on the thermoelectric figure of merit (ZT = S²σTκ⁻¹) of Ruddlesden–Popper phase SrO(SrTiO₃)_n (or Sr_n+1Ti_nO_3n+1), measurements were conducted for several thermoelectric parameters, e.g. electrical conductivity (σ), Seebeck coefficient (S) and thermal conductivity (κ), of (Sr_1−xRE_x)_n+1Ti_nO_3n+1 (n = 1 or 2, RE (rare earth): La or Nd, x = 0.05 and 0.1) dense ceramics prepared by a conventional solid-state reaction and hot-pressing technique. Crystal structures of the resultant ceramics were represented as (Sr_1−xRE_x)_n+1 Ti_nO_3n+1 evaluated by powder X-ray diffraction followed by the Rietveld analysis. All the ceramics exhibited electrical conductivity and the σ values simply depended on the dopant concentration, indicating that both La³⁺ and Nd³⁺ ions act as electron donors. The |S| values increased with temperature due to decrease in the chemical potential. Significant reduction of the κ values was observed as compared to cubic-perovskite SrTiO₃. The ZT value increased with temperature and reached 0.15 at 1000 K for (Sr_0.95La_0.05)₃Ti₂O₇.     

AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Catalytic methane combustion and CO oxidation were investigated over AFeO₃ (A=La, Nd, Sm) and LaFe_1−xMg_xO₃ (x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe⁴⁺ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m² g⁻¹ range. XRD analysis showed that LaFeO₃, NdFeO₃, SmFeO₃ and LaFe_1−xMg_xO₃ (x·0.3) are single phase perovskite-type oxides. Traces of La₂O₃, in addition to the perovskite phase, were detected in the LaFe_1−xMg_xO₃ catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO₃ of a very small fraction of Fe⁴⁺ which reduces to Fe³⁺. The fraction of Fe⁴⁺ in the LaFe_1−xMg_xO₃ samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO₂ below 973 and 773 K, respectively. For the AFeO₃ materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe_1−xMg_xO₃ catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase (x=0.3). Concerning the CO oxidation, the order of activity for the AFeO₃ materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe_1−xMg_xO₃ catalysts decreases at high magnesium content.     

Structure and microwave dielectric properties of Nd(2−x)/3LixTiO3

The structure evolution, and microwave dielectric properties of Nd_(2−x)/3Li_xTiO₃ ceramics (0 ≤ x ≤ 0.5) were investigated in this paper. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results show that samples with x = 0.2–0.4 exhibit single phase. Multi-phases of Nd₂Ti₂O₇, Nd_2/3TiO₃ and Nd₂Ti₄O₁₁ were observed when x = 0 and 0.1. The concentration and ordering degree of A-site decrease with the increase of x value. The dielectric constant increases up to x = 0.2 and then decreases with the further increase of x value. The Qf value decreases with the increase of x value. The temperature coefficient of resonant frequency exhibits negative value and the absolute value decreases greatly with the decrease of x value.     

Nanosized perovskite-type oxides La1−xSrxMO3−δ (M = Co, Mn; x = 0, 0.4) for the catalytic removal of ethylacetate

Nanometer perovskite-type oxides La_1−xSr_xMO_3−δ (M = Co, Mn; x = 0, 0.4) have been prepared using the citric acid complexing-hydrothermal-coupled method and characterized by means of techniques, such as X-ray diffraction (XRD), BET, high-resolution scanning electron microscopy (HRSEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and temperature-programmed reduction (TPR). The catalytic performance of these nanoperovskites in the combustion of ethylacetate (EA) has also been evaluated. The XRD results indicate that all the samples possessed single-phase rhombohedral crystal structures. The surface areas of these nanomaterials ranged from 20 to 33 m² g⁻¹, the achievement of such high surface areas are due to the uniform morphology with the typical particle size of 40–80 nm (as can be clearly seen in their HRSEM images) that were derived with the citric acid complexing-hydrothermally coupled strategy. The XPS results demonstrate the presence of Mn⁴⁺ and Mn³⁺ in La_1−xSr_xMnO_3−δ and Co³⁺ and Co²⁺ in La_1−xSr_xCoO_3−δ, Sr substitution induced the rises in Mn⁴⁺ and Co³⁺ concentrations; adsorbed oxygen species (O⁻, O₂⁻, or O₂²⁻) were detected on the catalyst surfaces. The O₂-TPD profiles indicate that Sr doping increased desorption of the adsorbed oxygen and lattice oxygen species at low temperatures. The H₂-TPR results reveal that the nanoperovskite catalysts could be reduced at much lower temperatures (<240 °C) after Sr doping. It is observed that under the conditions of EA concentration = 1000 ppm, EA/oxygen molar ratio = 1/400, and space velocity = 20,000 h⁻¹, the catalytic activity (as reflected by the temperature (T_100%) for EA complete conversion) increased in the order of LaCoO_2.91 (T_100% = 230 °C) ≈ LaMnO_3.12 (T_100% = 235 °C) < La_0.6Sr_0.4MnO_3.02 (T_100% = 190 °C) < La_0.6Sr_0.4CoO_2.78 (T_100% = 175 °C); furthermore, there were no formation of partially oxidized by-products over these catalysts. Based on the above results, we conclude that the excellent catalytic performance is associated with the high surface areas, good redox properties (derived from higher Mn⁴⁺/Mn³⁺ and Co³⁺/Co²⁺ ratios), and rich lattice defects of the nanostructured La_1−xSr_xMO_3−δ materials.     

Microwave dielectric properties and co-firing with copper of (Bi1−xCux)(Nb1−xWx)O4 ceramics

Effect of substitution of CuO and WO₃ on the microwave dielectric properties of BiNbO₄ ceramics and the co-firing between ceramics and copper electrode were investigated. The (Bi_1−xCu_x)(Nb_1−xW_x)O₄ (x = 0.005, 0.01, 0.015, 0.02) composition can be densified between 900 and 990 °C. The microwave dielectric constants lie between 36 and 45 and the pores in ceramics were found to be the main influence. The Q values changes between 1400 and 2900 with different x values and sintering temperatures while Q_f values lie between 6000 and 16,000 GHz. The microwave dielectric losses, mainly affected by the grain size, pores, and the secondary phase, are discussed. The (Bi_1−xCu_x)(Nb_1−xW_x)O₄ ceramics and copper electrode was co-fired under N₂ atmosphere at 850 °C and the EDS analysis showed no reaction between the dielectrics and copper electrodes. This result presented the (Bi_1−xCu_x)(Nb_1−xW_x)O₄ dielectric materials to be good candidates for LTCC applications with copper electrode.     

Microwave dielectric characteristics of ceramics in Mg2SiO4–Zn2SiO4 system

(Mg_1−xZn_x)₂SiO₄ ceramics were prepared and characterized. The densification temperatures of the present ceramics are much lower than those for Mg₂SiO₄ and Zn₂SiO₄ end-members. Small solid solution limits of Zn in Mg₂SiO₄ and Mg in Zn₂SiO₄ are observed, and the bi-phase structure is confirmed in (Mg_1−xZn_x)₂SiO₄ ceramics with x = 0.1–0.9. Even though, it is clear that the Qf value of Zn₂SiO₄ ceramics can be significantly improved together with a suppressed temperature coefficient of resonant frequency τ_f by substituting Mg for Zn. (Mg_0.4Zn_0.6)₂SiO₄ ceramics indicate a good combination of microwave dielectric characteristics: _r = 6.6 Qf = 95,650 GHz, and τ_f = −60 ppm/°C.     

Effect of glass additions on the sintering and microwave properties of composite dielectric ceramics based on BaO–Ln2O3–TiO2 (Ln = Nd, La)

Ten weight percent BBZS (Bi₂O₃, B₂O₃, ZnO and SiO₂) glass was added to x(Ba₄Nd_9.333Ti₁₈O₅₄) − (1 − x)(BaLa₄Ti₄O₁₅) (BNLT, 0 ≤ x ≤ 1) composite dielectric ceramics to lower their sintering temperature whilst retaining microwave properties useful for low temperature co-fired ceramic and antenna core technology. With the addition of 10 wt% BBZS glass, dense BNLT composite ceramics were produced at temperatures between 950 and 1140 °C, depending on composition (x), an average reduction of sintering temperature by 350 °C. X-ray diffraction, scanning and transmission electron microscopy and Raman spectroscopy studies revealed that there was limited inter-reaction between BLT/BNT and the BBZS glass. Microwave property measurement showed that the addition of BBZS glass to BNLT ceramics had a negligible effect on _r and τ_f, although deterioration in the measured quality factor (Qf) was observed. The optimised composition (xBNT − (1 − x)BLT)/0.1BBZS (x = 0.75) had _r  61, τ_f  38 ppm/°C and Qf  2305 GHz.     

AMnO3 (A=La, Nd, Sm) and Sm1−xSrxMnO3 perovskites as combustion catalysts: structural, redox and catalytic properties

Catalytic combustion of methane has been investigated over AMnO₃ (A = La, Nd, Sm) and Sm_1−xSr_xMnO₃ (x = 0.1, 0.3, 0.5) perovskites prepared by citrate method. The catalysts were characterized by chemical analysis, XRD and TPR techniques. Catalytic activity measurements were carried out with a fixed bed reactor at T = 623–1023 K, space velocity = 40 000 N cm³ g⁻¹ h⁻¹, CH₄ concentration = 0.4% v/v, O₂ concentration = 10% v/v.

Specific surface areas of perovskites were in the range 13–20 m² g⁻¹. XRD analysis showed that LaMnO₃, NdMnO₃, SmMnO₃ and Sm_1−xSr_xMnO₃ (x = 0.1) are single phase perovskite type oxides. Traces of Sm₂O₃ besides the perovskite phase were detected in the Sm_1−xSr_xMnO₃ catalysts for x = 0.3, 0.5. Chemical analysis gave evidence of the presence of a significant fraction of Mn(IV) in AMnO₃. The fraction of Mn(IV) in the Sm_1−xSr_xMnO₃ samples increased with x. TPR measurements on AMnO₃ showed that the perovskites were reduced in two steps at low and high temperature, related to Mn(IV) → Mn(III) and Mn(III) → Mn(II) reductions, respectively. The onset temperatures were in the order LaMnO₃ > NdMnO₃ > SmMnO₃. In Sm_1−xSr_xMnO₃ the Sr substitution for Sm caused the formation of Mn(IV) easily reducible to Mn(II) even at low temperature. Catalytic activity tests showed that all samples gave methane complete conversion with 100% selectivity to CO₂ below 1023 K. The activation energies of the AMnO₃ perovskites varied in the same order as the onset temperatures in TPR experiments suggesting that the catalytic activity is affected by the reducibility of manganese. Sr substitution for Sm in SmMnO₃ perovskites resulted in a reduction of activity with respect to the unsubstituted perovskite. This behaviour was related to the reduction of Mn(IV) to Mn(II), occurring under reaction conditions, hindering the redox mechanism.     

Electrical resistivity and thermopower of (La1−xSrx)MnO3 and (La1−xSrx)CoO3 at elevated temperatures

Electrical resistivity and Seebeck (S) measurements were performed on (La_1−xSr_x)MnO₃ (0.02x0.50) and (La_1−xSr_x)CoO₃ (0x0.15) in air up to 1073 K. (La_1−xSr_x)MnO₃ (x0.35) showed a metal-to-semiconductor transition; the transition temperature almost linearly increased from 250 to 390 K with increasing Sr content. The semiconductor phase above the transition temperature showed negative values of S. (La_1−xSr_x)CoO₃ (0x0.10) showed a semiconductor-to-metal transition at 500 K. Dominant carriers were holes for the samples of x0.02 above room temperature. LaCoO₃ showed large negative values of S below ca. 400 K, indicative of the electron conduction in the semiconductor phase.     

Methane combustion and CO oxidation on LaAl1−xMnxO3 perovskite-type oxide solid solutions

Structural, redox and catalytic deep oxidation properties of LaAl_1−xMn_xO₃ (x=0.0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions prepared by the citrate method and calcined at 1073 K were investigated. XRD analysis showed that all the LaAl_1−xMn_xO₃ samples are single phase perovskite-type solid solutions. Particle sizes and surface areas (SA) are in the 280–1180 Å and 4–33 m² g⁻¹ ranges, respectively. Redox properties and the content of Mn⁴⁺ were derived from temperature programmed reduction (TPR) with H₂. Two reduction steps are observed by TPR for pure LaMnO₃, the first attributed to the reduction of Mn⁴⁺ to Mn³⁺ and the second due to complete reduction of Mn³⁺ to Mn²⁺. The presence of Al in the LaAl_1−xMn_xO₃ solid solutions produces a strong promoting effect on the Mn⁴⁺→Mn³⁺ reducibility and inhibits the further reduction to Mn²⁺. Both for methane combustion and CO oxidation all Mn-containing perovskites are much more active than LaAlO₃, so pointing to the essential role of the transition metal ion in developing highly active catalysts. Partial dilution with Al appears to enhance the specific activity of Mn sites for methane combustion.     

Synthesis, characterization and catalytic activity for NO–CO reaction of Pd–(La, Sr)2MnO4 system

La_xSr_2−xMnO₄ (0 ≤ x ≤ 0.8) oxides were synthesized and single-phase K₂NiF₄-type oxides were obtained in the range of 0.1 ≤ x < 0.5. The catalytic activity of La_xSr_2−xMnO₄ for NO–CO reaction increased with increasing x in the range of solubility limit of La. La_0.5Sr_1.5MnO₄ showed the highest activity among La_xSr_2−xMnO₄ prepared in this study, but its activity was inferior to perovskite-type La_0.5Sr_0.5MnO₃. Among the Pd-loaded catalysts, however, Pd/La_0.8Sr_1.2MnO₄ showed the higher activity and the selectivity to N₂ than Pd/La_0.5Sr_0.5MnO₃ and Pd/γ-Al₂O₃. The excellent catalytic performance of Pd/La_0.2Sr_1.2MnO₄ could be ascribable to the formation of SrPd₃O₄ which was detected by XRD in the catalyst but not in the other two catalysts.     

Synthesis and properties of Bi0.5(Na1−x−yKxAgy)0.5TiO3 lead-free piezoelectric ceramics

Bi_0.5(Na_1−x−yK_xAg_y)_0.5TiO₃ piezoelectric ceramics were prepared by conventional ceramic processes. X-ray diffraction patterns show a pure perovskite structure, indicating that the K⁺ and Ag⁺ ions substitute for the Na⁺ ions in Bi_0.5Na_0.5TiO₃. The temperature dependence of the dielectric constant and dissipation factor shows all ceramics to experience two phase transitions: from ferroelectric to anti-ferroelectric and from anti-ferroelectric to paraelectric. The transition temperature from ferroelectric to anti-ferroelectric and the temperature at which the dielectric constant reaches its maximum value decrease with the increase of K⁺ amount. At room temperature, the ceramics containing 17.5–20 mol% K⁺ and 2 mol% Ag⁺ exhibit high piezoelectric constant (d₃₃ = 180 pC/N) and high electromechanical coupling factor (k_p = 35%).     

Formation of LiNixCo1−xO2 by decarbonization of organic gel precursors through treatment with nitric acid and hydrogen peroxide

We have used a complex sol–gel process to synthesize a family of compounds LiNi_xCo_1−xO₂ (x = 0, 0.25, 0.5, 0.75, 1). These compounds are candidates for electrode materials in high-energy-density batteries. Starting sols were prepared from xNi²⁺ + (1 − x) Co²⁺ acetates/ascorbic acid aqueous solutions by alkalizing with LiOH and NH₃. With thermal treatment in air, nickel carbonates formed in quantities roughly proportional to Ni concentration. The carbonate impurities could not be fully removed by heating in air to high temperatures. Because formation of pure layered oxides was inhibited by the presence of the carbonates, we developed a new way to remove them from just-formed precursors by treating the intermediate phases (those formed after calcination at 750 °C) with concentrated HNO₃ and H₂O₂. All resulting powders were phase pure by X-ray diffraction and were easily friable. Various electrochemical properties of compacts prepared from these powders were measured.     

Comparative study of lithium ion conductors in the system Li1+xAlxA2−x (PO4)3 with A=Ti or Ge and 0≤x≤0·7 for use as Li sensitive membranes

Preparations and physico-chemical characterizations of NASICON-type compounds in the system Li_1+xAl_xA_2−x^IV(PO₄)₃ (A^IV=Ti or Ge) are described. Ceramics have been fabricated by sol-gel and co-grinding processes for use as ionosensitive membrane for Li⁺ selective electrodes. The structural and electrical characteristics of the pellets have been examined. Solid solutions are obtained with Al/Ti and Al/Ge substitutions in the range 0≤x≤0·6. A minimum of the rhombohedral c parameter appears for x about 0·1 for both solutions. The grain ionic conductivity has been characterized only in the case of Ge-based compounds. It is related to the carrier concentration and the structural properties of the NASICON covalent skeleton. The results confirm that the Ti-based framework is more calibrated to Li⁺ migration than the Ge-based one. A grain conductivity of 10⁻³ S cm⁻¹ is obtained at 25°C in the case of Li_1·3Al_0·3Ti_1·7(PO₄)₃. A total conductivity of about 6×10⁻⁵ S cm⁻¹ is measured on sintered pellets because of grain boundary effects. The use of such ceramics in ISE devices has shown that the most confined unit cell (i.e. in Ge-based materials) is more appropriate for selectivity effect, although it is less conductive.©     

Effect of structural and electrochemical properties of different Cr-doped contents of Li[Ni1/3Mn1/3Co1/3]O2

Layered Li[Ni_(1−x)/3Mn_(1−x)/3Co_(1−x)/3Cr_x]O₂ materials with x = 0, 0.01, 0.02, 0.03, 0.05 are prepared by a solid-state pyrolysis method. The oxide compounds were calcined with various Cr-doped contents, which result in greater difference in morphological (shape, particle size and specific surface area) and the electrochemical (first charge profile, reversible capacity and rate capability) differences. The Li[Ni_(1−x)/3Mn_(1−x)/3Co_(1−x)/3Cr_x]O₂ powders were characterized by means of X-ray diffraction (XRD), charge/discharge cycling, cyclic voltammetry, and SEM. XRD experiment revealed that the Li[Ni_(1−x)/3Mn_(1−x)/3Co_(1−x)/3Cr_x]O₂ (x = 0, 0.01, 0.02, 0.03, 0.05) were crystallized to well layered -NaFeO₂ structure. The first specific discharge capacity and coulombic efficiency of the electrode of Cr-doped materials were higher than that of pristine material. When x = 0.02, the sample showed the highest first discharge capacity of 241.9 mAh g⁻¹ at a current density of 30 mA g⁻¹ in the voltage range 2.3–4.6 V, and the Cr-doped samples exhibited higher discharge capacity and better cycleability under medium and high current densities at room temperature.     

Ce1−xCuxO2−x/Al2O3/FeCrAl catalysts for catalytic combustion of methane

A series of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts (x = 0–1) were prepared. The structure of the catalysts was characterized using XRD, SEM and H₂-TPR. The catalytic activity of the catalysts for the combustion of methane was evaluated. The results indicated that in the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts the surface phase structure were the Ce_1−xCu_xO_2−x solid solution, -Al₂O₃ and γ-Al₂O₃. The surface particle shape and size were different with the variety of the molar ratio of Ce to Cu in the Ce_1−xCu_xO_2−x solid solution. The Cu component of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts played an important role to the catalytic activity for the methane combustion. There were the stronger interaction among the Ce_1−xCu_xO_2−x solid solution and the Al₂O₃ washcoats and the FeCrAl support.     

Preparation and characterisation of SrTi1−x−yZrxMnyO3 solid solution powders in relation to their use in combustion catalysis

Mixed oxides with compositions SrTi_1−x−yZr_xMn_yO₃, with 0 ≤ x ≤ 1 and 0 ≤ y ≤ 0.2 have been prepared with a conventional coprecipitation method. Some of them are constituted by very pure perovskite-type solid solution phases, with tetravalent Zr and Mn substituting for Ti in the B site. The addition of Zr to SrTiO₃ tends to increase the surface areas, while the insertion of Mn tends to decrease it. Mn-containing materials are active in the catalytic combustion of 1% methane in air at temperatures higher than 700 K and can be competitive with pure manganite perovskites like LaMnO₃ in spite of the lower Mn content. Pyridine adsorption experiments show that medium strength Lewis acid sites are located at the surface of these materials, and could be involved in the hydrocarbon CH bond activation.     

Hydrotreating properties of mixed NbxMo1−xS2 alumina supported catalysts

Niobium-molybdenum disulfide solid solution (Nb_xMo_1−xS₂) has been prepared in a dispersed state on gamma alumina. The existence of this solid solution supported on alumina carrier has been proven with the help of EXAFS technique. The catalytic properties of these materials have been studied in hydrogenation and hydrodesulfurization reactions. Interestingly, as already observed for niobium sulfide, the activity of the Nb_xMo_1−xS₂ solid solution (HDS of DBT, P_tot=33 bar) is not decreased in the presence of H₂S up to p(H₂S)=200 Torr, at least up to x=0.4.     

The homogeneity range and the microwave dielectric properties of the BaZn2Ti4O11 ceramics

Ceramics with a composition close to BaZn₂Ti₄O₁₁ were synthesized according to various substitutional mechanisms in order to verify an existence of a homogeneity range in the vicinity of this composition. Structural and microstructural investigations showed that the crystal structure of BaZn₂Ti₄O₁₁ was formed in the homogeneity range corresponding to the formula BaZn_2 − xTi₄O_11 − x (0 < x < 0.1). Densely sintered BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) ceramics exhibited a dielectric constant around 30, τ_f = −30 ppm/K and high Q × f values, which increased from 68,000 GHz at x = 0 to 83,000 GHz at x = 0.05. Structurally, the deficiency of Zn in BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) resulted in a slight decrease in the unit-cell volume. The influence of secondary phases in the BaZn₂Ti₄O₁₁-based materials on the microwave dielectric properties was also investigated. A presence of small amounts of ZnO, BaTiO₃, hollandite-type solid solutions (Ba_xZn_xTi_8 − xO₁₆) and BaTi₄O₉ caused a decrease in Q × f values.