Enhanced thermoelectric performance and high-temperature thermal stability of p-type Ag-doped β-Zn4Sb3

Here we report that the thermoelectric properties of bulk β-Zn4Sb3 can be improved in the range 300–575 K by Ag doping at the Zn sites. Proper Ag doping leads to decreased electrical resistivity and increased Seebeck coefficient, thus resulting in a large improvement in power factor. The figure of merit, zT, has an obvious enhancement due to Ag doping although the thermal conductivity is slightly increased. (Zn0.9925Ag0.0075)4Sb3 exhibits a promising zT of ∼1.2 at 575 K, which is superior to most previously reported p-type doped Zn4Sb3 materials. Furthermore, the high-temperature thermal stability is studied in detail. The (Zn0.9925Ag0.0075)4Sb3 bulk sample does not decompose even when the temperature is elevated to 793 K in vacuum. When the bulk sample is heated to 573 K in air, (Zn0.9925Ag0.0075)4Sb3 is also stable, unlike undoped Zn4Sb3 where Zn whiskers come out of the surface. In-house in situ powder X-ray diffraction (PXRD) and multi-temperature synchrotron PXRD (up to 793 K) reveal that the undoped Zn4Sb3 powder sample starts decomposing into ZnSb at 473 K if exposed to the air and it is fully decomposed into ZnSb, ZnO, and Sb after cooling down from 793 to 300 K. However, there is ∼24 wt% Zn4Sb3 preserved in the (Zn0.995Ag0.005)4Sb3 powder sample after the same heat treatment, while only ∼6 wt% Zn4Sb3 remains in (Zn0.99Ag0.01)4Sb3. The above result indicates that proper Ag doping leads to enhanced high-temperature thermal stability in β-Zn4Sb3. This work thereby suggests Ag-doped Zn4Sb3 bulk material as a promising candidate for thermoelectric applications in terms of enhanced performance as well as improved high-temperature thermal stability

Experimental Investigation of Zinc Antimonide Thin Film Thermoelectric Element over Wide Range of Operating Conditions

Zinc antimonide compounds are among the most efficient thermoelectric (TE) materials with exceptional low thermal conductivity at moderate temperatures up to 350 °C. This study aims to evaluate the performance of a zinc antimonide thin film TE deposited on an insulating substrate, while the heat flows in plane with the thin film. At first, the effect of applying different temperatures at the hot side of the specimen is investigated to reach steady state in an open circuit analysis. Then, the study focuses on performance and stability analysis of the thermoelectric element operating under different resistive loads and over a wide range of operating temperatures from 160 to 350 °C. The results show that at a hot side temperature equal to 275 °C the Seebeck coefficient (α) reaches its maximum value (242 µV K−1), which is comparable to that of bulk materials reported in the literature. According to a variation of the load resistance, the maximum power output, that is a function of temperature, occurs at 170.25 Ω. The maximum power is 8.46 µW corresponding to a cold and hot side temperature of ≈30 and 350 °C, respectively

Experimental setup for high-temperature in situ studies of crystallization of thin films with atmosphere control

Understanding the crystallization process for chemical solution deposition (CSD) processed thin films is key in designing the fabrication strategy for obtaining high-quality devices. Here, an in situ sample environment is presented for studying the crystallization of CSD processed thin films under typical processing parameters using near-grazing-incidence synchrotron X-ray diffraction. Typically, the pyrolysis is performed in a rapid thermal processing (RTP) unit, where high heating rates, high temperatures and atmosphere control are the main control parameters. The presented in situ setup can reach heating rates of 20°C s−1 and sample surface temperatures of 1000°C, comparable with commercial RTP units. Three examples for lead-free ferroelectric thin films are presented to show the potential of the new experimental set-up: high temperature, for crystallization of highly textured Sr0.4Ba0.6Nb2O6 on a SrTiO3 (001) substrate, high heating rate, revealing polycrystalline BaTiO3, and atmosphere control with 25% CO2, for crystallization of BaTiO3. The signal is sufficient to study a single deposited layer (≥10 nm for the crystallized film) which then defines the interface between the substrate and thin film for the following layers. A protocol for processing the data is developed to account for a thermal shift of the entire setup, including the sample, to allow extraction of maximum information from the refinement, e.g. texture. The simplicity of the sample environment allows for the future development of even more advanced measurements during thin-film processing under non-ambient conditions

Thermal evolution of the crystal structure and phase transitions of KNbO3

The thermal evolution of the crystal structure and phase transitions of KNbO3 were investigated by high-temperature powder X-ray diffraction and Rietveld refinement of the diffraction data. Two phase transitions from orthorhombic (Amm2) to tetragonal (P4mm) and from tetragonal to cubic (Pm3¯m) were confirmed, both on heating and cooling. Both phase transitions are first order based on the observed hysteresis. The mixed displacive and order–disorder nature of the tetragonal to cubic transition is argued based on symmetry and apparent divergence of the atomic positions from pseudocubic values. The transition between the orthorhombic and tetragonal phase shows no temperature-dependence for atomic positions and only thermal expansion of the unit cell parameters and is thus discussed in relation to a lattice dynamical instability

Mechanisms for texture in BaTiO3 thin films from aqueous chemical solution deposition

The prototype piezoelectric material BaTiO3 is widely used in e.g., capacitators. Chemical solution deposition (CSD) of BaTiO3 films is a simple and environmentally friendly processing route, but insight in the crystallization process is crucial to tailor the film properties. In this work, the influence of the annealing conditions on the crystallization behavior of BaTiO3 thin films from aqueous chemical solution deposition is presented. In situ synchrotron X-ray diffraction was used to reveal the phase evolution, crystallization of the films, and to study how the degree of crystallographic texture in the polycrystalline films evolved. Our results revealed that the formation of an intermediate metastable oxycarbonate phase is critical for the formation of BaTiO3 thin films prepared by aqueous CSD. The pyrolysis products present in the film before crystallization determine the degree of preferential orientation and by tuning the heating program, especially the heating rate through nucleation (<0.2 °C/s), control of the microstructure and degree of preferential orientation in the films was demonstrated

Revealing the Slow Decomposition Kinetics of Type-I clathrate Ba8Ga16Ge30

Inconsistencies in high temperature thermoelectric property measurements of Ba8Ga16Ge30 have prompted our study on the thermal stability of this heavily studied inorganic clathrate. Using X-ray diffraction, thermal analysis, and imaging techniques on both powder and spark plasma sintered pelletized samples, we probe the structure and decomposition characteristics of this important high temperature thermoelectric material. We demonstrate that the decomposition of Ba8Ga16Ge30 is extremely dependent on the heating conditions employed and, as a result of the slow decomposition kinetics of the clathrate, reveal that the true stability of this system has been overlooked in the extensive literature available. Loss of Ga and Ge from the clathrate cage is evident in all high temperature experiments under both air and inert environments. This study serves to highlight that the underlying structural chemistry and stability of thermoelectric materials at high temperature needs to be considered in parallel with the thermoelectric properties which constitute the figure of merit. Only then will reliable thermoelectric modules for real applications be realized

Tailoring preferential orientation in BaTiO3-based thin films from aqueous chemical solution deposition

Ferroelectric properties of thin films can be enhanced by crystallographic texture. In this work, we report on how heat treatment of films can be designed to tailor the degree of preferential orientation in BaTiO3-based thin films from aqueous chemical solution deposition. In situ synchrotron X-ray diffraction in combination with Rietveld refinements was used to study the crystallization process of films from a single deposition and to give an in depth characterization of the crystallographic texture of the films. Transmission electron microscopy was employed to evaluate the microstructure and degree of preferred orientation in thicker films from multiple depositions. Texture was induced in the multilayer films by a repeated annealing process. Cube-on-cube growth was demonstrated to occur in both single and multi-layered films provided the heating program was designed to give limited nucleation and growth below the threshold for where cube-on-cube growth is favoured, resulting in a very high degree of preferred orientation. The cube-on-cube grown films were relaxed with respect to the lattice unit cell mismatch between the film and the substrate, where strain and relaxation depend on the film thickness. Texture and cube-on-cube growth were demonstrated on several types of single crystal substrates. Calcium and zirconium substitution did not alter the crystallization process, but zirconium decreased the texture formation kinetics. The ferroelectric response was strongest in the films with a high degree of preferred orientation

Location of Cu2+ in CHA zeolite investigated by X-ray diffraction using the Rietveld/maximum entropy method

Accurate structural models of reaction centres in zeolite catalysts are a prerequisite for mechanistic studies and further improvements to the catalytic performance. The Rietveld/maximum entropy method is applied to synchrotron powder X-ray diffraction data on fully dehydrated CHA-type zeolites with and without loading of catalytically active Cu2+ for the selective catalytic reduction of NOx with NH3. The method identifies the known Cu2+ sites in the six-membered ring and a not previously observed site in the eight-membered ring. The sum of the refined Cu occupancies for these two sites matches the chemical analysis and thus all the Cu is accounted for. It is furthermore shown that approximately 80% of the Cu2+ is located in the new 8-ring site for an industrially relevant CHA zeolite with Si/Al = 15.5 and Cu/Al = 0.45. Density functional theory calculations are used to corroborate the positions and identity of the two Cu sites, leading to the most complete structural description of dehydrated silicoaluminate CHA loaded with catalytically active Cu2+ cations

Optimized Carbonation of Magnesium Silicate Mineral for CO₂ Storage

The global ambition of reducing the carbon dioxide emission makes sequestration reactions attractive as an option of storing CO₂. One promising environmentally benign technology is based on forming thermodynamically stable carbonated minerals, with the drawback that these reactions usually have low conversion rates. In this work, the carbonation reaction of Mg rich olivine, Mg₂SiO₄, under supercritical conditions has been studied. The reaction produces MgCO₃ at elevated temperature and pressure, with the addition of NaHCO₃ and NaCl to improve the reaction rates. A sequestration rate of 70% was achieved within 2 h, using olivine particles of sub-10 μm, whereas 100% conversion was achieved in 4 h. This is one of the fastest complete conversions for this reaction reported to date. The CO₂ sequestration rate is found to be highly dependent on the applied temperature and pressure, as well as the addition of NaHCO₃. In contrast, adding NaCl was found to have limited effect on the reaction rate. The roles of NaHCO₃ and NaCl as catalysts are discussed and especially how their effect changes with increased olivine particle size. The products have been characterized by Rietveld refinement of powder X-ray diffraction, scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) spectroscopy revealing the formation of amorphous silica and micrometer-sized magnesium carbonate crystals