Drop Solution Calorimetry Research Articles

Thermodynamics of radiation induced amorphization and thermal annealing of Dy2Sn2O7 pyrochlore

Thermodynamics and annealing behavior of swift heavy ion amorphized Dy2Sn2O7 pyrochlore were studied. Its amorphization enthalpy, defined as the total energetic difference between the irradiation amorphized and undamaged Dy2Sn2O7 states, was determined to be 283.6 ± 6.5 kJ/mol by high temperature oxide melt drop solution calorimetry. It has been an enigma that stannate and some other pyrochlores do not follow the general rA/rB−radiation resistance relation seen in most pyrochlore systems. In this work, we use the amorphization enthalpy, which reflects all the complex chemical and structural characteristics, as a more effective parameter to correlate the radiation damage resistance of pyrochlores with their compositions. It successfully explains the superior radiation damage resistance of the stannate pyrochlores compared with titanate pyrochlores. Differential scanning calorimetry (DSC) reveals a strong exothermic event starting at 978 K, which is attributed to long-range recrystallization based on X-ray diffraction (XRD) analysis, similar to the effect previously observed in Dy2Ti2O7. A second pronounced heat event beginning at ∼1148 K, which results from local structural rearrangement, is clearly decoupled from the first event for irradiated Dy2Sn2O7. Both the heat releases measured by DSC on heating to 1023 and 1473 K, and the excess enthalpies of the annealed samples indicate that the recovery to the original, ordered state was not fully achieved up to even 1473 K, despite XRD showing the apparent restoration of crystalline pyrochlore structure. The remaining metastability may be attributed to local disorder in the form of weberite-like short-range domains in the recrystallized material. Intriguingly, the second event for different pyrochlores generally starts at similar temperatures while the onset of the long range recrystallization is compositionally dependent. The amorphization and thermal annealing behavior observed in irradiated Dy2Sn2O7 may provide insights into the general mechanisms of radiation damage and recovery of pyrochlores relevant to their nuclear applications.

Thermodynamic properties of ZrSiO4 zircon and reidite and of cotunnite-type ZrO2 with application to high-pressure high-temperature phase relations in ZrSiO4

Enthalpies of high-pressure phase transitons in ZrSiO4 were measured by high-temperature drop-solution calorimetry. The enthalpies for the zircon-reidite transition in ZrSiO4 and for dissociation of reidite to cotunnite-type ZrO2 + SiO2 stishovite were obtained to be 27.5 ± 3.3 and 50.3 ± 3.0 kJ/mol, respetively, at 298 K. By thermal relaxation method, low-temperature isobaric heat capacities of ZrSiO4 reidite and cotunnite-type ZrO2 were measured, giving standard entropies at 298 K of 77.29 ± 0.04 and 49.10 ± 0.04 J/mol·K, respectively. Combining the measured enthalpies and entropies with other available thermophysical data, the transition boundaries in ZrSiO4 were calculated. The calculated zircon-reidite transiton boundary is consistent with that by in situ high-pressure expreiments by Ono et al. (2004a) within the errors. The calculated dissociation boundary of reidite has a smaller slope than that by high-pressure quench expreiments by Tange and Takahashi (2004). The equilibrium zircon-reidite transition pressure is calculated to be 8 GPa at 298 K, which is by 12–15 GPa lower than the transition pressures observed by room-temperature static compression experiments. Our results suggest that zircon transforms to reidite and dissociates into the two phases at 330 and 610 km depths, respectively, along the normal mantle geotherm. Combining the calculated phase relations with kinetics estimated from the first principles calculation may provide some insights to P, T conditions which reidites found in impact ejecta layers and craters experienced.

Enthalpy of formation of U3Si2: A high-temperature drop calorimetry study

U3Si2 is presently receiving consideration as a high density light water reactor fuel. A reliable knowledge of the formation enthalpy of U3Si2 not only helps study the thermal stability but also facilitate the modeling efforts by serving as a benchmark parameter for thermodynamic calculations of phase equilibria at high temperatures. Previous high temperature thermal analysis on U3Si2 laid the basis for us to conduct two types of high-temperature drop calorimetric measurements to determine its enthalpy of formation: oxide-melt drop-solution calorimetry and transposed temperature drop calorimetry, from which the results obtained are consistent. The determined standard enthalpy of formation of U3Si2 per mole atom, −33.2 ± 3.1 kJ/mol⋅at.%, is in good agreement with previously reported values obtained by other techniques. Our drop calorimetry methods will be used for thermodynamic studies of other U-Si compounds whose enthalpies of formation are not available.

A thermodynamic investigation of the Li–Sb system

Many standard thermodynamic data in the binary system Li–Sb can be found in literature; however, they are often derived from electrochemical measurements taken at higher temperatures. The uncertainties associated with the extrapolation of these high-temperature data to room temperature are, however, inherently large. Therefore, a comprehensive investigation of the thermodynamic properties in the Li–Sb system was conducted in this work to generate more reliable data. Four different experimental techniques were used for the investigations. The heat capacities for both binary compounds, Li2Sb and Li3Sb, were measured for the very first time. In addition, the enthalpies of formation for both compounds were determined by drop solution and direct reaction calorimetry. Furthermore, Knudsen effusion mass spectrometry was performed to measure partial enthalpies and activities of Sb.

Thermochemical stability of Li–Cu–O ternary compounds stable at room temperature analyzed by experimental and theoretical methods

AbstractCompounds in the Li–Cu–O system are of technological interest due to their electrochemical properties which make them attractive as electrode materials, i. e., in future lithium ion batteries. In order to select promising compositions for such applications reliable thermochemical data are a prerequisite. Although various groups have investigated individual ternary phases using different experimental setups, up to now, no systematic study of all relevant phases is available in the literature. In this study, we combine drop solution calorimetry with density function theory calculations to systematically investigate the thermodynamic properties of ternary Li–Cu–O phases. In particular, we present a consistently determined set of enthalpies of formation, Gibbs energies and heat capacities for LiCuO, Li2CuO2 and LiCu2O2 and compare our results with existing literature.

Enthalpies of formation of layered LiNixMnxCo1–2xO2 (0 ≤ x ≤ 0.5) compounds as lithium ion battery cathode materials

AbstractLayer-structured mixed transition metal oxides with the formula LiNixMnxCo1–2xO2 (0 ≤ x ≤ 0.5) are considered as important cathode materials for lithium-ion batteries. In an effort to evaluate the relative thermodynamic stabilities of individual compositions in this series, the enthalpies of formation of selected stoichiometries are determined by high temperature oxide melt drop solution calorimetry and verified by ab-initio calculations. The measured and calculated data are in good agreement with each other, and the results show that LiCoO2–LiNi0.5Mn0.5O2 solid solution approaches ideal behavior. By increasing x, i. e. by equimolar substitution of Mn4+ and Ni2+ for Co3+, the enthalpy of formation of LiNixMnxCo1–2xO2 from the elements becomes more exothermic, implying increased energetic stability. This conclusion is in agreement with the literature results showing improved structural stability and cycling performance of Ni/Mn-rich LiNixMnxCo1–2xO2 compounds cycled to higher cut-off voltages.

Thermodynamic Investigation of Selected Compositions in the LiNiO2-LiMnO2-LiCoO2 System As Lithium Ion Battery Cathode Materials

The choice of cathode material is critical for determining the performance and safety of lithium ion batteries. Therefore, the development and optimization of new chemistries for cathodes of lithium ion cells is of major importance. Layered structured lithium mixed transition metal oxides with the general formula of LiNixMnyCozO2 (x + y + z = 1) and an α-NaFeO2-type layered structure (space group ) are attractive intercalation type cathode materials which overcome the limitations of LiCoO2, the most commonly used cathode for lithium ion batteries in portable applications. These NMC materials not only are isostructural to LiCoO2, but also offer comparatively higher reversible capacity, lower cost, and improved thermal stability, which are important characteristics for large scale applications. Although the structural characteristics and electrochemical behavior of many promising compositions in the NMC system have been investigated to some extent, there are very few studies devoted to the thermodynamic properties of this class of materials. The enthalpies of formation of LiNi1−xCoxO2 compounds were determined by Wang and Navrotsky [1] using high temperature oxide solution calorimetry and the enthalpy of formation of LiNi1/3Mn1/3Co1/3O2 and its chemically delithiated phases were measured by Idemoto and Matsui [2] using acid solution calorimetry. In the present work, with the aim of investigating the effect of cation substitution on thermodynamic stability of NMC materials, the standard enthalpies of formation of two different series of sol-gel made NMC samples, namely LiNixMnxCo1-2xO2 (0 ≤ x ≤ 0.5) and LiNi0.8-yMnyCo0.2O2 (0 ≤ y ≤ 0.4), are determined by high-temperature oxide-melt drop solution calorimetry. The compositions are selected to include the technologically relevant NMC111, NMC442, NMC532 and NMC622 compounds. It is shown that LiNixMnyCozO2 with layered structure is a nearly ideal solid solution of LiCoO2, LiNiO2 and LiNi0.5Mn05O2. Our calorimetry results show that the standard enthalpies of formation of the compounds from the elements vary almost linearly with composition in the respective area. Furthermore, based on our experimental heat of formation data and available low temperature heat capacity data for LiCoO2, LiNiO2 and LiNi0.5Mn05O2, a model for the variation of the Gibb’s free energy of formation and equilibrium potentials of the NMCs with composition at room temperature is presented. In addition, key electrochemical tests were performed to establish a relationship between the thermodynamic properties and electrochemical performance of the cathode materials. This work should help clarify the structure–property relationships in these compounds. M.Wang, & A. Navrotsky, Solid State Ionics 166 (2004), 167-173.Y.Idemoto & T. Matsui, Solid State Ionics 179 (2008), 625-635.

Calorimetric measurement of the standard enthalpy of formation of ZnSb at 298 K

After a critical review of the literature data on the standard enthalpy of formation of ZnSb, discrepancies between various experimental studies are highlighted. Moreover, experimental values disagree with values calculated by ab initio methods. As many of the experimental methods used suffer from some serious drawbacks, a new determination of the standard enthalpy of formation of ZnSb by an alternative calorimetric method, drop solution calorimetry in liquid tin, has been performed. Two different synthesis methods have been used to obtain a pure ZnSb phase and drop solution experiments were performed at 665 and at 974K. The standard enthalpy of formation values derived from these experiments are − 6.1±2.5kJmol of atoms−1 for the ZnSb sample prepared by ball milling and − 7.9±0.4kJmol of atoms−1 for the ZnSb sample prepared by melting. These results are discussed and compared to literature data.

Enthalpies of formation of polyhalite: A mineral relevant to salt repository

Polyhalite is an important coexisting mineral with halite in salt repositories for nuclear waste disposal, such as Waste Isolation Pilot Plant (WIPP) in Carlsbad, New Mexico. The thermal stability of this mineral is a key knowledge in evaluating the integrity of a salt repository in the long term, as water may release due to thermal decomposition of polyhalite. Previous studies on structural evolution of polyhalite at elevated temperatures laid the basis for detailed calorimetric measurements. Using high-temperature oxide-melt drop-solution calorimetry at 975K with sodium molybdate as the solvent, we have determined the standard enthalpies of formation from constituent sulfates (ΔH°f,sul), oxides (ΔH°f,ox) and elements (ΔH°f,ele) of a polyhalite sample with the composition of K2Ca2Mg(SO4)4·1.95H2O from the Salado formation at the WIPP site. The obtained results are: ΔH°f,sul=−152.5±5.3kJ/mol, ΔH°f,ox=−1926.1±10.5kJ/mol, and ΔH°f,ele=−6301.2±9.9kJ/mol. Furthermore, based on the estimated formation entropies of polyhalite, its standard Gibbs free energy of formation has been derived to be in the range of −5715.3±9.9kJ/mol to −5739.3±9.9kJ/mol. These determined thermodynamic properties provide fundamental parameters for modeling the stability behavior of polyhalite in salt repositories.

Thermodynamic investigation of uranyl vanadate minerals: Implications for structural stability

Understanding the crystal chemistry, materials properties, and thermodynamics of uranyl minerals and their synthetic analogs is an essential step for predicting and controlling the long-term environmental behavior of uranium. Uranyl vanadate minerals are relatively insoluble and widely disseminated within U ore deposits and mine and mill tailings. Pure uranyl vanadate mineral analogs were synthesized for investigation using high-temperature drop solution calorimetry. Calculated standard-state enthalpies of formation were found to be −4928.52 ± 13.90, −5748.81 ± 13.59, and −6402.88 ± 21.01, kJ/mol for carnotite, curienite, and francevillite, respectively. Enthalpies of formation from binary oxides for uranyl vanadate minerals exhibit a positive linear correlation as a function of the acidity of oxides. Normalized charge deficiency per anion (NCDA) is presented to relate bonding requirements of the structural units and interstitial complexes. An exponential correlation was observed between NCDA and energetic stability (enthalpy of formation from binary oxides) for the studied minerals. Additionally, NCDA and oxide acidity exhibit an exponential correlation where decreasing oxide acidity results in an exponential decrease in NCDA. The number of occurrences of uranyl vanadate mineral species are found to correlate with both enthalpy of formation from oxides and NCDA.

Standard enthalpy of reaction for the reduction of Co3O4 to CoO

Two different high-temperature calorimetry methods were used to determine the enthalpy of reaction for the reduction of Co3O4 to CoO. This reaction is particularly interesting for assessing the thermodynamics of the first step of the electrochemical half-cell reaction for conversion anodes based on Co3O4 at very low discharge currents. The enthalpy of reaction for the reduction of Co3O4 to CoO was determined to be ΔrH°=205.14±2.48kJ/mol and ΔrH°=204.46±3.17kJ/mol from transposed temperature drop and high-temperature oxide melt drop solution calorimetry, respectively. Using this data, the enthalpy of reaction for the first step of the half-cell reaction for Co3O4 at very low discharge currents involving the formation of Li2O and CoO was calculated as ΔrH°=−393.60±3.67kJ/mol and ΔrH°=−394.28±4.16kJ/mol from the transposed temperature drop and the high-temperature oxide melt drop solution calorimetry, respectively.

Phase stability in scandia‐zirconia nanocrystals

AbstractScandia‐zirconia system has great technological interest as it has the highest ionic conductivity among doped zirconia ceramics. However, polymorphism is the most important limiting factor for application of this material. Considering that there is a strong grain size dependence on phase transitions in this class of materials, mapping out the stable polymorph as a function of grain size and composition may lead to more efficient compositional design. In this article, the phase stability of zirconia‐scandia nanocrystals was investigated based on experimental thermodynamic data. Exploiting recent advances in microcalorimetry, reliable surface energy data for five polymorphs of scandia‐zirconia system: monoclinic, tetragonal, cubic, rhombohedral β and γ are reported for the first time. Combining surface energy values with bulk enthalpy data obtained from oxide melt drop solution calorimetry allowed us to create a predictive phase stability diagram that shows the stable zirconia polymorph as a function of composition and grain size of the specimen within the range of 0‐20 mol% scandia.

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

AbstractBlast‐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.

The Li-Sb phase diagram part I: New experimental results

Although several versions of the phase diagram for the Li-Sb system exist, and critical assessments of the data have been performed, there is still a lack of fundamental experimental data, especially with regard to the melting behavior of the liquid phase. Therefore, the aim of this work is to establish the phase relations in the Li-Sb system based on reliable experimental techniques. 27 Li-Sb samples were synthesized and powder X-Ray diffraction, differential thermal analysis and drop solution calorimetry were performed. On the basis of the obtained results, the Li-Sb phase diagram was reconstructed. In particular, the transition temperatures for the monovariant and invariant reactions were revised to be in agreement with the experimental data. The existence of two intermetallic compounds Li3Sb and Li2Sb and their reported structures could also be confirmed. However, the cubic to hexagonal transition temperature of Li3Sb was omitted, since the cubic polymorph was found to be a meta-stable, pressure stabilized modification.

Enthalpies of Formation of Layered LiNixMnxCo1-2xO2 Compounds As Promising Li-Ion Battery Cathode Materials

Layer structured lithium mixed transition metal oxides with the general formula of LiNixMnyCozO2 (x+y+z=1) and theoretical capacity of approximately 280 mAh/g are considered as promising intercalation type positive electrode materials for Lithium-ion batteries. Owing to their lower costs and higher capacities, LiNixMnyCozO2 (NMCs) are extensively investigated for electric vehicles and grid storage applications as suitable substitutes for LiCoO2, the most widely used cathode material for Li-ion batteries. Several studies have been devoted to investigating the structure and electrochemical behavior of different compositions in the LiNixMnxCo1-2xO2 (0≤x≤0.5) solid solution series. However, there are very limited thermochemical data reported on these compounds. The enthalpies of formation of LiNi1−xCoxO2 compounds were determined by Wang et al. [1] using high temperature oxide solution calorimetry and the enthalpy of formation of LiNi1/3Mn1/3Co1/3O2 and its delithiated phases were measured by Idemoto et al. [2] using acid solution calorimetry. The aim of this work is therefore to determine the enthalpy of formation of selected compositions in LiNixMnxCo1-2xO2(0≤x≤0.5) series by calorimetric measurements in order to study the relative thermodynamic stabilities of these phases and further clarify structure–property relationships in these compounds. In this study, single phase LiNixMnxCo1-2xO2 for x=1/6, 1/3, 0.4, 0.5 were synthesized according to the sol-gel method using metal acetates and adipic acid as a chelating agent. The chemical compositions of the samples after the final heat treatment were measured by inductively coupled optical emission spectroscopy (ICP-OES). In addition, powder X-ray diffraction (XRD) was performed to determine phase impurities and lattice parameters of the respective compounds. Subsequently, the standard enthalpies of formation of the compounds were measured by employing high temperature oxide melt drop solution calorimetry using a Setaram AlexSys-1000 Tian–Calvet calorimeter (Setaram Instrumentation, Caluire-et-Cuire, France) with a sodium molybdate (3Na2O.4MoO3) solvent maintained at approximately 700°C. The powder-XRD results show that all compositions were single phase adopting the α-NaFeO2 type structure (space group: , No. 166). As x in LiNixMnxCo1-2xO2 increases, the a and c lattice parameters increase whereas the c/a ratio decreases. This can be explained by the difference in ionic radii between Ni2+ and Co3+ and the fact that Co3+ which is situated on the transition metal layer is continuously substituted by equal amounts of Mn4+ and Ni2+. The results of our high temperature oxide melt solution calorimetry measurements show that the standard enthalpies of formation of the LiNixMnxCo1-2xO2 compounds from the elements become more exothermic with increasing x. Therefore, contrary to the work of Idemoto et at. [2], we show that the investigated NMC compounds are energetically more stable than LiCoO2. The increasing thermodynamic stability is caused by the substitution of Co3+ with equal amounts of Mn4+ and Ni2+cations on the transition metal layer (3b sites). Keyword: Lithium nickel manganese cobalt oxide, Layered structure, Oxide solution calorimetry, Enthalpy of formation, Lithium-ion battery References Wang, M. & Navrotsky, A. Enthalpy of formation of LiNiO2, LiCoO2 and their solid solution, LiNi1-xCoxO2. Solid State Ionics 166 (2004), 167-173.Idemoto, Y., & Matsui, T. Thermodynamic stability, crystal structure, and cathodic performance of Lix(Mn1/3Co1/3Ni1/3)O2 depend on the synthetic process and Li content. Solid State Ionics 179 (2008), 625-635.

Thermodynamic Stability of Low‐k Amorphous SiOCH Dielectric Films

Si–O–C‐based amorphous or nanostructured materials are now relatively common and of interest for numerous electronic, optical, thermal, mechanical, nuclear, and biomedical applications. Using plasma‐enhanced chemical vapor deposition (PECVD), hydrogen atoms are incorporated into the system to form SiOCH dielectric films with very low dielectric constants (k). While these low‐k dielectrics exhibit chemical stability as deposited, they tend to lose hydrogen and carbon (as labile organic groups) and convert to SiO2 during thermal annealing and other fabrication processes. Therefore, knowledge of their thermodynamic properties is essential for understanding the conditions under which they can be stable. High‐temperature oxidative drop solution calorimetry measurement in molten sodium molybdate solvent at 800°C showed that these materials possess negative formation enthalpies from their crystalline constituents (SiC, SiO2, C, Si) and H2. The formation enthalpies at room temperature become less exothermic with increasing carbon content and more exothermic with increasing hydrogen content. Fourier transform infrared spectroscopy (FTIR) spectroscopy examined the structure from a microscopic perspective. Different from polymer‐derived ceramics with similar composition, these low‐k dielectrics are mainly comprised of Si–O(C)–Si networks, and the primary configuration of carbon is methyl groups. The thermodynamic data, together with the structural analysis suggest that the conversion of sp2 carbon in the matrix to surface organic functional groups by incorporating hydrogen increases thermodynamic stability. However, the energetic stabilization by hydrogen incorporation is not enough to offset the large entropy gain upon hydrogen release, so hydrogen loss during processing at higher temperatures must be managed by kinetic rather than thermodynamic strategies.

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 γ1, γ2, 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.

Phase Stability in Calcia‐Doped Zirconia Nanocrystals

Calcia‐doped zirconia exhibits all of the polymorphism seen in the yttria‐doped zirconia ceramics, but can be produced at lowered costs and in greater abundance due to the accessibility of Calcium precursors in comparison to Yttrium. Although with great challenges, there exists an opportunity to replace yttria with calcia in applications such as ionic conductors where phase stability is critical. There is a dearth of surface characterization to enable design and prediction of the polymorphism in nanoparticulate calcia–zirconia. With recent advances in water adsorption microcalorimetry, one can accurately probe surface energies of the four zirconia polymorphs: monoclinic, tetragonal, cubic, and amorphous. The surface energies can then be coupled with bulk enthalpies extracted from oxide melt drop solution calorimetry to create a nanocrystalline phase stability diagram similar to its bulk counterpart. We report here the surface and bulk thermodynamic data on polymorphs of calcia–zirconia with composition ranging from 0 to 20 mol% calcia and use it to build a nanophase diagram for this system. The effect of the humidity in the phase stability diagram trends is also addressed and demonstrated to minimize the effect of the surface energies in the overall polymorphism trends.

Experimental determination of the thermodynamic properties of the Laves phases in the Cr–Fe–Nb system

Standard enthalpies of formation, ΔfH, of the Laves-phases within their solid solutions along the Cr2Nb–Fe2Nb section were measured using drop solution calorimetry for the first time. The enthalpy values change linearly from −2.9kJmol−1atom−1 for binary C15-Cr2Nb to −13.6kJmol−1atom−1 for C14-Fe2Nb. The obtained values are compared with some earlier results on the binary compounds gathered mostly from ab initio calculations.

Thermodynamic Stability of SnO2Nanoparticles: The Role of Interface Energies and Dopants

The stability of nanoparticles is strongly dependent on the thermodynamics of interfaces. Providing reliable data on surface and grain boundary energies is therefore of key importance for predicting and improving nanostability. In this work, we used a combination of high-temperature oxide melt drop solution calorimetry and water adsorption microcalorimetry to demonstrate the effect of a dopant (manganese) on both surface and grain boundary energies of SnO2, and discussed the impacts on the average particle size at a given temperature. The results show a significant decrease in the grain boundary energy with increasing manganese content and a concomitant moderate decrease in the surface energy, consistently with segregation enthalpy values acquired from an analytical fitting model. The results explain the measured increase in stability with increasing dopant content (smaller sizes) and suggest the grain boundary energy has a much more important role in defining particle stability than previously supposed.