Distribution of Σ3 misorientations in polycrystalline strontium titanate

SrTiO₃ is a potential electrolyte material for solid-oxide fuel cells due to its high ion conductivity. Grain boundaries (GBs) play an important role in bulk ion conductivity. It has been reported that the resistance of Σ3 GBs is much lower than the general GB in SrTiO₃. In order to clarify the conflicting reports on the prevalence of Σ3 GBs in SrTiO₃, grain size and GB misorientations in Nb-doped and undoped SrTiO₃ have been investigated as a function of annealing time. The observations suggest that the prevalence of both low-angle GBs and Σ3 GBs is strongly correlated to both abnormal grain growth and annealing time. In particular, the Σ3 GB population approaches that predicted by the Mackenzie random distribution when abnormal grain growth dominates.     

The influence of grain boundary misorientation on ionic conductivity in YSZ

Molecular dynamics (MD) was used to investigate the structure and ion transport properties of three interfaces in 8 mol% yttria-stabilized zirconia (YSZ); namely the Σ5 (310)/[001] and Σ13 (320)/[001] tilt grain boundaries and Σ5 (111) 60° twist grain boundary. Atomic interactions were described by a potential function of the Buckingham form. Diffusion rates of oxide ions in the grain boundary containing systems showed that the tilt grain boundaries reduce the overall ionic conductivity relative to a single crystal of 8 mol% YSZ, while the Σ5 twist boundary is able to support rapid diffusion and increases the total conductivity. The effect of segregation of dopant ions to the boundary regions was also investigated.     

Polyol hydrogenolysis on supported Pt catalysts: Comparison between glycerol and 1,2-propanediol

Pt-based catalysts supported on TiO₂ and SIRAL 20 (Al₂O₃–20 wt.%SiO₂) were prepared and characterized by H₂ chemisorption, FTIR of adsorbed pyridine and 3,3-dimethyl-1-butene isomerization. The catalysts were evaluated for the transformation under aqueous phase of glycerol and 1,2-propanediol at 210 °C, under 60 bar as total pressure (H₂ atmosphere). Under similar conditions, 1,2-propanediol is easier converted than glycerol, indicating that in the glycerol transformation process in aqueous phase, the 1,2-propanediol reactivity is inhibited by the presence of glycerol. This behavior is explained by a strong adsorption of glycerol compared to 1,2-propanediol on the catalyst surface.     

Glycerol Hydrogenolysis to 1,2-Propanediol by Cu/ZnO/Al2O3 Catalysts

The effect of preparation methods on the Cu/ZnO/Al₂O₃ catalyst structure and catalytic activity on liquid glycerol hydrogenolysis to 1,2-propanediol has been investigated. The physicochemical properties of the catalysts were characterized by BET, XRD, TG/DTA, NH₃-TPD and TPR. The experimental results showed that the catalyst prepared by an oxalate gel–coprecipitation had the highest activity. At 200 °C and 400 psi hydrogen pressure, the glycerol conversion and 1,2-propanediol selectivity catalyzed by the Cu/ZnO/Al₂O₃ catalyst prepared via oxalate gel–coprecipitation were 92.3 and 94.5 % respectively. It was found that the 1,2-propanediol selectivity was dependent on hydrogen pressure and the un-desired by-products were mainly due to the side reactions caused by the presence of the intermediate acetol.     

Glycerol Hydrogenolysis to 1,2-Propanediol by Cu/ZnO/Al2O3 Catalysts

The effect of preparation methods on the Cu/ZnO/Al₂O₃ catalyst structure and catalytic activity on liquid glycerol hydrogenolysis to 1,2-propanediol has been investigated. The physicochemical properties of the catalysts were characterized by BET, XRD, TG/DTA, NH₃-TPD and TPR. The experimental results showed that the catalyst prepared by an oxalate gel–coprecipitation had the highest activity. At 200 °C and 400 psi hydrogen pressure, the glycerol conversion and 1,2-propanediol selectivity catalyzed by the Cu/ZnO/Al₂O₃ catalyst prepared via oxalate gel–coprecipitation were 92.3 and 94.5 % respectively. It was found that the 1,2-propanediol selectivity was dependent on hydrogen pressure and the un-desired by-products were mainly due to the side reactions caused by the presence of the intermediate acetol.

     

Glycerol dehydroxylation in hydrogen on a Raney cobalt catalyst

Glycerol dehydroxylation on a Raney cobalt catalyst in hydrogen was studied. It was found that with an increase in temperature from 140 to 200°C at a hydrogen pressure of 30 MPa, glycerol conversion increases from 14 to 97%. The glycerol is completely converted in 20 h, and the yield of 1,2-propanediol is 40%. With an increase in H₂ pressure from 3 to 8 MPa, the glycerol conversion increases from 34 to 95%, and the yield of 1,2-propanediol increases from 18 to 38%. The maximum yield of 1,2-propanediol is 44% at 200°C and a hydrogen pressure of 3 MPa. The glycerol dehydroxylation in hydrogen on heterogeneous catalysts can be considered a promising method of glycerol conversion when glycerol is a byproduct of biodiesel manufacture from vegetable oil and animal fats.     

Reactive FAST/SPS sintering of strontium titanate as a tool for grain boundary engineering

A high-pressure FAST/Spark Plasma Sintering method was used to produce dense SrTiO₃ ceramics at temperatures of 1050 °C, more than 250 °C below typical sintering temperatures. Combining SPS with solid-state reactive sintering further improves densification. The process resulted in fine-grained microstructures with grain sizes of ∼300 nm. STEM-EDS was utilized for analyzing cationic segregation at grain boundaries, revealing no cationic segregation at the GBs after SPS. Electrochemical impedance spectroscopy indicates the presence of a space charge layer. Space charge thicknesses were calculated according to the plate capacitor equation and the Mott-Schottky model. They fit the expected size range, yet the corresponding space charge potentials are lower than typical values of conventionally processed SrTiO₃. The low space charge potential was associated to low cationic GB segregation after SPS and likely results in better grain boundary conductivity. The findings offer a path to tailor grain boundary segregation and conductivity in perovskite ceramics.     

Hydrogenolysis of Glycerol to Propanediols Over Highly Active Ru–Re Bimetallic Catalysts

Several supported Ru–Re bimetallic catalysts (Ru–Re/SiO₂, Ru–Re/ZrO₂, Ru–Re/TiO₂, Ru–Re/H-β, Ru–Re/H–ZSM5) and Ru monometallic catalysts (Ru/SiO₂, Ru/ZrO₂, Ru/TiO₂, Ru/H-β, Ru/H–ZSM5) were prepared and their catalytic performances were evaluated in the hydrogenolysis of glycerol to propanediols (1,2-propanediol and 1,3-propanediol) with a batch type reactor (autoclave) under the reaction conditions of 160 °C, 8.0 MPa and 8 h. Compared with Ru monometallic catalysts, the Ru–Re bimetallic catalysts showed much higher activity in the hydrogenolysis of glycerol, and Re exhibited obvious promoting effect on the performance of the catalysts. The supported Ru monometallic catalysts and Ru–Re bimetallic catalysts were characterized by N₂ adsorption/desorption, XRD, TEM-EDX, H₂-TPR and CO chemisorption for obtaining some physicochemical properties of the catalysts, such as specific surface areas, crystal phases, morphologies/microstructure, reduction behaviors and dispersion of Ru metal. The results of XRD and CO chemisorption indicate that the addition of Re component could improve the dispersion of Ru species on supports. The measurements of H₂-TPR revealed that the coexistence of Re and Ru components on supports changed the respective reduction behavior of Re or Ru alone on the supports, indicating the existence of synergistic effect between Ru and Re species on the bimetallic catalysts. The hydrogenolysis of some products (such as 1,2-propanediol, 1,3-propanediol, 1-propanol and 2-propanol) were also examined over Ru and Ru–Re catalysts for evaluating influence of Re–Re on the reaction routes during glycerol hydrogenolysis. The results showed that over Ru–Re catalysts, glycerol was favorable to be converted to 1,2-propanediol, but not favorable to ethylene glycol, while 1,2-propanediol and 1,3-propanediol were favorable to be converted to 1-propanol. The influence of glycerol concentration in its aqueous solution on the catalytic performance was also evaluated over Ru and Ru–Re catalysts.     

Effect of nanostructure on the thermal conductivity of La-doped SrTiO3 ceramics

A series of La-doped (10 at.%) SrTiO₃ ceramics with grain size ranging from 6 μm to 24 nm was prepared from nanocrystalline powders using high-pressure field assisted sintering (HP-FAST). A progressive reduction of thermal conductivity κ with decreasing grain size was observed. At room temperature, κ of the ceramic with grain size of 24 nm (1.2 W m⁻¹ K⁻¹) is one order of magnitude lower than that of undoped single crystals. The strong suppression of κ can be ascribed to (i) the high concentration of lattice defects, (ii) the increasing contribution of grain boundaries to phonon scattering when the grain size is decreased to the nanoscale and (iii) a moderate amount (10–15 vol.%) of nanopores. These results demonstrate that nanostructuration can be a successful strategy to attain a considerable reduction of κ in heavily doped bulk oxide ceramics. The low electrical conductivity of the La:SrTiO₃ nanoceramics represents a major obstacle for thermoelectric applications.     

Effect of annealing temperature in carbon powder on thermoelectric properties of the SrTiO3−δ single crystal in the [111] crystal orientation

SrTiO₃ is a promising thermoelectric material for applications in harsh environments, owing to its excellent thermoelectric (TE) properties and high-temperature stability. The effect of annealing in carbon powders on the TE properties of SrTiO_3?δ in the [111] direction was experimentally investigated and analysed using first-principles calculation. The electrical conductivity of the SrTiO_3?δ single crystals in the [111] direction increases with an increase in annealing temperature in the range of 1573–1673 K, which is opposite to that in the [510] direction as shown in a previous experiment. This is attributed to the different variation trend of carrier concentration. First-principle calculation results show that the carrier concentration in the [111] direction increases with increasing oxygen vacancy concentration, but it is the opposite in the [510] direction. Strong anisotropy of carrier concentration should be caused by the difference in atomic arrangement, making difference in the amount of free electrons that maybe constrained at high oxygen vacancy in different directions. Although the SrTiO_3?δ single crystal annealed at 1573 K shows a lower electrical conductivity, it obtains higher power factor with the maximum value of 2.24 mW m^?1 K^?2 at 323 K, which is predominantly due to its higher effective mass. It also yields the highest ZT value of 0.18 at 873 K owing to the lower thermal conductivity. The results of this study are imperative for the design and development of perovskite TE materials.     

Hydrogenolysis of Glycerol to Propanediol Over Ru: Polyoxometalate Bifunctional Catalyst

Ruthenium-doped (5 wt%) acidic heteropoly salt Cs_2.5H_0.5[PW₁₂O₄₀] (CsPW) is an active bifunctional catalyst for the one-pot hydrogenolysis of glycerol to 1,2-propanediol (1,2-PDO) in liquid phase, providing 96% selectivity to 1,2-PDO at 21% glycerol conversion at 150 °C and an unprecedented low hydrogen pressure of 5 bar. Rhodium catalyst, 5%Rh/CsPW, although less active, shows considerable selectivity to 1,3-PDO (7.1%), with 1,2-PDO being the main product (65%).     

Measurement of conductivity profiles in acceptor-doped strontium titanate

Spatially resolved microcontact impedance spectroscopy using circular microelectrodes is applied to acceptor-doped SrTiO₃. It is demonstrated that local bulk conductivities can be obtained with a spatial resolution down to the 10 μm range. In particular, conductivity profiles in electrocolored samples are measured, revealing a very characteristic profile shape. An existing model for the resistance degradation in SrTiO₃ could be confirmed by this experiment. Measurements in polycrystalline material yield conductivity variations within single grains which can be related to oxygen vacancy blocking at grain boundaries and the corresponding stoichiometry variations.     

Vapor-phase hydrogenolysis of glycerol to 1,2-propanediol using a chromium-free Ni-Cu-SiO2 nanocomposite catalyst

A Ni-Cu-SiO₂ nanocomposite was studied as a catalyst for vapor-phase glycerol hydrogenation to produce 1,2-propanediol (1,2-PDO). Substitution of a small amount (3 wt%) of Ni for Cu is beneficial for decreasing the Cu particle size, which would be advantageous for attaining higher 1,2-PDO selectivity and higher glycerol conversion. 92% 1,2-PDO selectivity and 100% glycerol conversion were obtained at 220 °C, 30 bar, and a weight hourly space velocity of 0.5 h^− 1 over Ni(3)-Cu(77)-SiO_2, which were nearly identical to those obtained with the conventional copper chromite (CuO-Cr₂O₃) catalyst. Therefore, the present Ni(3)-Cu(77)-SiO₂ nanocomposite is regarded as a green and efficient catalyst for glycerol conversion into more valuable 1,2-PDO.     

Synthesis and characterization of zirconia toughened alumina ceramics prepared by co-precipitation method

Undoped and 3 mol% yttrium doped ZrO₂–Al₂O₃ composite powders with partially stabilized ZrO₂ (PSZ) content varying from 0 to 30 wt% were prepared by a co-precipitation route using inorganic precursors Al(NO₃)₃, ZrOCl₂ and Y(NO₃)₃. The precipitates were characterized by DTA and subsequently calcined at 1200 °C for 4 h to achieve fine grained composite powders. The calcined powders were characterized by FTIR and XRD. In order to enhance the sinterability, the calcined powders were wet milled in a high energy ball mill. Powders were uniaxially pressed to form pellets and sintered at 1600 °C for 5 h to achieve greater than 96% relative density. Microstructural analysis of the sintered compacts revealed the uniform distribution of the zirconia particles among the alumina matrix. It was also observed that the faceted intergranular zirconia grains were present at the grain boundaries and junctions in the alumina matrix. Vickers indentation was carried out at 1 kgf load for hardness and 2 kgf load for estimating the critical stress concentration factor (K_c). Microscopic studies of the indented samples showed that cracks were propagating around the grain boundaries. Highest K_c ∼8.40 ± 0.4 MPa√m and hardness ∼16.31 ± 0.58 GPa was obtained for the 30 wt% PSZ-Al₂O₃ composite. The sintered density and critical stress intensity factor (K_c) achieved were compararble to that achieved earlier by hot press and SPS.     

Hydrogen-induced degradation in SrTiO3-based grain boundary barrier layer ceramic capacitors

Hydrogen-induced degradation in SrTiO₃-based grain boundary barrier layer ceramic capacitors was studied through electrochemical hydrogen charging, in which the capacitors were placed in 0.01 M NaOH solution with hydrogen deposited on their electrodes from the electrolysis of water. The properties of the capacitors were greatly degraded after 0.5 h of treatment: The capacitance was dramatically decreased and the dielectric loss was dramatically increased over the frequency range of 10²–10⁵ Hz, the leakage current was increased by orders of magnitude. It was proposed that atomic hydrogen diffused relatively easily along the grain boundaries and induced a reduction reaction to the grain boundary layer, which resulted in the degradation observed. Hydrogen-induced degradation is more serious in SrTiO₃-based grain boundary barrier layer ceramic capacitors than in other ceramic capacitors and great efforts should be made to prevent hydrogen-induced degradation in them.     

Effect of SrTiO3 content on the growth of (100−x)(K0.5Na0.5)NbO3–xSrTiO3 lead-free piezoelectric single crystals grown by the solid-state crystal growth method

In the present work, the single crystal growth by solid-state crystal growth of (100?x)(K_0.5Na_0.5)NbO₃–xSrTiO₃, where x?=?0,1,2,3?mol-%, has been examined in order to study the effect of SrTiO₃ content on single crystal growth. Powders were prepared by the conventional mixed oxide method. <001> KTaO₃ seed crystals were buried in the powders, pressed into pellets and sintered at 1100°C for 1, 3, 5 and 10?h. Single crystals of the ceramic compositions grew onto the seeds. For the (K_0.5Na_0.5)NbO₃ sample, both single crystal growth and abnormal grain growth in the matrix began to take place within 1?h. As the amount of SrTiO₃ increased, the onset of both single crystal growth and abnormal grain growth were delayed. The effect of SrTiO₃ addition on the single crystal and matrix grain growth behaviour is explained in terms of the mixed control theory of grain growth.     

Grain growth and segregation in Fe-doped SrTiO3: Experimental evidence for solute drag

In functional ceramics, the impact of dopants on bulk crystals is generally well understood. Their impact on grain boundaries is less well known. The present study investigates the impact of acceptor dopants on grain growth in strontium titanate. Scanning electron microscopy and analytical (scanning) transmission electron microscopy have been used to gain knowledge on Fe segregation behavior, grain sizes, and grain size distributions of SrTiO₃. While undoped microstructures show normal grain growth at low temperatures (<1350 °C), doped microstructures evolve bimodally. With increasing acceptor dopant concentration, an increasing population of small grains develops. It is shown that Fe segregates to the grain boundaries due to its negative charge and a positive boundary potential. Thus, the experimental findings seem to be well explained by the theory of solute drag: The diffusion of segregated defects (‘solutes’) at grain boundaries can retard grain boundary migration.     

Effects of SmF3 addition on aluminum nitride ceramics via pressureless sintering

Aluminum nitride (AlN) ceramics with dense structure, high thermal conductivity, and exceptional mechanical properties were fabricated by pressureless sintering with a novel non-oxide sintering additive, samarium fluoride (SmF₃). The results showed that the use of a moderate amount of SmF₃ promoted significant densification of AlN and removed the oxygen impurity. This led to the formation of fine and isolated secondary phase that cleaned the grain boundaries and increased the contact between AlN grains, remarkably enhancing thermal conductivity. Furthermore, SmF₃ also exhibited grain refinement and grain boundary strengthening effects similar to traditional sintering additive, samarium oxide (Sm₂O₃), leading to high mechanical properties in SmF₃-doped AlN samples. The most optimal characteristics (thermal conductivity of 190.67 W·m⁻¹·K⁻¹, flexural strength of 403.86 ± 18.27 MPa, and fracture toughness of 3.71 ± 0.19 MPa·m^1/2) were achieved in the AlN ceramic with 5 wt% SmF₃.     

Influence of additives on the purity of tetragonal phase and grain size of ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP)

A comprehensive study on the influence of typical additives on zirconia (ZrO₂) crystallization was presented. Zirconium nitrate pentahydrate (Zr(NO₃)₄·5H₂O) and cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O) were employed as reagents, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) or glycerol were adopted as additives, and ammonia water was adopted as pH regulator. The ZrO₂ powders were prepared by hydrothermal method. The crystal phase purity, grain size and micro morphology of the ZrO₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) to investigate the influence of EDTA-2Na, glycerol and Ce⁴⁺ content on the purity of tetragonal phase and the grain size of ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP). It was found that EDTA-2Na could decrease the purity of tetragonal phase and alter the grain size of Ce-TZP nonlinearly, while glycerol could not decrease the purity of tetragonal phase and the grain size of Ce-TZP, and the grain size was not linear with the amount of glycerol; Doping Ce⁴⁺ could increase the purity of tetragonal phase of zirconia but could not decrease the grain size, and the grain size was not linear with the Ce⁴⁺ content; In addition, it was indicated that EDTA-2Na and glycerol could not improve the distribution uniformity of Ce⁴⁺. This study is expected to have provided a novel path to achieve tailored ZrO₂ crystals with reduced low-temperature degradation.     

Processing and thermal properties of SrTiO3 /Ti3AlC2 ceramic nanocomposites

Modulating the thermal conductivity has been a pragmatic approach for the development of high-performance thermoelectric material and thereby a step forward towards commercialization. Despite some efforts, the reduction in thermal conductivity of SrTiO₃ ceramic has not been fully realized. In this work, Ti₃AlC₂ in 3, and 7 vol% were uniformly incorporated in SrTiO₃ through nanostructured powder processing. The pristine SrTiO₃ and composites powders were consolidated by the spark plasma sintering at 1200 °C under uniaxial pressure of 50 MPa. Thermal properties of the bulk samples were evaluated from room temperature to 750 K through laser flash analysis. The thermal conductivity of SrTiO₃ based composites decreases substantially with the addition of nanostructured Ti₃AlC₂ from the pristine SrTiO₃ bulk sample. The reduction in thermal conductivity of 7 vol% composites is more than 30% at room temperature and even higher at elevated temperatures from the SrTiO₃. The interface thermal resistance was estimated which indicates a dominant role in diminishing the thermal conductivities of the composites. The results suggest that the addition of Ti₃AlC₂ as a second phase and nanostructuring through ball milling has significantly altered the phonon scattering mechanisms through multiple factors and thereby contributed to reducing effective thermal conductivities of the composites. This, work provide a scalable and economical route for the development of high-performance thermoelectric material.