Heat Capacity Curves Research Articles

Adsorption and freezing of diblock copolymers on stripe-patterned surfaces: A scaling analysis

We present the results of scaling analysis of diblock copolymers adsorbed on stripe-patterned surfaces of various widths. Our previous studies [K. Sumithra and E. Straube, J. Chem. Phys. 125, 154701 (2006)] show that the adsorption of diblock copolymer on patterned surfaces yields two peaks in the specific heat capacity, thereby indicating two transition. In the current study, we characterize these two transitions. The scaling of the adsorption energy data proves that the first peak in the heat capacity curve is, in fact, associated with the adsorption transition. We found that for this transition the classical scaling laws are obeyed and that the critical crossover exponent is unaltered with respect to the case of homogeneous polymers. However, we found a change in the scaling exponent in the case of parallel component of the radius of gyration. It is evident from the scaling analysis of the parallel component of the radius of gyration that the chain is stretched along the direction of the stripes. The scaling plot shows, for (square root <Rg parallel2>)/Nnu, an exponent of approximately 0.55 which is much different from that expected of a self-avoiding chain (nud=2-nu)/phi which is 0.25. The observed value is closer to an exponent of (nud=1-nu)/phi=0.69, for a completely stretched chain in one dimension. The perpendicular component of the radius of gyration <Rg perpendicular2> shows deviation from the power law and the slope is steeper than the expected value of -2. We have also defined an order parameter to characterize the second transition and have found that it corresponds to a freezing transition where there are only a few dominant conformations. The perpendicular component of the radius of gyration also supports this information.

Molar Heat Capacities, Thermodynamic Properties, and Thermal Stability of trans-4-(Aminomethyl)cyclohexanecarboxylic Acid

The low-temperature heat capacities of trans-4-(aminomethyl)cyclohexanecarboxylic acid were measured with an adiabatic calorimeter over the temperature range from (82 to 378) K. The heat capacity curve was smooth and continuous, which indicated that the compound was stable over the whole temperature range. Thermal decompositions of the compound were studied by thermogravimetry (TG) and the possible mechanism of the thermal decomposition was deduced. The energy of combustion of the compound was determined, and the standard enthalpy of combustion, ΔcHϑ, and standard enthalpy of formation, ΔfHϑ, were calculated.

Density functional analysis of the structural evolution of Gan (n=30–55) clusters and its influence on the melting characteristics

Recent experimental results have reported surprising variations in the shapes of the heat capacity curves and melting temperatures of gallium clusters in the size range of 30-55 atoms [G. A. Breaux et al., J. Am. Chem. Soc. 126, 8628 (2004)]. In the present work, we have carried out an extensive density functional investigation on ten selected clusters in the above mentioned size range. In particular, we have analyzed the ground state geometry and the nature of bonding in these clusters using electron localization function. We demonstrate that the existence or otherwise of a large island of atoms bonded with similar strength (i.e., the local order) in the ground state geometry is responsible for the variation in the shape of the heat capacity curve. We attribute the observed higher melting temperatures of some of the clusters (viz., Ga45-Ga48) to the presence of a distinct core and strong covalent bonds between the core and surface atoms. The present work clearly demonstrates that it is possible to understand the general trends observed in the heat capacity curves across the entire series on the basis of the analysis of their ground state.

Thermoelectric properties and electronic structure of Zintl compound BaZn2Sb2

Polycrystalline sample of the title compound was prepared and its thermoelectric properties from 2to675K were investigated. This Zintl compound shows rather low thermal conductivity, 1.6Wm−1K−1, at room temperature. The value of its thermoelectric figure of merit ZT reaches 0.31 at 675K. Its electronic structure, calculated by ab initio methods, suggests that the electrical transport are mainly ascribe to [Zn2Sb2] framework for p-type BaZn2Sb2. The heat capacity curve at low temperature was fitted lineally to obtain Debye temperature (about 208K). It provides the authors with a host lattice for modification and optimization the thermoelectric properties through substitution and/or doping.

Low-temperature heat capacities of [Er 2( l-Glu) 2(H 2O) 6](ClO 4) 4·6H 2O(s)

A coordination compound of erbium perchlorate with l-α-glutamic acid, [Er 2( l-Glu) 2(H 2O) 6](ClO 4) 4·6H 2O(s), was synthesized. By chemical analysis, elemental analysis, FTIR, TG/DTG and comparison with relevant literature, its chemical composition and structure were established. The purity was found to be >99.0%, without melting point. Low-temperature heat capacities were measured by a precision automated adiabatic calorimeter from 78 to 318 K. An endothermic peak in the heat-capacity curve was observed over the temperature region of 290–316 K, which was ascribed to a solid-to-solid phase transition. The temperature T trans, the enthalpy Δ trans H m and the entropy Δ trans S m of the phase transition for the compound were determined to be: 308.73 ± 0.45 K, 10.49 ± 0.05 kJ mol −1 and 33.9 ± 0.2 J K −1 mol −1. Polynomial equation of heat capacities as a function of the temperature in the region of 78–290 K was fitted by the least square method.

Low‐Temperature Heat Capacities and Thermodynamic Properties of Octahydrated Barium Dihydroxide, Ba(OH)2·8H2O(s)

AbstractLow‐temperature heat capacities of octahydrated barium dihydroxide, Ba(OH)2·8H2O(s), were measured by a precision automated adiabatic calorimeter in the temperature range from T=78 to 370 K. An obvious endothermic process took place in the temperature range of 345–356 K. The peak in the heat capacity curve was correspondent to the sum of both the fusion and the first thermal decomposition or dehydration. The experimental molar heat capacities in the temperature ranges of 78–345 K and 356–369 K were fitted to two polynomials. The peak temperature, molar enthalpy and entropy of the phase change have been determined to be (355.007±0.076) K, (73.506±0.011) kJ·mol−1 and (207.140±0.074) J·K−1·mol−1, respectively, by three series of repeated heat capacity measurements in the temperature region of 298–370 K. The thermodynamic functions, (HT−H298.15 K) and (ST−S298.15 K), of the compound have been calculated by the numerical integral of the two heat‐capacity polynomials. In addition, DSC and TG‐DTG techniques were used for the further study of thermal behavior of the compound. The latent heat of the phase change became into a value larger than that of the normal compound because the melting process of the compound must be accompanied by the thermal decomposition or dehydration of 7H2O.

Magnetic excitations and heat capacity of fayalite, Fe2SiO4

We have used inelastic neutron-scattering measurements to study the magnetic excitations in the antiferromagnetic and paramagnetic phases of polycrystalline fayalite, Fe2SiO4. Sharp, nondispersing excitations are found in the ordered state, at 3.3, 5.4, 5.9, and 11.4 meV, and are interpreted as arising from the spin-orbit manifold of the Fe2+ ions. These excitations are increasingly damped with increasing temperature, merging into a quasielastic continuum near the 65 K Neel temperature, although their energy does not vary with temperature. We have calculated the contribution of the heat capacity arising from these magnetic excitations and found that it compares favorably with the magnetic heat capacity deduced experimentally. Our analysis indicates that the M1 and M2 sites behave distinctly. The M1 site behaves quasi-locally and appears in the heat capacity as a Schottky anomaly that explains the shoulder in the heat capacity curve near 20 K, while the M2 site contributes predominantly to the critical lambda anomaly. The behavior of fayalite illuminates the nature of magnetic states in several related minerals, including others that also show shoulders and lambda anomalies in the heat capacity (tephroite), those that show only lambda anomalies (cobalt olivine and liebenbergite), and those that show only non-lambda anomalies (bronzite, anthophyllite, and almandine). We find no evidence to support the recent claim that some transition metal silicate and germanate olivines exhibit strong geometric frustration.

Intermediates in the melting transitions of aluminum nanoclusters

The author uses heat capacity data for aluminum cluster ions, Aln+, obtained in the laboratory of Breaux et al. [Phys. Rev. Lett. 94, 17340 (2005)] to determine whether or not intermediate species are present in the transition from the solidlike form of the clusters present at low temperatures to the liquidlike form present at high temperatures. He gives a general method on how to test for the presence of such intermediates and how to calculate their probabilities and thermodynamics as a function of temperature. In addition he uses energy distribution functions, using the maximum-entropy method that he developed previously, to substantiate the presence or absence of intermediates. As examples of the method he treats n=53 and n=79 clusters both of which exhibit marked maxima in the temperature dependence of their heat capacity curves, indicating strong order-disorder transitions. He find that in the melting transition n=53 clusters have no intermediates while the melting of n=79 clusters is dominated by intermediate species.

Dopant-induced stabilization of silicon clusters at finite temperature

With the advances in miniaturization, understanding and controlling properties of significant technological systems like silicon in nano regime assumes considerable importance. It turns out that small silicon clusters in the size range of 15-20 atoms are unstable upon heating and in fact fragment in the temperature range of 1200 K to 1500 K. In the present work we demonstrate that it is possible to stabilize such clusters by introducing appropriate dopant (in this case Ti). Specifically, by using the first principle density functional simulations we show that Ti doped Si$_{16}$, having the Frank-Kasper geometry, remains stable till 2200 K and fragments only above 2600 K. The observed melting transition is a two step process. The first step is initiated by the surface melting around 600 K. The second step is the destruction of the cage which occurs around 2250 K giving rise to a peak in the heat capacity curve.

Absence of the heat capacity anomaly in the Pb-free relaxorBaTi0.65Zr0.35O3

A relaxor ferroelectrics, $\mathrm{Ba}{\mathrm{Ti}}_{0.65}{\mathrm{Zr}}_{0.35}{\mathrm{O}}_{3}$ (BTZ35), which was free from lead, was synthesized and the physicothermal properties were measured. The heat capacity was measured using a homemade adiabatic calorimeter at temperatures from $13\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}420\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the dielectric constant measurements were carried out between $1\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$ and $1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ at temperatures from $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}420\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. No anomaly was detected in the heat capacity curve for the whole temperature range covered in the present experiments, while a large and broad peak was observed in the dielectric constant curve. The dielectric constant showed the relaxor type frequency dependence. The results of the present experiments were compared with our previous studies on a typical relaxor of lead-based complex perovskite oxide, $\mathrm{Pb}{\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}{\mathrm{O}}_{3}$ (PMN), which showed a large heat capacity anomaly as well as a large broad peak of dielectric constant due to formation of ferroelectric polar nanoregion in the crystal. Some differences between the two compounds, BTZ35 and PMN, were compared and the mechanism was discussed.

Heat capacity and physical transitions in collagen and solubility of water in it

The heat capacities of cattle and poultry collagen samples were measured by adiabatic calorimetry in the range 80–330 K. The temperatures of physical transitions in these samples were determined. Endothermic relaxation transitions (γ1, γ2, and β) were observed in the heat capacity curves of the studied collagen samples. The solubility of water in collagen samples obtained from various sources was determined by calorimetry from the enthalpy of melting of the “free” water phase.

Low-Temperature Heat Capacities and Standard Molar Enthalpy of Formation of the Solid-State Coordination Compound trans-Cu(Gly)2·H2O(S) (Gly = l-α-Glycine)

Low-temperature heat capacities of the solid coordination compound trans-Cu(Gly)2·H2O(s) were measured by a precision automated adiabatic calorimeter over the temperature range from T = 78 K to T = 381 K. The initial dehydration temperature of the coordination compound was determined to be TD = 329.50 K, by analysis of the heat-capacity curve. The experimental values of the molar heat capacities in the temperature region of 78−328 K were fitted to a polynomial equation of heat capacities (Cp,m) with the reduced temperatures (X), [X = f(T)], by a least-squares method. The smoothed molar heat capacities and thermodynamic functions of the complex trans-Cu(Gly)2·H2O(s) were calculated based on the fitted polynomial. The smoothed values of the molar heat capacities and fundamental thermodynamic functions of the sample relative to the standard reference temperature 298.15 K were tabulated with an interval of 5 K. Enthalpies of dissolution of {Cu(Ac)2·H2O(s) + 2Gly(s)} [ΔsHmϑ (1)] and HAc(l) [ΔsHmϑ (2)] in 100.00 mL of 2 mol·dm-3 HCl and trans-Cu(Gly)2·H2O(s) [ΔsHmϑ (3)] in the solvent [HAc(l) + 2 mol·dm-3 HCl] at T = 298.15 K were determined to be ΔSHmϑ (1) = (13.59 ± 0.03) kJ·mol-1, ΔSHmϑ (2) = −(0.93 ± 0.03) kJ·mol-1, and ΔSHmϑ (3) = −(14.64 ± 0.02) kJ·mol-1 by means of an isoperibol solution-reaction calorimeter. The standard molar enthalpy of formation of the compound was determined as: ΔfHm0(trans-Cu(Gly)2·H2O, s, 298.15 K) = −(1251.6 ± 1.0) kJ·mol-1, from the enthalpies of dissolution and other auxiliary thermodynamic data through a Hess thermochemical cycle.

Isochoric heat capacity and coexistence curve of methyl alcohol in the neighborhood of the critical point

A high-temperature adiabatic calorimeter is used to experimentally measure the isochoric heat capacity Cυ of methyl alcohol in the temperature range from 474.39 to 531.63 K along 22 isochores which cover the density range from 136.1 to 750.1 kg/m3. The data on Cυ are used to determine the curve of liquid-vapor phase equilibrium for methyl alcohol. The thus obtained data are described by scaling equations. Comparison is made with results of other researchers.

Low-temperature heat capacities and standard molar enthalpy of formation of the complex

Abstract Low-temperature heat capacities of a solid complex Zn(Val)SO4·H2O(s) were measured by a precision automated adiabatic calorimeter over the temperature range between 78 and 373 K. The initial dehydration temperature of the coordination compound was determined to be, TD=327.05 K, by analysis of the heat-capacity curve. The experimental values of molar heat capacities were fitted to a polynomial equation of heat capacities (Cp,m) with the reduced temperatures (x), [x=f (T)], by least square method. The polynomial fitted values of the molar heat capacities and fundamental thermodynamic functions of the complex relative to the standard reference temperature 298.15 K were given with the interval of 5 K. Enthalpies of dissolution of the [ZnSO4·7H2O(s)+Val(s)] (Δsol Hm,l0) and the Zn(Val)SO4·H2O(s) (Δsol Hm,20) in 100.00 mL of 2 mol dm−3 HCl(aq) at T=298.15 K were determined to be, Δsol Hm,l0=(94.588±0.025) kJ mol−1 and Δsol Hm,20=–(46.118±0.055) kJ mol−1, by means of a homemade isoperibol solution–react...

Crystal structure and specific heat of GdCuGe

A single crystal study of GdCuGe revealed full copper–germanium ordering: NdPtSb type, space group P6 3 mc, a = 423.2 ( 1 ) , c = 753.7 ( 2 ) pm , w R 2 = 0.0443 , 414 F 2 values and 10 variables. The copper and germanium atoms build up two-dimensional networks of ordered [Cu 3Ge 3] hexagons with Cu–Ge distances of 244 pm. Consecutive [Cu 3Ge 3] layers are rotated by 60° around the perpendicular c-axis with respect to each other. The [Cu 3Ge 3] hexagons show a weak puckering. Heat capacity measurements on the polycrystalline sample of GdCuGe establishes antiferromagnetic ordering around 14 K in agreement with reports in the literature. The curves of heat capacity measured in different applied fields crosses each other at two well-defined points exhibiting a behavior usually associated with heavy fermion compounds. The results of the structural analysis and heat capacity measurements are discussed in the light of these interesting observations for a gadolinium intermetallic.

Crystal Structure of the Biglycan Dimer and Evidence That Dimerization Is Essential for Folding and Stability of Class I Small Leucine-rich Repeat Proteoglycans

Biglycan and decorin are two closely related proteoglycans whose protein cores contain leucine-rich repeats flanked by disulfides. We have previously shown that decorin is dimeric both in solution and in crystal structures. In this study we determined whether biglycan dimerizes and investigated the role of dimerization in the folding and stability of these proteoglycans. We used light scattering to show that biglycan is dimeric in solution and solved the crystal structure of the glycoprotein core of biglycan at 3.40-angstroms resolution. This structure reveals that biglycan dimerizes in the same way as decorin, i.e. by apposition of the concave inner surfaces of the leucine-rich repeat domains. We demonstrate that low concentrations of guanidinium chloride denature biglycan and decorin but that the denaturation is completely reversible following removal of the guanidinium chloride, as assessed by circular dichroism spectroscopy. Furthermore, the rate of refolding is dependent on protein concentration, demonstrating that it is not a unimolecular process. Upon heating, decorin shows a single structural transition at a T(m) of 45-46 degrees C but refolds completely upon cooling to 25 degrees C. This property of decorin enabled us to show both by calorimetry and light scattering that dimer to monomer transition coincided with unfolding and monomer to dimer transition coincided with refolding; thus these processes are inextricably linked. We further conclude that folded monomeric biglycan or decorin cannot exist in solution. This implies novel interrelated functions for the parallel beta sheet faces of these leucine-rich repeat proteoglycans, including dimerization and stabilization of protein folding.

Formation of metastable crystals of [C 4mim][NTf 2] and [C 6mim][NTf 2

Heat capacities of crystalline 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C 4mim][NTf 2] and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C 6mim][NTf 2] in the range 80 K– T fus were measured in an adiabatic calorimeter. Anomalies in the heat-capacity curves for the both compounds occurred near 240 K. Positions of the anomalies depended on thermal history of the samples. More stable crystals had higher heat capacities in the range 220–260 K. Below 200 K heat capacities of all the crystals of the same compound were indistinguishable.

Redetermination of the thermodynamic properties of the solid–solid transition of adamantane by adiabatic calorimetry to investigate the suitability as a reference material for low-temperature DSC-calibration

Sub-ambient calibration of DSCs is a problem because there are few reference substances, and every available substance has its drawbacks. One of the possibilities is the low-temperature solid–solid transition of adamantane. High-accuracy adiabatic calorimetry results are, however, only available from one source. Therefore it was decided to measure the transition with our adiabatic calorimeter. Although our results show some deviation from the results reported in the literature source – our transition temperature (208.0 K) is 0.6 K lower and our enthalpy of transition (3213 J mol −1) is 5% smaller – we strongly believe this transition can be useful for the calibration of DSCs. Because the heat capacity of the low-temperature phase significantly increases well below the transition temperature, we propose to use a linear base line drawn as a tangent to the (almost) linear heat capacity curve above the transition region and start the integration at the temperature at which the drawn base line crosses the measured curve (at about 190 K). With this integration we found a heat of transition of 3094 J mol −1 (=22.7 J g −1).

Correlation between low frequency vibrational excitations and glass transition dynamics in amorphous semiconductors Ge1−xSx

The structural relaxation in the glass transition of amorphous semiconductors Ge1−xSx (0.63⩽x⩽0.90) has been investigated by differential scanning calorimetry. The heat capacity curves were analyzed by the Moynihan formulation with a Kohlrausch–Williams–Watts (stretched exponential) relaxation function and a Narayanaswamy relaxation time. A fractal dimension was derived from the nonexponentiality of the relaxation using a free-volume theory, and was found to decrease with the addition of S from about 2.1 for x=0.63 to about 1.8 for x=0.90. A thermokinetic dimension was also calculated from the activation energy of the relaxation using dimensionally-expanded Kissinger plots. We have found that the two dimensions are in good agreement with the fractal dimension obtained from the low frequency Raman scattering spectra. The observations suggest that the change in the fractal dimension is associated with the lowering of the network dimensionality due to the weakening of bond constraints with the addition of S.

Level densities and thermodynamical quantities of heatedMo93−98isotopes

Level densities for $^{93\ensuremath{-}98}\mathrm{Mo}$ have been extracted using the ($^{3}\mathrm{He}$,$\ensuremath{\alpha}\ensuremath{\gamma}$) and ($^{3}\mathrm{He}$,$^{3}\mathrm{He}$${}^{'}\ensuremath{\gamma}$) reactions. From the level densities thermodynamical quantities such as temperature and heat capacity can be deduced. Data have been analyzed by utilizing both the microcanonical and the canonical ensemble. Structures in the microcanonical temperature are consistent with the breaking of nucleon Cooper pairs. The $\mathsf{S}$ shape of the heat capacity curves found within the canonical ensemble is interpreted as consistent with a pairing phase transition with a critical temperature for the quenching of pairing correlations at ${T}_{c}~0.7\ensuremath{-}1.0$MeV.