Search results for "enthalpy"
showing 10 items of 199 documents
Thermodynamics of a small system in a μT reservoir
2011
Abstract Due to advances in experimental techniques operating at the nanoscale, it is possible to compute properties from density fluctuations by studying ‘snapshots’ of particle configurations. Thermodynamics on a small scale is different from thermodynamics in bulk systems. We show how the molar enthalpy h and the inverse thermodynamic correction factor Γ - 1 depend on system size and how these properties can be computed from fluctuations at the nanoscale. We find a 1/ L finite size effect for all thermodynamic quantities for a small system in contact with a reservoir, where L is the length of the system in a single dimension.
Si self-diffusion in cubic B20-structured FeSi
2008
Self-diffusion of implanted 31Si in the e-phase FeSi (cubic B20-structure) has been determined in the temperature range 660–810 °C using the modified radiotracer technique. With an activation enthalpy of 2.30 eV and a pre-exponential factor of 15×10−8 m2 s−1 the silicon diffusivity was found to be slightly slower than Ge impurity diffusion in FeSi. This difference is proposed to originate from attractive elastic interactions prevailing between the slightly oversized Ge atoms and the Si sublattice vacancies. The results confirm the argument that 71Ge radioisotopes may be used to substitute the short-lived 31Si radiotracers when estimating self-diffusion in silicides.
A CCHP system fed by low enthalpy sources in Mediterranean area
2018
Climate change and energy consumption increasingly call for a better use of energy sources. Renewable sources are still growing in the power production of many countries, so leading to the reduction of CO2 emissions. Further restrictions will be applied to the use of fossil fuels with a lot of consequences; for example, some German cities are all contemplating bans related to diesel pollution, and on the other hand some of the biggest car producers are planning to stop the diesel engines production in a few years. At the same time, some restrictions are applied also to many refrigerating fluids, in order of limiting the ODP and GWP values. This restriction leads to research new fluids and/o…
A CCHP system fed by low enthalpy sources in Mediterranean area
2018
Climate change and energy consumption increasingly call for a better use of energy sources. Renewable sources are still growing in the power production of many countries, so leading to the reduction of CO2 emissions. Further restrictions will be applied to the use of fossil fuels with a lot of consequences; for example, major German cities such as Stuttgart, Dusseldorf and Hamburg are all contemplating bans related to diesel pollution, and on the other hand some of the biggest car producers - FCA, Toyota, etc. - are planning to stop the diesel engines production in a few years. At the same time, some restrictions are applied also to many refrigerating fluids, in order of firstly limiting th…
Thermoanalytical studies on phases of D-mannitol
1993
Abstract D-mannitol exists in several polymorphic crystalline forms. The conditions of crystallization are not well-defined, and in some cases mixtures of the different forms are obtained. In the DSC study the different forms melted nearly at the same temperature (166±°C). The heat of fusion of the k form was lower than those of the other forms. With slow heating rate (2 ° C min ) the δ form changed to k between 140 and 165 °C. The crystallization of D-mannitol from the melt gave from one to seven peaks in the DSC between 95 and 126 °C depending on cooling rates and the amounts of sample.
Elastic neutron scattering of dry and rehydrated trehalose coated carboxy-myoglobin
2008
We report here a comparison between the hydrogen atoms mean square displacements measured by elastic neutron scattering on trehalose coated carboxy-myoglobin, at ILL on the backscattering spectrometers IN13 and IN16. An inconsistency is observed when comparing the mean square displacements measured on the two spectrometer, on samples of identical composition, since they resulted of larger amplitude on IN13 (either in condition of drought or after overnight rehydration under 75% D2O atmosphere), notwithstanding the lower time window accessible on this instrument with respect to IN16. Such inconsistency disappears when the data obtained on this last spectrometer are analyzed in two separate r…
Thermal- and photoinduced spin-state switching in an unprecedented three-dimensional bimetallic coordination polymer.
2005
The compound {Fe(pmd)[Ag(CN)2][Ag2(CN)3]} (pmd=pyrimidine) was synthesized and characterized. Magnetic, calorimetric and single crystal visible spectroscopic studies demonstrate the occurrence of a two-step high-spin (HS) right arrow over left arrow low-spin (LS) transition. The critical temperatures are T(c1)=185 and T(c2)=148 K. Each step involves approximately 50 % of the iron centers, with the low-temperature step showing a hysteresis of 2.5 K. The enthalpy and entropy variations associated with the two steps are DeltaH(1)=3.6+/-0.4 kJ mol(-1) and DeltaS(1)=19.5+/-3 J K(-1) mol(-1); DeltaH(2)=4.8+/-0.4 kJ mol(-1) and DeltaS(2)=33.5+/-3 J K(-1) mol(-1). Photomagnetic and visible spectros…
Effect of T-R conformational change on sickle-cell hemoglobin interactions and aggregation
2004
We compare the role of a conformational switch and that of a point mutation in the thermodynamic stability of a protein solution and in the consequent propensity toward aggregation. We study sickle-cell hemoglobin (HbS), the beta6 Glu-Val point mutant of adult human hemoglobin (HbA), in its R (CO-liganded) conformation, and compare its aggregation properties to those of both HbS and HbA in their T (unliganded) conformation. Static and dynamic light scattering measurements performed for various hemoglobin concentrations showed critical divergences with mean field exponents as temperature was increased. This allowed determining spinodal data points T(S)(c) by extrapolation. These points were …
Water properties in fern spores: sorption characteristics relating to water affinity, glassy states, and storage stability.
2007
Ex situ conservation of ferns may be accomplished by maintaining the viability of stored spores for many years. Storage conditions that maximize spore longevity can be inferred from an understanding of the behaviour of water within fern spores. Water sorption properties were measured in spores of five homosporeous species of ferns and compared with properties of pollen, seeds, and fern leaf tissue. Isotherms were constructed at 5, 25, and 45 degrees C and analysed using different physicochemical models in order to quantify chemical affinity and heat (enthalpy) of sorption of water in fern spores. Fern spores hydrate slowly but dry rapidly at ambient relative humidity. Low Brunauer-Emmet-Tel…
The reaction enthalpy of hydrogen dissociation calculated with the Small System Method from simulation of molecular fluctuations.
2014
We show how we can find the enthalpy of a chemical reaction under non-ideal conditions using the Small System Method to sample molecular dynamics simulation data for fluctuating variables. This method, created with Hill's thermodynamic analysis, is used to find properties in the thermodynamic limit, such as thermodynamic correction factors, partial enthalpies, volumes, heat capacities and compressibility. The values in the thermodynamic limit at (T,V, μj) are then easily transformed into other ensembles, (T,V,Nj) and (T,P,Nj), where the last ensemble gives the partial molar properties which are of interest to chemists. The dissociation of hydrogen from molecules to atoms was used as a conve…