Surface Energetics of Nanoscale LaMnO3+δ Perovskite

Perovskite type LaMnO₃ and related materials are important compounds with many useful and unique physical and chemical properties. There is a lack of experimental thermochemical data on the energetics of LaMnO₃ nanoparticles. In this work, a series of LaMnO_3+δ samples were synthesized via the citrate method and calcined at 700°C–1050°C. All samples displayed rhombohedral structure (X‐ray diffraction) with similar oxygen stoichiometry 3+δ = 3.16–3.18 (iodometric titration coupled with gravimetric analysis). The BET surface area varied from null for bulk sample to 6.88 ± 0.08 × 10³ m²/mol for the sample calcined at 700°C. The water content varied linearly with the surface area with the highest value being 2.34 wt%. The chemisorbed water adsorption enthalpy was ?63.0 ± 4.1 kJ/mol with the chemisorbed water coverage of 8 H₂O/nm². High‐temperature oxide melt drop solution calorimetry, performed in sodium molybdate at 702°C, yielded enthalpy of formation from La₂O₃, Mn₂O₃, and O₂ of bulk LaMnO_3.16 of ?77.85 ± 1.94 kJ/mol. After correction of drop solution enthalpies of nanometric samples for water content, the calorimetric data were used to calculate the surface energy of LaMnO_3+δ. The energy of the anhydrous surface was 2.27 ± 0.29 J/m², and that of the hydrous surface was 2.02 ± 0.27 J/m². These values are higher than the surface energies of LaMnO_3.00 predicted elsewhere by theoretical methods, probably due to the different oxygen content and possibly more complex surface structure and exposed surface planes. The measured surface energy of LaMnO_3+δ lies between the values reported recently for BaTiO₃ and PbTiO₃ and close to the reported values for MnO₂. This suggests that LaMnO_3+δ surface is predominantly MnO₂‐terminated, in line with the trends predicted by theoretical calculations.     

Direct Measurement of Fusion Enthalpy of LaAlO3 and Comparison of Energetics of Melt,Glass, and Amorphous Thin Films

Enthalpy of fusion and melting temperature of perovskite LaAlO₃ were measured using thermal analysis method as 124 ± 10 kJ/mol at 2134 ± 10°C, providing a value of 52 ± 4 J·(mol·K)^?1 for entropy of fusion. Crystallization enthalpy of amorphous LaAlO₃ thin films was found to change from ?24 to ?17 kJ/mol with decrease in film thickness from 100 to 20 nm. Differences in energetics of amorphous LaAlO₃ films and glass cannot be explained exclusively by surface energy contribution but must reflect differences in structure between films and glasses in this system.     

On the Effect of Lithium on the Energetics and Thermal Stability of Nano‐Sized Nonstoichiometric Magnesium Aluminate Spinel

The thermal stability of Li‐doped nonstoichiometric nano‐sized magnesium aluminate spinel, synthesized using a combustion synthesis method, was studied using XRD, FTIR, and high‐temperature differential scanning calorimetry. Li content within the magnesium aluminate spinel was determined to be a function of crystallite size and stoichiometry. For smaller crystallite sizes and higher Mg deficits, a greater amount of lithium could be incorporated into the structure as a solid solution between LiAl₅O₈ and MgO·nAl₂O₃ spinel, where n is the ratio between Al₂O₃ and MgO. By assessing the intensities of the IR γ_1, γ_2, and γ₅ modes, the degree of structural disorder (i.e., the inversion parameter and lithium occupancy) was defined. The results indicated that the as‐synthesized materials were heavily disordered. The surface enthalpy of the MgO·1.06Al₂O₃, 1.51 ± 0.15 J/m², is in good agreement with the reported value for the same composition, 1.8 ± 0.3 J/m², measured using high‐temperature drop solution calorimetry_. The surface enthalpies of MgO·1.21Al₂O₃ and 0.20 at.% Li–MgO·1.21Al₂O₃ were 1.17 ± 0.15 and 1.05 ± 0.12 J/m, respectively.     

CaTiO3 linear dielectric ceramics with greatly enhanced dielectric strength and energy storage density

CaTiO₃ is a typical linear dielectric material with high dielectric constant, low dielectric loss, and high resistivity, which is expected as a promising candidate for the high energy storage density applications. In the previous work, an energy density of 1.5 J/cm³ was obtained in CaTiO₃ ceramics, where the dielectric strength was only 435 kV/cm. In fact, the intrinsic dielectric strength of CaTiO₃ is predicted as high as 4.2 MV/cm. Therefore, it should be a challenge issue to enhance the dielectric strength and energy storage density of CaTiO₃ ceramics by optimizing the microstructures. In the present work, dense CaTiO₃ ceramics with fine and uniform microstructures are prepared by spark plasma sintering, and the greatly enhanced dielectric strength (910 kV/cm) and energy storage density (6.9 J/cm³) are obtained. This can be ascribed to the improved resistivity and thermal conductivity, associated with the fine and uniform microstructures. The different post‐breakdown features of CaTiO₃ ceramics prepared by different process well interpret why the enhanced dielectric strength is achieved in the SPS sample. The energy storage density can be further improved to 11.8 J/cm³ by introducing the amorphous alumina thin films as the charge blocking layer, where the dielectric strength is 1188 kV/cm.     

Preparation and characterization of nanocrystalline and mesoporous strontium titanate thin films at room temperature

The low temperature perovskite-type strontium titanate (SrTiO₃) thin films and powders with nanocrystalline and mesoporous structure were prepared by a straightforward particulate sol–gel route. The prepared sol had a narrow particle size distribution with hydrodynamic diameter of about 17 nm. X-ray diffraction (XRD) revealed that the synthesized powders had a perovskite-SrTiO₃ structure with preferable orientation growth along the (1 0 0) direction. TEM images showed that the average crystallite size of the powders annealed in the range 300–800°C was around 8 nm. FE-SEM analysis and AFM images revealed that the deposited thin films had mesoporous and nanocrystalline structure with the average grain size of 25 nm at 600°C. Based on Brunauer–Emmett–Taylor (BET) analysis, the synthesized powders showed mesoporous structure with BET surface area in the range 92–75 m²/g at 400–600°C. One of the smallest crystallite sizes and one of the highest surface areas reported in the literature were obtained, which can be used in many applications, such as photocatalysts.     

Anisotropic Grain Boundary Mobility in Undoped and Doped Alumina

The grain boundary mobility of polycrystalline alumina (α‐Al₂O₃), and the effective grain boundary mobility of the basal (0001) plane as it grew into polycrystalline alumina, was determined for undoped alumina, alumina doped with 23 ppm MgO, and alumina doped with 13 ppm CaO at 1600°C. Doping with MgO at a level below the solubility limit decreased the grain‐boundary mobility from 2.7 × 10^?15 to 1.5 × 10^?15 m²/s, and doping with CaO at a level below the solubility limit increased the mobility to 3.5 × 10^?15 m²/s. For the undoped samples at 1600°C, the activation energy for the average grain boundary mobility was 372 ± 39 kJ/mol. The mobility of the (0001) plane growing into alumina doped with MgO at a level below the solubility limit decreased to 1.1 × 10^?15 m²/s compared with the mobility of the (0001) plane growing into undoped alumina (2.5 × 10^?15 m²/s), and the mobility of the (0001) plane growing into alumina doped with CaO (below the solubility limit) increased to 3.2 × 10^?15 m²/s. The activation energy for the mobility of the (0001) plane was 483 ± 76 kJ/mol. Although a measured Ca excess of 2.6 Ca/nm² at the boundary between the (0001) plane and CaO‐doped alumina is correlated with an increased mobility, the platelike morphology of CaO‐doped polycrystalline alumina is associated with an increased mobility of nonbasal planes.     

Microstructure and dielectric properties of SrTiO3 ceramics by controlled growth of silica shells on SrTiO3 nanoparticles

SrTiO₃@SiO₂ nanopowder was synthesized via a core-shell nano-scale technique that is known as the Stöber process. The effect of the SiO₂ concentration on microstructure, dielectric response and energy storage properties of SrTiO₃@SiO₂ ceramics was investigated. Transmission electron microscopy (TEM) results confirmed the formation of core–shell nanostructures with controlled shell thicknesses between 2 nm and 13 nm. After increasing SiO₂, a secondary phase with Sr₂TiSi₂O₈ appeared due to inter-diffusion reactions between the SrTiO₃ core and SiO₂ shells during the sintering process. The results show that both breakdown strength and energy density improved apparently. The homogeneous coating of silica on ST cores is considered to dominate the contribution to improved breakdown strength. The composition for SrTiO₃ coated with 2.5 wt% SiO₂ shows the maximum energy storage density (1.2 J/cm³) and a breakdown strength of 310 kV/cm. The former is higher than for pure SrTiO₃ (0.19 J/cm³). Measurements of the dielectric performance indicate that the SrTiO₃@SiO₂ ceramics possess good bias stabilities compared to pure ST ceramics.     

Thermo-analysis of nanocrystalline TiO2 ceramics during the whole sintering process using differential scanning calorimetry

The thermodynamics of nanocrystalline TiO₂ ceramics during the whole sintering process were analyzed using differential scanning calorimetry (DSC) for different heating rates (10, 20 and 30 °C/min). The raw TiO₂ powder was also studied comparably. The DSC and specific heat capacity (C_p) were also studied. The results show that there is no obvious endothermic or exothermic peak at the stage where the maximum densification rate occurred for TiO₂ ceramics. The ordering process induced by the microstructural densification counteracts the disordering process induced by increasing the sintering temperature. The sintering process is a result of combination of the ordering process and the disordering one. The activation energy of nanometer TiO₂ ceramics determined by Kissinger method is 103.8 kJ/mol.     

Enthalpy of formation and dehydration of lithium and sodium zeolite beta

Zeolite Li-BEA and Na-BEA with Si/Al = 3–4 were synthesized by alumination and ion exchange, then characterized by XRD, TG–DSC and NMR. The enthalpies of formation and dehydration of Li and Na ion exchanged zeolite beta are investigated by high temperature oxide melt solution calorimetry. For Li-BEA, the formation enthalpies of formation from oxides at 25 °C are 25.6 ± 1.7 kJ/mol TO₂ for the dehydrated zeolite and −8.45 ± 0.94 kJ/mol TO₂ for the fully hydrated zeolite; for Na-BEA they are −2.4 ± 0.6 kJ/mol TO₂ for the dehydrated and −17.8 ± 1.0 kJ/mol TO₂ for the fully hydrated zeolite. The integral dehydration enthalpy at 25 °C is 33.2 ± 1.8 kJ/mol H₂O for Li-BEA and 16.5 ± 1.1 kJ/mol H₂O for Na-BEA. The partial molar dehydration enthalpies of both Li-BEA and Na-BEA are a linear function of water content. Molecular mechanics simulations explore the cation and water molecule positions in the framework at several water contents.     

Nonstoichiometry and relaxation kinetics of nanocrystalline mixed praseodymium–Cerium oxide Pr0·7Ce0·3O2−x

The oxygen deficiency and kinetics of oxygen uptake and release of nanocrystalline mixed praseodymium–cerium oxide with composition Pr_0·7Ce_0·3O_2−x were investigated by combining coulometric titration and potentiometric measurements using stabilised zirconia oxygen concentration cells. The P(O₂) versus composition isotherms indicate a two-phase region at high P(O₂) [P(O₂)>0·1 bar at 560°C] and a single-phase region at lower P(O₂). The oxygen pressure dependence in the homogeneous region can be described by a power law with an exponent (−1/6), in accordance with doubly charged oxygen vacancies as majority defects. The enthalpy of reduction amounts to (2·9±0·3) eV. The chemical diffusion coefficients are of the order of 10⁻⁶ cm² s⁻¹ at 640°C with an activation energy of ≈0·3 eV. The low activation energy for diffusion may be related to the high density of interface sites in the nanocrystalline material.     

Oxidation of ethane on high specific surface SmCoO3 and PrCoO3 perovskites

An adapted sol–gel method allowed synthesizing SmCoO₃ and PrCoO₃ oxides with high specific surface (ca. 28 m² g⁻¹) and a relatively clean perovskite phase at 600 °C, a temperature much lower than the one required in ceramic methods. The perovskites were investigated as catalysts for the oxidation of ethane in the temperature range 300–400 °C. Both catalysts were very active: ethane was activated already at 300 °C, i.e., 100 °C below the temperatures previously reported for perovskites. The main product was CO₂ on both catalysts, but on PrCoO₃ oxidehydrogenation (ODH) to ethylene was observed already at 300 °C, with the low selectivity. Even so, this was quite unusual for simple perovskites, and for such a low temperature. TPR data showed that praseodymium decreases the reducibility of Co³⁺ in the perovskite, what could explain the observed ODH, and suggest it proceeds via a Mars–van Krevelen mechanism. Kinetic study showed a similar apparent activation energy for both catalysts (ca. 80 kJ/mol), but a difference in the nature of the participating oxygen species: while on PrCoO₃ both adsorbed and lattice species contribute to the reaction, on SmCoO₃ contribution of adsorbed species is practically negligible, due to its very high oxygen lability. The results show that these simple perovskites may be promising catalysts for ethane oxidation at relatively low temperatures.     

Low‐temperature synthesis of nanocrystalline hydroxyapatite: Effect of Mg and Sr content

Nanocrystalline Mg‐ or Sr‐containing hydroxyapatite powders were synthesized through low‐temperature chemical precipitation. The most significant factor for reduction in particle sizes included adjusting the reaction temperature between 0°C and 50°C. Syntheses products were characterized using several analytical tools to determine purity and influence of added amount (up to 15 mol%) of Mg or Sr on the composition and structure. Qualitative analysis by Fourier transform infrared spectroscopy and low intensity, broad X‐ray diffraction peaks indicated the presence of nanocrystalline and/or amorphous hydroxyapatite in all the products. Moreover, a significant decrease in the crystallinity was observed with increasing Mg (up to 2.8 ± 0.3 wt%) and Sr (up to 9.6 ± 1.0 wt%) concentration. N₂ adsorption and scanning electron microscopy characterizations confirmed the nanocrystalline nature of the synthesized products. The synthesized products had nanosized spherical‐like particle morphology with the specific surface area ranging from 89 ± 7 to 150 ± 20 m²/g.     

High Reactive MgO‐Y2O3 Nanopowders via Microwave Combustion Method and Sintering Behavior

To decrease the sintering temperature of MgO‐Y₂O₃ composites to avoid undesired grain coarsening, high reactive MgO‐Y₂O₃ nanopowders were synthesized via microwave combustion method. The degree of combustion was enhanced effectively by adding an extra oxidant ammonium nitrate. The as‐synthesized MgO‐Y₂O₃ nanopowders, ~18 nm in size, showed high specific surface area of 64.55 m²/g and low agglomeration. Relative density of 98% was obtained when sintered at a low sintering temperature of 1350°C. The high reactivity can be attributed to the lower activation energy Q (131.13 kJ/mol), compared with samples without extra oxidant (192.97 kJ/mol).     

Hydrothermal synthesis of Litchi-like SrTiO3 with the help of ethylene glycol

Templated by TiO₂ microspheres , litchi-like SrTiO₃ crystals with a narrow size distribution and monodispersity were synthesized through the combination of regulating the ethylene glycol concentration during the hydrothermal process and the post heat treatment. The results show that when the volume ratio of water and ethylene glycol reached 10:70, microsized SrCO₃ was firstly formed under the hydrothermal process, and then the litchi-like SrTiO₃ powder was obtained after the postheat treatment at 700°C for 4 hours, which shows a large specific surface area of 37.41 m²/g. It is found that the concentration of ethylene glycol aqueous solution plays an important role on the morphology of the SrTiO₃ crystals, and the slightly higher viscosity and proper amount of OH hydroxyl groups facilitate the formation of the micrometer spherical hierarchical surface.     

Construction of CaTiO3 nanosheets with boron nitride quantum dots as effective photogenerated hole extractor for boosting photocatalytic performance under simulated sunlight

Herein, titanate-based perovskite CaTiO₃ nanosheets were successfully designed via boron nitride quantum dots (BNQDs) to fabricate CaTiO₃/BNQDs catalyst. The as-fabricated composite catalysts were analysed by transmission electron microscope (TEM), scanning electron microscopy coupled with energy dispersive spectrometry (SEM-EDS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), X-ray diffraction (XRD), UV–vis spectroscopy (UV-DRS), photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) techniques. SEM-Mapping analysis showed that the boron and nitrogen elements dispersed well over the CaTiO₃ surface which was useful for building electronic channels for rapid transport of photo-induced charge pairs. TEM images verified the attachment of BNQDs around the surface of host CaTiO₃ forming intimate interface while the distribution of chemical states was observed by XPS analysis demonstrating strong coupling effect between BNQDs and CaTiO₃ through Ti–O–N and Ti–O–B bonds. Moreover, PL and light absorption properties enhanced with the quantum confinement effect of BNQDs. As expected, the photocatalytic degradation rate of CaTiO₃/BNQDs was increased to k_app = 0.015 min^{? 1} with optimum BNQDs loading, which was 2.31 times folder than that of bare CaTiO₃ (0.006 min^{? 1}). The enhanced photocatalytic efficiency was observed for CaTiO₃/BNQDs than pristine perovskite on account of formation of electron tapping sites, decreased band gap energy and hindered recombination rate. On the other hand, in the presence of H₂O₂, the degradation percentage increased from 88.5% to 92.1% at the end of 120 min of irradiation while 96.8% of TC was quickly degraded within 60 min after activating with peroxymonosulfate which created strong sulphate radicals. Radical trapping tests indicated that the photo-generated holes were the primary active species in the photocatalytic mechanism. Moreover, CaTiO₃/BNQDs catalyst showed excellent stability in recycling tests. Besides, the possible degradation mechanism was proposed. This study shed light on the significance of BNQDs in the enhancement of the photocatalytic activities of titanate-based perovskite for effective degradation of tetracycline antibiotic in contaminated water.     

High‐temperature calorimetric study of oxide component dissolution in a CaO–MgO–Al2O3–SiO2 slag at 1450°C

Blast‐furnace slags are formed, as iron ore is reduced to metal, as a molten a mixture of refractory and not easily reducible oxides, largely silica, alumina, lime, and magnesia. Their relatively low silica content makes them basic and poor glass formers. Their thermodynamic properties, though important for modeling their formation and reactivity, as well as furnace heat balance, are poorly known. Solution calorimetry of small amounts of solid oxides in a molten oxide solvent at high temperature (up to about 1500°C) permits direct assessment of energetics of dissolution. The enthalpies of solution of slag forming oxides: CaO, SiO₂, Al₂O₃, MgO, and Fe₂O₃ in a simplified model slag of composition: CaO (45.9 mol%), SiO₂ (35.1 mol%), Al₂O₃ (8.3 mol%), MgO (10.7 mol%) were measured by high‐temperature drop solution calorimetry at 1450°C. For this slag composition, enthalpies of solution become more exothermic in the order: Fe₂O₃ (279.3 ± 20.8 kJ/mol), MgO (56.7 ± 9.1 kJ/mol), Al₂O, (41.6 ± 11.3 kJ/mol), CaO (?4.3 ± 2.3 kJ/mol), and SiO₂, (?20.4 ± 4.4 kJ/mol), reflecting the relatively basic character of this low‐silica melt. Within these fairly large experimental errors, characteristic of calorimetry at this high temperature, there is little or no discernible concentration dependence for these heats of solution. The trends seen for these five solutes parallel those seen for heats of solution of the same oxides in other melts at various temperatures, with changes in magnitude reflecting the differences in acid‐base character of the melts. The new data for quartz show systematic behavior which extends the range of basicity studied for the enthalpy of dissolution of silica. The results provide reliable data for future modeling of the thermal balance of steel‐making furnaces and geologic and ceramic systems.     

Microwave‐assisted Hydrothermal Synthesis of Single‐crystal Nanorods of Rhabdophane‐type Sr‐doped LaPO4·nH2O

A novel, simple, soft, and fast microwave‐assisted hydrothermal method was used for the preparation of single‐crystal nanorods of hexagonal rhabdophane‐type La_1?xSr_xPO_4?x/2·nH₂O (x = 0 or 0.02) from commercially available La(NO₃)₃·6H₂O, Sr(NO₃)₂, and H₃PO₄. The synthesis was conducted at 130°C for 20 min in a sealed‐vessel microwave reactor specifically designed for synthetic applications, and the resulting products were characterized using a wide battery of analytical techniques. Highly uniform, well‐shaped nanorods of LaPO₄·nH₂O and La_0.98Sr_0.02PO_3.99·nH₂O were readily obtained, with average length of 213 ± 41 nm and 102 ± 25 nm, average aspect ratio (ratio between length and diameter) of 21 ± 9 and 12 ± 5, and specific surface area of 45 ± 2 and 51 ± 1 m²/g, respectively. In both cases, the single‐crystal nanorods grew anisotropically along their c crystallographic‐axis direction. At 700°C, the hexagonal rhabdophane‐type phase has already transformed into the monoclinic monazite‐type structure, although the undoped and Sr‐doped nanorods retain their morphological features and specific surface area during calcination.     

Low thermal conductivity of SrTiO3−LaTiO3 and SrTiO3−SrNbO3 thermoelectric oxide solid solutions

Electron-doped SrTiO₃ has been attracting attention as oxide thermoelectric materials, which can convert wasted heat into electricity. The power factor of the electron-doped SrTiO₃, including SrTiO₃-LaTiO₃ and SrTiO₃-SrNbO₃ solid solutions, has been clarified. However, their thermal conductivity (κ) has not been clearly identified thus far. Only a high κ (>12 W m⁻¹ K⁻¹) has been assumed from the electron contribution based on Wiedemann–Franz law. Here, we show that the κ of the electron-doped SrTiO₃ is lower than the assumed κ, and its highest ZT exceeded 0.1 at room temperature. The κ slightly decreased with the carrier concentration (n) when n is below 4 × 10²¹ cm⁻³. In the case of SrTiO₃-SrNbO₃ solid solutions, an upturn in κ was observed when n exceeds 4 × 10²¹ cm⁻³ due to the contribution of conduction electron to the κ. On the other hand, κ decreased in the case of SrTiO₃-LaTiO₃ solid solutions probably due to the lattice distortion, which scatters both electrons and phonons. The highest ZT was 0.11 around n = 1 × 10²¹ cm⁻³. These findings would be useful for the future design of electron-doped SrTiO₃-based thermoelectric materials.     

Thermal behavior of Al/MoO3 xerogel nanocomposites

Sol–gel process using molybdenum alkoxides was employed to prepare Al/MoO₃ xerogel nanocomposites as a thermite with better performance by improvement of interfacial contact area between the oxidizer and fuel. Micromorphology and thermite reaction characteristics of Al/MoO₃ xerogel nanocomposites were analyzed by scanning electron microscopy (SEM) and thermogravimetry/differential scanning calorimetry (TG/DSC), respectively. In the present Al/MoO₃ xerogel system, it was found that exothermic enthalpy increases as the Al/Mo mole ratio increases and then decreases when Al/Mo mole ratio is larger than 6 indicating that optimum mole ratio of Al/Mo is 6 with reaction enthalpy of 420.58 J/g.     

Mesoporous silica synthesis: Energetics of interaction between framework and structure directing agent

The thermodynamics of mesoporous silicas (MCM-41, MCM-48, SBA-15, and SBA-16) were studied by solution calorimetry at 323 K in 25% aqueous HF. The enthalpies of formation were determined for calcined mesoporous silica (MS) and organic structure-directing agent (SDA) occluded samples (SDA: n-hexadeciltrimethylammonium bromide or CTAB, Pluronic P123, and Pluronic F127). The following are the measured interaction enthalpies between the MS and SDA: MCM-41/CTAB, −6.1 kJ/mol SiO₂; MCM-48/CTAB, −12.3 kJ/mol SiO₂; SBA-15/P123, −19.7 kJ/mol SiO₂; SBA-16/F127, −19.9 kJ/mol SiO₂. Per unit surface area, these interactions are −0.08, −0.15, −0.43, and −0.40 J/m², respectively. Though these SDA-framework interaction energies are still small in magnitude, they are somewhat more exothermic than those in silica zeolite formation, reflecting the greater metastability of the MS materials and the role of the long chain SDA in stabilizing and space-filling the large pores. The cubic MS (SBA) show stronger SDA interactions than the hexagonal (MCM). The interaction energies confirm a complex landscape of many competing structures of similar energy; with the role of SDA kinetic in selecting a specific structure rather than energetic in strongly stabilizing a given state, as has already been noted for zeolites. The enthalpies of the calcined MS relative to quartz determined by HF solution calorimetry in this study are in excellent agreement with those determined previously by high temperature oxide melt solution calorimetry.