Curie temperature standards for thermogravimetry: the effect of magnetic field strength and comparison with melting point standards using Ni and Pb

Thermogravimetric (TG) runs were made with a Ni sample over a wide range of heating and cooling rates using an electromagnet at three different field strengths. The extrapolated onsets and endpoints of the weight change induced by the magnetic field gradient were obtained from both TG and the differential curves (DTG). The onset upon heating and endpoint on cooling are very clearly dependent upon magnetic field strength and hence unsuitable for temperature calibration using this method. The endpoint upon heating and onset upon cooling, however, showed only a slight dependence upon the cube root of the magnetic field strength. Observed values of T_c showed a linear dependence upon heating or cooling rates which extrapolated to a common point at zero rate. The value of T_c derived from extrapolation to zero rate and then zero field is 339.15°C, which is 16°C below the literature value.Simultaneous measurements of the melting point (T_m) of Pb and the T_m of Ni using a different furnace/thermocouple/magnet combination indicated a value of T_m = 311.8°C and T_c = 343.1°C. The value of T_m is 15.7°C below the value from the International Temperature Scale. The value of T_c is uncorrected to zero magnetic field strength, which would probably lower it several degrees. It would appear that both methods of temperature calibration are suitable for TG and the choice is best determined by the convenience with the particular apparatus involved and the temperature range desired.     

Molecular orbital studies on ice-like structures

Molecular orbital (MO) calculations were carried out on a series of ice-I _h and ice-I _c type lattices. These lattices were assigned dimensions which approximate conditions in liquid water (O-O = 2.86 Å) and, in addition, the dimensions of the ice-I _h lattices were changed to ascertain the influence of lattice expansion and contraction. Component parts of several lattices were investigated as were lattices lacking one or two individual monomeric units. Results are in accord with current experimental approximations. As the ice-I _h lattice is expanded, the stabilization energy diminishes. In the liquid model, ice-I _h structures are generally more stable than those of ice-I _c ; component rings of the lattice models of ice-I _c determine the stabilities of these models, and a stable ring system can stabilize an unstable system. An ice-I _h lattice model lacking a single monomer is stable. The trend of charges at the oxygen centres closely correlates with the charges on the central oxygen atom of the appropriate trimer.     

A novel online visualization system for observing polymer extrusion foaming

Cell nucleation and premature cell growth in extrusion foaming are critical to elaborate the morphology of final foams. These courses happen in the extrusion die which has been unknown for real extrusion foaming process. In this study, a novel visualization system was developed to online observe the cell nucleation and evolution behavior in the extrusion die. The cell evolution and real time pressure along the flow direction could also be obtained with the system. It was found that the solution pressure P_solution was influential to the critical nucleation radius R_cr and the nucleation rate N₀. The higher screw rotating speed corresponded to higher P_solution, lower in die N₀ and less premature cells, while higher cell density for extrude foams. In addition, premature cells with radius over critical break radius R_crb would break into several small cells under sufficient stress gradient in the extrusion flow field.     

Quantitative estimation of the discontinuous function of the solvent in aqueous electrolyte solutions

The adiabatic compression method is used to determine quantitative solvation parameters such as hydration numbers h, the molar adiabatic compressibility of hydrated complexes β _h V _h , the volume V _1h and compressibility β_1h of water in ion hydration shells, and several other properties in the temperature range 278.15–323.15 K. Hydration numbers are used to determine the verified activity coefficient of the solvent, γ _R . The concentration dependence of the coefficient is shown to be a discontinuous function, with the discontinuity point corresponding to the complete solvation boundary.     

Structure and dynamics of the lowest triplet state in p-benzoquinone: I. An isotope effect study on the optical absorption,emission and ODMR spectra

The results of detailed spectroscopic experiments on the lowest nπ* triplet state of p-benzoquinone-h₄, -dh₃, 2,6-d₂h₂, -d₄ and -CH₃ in mixed and isotopic mixed crystals are presented and analyzed. The origin of the lowest B_1g (nπ*) singlet-triplet transition in p-benzoquinone-h₄ (PBQ-h₄) is shown to be induced by asymmetric isotopic substitution and the oscillator strength of this origin is seen to be accounted for by a corresponding decrease in intensity of a level 16.9 cm^?1 higher in energy in the pure PBQ-h₄ crystal. The combined oscillator strength of these close lying levels is measured and found to be almost independent of deuteration.These results are discussed in reference to the previously proposed double minimum potential model for the lowest nπ* triplet state in PBQ-h₄ and the applicability for this model is critically examined.Optical absorption experiments on heavily doped isotopic mixed crystals of PBQ-h₄ in PBQ-d₄ show hydrogen (deuterium) bounding effects between translational inequivalent molecules to be primarily responsible for the observed cluster states. These hydrogen bounding effects also induce the electronic origin of the B_1g (nπ*) triplet state in case of a translational inequivalent dimer.A detailed vibrational analysis of the phosphorescence spectrum of PBQ-h₄ in a PBQ-d₄ host crystal at 1.8 K is presented and it is shown that the unobserved origin of the B_1g (nπ*) triplet state of PBQ-h₄ is located at 18609 ± 1 cm^?1 and that the inversion splitting in this lowest excited state amounts to 21 ± 1 cm^?1 in this mixed crystal system. An isotope effect is study on the vibronic structure in the emission spectrum further indicates that the excited state structure of PBQ is isotope dependent.The observed large isotope effect on the ZFS parameters of the lowest triplet state of PBQ-h₄ is demonstrated to be an intramolecular phenomenon and explained as an isotope dependent spin-orbit contribution to the ZI-S parameters, induced by localization of the nπ* excitation on oxygen.Finally the dynamics of energy migration in the dilute PBQ-h₄ in PBQ-d₄ isotopic mixed crystal is probed by concentration and temperature dependent phosphorescence intensity measurements and it is suggested that trap-exciton band communication effects are of importance in this system.     

Solvation of magnesium chloride and sulfate in aqueous solutions at 278.15–323.15 K

Isoentropy compressibilities of aqueous magnesium chloride and sulfate were determined based on precision measurements of ultrasound velocity, density, and isobaric heat capacity at low to high concentrations at 278.15–323.15 K. The hydration numbers h and the molar parameters of volume and compressibility were calculated based on thermodynamically correct equations for hydration complexes (V _h , β _h V _h ), water in the hydration shell (V _1h , β_1h V _1h ), and the void containing a stoichiometric mixture of ions (V _2h , β_2h V _2h ). The h and β _h V _h values were found to be independent of temperature; the molar compressibility of the hydration sphere (β_1h V _1h ) and the stoichiometric mixture of ions without a hydration shell (β_2h V _2h ) were independent of the concentration under the stated conditions. The effect of the electrostatic field of ions on the temperature dependence of the molar volume of water in the hydration sphere was more significant than the effect of pressure on the temperature dependence of the molar volume of bulk water. This is attributed to changes in the dielectric constant of water in the vicinity of the electrolyte ions.     

Two photon excitation spectrum of benzene and benzene-d6 in the gas phase: Assignment of inducing modes by hot band analysis

The two-photon absorption spectrum for benzene-h₆ and -d₆ is reported and assigned, leading to the frequency assignment for hitherto unknown vibrations in the electronically excited state. In addition absorption has been measured from hot bands in the ground state. This latter technique has allowed us to make an unequivocal assignment of the vibrational modes responsible for inducing the otherwise forbidden two-photon process. The result is in disagreement with current theory, for this prototype two-photon spectrum in the gas phase.     

DSC calibration in the study of shape memory alloys

The unusual mechanical properties (i.e. shape memory effect and superelasticity) of shape memory alloys (SMA) rely on the thermoelastic martensitic transformation (TMT) which is a first-order solid-solid, non-diffusive phase transition, athermal in character. Differential scanning calorimetry (DSC) is often used as a convenient method of investigating the thermal properties ofSMAs. The common practice of standard temperature calibration, required for a correct instrument performance, is here critically discussed in relation to the study of both the direct exothermic transformation on cooling, and the reverse endothermic transformation on heating in a NiTiSMA. The DSC results show that, with the standard temperature calibration, the instrument is calibrated on heating but un-calibrated on cooling. A general method is advanced to overcome this problem, intrinsically related to the dynamic character of DSC.     

Effect of free solvent on the solvation of electrolytes and nonelectrolytes

An approach is developed for analyzing hydration of sodium and magnesium sulfate electrolytes over a wide range of concentrations up to 1.8 mol/L at temperatures from 278.15 to 318.15 K. The model of free water (water that does not enter hydrate complexes) is used to analyze the hydration numbers, mean compressibility of water in the hydration sphere, its temperature dependence, and several other solvation parameters that are determined with difficulty. A comparative analysis of the behavior of the β_h V _h = f(h) function is carried out for zwitterions and electrolytes. This function has an identical slope for amino acids, halides, and nitrates; that is, the adiabatic compressibility of water in the hydrate complex of an electrolyte or nonelectrolyte (β_1h V1h) is independent of the solution concentration over a wide range of compositions. This suggests that the clathrate solvation mechanism operates both for a stoichiometric ion mixture and for zwitterions. Aqueous solutions of sodium sulfate and magnesium sulfate behave quite differently: they appreciably deviate from the solutions of the specified systems.     

Analysis of the decay of picosecond fluorescence in semiconductors: Criteria for the presumption of electroneutrality during the decay of an exponential electron-hole profile

When an exponential profile of electron-hole pairs is photogenerated (in a semiconductor) with a delta-function light pulse, unequal diffusion coefficients of holes and electrons (i.e. D_e≠ D_h) effect deviations from electroneutrality as electrons and holes diffuse into the bulk semiconductor. These deviations will in turn effect errors in the analysis of data (e.g. time resolved fluorescence) when using theory based on the presumption of electroneutrality. We deduce here the experimental conditions required for an effective electroneutrality to be maintained during the course of an experiment. Analyses were carried out using computer simulations without the presumption of electroneutrality and the analytic solution with the presumption of electroneutrality. The differences in the measured fluorescences predicted by the two computations are characterized as a function of a variety of experimental parameters and physical properties: intensity (of the excitation pulse), the absorption of the exciting and emitted light, the the ratio D_h/D_e, bulk dielectric constant of the semiconductor, bulk and surface recombination kinetics. It is shown that a conditon of adequate electroneutrality can be effectively attained when a well defined a minimum number of electron-hole pairs is generated; an upper limit of the number of e^?–k⁺ pairs is also established in order to avoid an intolerable temperature pulse.     

Quantitative characteristics of hydration in solutions of sodium sulfate and sodium chloride at 278.15–323.15 K

The solvation parameters of aqueous solutions of sodium chloride and sodium sulfate were studied on the basis of published density and ultrasound velocity data. Correct thermodynamic relations for temperature variation from 278.15 to 323.15 were used to determine quantitative parameters of solvation, in particular, the hydration numbers h, the molar adiabatic compressibility of hydrate structures β _h V _h , the volume V _1h and compressibility β _1h of water in the hydration shells of ions, and others. h and β _h V _h do not depend on temperature in the range of parameters studied, and electrostriction compression about the ions has a more pronounced effect on the structure than mere pressure change.     

Ab initio study on the stability,geometry and electronic structure of the cyclopropenyl system

Results of an LCAO-MO-SCF investigation into the stability and geometry of the cyclopropenyl cation (C₃H₃⁺), anion (C₃H₃^?) and radical (C₃H₃^o) are presented. By independently varying the two relevant bond angles, the shape of the potential energy curves in the corresponding two-parameter space for these three species has been obtained. It is found that the cation is most stable in the D_3h nuclear configuration, while the anion has minimum energy in the C_s symmetry. In the specific case of the C₃H₃ radical, which exhibits an orbital, as well as spin-degeneracy, in the D_3h configuration, a strong Jahn-Teller effect is observed, leading to estimates for the non-symmetrical equilibrium configuration at C_2v and for the distortion energy of 12 kcal/mole from the D_3h symmetry. In terms of the population analysis and the contour diagrams, the electronic charge distribution has also been studied for these species in their most stable configurations.     

Differential double-layer capacitance of a mercury-drop electrode attached to a platinum wire: Frequency dispersion due to the measuring bridge and to the geometry at pendant and sessile electrodes

The frequency dependence of the double-layer capacitance has been studied at the interface Hg/0.116 M KCl in water at ?1.200 V vs. a reversible calomel electrode at 25°C. Two drop electrodes attached to 0.1-mm platinum wire have been used, one sessile with a narrow wedge, the other pendant with a widely open wedge between drop surface and support. The operation of the a.c. resistor-ratio arms bridge has been analysed in a manner generally applicable to series-R, C measurements. On that basis the effect of stray capacitances could be compensated and the frequency dispersion due to the measuring system reduced to a minimum. Both electrodes show a low-frequency dispersion of the measured series capacitance C(ω) and resistance R(ω), ω being the angular frequency. This dispersion has been analysed with a simple R, C network. In series with the bulk resistance R₃ this network has two mutually parallel series R, C's: R₁, C₁ of the drop itself and R₂, C₂ of its edge (neck). From the actual C(ω), R(ω) data follow correct capacitance data C₁, while R₂ and C₂ show a relatively stronger dependence on frequency and kind of electrode (pendant or sessile). Moreover, a sessile electrode exhibits a wedge or shielding effect above a much smaller ω (=ω_h >ω₁, the frequency above which the edge effect becomes insignificant) than a pendant electrode. Conclusions drawn from this information are: in the frequency domain (ω_h?ω₁) diffusive mass transport may be studied without interference from any geometric effect, which is probably also true at small electrolyte concentrations. This study should be performed with a pendant electrode, since it has the largest (ω_h?ω₁) domain.     

Zur Definition von Selektivität,Spezifität und Empfindlichkeit von Analysenverfahren

The three metric, functional concepts named in the title can be defined without arbitrariness and their values may be calculated for any multicomponent-analytical procedure from the relation of the measurable physical quantities x _ito the contents c _kwhich are to be determined. This relation is mathematically a “mapping” (achieved by systems of functions). The system of “analytical functions” (x _i→ c _k) is the inverse of the system of “calibration functions” (c _k→ x _i) which alone can be directly gained by experiments. The ‘calibration matrix” (γik) whose elements γ_ik are the “partial sensitivities” \(\tfrac{{\partial x_i }}{{\partial c_k }}\) represents the system in first approximation but only locally, i.e. for the respective constitution of the sample. The “sensitivity” of the analytical procedure as a whole, is numerically given by the determinant of this matrix; the “selectivity” is derived from the condition that the inversion of the calibration system to the analytical system (of functions) shall be possible by an iteration process. “Specificity” is defined in analogy to selectivity. In the appendix it is explained why selectivity is the strongest means to reduce the expenditure for the complete calibration of multicomponent analyses to a realistic and tolerable degree.     

DSC Calibration During Cooling. A survey of possible compounds

A number of compounds is investigated for DSC calibration during cooling. Adamantane and Zn show fast reversible transitions and can be applied both for temperature and for heat calibrations. A third compound, namely 4,4’-azoxyanisole, has a liquid crystal to isotropic liquid transition at 409K. This compound can be used for temperature calibration. Heat calibration with this compound is more problematic because of the small heat effect and the construction of the baseline. Other compounds like NaNO₃, In, Hg and Pb, show a slight supercooling. Nevertheless they can be used for heat calibration. The use of large samples of NaNO₃ and In gives the possibility to construct the equilibrium onset temperatures of the cooling peaks, so these two compounds are also appropriate for temperature calibration on cooling. This revised version was published online in July 2006 with corrections to the Cover Date.     

Development of a calorimetric sensor for medical application

A calorimetric sensor has been developed for local and direct measurement of calorific dissipations in different parts of the human body. The constructed prototype has a detection surface of 36 cm². The calibration of the sensor is based on a semi-empirical model that permits to simulate the operation of the device, making easier an operational functioning method. The device is modeled as a system with two inputs and two outputs. The inputs are the calorific power (W) that is intended to be measured and the power (W _pid) that dissipates a resistance, keeping constant the thermostat temperature through the use of a PID controller. The outputs are the thermostat temperature (T _pid) and the calorimetric signal (y) that provides the thermopile that is in contact with the body.     

Scanning microcalorimetry at high cooling rate

Heat capacity measurements at fast cooling and heating were realized for linear polyethylene NBS SRM (standard reference material) 1484 sample, ca. 120 ng, in the melting-crystallization region. A commercial vacuum sensor, thermal conductivity gauge TCG-3880, Xensor Integrations, was utilized as a cell for a micro-calorimeter suitable for such measurements. The cell consists of a thin-film Si₃N_x membrane with a film-thermopile and a film-heater, which are formed at the membrane center. The current at the heater as well as the signal from the thermopile were monitored in real time during fast scanning of temperature of the central part of the membrane. The measurements were performed in an ambient gas, so that controlled cooling and heating rates up to 5×10³ K/s were achieved. As conditions were not adiabatic, the heat leakage from the sample was calibrated and was taken into account for heat capacity measurements. A simple calibration algorithm was developed for such measurements. Thus, a step towards ultra fast cooling scanning calorimetry was made.     

Study of some interelectronic properties in helium-like atoms

By means of the optimumM-term Hylleraastype wavefunctions with 1≦M≦6 we study various interelectronic properties of the Helium-like atoms with nuclear chargeZ=1, 2, 3, 5 and 10. Leth(u) denote the spherically averaged electron-pair density of a finite many-electron system. Firstly we found that the intracule functionh(u)/u ^α of the above-mentioned atoms is (i) monotonically decreasing from the origin for α≥α₁ and (ii) convex for α≥α₂, where α₁ and α₂ are positive constants which depend onZ andM. Then we show that the electron-electron cusp condition, i.e. thath′(0)=h(0), may be extended in the sense that the inequalityh(u)?h′(u)≧0 is valid for anyu≥0. Thirdly, it is shown that the inequalities involving three interelectronic moments 〈u ⁿ 〉 recently found by the authors are, at times, of great quality. Finally the goodness of some bounds to the characteristics of the maximum ofh(u) and to the total interelectronic repulsion energy is discussed in detail.     

Evidence of solvation of aqueous electrolytes in a range from diluted to concentrated solutions

Parameters of solvation of strong electrolytes in aqueous solutions have been investigated based on literature data on their densities, thermal capacities, and rates of ultrasound propagation. By using appropriate thermodynamic equations, such quantitative parameters of solvation as hydration number (h), molar adiabatic compressibility of the hydrated forms (β _h V _h ), volume (V _1h ), and compressibility (β_1h ) of water in the hydrated shells of ions have been determined in the temperature interval from 278.15 to 323.15 K. It has been demonstrated that the h and β _h V _h values are temperature-independent within the studied interval and that electrostrictive compression near ions has a stronger effect on their structure than a simple change in pressure.     

Electron diffraction and quantum-chemical studies of the conformational properties of the 1,3-benzenedisulfochloride molecule

A combined electron diffraction (T = 394(5) K) and quantum-chemical (MP2/6-31G**) study has been performed to investigate the molecular structure of 1,3-benzenedisulfochloride (1,3-BDSC). The 1,3-BDSC molecule was found to exist as the trans (I) and cis (II) stable conformers where the planes containing S-Cl bonds are perpendicular to the plane of the benzene ring. The energy of conformer I is 0.13 kJ/mol lower than that of conformer II. The mutual effect of the sulfochloride groups was found to be absent, which is evident from the coincident bond lengths and angles in the two conformers. The main structural parameters of the conformers are r _h1(C-H)_av = 1.103(4) Å, r _h1(C-C)_av = 1.401(3) Å, r _h1(C-S) = 1.767(4) Å, r _h1(S=O) = 1.422(3) Å, r _h1(S-Cl) = 2.048(4) Å, ∠Cl-S-O = 106.6(2)°, ∠C-S-Cl = 100.4(5)°, ∠ O-S-O = 123.2(5)°.