Thermal stability, microstructure and crystallization kinetics of melt-spun Zr-Ti-Cu-Ni metallic glass

Melt-spun Zr_63.33Ti_8.89Cu_15.45Ni_12.33 glassy ribbons display a double-step devitrification behavior characterized by the precipitation of a metastable quasicrystalline phase in the first stage of the crystallization process, followed by the formation of crystalline phases in the following crystallization event. Investigation of the crystallization kinetics reveal that the initial part of the glass-to-quasicrystalline transformation (x ≤ 55 vol.%) occurs by diffusion controlled growth with an increasing nucleation rate (Avrami exponent n ≥ 2.5), whereas the later stage of the transformation (x > 55 vol.%) is dominated by the growth of the formed nuclei rather than by the generation of new nuclei (2.0 ≤ n ≤ 2.5). The activation energy for quasicrystallization is 360 kJ/mol, which is comparable to the values reported for other quasicrystal-forming Zr-based metallic glasses.     

Thermal properties of cerium and thorium tellurates

Enthalpy increment measurements on CeTe₂O₆ (s) and ThTe₂O₆ (s) were carried out using a Calvet micro-calorimeter. The enthalpy increment values were least squares analyzed, with the constraint that H⁰(T)−H⁰ (298.15 K) at 298.15 equals 0 and C_p⁰ (298.15 K) is equal to the value estimated by Kellog’s method.The dependence of the enthalpy increment with temperature can be given as:

H⁰(T)−H⁰ (298.15 K)(J mol⁻¹)=189.95 T (K)+15.226×10⁻³ T² (K) +15.414×10⁵/T (K)−63157 (CeTe₂O₆ (s), 391.5–848.0 K)

H⁰(T)−H⁰ (298.15 K)(J mol⁻¹)=191.34 T (K)+14.993×10⁻³ T² (K) +14.668×10⁵/T (K)−63300 (ThTe₂O₆ (s), 391.5–909.3 K)

Molar heat capacity C_p⁰(T), S(T) and Gibb’s free energy functions were evaluated.     

A brief history of heat measurements by calorimetry with emphasis on the thermochemistry of metallic and metal-nonmetal compounds

A brief summary of the history of heat measurements is assessed from the Graeco-Roman time period to the first quantitative heat measurement in the 18th century. The concept of heat as energy is emphasized as a milestone in the development.The development of the modern calorimeters are summarized. Illustrations show the different types of equipment as designed for a variety of aims and purpose. In chronological order we illustrated the calorimeters by Lavoisier(1789), by Kawakami(1927), by Kubaschewski and Walter(1939), by Calvet(1950), by Kleppa(1955–2006), by Ticknor and Bever(1954), by Gachon(1979), by Cacciamani(1995), by Jung(2003) and by Navrotsky(2016).We discussed the theoretical and practical significance of calorimetry. The types of calorimetry measurements are briefly described. We also illustrated the international aspect of the use of this technique by showing some of the countries having actively participated in this effort.As a conclusion we compared to what extent the experimental measurements agree with theoretical predictions.     

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 974 K. The standard enthalpy of formation values derived from these experiments are − 6.1±2.5 kJ mol of atoms⁻¹ for the ZnSb sample prepared by ball milling and − 7.9±0.4 kJ mol of atoms⁻¹ for the ZnSb sample prepared by melting. These results are discussed and compared to literature data.     

NMR and calorimetric investigation of water in a superabsorbing crosslinked network based on cellulose derivatives

In this study we have investigated the state of water in a superabsorbing network based on hydroxyethylcellulose (HEC) and carboxymethylcellulose sodium salt (CMCNa) crosslinked with divinylsulfone (DVS).This type of network, at low degree of crosslinking, when exposed to distilled water is able to form a stable hydrogel containing an amount of water as high as 1000 times its own weight.De-hydrated/re-hydrated networks, containing different amounts of absorbed water, have been studied using differential scanning calorimetry (DSC) and NMR relaxometric methods. DSC analysis allowed the evaluation of freezable and non-freezable fractions of absorbed water showing also the presence of two types of freezable water. On the other hand, NMR relaxometry evidenced the presence of two hydration shells, characterized by a different mobility, which in both cases is lower than that of bulk water.An excellent quantitative agreement was found in the determination of the amount of freezable water using the two techniques.A comparison of the state of water in the crosslinked network and in the corresponding uncrosslinked mechanical mixture shows that in the last case micro-heterogeneity arises.     

Characterization of stoichiometric adsorption complexes in H-ZSM-5 using microcalorimetry

We have used microcalorimetry to examine the stoichiometric adsorption complexes formed by ammonia, pyridine, and isopropylamine at the A1 sites in H-ZSM-5. The results show that the heats of adsorption for each adsorbate are constant up to a coverage of one molecule /Al, implying that all of the acid sites in H-ZSM-5 are equivalent in strength. The adsorption energies of the complexes formed by ammonia, pyridine, and isopropylamine were found to be 150, 200, and 240 kJ/mol, respectively. Comparison of these values with a simple potential-energy model which assumes that the heat of adsorption scales linearly with gas phase proton affinities suggests that proton transfer dominates the interaction between the adsorbate and the acid site, but that other factors must also be included.     

Sodium silicate-based, alkali-activated slag mortars: Part I. Strength, hydration and microstructure

Alkali activation of ground granulated blast furnace slag (GGBFS) with sodium silicate gave clinker-free binders, with high strength and early strength development, although set times were short and somewhat variable. Isothermal calorimetry detected three heat evolution peaks (wetting, gelation of activator and bulk reaction of slag). X-ray diffraction (XRD) showed no crystalline products. Hydration was investigated by scanning electron microscopy (SEM; with quantitative image analysis) and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR). From early age, a uniform gel filled the initially water-filled space, and gradually densified as reaction proceeded. Microanalysis of outer product (OP) showed an Al-substituted C-S-H gel phase of widely variable (0.5-1.0) Ca/Si ratio. NMR showed long-chain substituted C-S-H with Al/Si ratio rising to 0.19 at 1 year, and also cross-linked material, consistent with a Ca- or Al-modified silica gel. Inner product (IP) regions around slag grains probably also contained hydrotalcite. Activation with KOH gave more rapid reaction of slag than for silicate activation, a less homogeneous microstructure, and lower strengths. The hydrates contained a substituted C-S-H gel of low Ca/Si ratio probably mixed with hydrotalcite, and occasional higher Al regions in the OP regions.     

Determining the “apparent” activation energy of concrete: Ea—numerical simulations of the heat of hydration of cement

The “apparent activation energy” of concrete—E_a—is usually determined by means of mechanical or calorimetric tests. This parameter, which characterises the sensitivity of concrete hydration processes to temperature, is required for estimating the compressive strength of concrete at early age by the “method of equivalent age.” The major aim of this paper is to show how numerical simulations of released heat under adiabatic conditions could be an alternative to experiments for determining E_a. The simulation model—the CHAL program—which is developed here, is based on the kinetic model of Avrami-Erofeev. The coupling by temperature between the different chemical reactions of the main anhydrous components of cement is taken into account by using the Arrhenius' law. The model leads to a better understanding of the polymineral characteristic of cement as regards heat of hydration and E_a determination. The “apparent” nature of E_a and its dependence on hydration degree can be underlined.     

Synthesis and characterization of lithium-carbon compounds for hydrogen storage

Three carbon materials were prepared for the synthesis of Li-C compounds, such as Li intercalated graphite. The materials were as-received high purity polycrystalline graphite (G), graphite milled under a hydrogen atmosphere (HG), and graphite milled an argon atmosphere (AG). With respect to the difference for them, HG preserved a better crystalline structure than AG. Each material was milled with Li, where the products are denoted as Li-G, Li-HG, and Li-AG. In XRD patterns of Li-G and Li-HG, the peaks corresponding to LiC₆ and LiC₁₂ were revealed, while no peaks were observed in the case of Li-AG. However, the formation of lithium carbide Li₂C₂ was suggested for Li-AG by a thermal analysis under an inert gas. After the hydrogenation, LiH was formed for all the compounds, and graphite was recovered for Li-G and Li-HG. Each hydrogenated compound desorbed H₂ with different profile by heating up to 500 °C. As a reaction product, Li₂C₂ was formed for the hydrogenated Li-HG and Li-AG. In the case of the hydrogenated Li-G with better crystalline structure, Li intercalated graphite were formed after the dehydrogenation. Therefore, it is concluded that the hydrogen absorption and desorption process of Li intercalated graphite was different from those of Li₂C₂.     

Influence of compositional ratio on microstructure and martensitic transformation of CuZr shape memory alloys

In the present work the effect of Cu/Zr atomic ratio on structural and calorimetric properties of this high temperature shape memory alloy (HTSMA) is studied. It has been discussed the changes induced by Cu/Zr ratio on the martensitic transformation temperatures and the corresponding transformation heats coupled with the phases microstructure. The modification of the Cu content in the range ±2% at, around the equiatomic composition, does not drastically change the thermal properties of the alloys. Moreover, the Cu/Zr ratio strongly influences the microstructure in terms of the presence and amount of the other characteristic phases, Cu₁₀Zr₇ and CuZr₂, in the place of the CuZr phase. The understanding of the basic properties of the binary system can be of great help for further investigations on CuZr based systems with other alloying elements.