Classical Solid State Method Research Articles

Crystal structures, Raman spectra, chemical bonds, and microwave dielectric properties of novel low-εr CaASbWO8 (A = Gd, La) ceramics with opposite temperature coefficients

In this work, two novel CaASbWO8 (A = Gd, La; named CGSWO and CLSWO) microwave dielectric ceramics were synthesized by a classical solid-state method. Remarkably, the outstanding microwave dielectric properties (εr = 12.97, Q×f = 45,034 GHz, and τf = −16.29 ppm/°C) were achieved for CGSWO ceramics sintered at 1440 °C, and CLSWO ceramics sintered at 1300 °C possessed the excellent microwave dielectric properties of εr = 9.89, Q×f = 61,326 GHz, and τf = 0.55 ppm/°C. Bond valence, ionic polarizability, Raman spectroscopy and Phillips–Vechten–Levine (P-V-L) theory were used to study the internal lattice vibration and chemical bonds in detail, and the dielectric properties were explored from the internal factors. In comparison to the εr, the εtheo of CGSWO was found to be consistently underestimated, whereas CLSWO exhibited a contrasting pattern. This discrepancy was primarily attributed to the rattling and compressed effects of A-site cation. According to the analysis of P-V-L theory, W-O bond played an important role in regulating the Q×f value. Moreover, the τf values of CGSWO and CLSWO ceramics were opposite, which provided an opportunity to manufacture microwave ceramic devices with good temperature stability.

Anomalous grain growth in sintered Bi2Ca2Co2−xCuxOy + Ag ceramic composites by Cu doping

Bi2Ca2Co2 − xCuxOy + 4 wt%Ag (with x between 0 and 0.20) thermoelectric materials were prepared through a sol-gel method via nitrates. Powder XRD patterns showed the presence of thermoelectric phase as the major one, accompanied by metallic silver. SEM observations identified three different secondary phases in the samples, together with an exaggerated grain growth with Cu doping. The microstructural modifications are reflected in an important decrease of electrical resistivity, ∼ 3% when compared to undoped samples, due to the reduction on the number of grain boundaries, and the increase of Co4+ in the conducting layer. The minimum values at 650 ºC were obtained in 0.10 Cu-doped samples, 69 mΩ cm, which are lower than the reported in sintered samples. On the other hand, Seebeck coefficient was only slightly affected and reached the highest values at 650 ºC in 0.15 Cu-doped samples, 215 µV/K, which is practically the same measured in undoped ones, 211 µV/K, and lower than those reported for sintered samples. Consequently, the highest PF at 650 ºC has been determined in 0.10 Cu-doped samples, which is around 50% higher than the obtained in samples prepared through the classical solid-state method, or the undoped ones prepared in this work.

Tuning thermoelectric properties of Bi2Ca2Co2Oy through K doping and laser floating zone processing

In the present study, thermoelectric Bi2Ca2-xKxCo2Oy ceramic materials (x = 0.0, 0.05, 0.075, 0.10, and 0.125) in different forms (called bulk, as-grown and annealed fibers) have been manufactured via a classical solid-state method and textured using the laser floating zone (LFZ) technique. The identification and characteristics of undoped and doped samples were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD patterns of all samples have shown great similarity, and the major peaks can be assigned to the Bi2Sr2Co2Oy thermoelectric phase, independently of the processing technique and K-doping. SEM-EDS have indicated the randomly oriented plate like grains of different sizes in bulk sample, evolving to longer and well-oriented grain structure through K-doping and LFZ. Because of the incongruent melting properties of compound, the high number of secondary phases formed in the as-grown samples. In order to reduce it, an annealing and K-doping process have been applied. The microstructural evolution is reflected on the electrical properties, and the lowest resistivity values are found in the annealed K-doped fibers. Seebeck coefficient is positive in all cases, pointing out to p-type conduction mechanism. These modifications led to PF values up to 0.162 mW/(K2m), obtained in 0.10 K-doped annealed fibers at 650 °C.

Reduction in Processing Time in Ca3Co4O9+δ Ceramics through Nanoprecursors Produced by an Easily Scalable and Environmentally Friendly Process.

Attrition milling is an easily scalable and environmentally friendly process used to produce Ca3Co4O9+δ nanoprecursors in a relatively short time. Sintered materials produced through the classical solid-state method, involving ball milling, show much larger grain sizes and slightly lower density than those obtained in samples produced from attrition-milled precursors. On the other hand, electrical resistivity has been drastically decreased, accompanied with a slight decrease in the Seebeck coefficient in samples obtained from these attrition-milled precursors. Moreover, the use of an attrition milling process leads to a very important reduction in processing time (around 75%), together with a slight power factor improvement of around 10%, when compared to the classically prepared samples.

Drastic enhancement of mechanical properties of Ca3Co4O9 by B4C addition

Ca3Co4O9 with B4C additions in different proportions (0, 0.10, 0.25, 0.5, and 0.75 wt.%) have been fabricated using the classical solid-state method. Powder XRD patterns have displayed that Ca3Co4O9 phase is the major one in all samples. Microstructural observations showed that B4C has been superficially oxidized, producing liquid B2O3 during sintering, and reacting with the Ca3Co4O9 grains to produce bridges between them. In spite of the increase of porosity, these bridges led to an important raise (more than two times) of mechanical properties when compared to the pristine materials. On the other hand, while B4C addition has not influenced S values, it has decreased electrical resistivity, thermal conductivity, and thermal expansion. Consequently, ZT values have been also increased, reaching 0.24 at 800 °C in 0.25 wt.% B4C containing samples, which is very close to the best values reported in the literature, and two times higher than the obtained in pure materials in this work.

High performance solid electrolyte-based NO2 sensor based on Co3V2O8 derived from metal-organic framework

In this paper, a well-performing solid electrolyte-based NO2 sensor was initially assembled by using Co3V2O8 derived from Co-V-MOF as sensitive electrode material and La10Si5.5Al0.5O26.75 as solid electrolyte. The sensor displayed excellent sensing behaviors towards NO2 by comparison with the sensor using Co3V2O8 fabricated from classical solid state method as electrode material at 575 °C. The sensor exhibited good response-recovery characteristics to different NO2 concentrations, with a sensitivity of 78.2 mV/decade in the NO2 concentration range of 50–500 ppm. The sensor also showed good repeatability, strong anti-interference ability to CH4, CO2, O2, NO and CO, as well as good long-term stability for 30 test days at 575 °C. In addition, the sensor had a low cross-sensitivity to water vapor in the relative humidity of 20%–98% at 575 °C. The good sensing behaviors could be attributed to the synergistic effect of highly disperse Co/V catalysis sites from Co-V-MOF as well as the larger three-phase boundary of Co3V2O8/ La10Si5.5Al0.5O26.75/NO2. The sensing mechanism of the sensor was investigated and it followed the mixed-potential model.

Improving bulk Ca3Co4O9 thermoelectric materials through Zr doping

ABSTRACTCa3Co4−xZrxOy polycrystalline ceramics with small Zr substitution have been prepared through the classical solid-state method. X-ray diffraction data have shown that all samples are composed only of Ca3Co4O9 and Ca3Co2O6 phases. Moreover, by increasing Zr substitution up to 0.07, Ca3Co2O6 phase content is decreased. Density measurements have revealed that all samples are very similar, with values around 74% of the theoretical density. Electrical resistivity is decreased in Zr-doped samples, with respect to the pure samples, while Seebeck coefficient is unchanged. Both factors lead to power factor values around 0.33 mW K−2 m−1 at 800°C in 0.07 Zr-doped samples, which are about 65% higher than those obtained for the undoped samples.

Luminescent properties under X-ray excitation of Ba(1−x)PbxWO4 disordered solid solution

A series of polycrystalline barium-lead tungstate Ba1-xPbxWO4 with 0 ≤ x ≤ 1 was synthesized using a classical solid-state method with thermal treatment at 1000°C. These materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier Transform Raman (FT-Raman) spectroscopy. X-ray diffraction profile analyses were performed using Rietveld method. These materials crystallized in the scheelite tetragonal structure and behaved as quasi ideal solid solution. Raman spectroscopy confirmed the formation of the solid solution. Structural distortions were evidenced in X-ray diffraction profiles and in vibration Raman spectra. The scanning electron microscopy experiments showed large and rounded irregular grains. Luminescence experiments were performed under X-ray excitation. The luminescence emission profiles have been interpreted in terms of four Gaussian components, with a major contribution of blue emission. The integrated intensity of luminescence reached a maximum value in the composition range x = 0.3–0.6, in relation with distortions of crystal lattice.

Preparation of high performance Bi2Sr2Co1.8Ox thermoelectric materials from nanosized precursors

ABSTRACTBi2Sr2Co1.8Ox thermoelectric (TE) materials were prepared by three different synthesis methods producing nanosized precursors: coprecipitation (with ammonium carbonate or oxalic acid) and attrition milling, which were compared with those obtained by the classical solid state method. Microstructure has shown that precursors produced by coprecipitation and attrition milling methods produced nanometric precursors much smaller than the typical sizes produced by the solid state route. The TE properties are in agreement with the microstructural features, leading to lower resistivity in all the samples, compared with the solid state ones, while Seebeck coefficient is practically unchanged in all cases. As a consequence, maximum power factor values of around four times higher than those obtained in the classical solid state method have been determined. Moreover, the highest power factor value at 650°C is higher than the best results obtained in as-grown textured materials produced by the laser floating zone technique.

Effect of precursors on the microstructure and electrical properties of Bi2Ba2Co2O x

Bi2Ba2Co2O x thermoelectric materials have been prepared through three different synthesis methods. The corresponding precursors were obtained from a coprecipitation method using oxalic acid, attrition milling, and classical solid state method (used as reference). Microstructural studies of precursors have shown that the ones produced by coprecipitation and attrition milling led to smaller grain sizes than the ones obtained through ball milling. Infrared spectroscopy has demonstrated that a thermal treatment at 450 °C totally decomposes the metallic oxalates, producing a mixture of Bi and Co oxides, and Ba carbonate, the same average composition of the other precursors. After sintering procedure, it has been determined that the small grain size precursors produce high-density samples, reflected in a decrease of electrical resistivity without significant modification of the Seebeck coefficient. As a consequence, power factor values are more than two times higher than the measured in samples prepared by the classical solid state method. Moreover, the highest power factor values are in the order of the best reported in textured materials, demonstrating the critical role of precursors in the final properties of these materials.

Ca2−xLi2xGeO4 solid solutions. Chemical composition range and flux crystal growth

A series of Ca2−xLi2xGeO4 specimens with 0 < 2x < 2, were synthesized by the classical solid state method. X-ray analysis revealed that for 0 < 2x < 0.6 only Ca2−xLi2xGeO4 solid solutions of Ca2GeO4 structure crystallized. The cell parameters of these solid solutions linearly decreased upon increasing the lithium concentration, which means that the solutions are in accordance with the Vegard's law. For 2x > 0.6 the specimens contained two phases: (i) Ca2−xLi2xGeO4 solid solutions with maximum lithium concentration approximately equal to that for 2x = 0.6 and minimum values of the cell parameters and (ii) Li2CaGeO4 phase. Li2O·MoO3–Ca2GeO4 high-temperature solutions were used to grow Ca2−xLi2xGeO4 solid solutions and Li2CaGeO4 single crystals by the high temperature solution (flux) method. Li2CaGeO4 crystals were grown in the concentration range 8–26 wt. % Ca2GeO4 in the temperature range 830–980 °C. Crystals with chemical compositions of the Ca2−xLi2xGeO4 solid solutions with different Li concentrations were grown in the concentration range 26–40 wt. % Ca2GeO4 in the temperature range 980–1090 °C.

Effect of Na doping on the Ca3Co4O9 thermoelectric performance

Ca3−xNaxCo4O9 polycrystalline thermoelectric ceramics with small amounts of Na (x=0, 0.01, 0.03, 0.05, 0.07, and 0.10) have been prepared using the classical solid state method. Microstructural characterization has shown that Na has been incorporated into the Ca3Co4O9 phase and that no Na-based secondary phases have been produced. It has also been found that Na addition promotes grain growth and favours sintering due to the formation of a small amount of liquid phase. Electrical resistivity decreases when Na content increases until 0.07Na addition while Seebeck coefficient is maintained practically unchanged. The improvement in electrical resistivity leads to higher power factor values than the usually obtained in samples prepared by more complex and expensive techniques.

Improvement of thermoelectric properties in Ca3Co4O9 ceramics by Ba doping

Ca3−xBaxCo4O9 bulk polycrystalline thermoelectric ceramics with x = 0.00, 0.01, 0.03 and 0.05 have been prepared through a classical solid state method. Microstructural characterizations of sintered samples have shown that Ba has been incorporated into the Ca3Co4O9 and/or Ca3Co2O6 structures and that no Ba-based secondary phases have been produced. The apparent density values increase when the amount of barium is raised with respect to the undoped samples, reaching a maximum value of around 77 % of the Ca3Co4O9 theoretical density, for samples with the highest amount of barium. Electrical resistivity of doped samples decreases in the whole measured temperature range with respect to the undoped ones, while Seebeck coefficient remains practically unchanged. In conclusion, the improvement in resistivity leads to power factor values which are higher than the ones measured in undoped samples, fact which makes this bulk polycrystalline compound a promising candidate for high-temperature power generation applications in oxidative environments.

Thermoelectric doping effect in Ca3Co4-xNixO9 ceramics

Ca3Co4-xNixO9 (x=0.01, 0.03, and 0.05) polycrystalline thermoelectric ceramics have been prepared by the classical solid state method. As a result of the Ni addition an increase in porosity has been detected. Moreover, the presence of Ni has been related with the increase of Ca2Co3O6 secondary phase and the appearance of a new NiO-CoO solid solution.However, for the 0.01-Ni doped samples an improvement in the thermoelectric performances has been measured. This effect has been related with a decrease in the resistivity values and an increase in the Seebeck coefficient.The raise in the power factor for the 0.01-Ni doped samples, compared with the undoped ones, is between 10 and 25% at 50 and 800°C respectively. Moreover, the maximum power at 800°C, around 0.25 mW/K2.m, is significantly higher than the best results obtained in Ni doped samples reported previously in the literature.

Thermoelectric properties in Ca3Co4−xMnxOy ceramics

Ca3Co4−xMnxOy polycrystalline thermoelectric ceramics with small amounts of Mn have been prepared by the classical solid state method. X-ray diffraction data have shown that Ca3Co4O9 is the major phase, with small amounts of the Ca3Co2O6 one. Moreover, they show that the Mn has been incorporated into these two phases. Electrical resistivity decreases, compared with the values for undoped samples, with Mn content until a minimum for the 0·03 doped ones, increasing for higher Mn substitution. Seebeck coefficient does not change in all the measured temperature range, independently of Mn content. The improvement in electrical resistivity leads ∼30% higher power factor values for the 0·03 Mn doped samples than that obtained in the undoped ones. The maximum power factor at 800°C, ∼0·28 mW K−2 m−1, is close to that obtained in much higher density samples, clearly indicating the good thermoelectric properties of these samples.

Decrease of electrical resistivity in Ca3Co4O9 thermoelectric ceramics by Ti doping

Ca3Co4−xTixO9 polycrystalline thermoelectric ceramics with small amounts of Ti have been prepared by the classical solid state method. X-ray diffraction data have shown that Ca3Co4O9 is the major phase, with small amounts of the Ca3Co2O6 one. Moreover, they show that Ti has been incorporated into these two phases. Electrical resistivity decreases, compared with the values for undoped samples, until 0.03-Ti doped ones. Further Ti addition produces an increase of resistivity with respect to the 0.03 Ti doped samples. Seebeck coefficient does not appreciably change in all the measured temperature range, independently of Ti content. The improvement in electrical resistivity leads to about 55 % higher power factor values for the 0.03 Ti-doped samples than that obtained in the undoped ones. The maximum power factor at 800 °C, around 0.33 mW/K2 m, is slightly higher than the obtained in higher density samples, clearly indicating the good thermoelectric performances of these doped samples.

Effect of synthesis methods on the Ca3Co4O9 thermoelectric ceramic performances

Three different synthesis methods producing nanometric grain sizes, coprecipitation with ammonium carbonate, oxalic acid, and by attrition milling have been studied to produce Ca3Co4O9 ceramics and compared with the classical solid state route. These three processes have produced high reactive precursors and all the organic material and CaCO3·have been decomposed in a single thermal treatment. Coprecipitation leads to pure Ca3Co4O9 phase, while attrition milling and classical solid state produce small amounts of Ca3Co2O6 secondary phase. Power factor values are similar for all three samples, being slightly lower for the ones produced by attrition milling. These values are much higher than the obtained in samples prepared by the classical solid state method, used as reference. The maximum power factor values determined at 800°C (~0.43mW/K2m) are slightly higher than the best reported values obtained in textured ones which also show much higher density values.

Doping effect in Ca3Co4−xZnxOy ceramics

Ca3Co4−xZnxOy (x = 0.01, 0.03, and 0.05) polycrystalline thermoelectric ceramics have been prepared by the classical solid state method. XRD data have shown that Ca3Co4O9 is the major phase, with small amounts of the Ca3Co2O6 one. Moreover, it has been found that the Zn has been incorporated into these two phases. Slight Zn doping decreases electrical resistivity compared with the values obtained in undoped samples. The minimum values have been obtained for the 0.01-Ni doped samples, increasing for further Zn substitution. Seebeck coefficient does not appreciably change in all the measured temperature range, independently of Zn content. The improvement in electrical resistivity leads to higher power factor values for the 0.01 Zn-doped samples (about 30 %) than for the undoped ones. The maximum power factor at 800 °C, around 0.27 mW/K2 m is significantly higher than the best results obtained in Zn doped samples reported in the literature.

Thermoelectric properties of rare earth doped Ca3-xRExCo4O9 (RE = Dy, Er, Gd, and Tb; x = 0, 0.01, 0.03, and 0.05)

Ca3-xRExCo4O9 thermoelectric ceramics with different rare earth (RE = Dy, Er, Gd, and Tb) substitution for Ca have been successfully produced by the classical solid state method. All these rare earths in small proportions produce the same effects on the thermoelectric phase, reflected on a similar and significant decrease of the electrical resistivity, compared with the undoped samples. The Seebeck coefficient slightly decreases with all the studied substitutions while the power factor values appreciably increase at high temperatures for small amounts (0.01) of doping elements. Further doping produces the decrease of power factor which indicates that the optimal doping level has been determined to be 0.01 for all the tested rare earths which produces, approximately, the same improvement, around 20 % of the power factor obtained in the undoped samples at 800 °C.

Structural, magnetic and electrochemical investigation of novel binary Na2 − x(Fe1 − yMny)P2O7 (0 ≤ y ≤ 1) pyrophosphate compounds for rechargeable sodium-ion batteries

Pyrophosphate cathodes have been recently reported as a competent family of insertion compounds for sodium-ion batteries. In the current study, we have investigated the binary Na2(Fe1−yMny)P2O7 (0≤y≤1) pyrophosphate family, synthesized by the classical solid-state method. They form a continuous solid solution maintaining triclinic P-1 (#2) symmetry. The local structural coordination differs mainly by different degrees of Na site occupancy and preferential occupation of the Fe2 site by Mn. The structural and magnetic properties of these mixed-metal pyrophosphate phases have been studied. In each case, complete Fe3+/Fe2+ redox activity has been obtained centered at 3V vs. Na. The Fe3+/Fe2+ redox process involves multiple steps between 2.5 and 3V owing to Na-cation ordering during electrochemical cycling, which merge to form a broad single Fe3+/Fe2+ redox peak upon progressive Mn-doping.