Expected Melting Temperature Research Articles

Optimization of a new interatomic potential to investigate the thermodynamic properties of hypo-stoichiometric mixed oxide fuel U1−y Pu y O2−x

The behaviour of stoichiometric U1−y Pu y O2 compounds used as nuclear fuel is relatively well understood. Conversely, the effects of stoichiometry deviation on fuel performance and fuel stability are intricate and poorly studied. In order to investigate what affect these have on the thermophysical properties of hypo-stoichiometric U1−y Pu y O2−x mixed oxide fuel, new interaction parameters based on the many-body CRG (Cooper–Rushton–Grimes) potential formalism were optimized. The new potential has been fitted to match experimental lattice parameters of U0.70Pu0.30O1.99 (O/M = 1.99) and U0.70Pu0.30O1.97 (O/M = 1.97), where M represents the total amount of metallic cations, through a rigorous procedure combining classical molecular dynamic and classical molecular Monte Carlo simulation methods. This new potential provides an excellent description of the U1−y Pu y O2−x system. Concerning lattice parameter, although fitted on only one Pu content (30%) and two stoichiometries (1.99 and 1.97), our potential allows good predictions compared to available experimental results as well as to available recommendations in wide ranges of O/M ratio, Pu content and temperature. For the U0.70Pu0.30O2−x hypo-stoichiometric system (30% Pu content and O/M ratio ranging from 1.94 to 2.00), some direct properties (lattice parameter and enthalpy) and some derivative properties (linear thermal expansion coefficient and specific heat) were systematically investigated from room temperature up to the expected melting temperatures and a good agreement with experiments is found. Moreover, our potential shows good transferability to the plutonium sesquioxide Pu2O3 system.

Melting of indium, tin, and zinc nanowires embedded in the pores of anodic aluminum oxide

The melting temperature of metal nanostructures embedded in the matrix is an essential thermodynamic characteristic and a key parameter of the processes of their transformation into semiconductor structures. In this work, great attention is paid to the investigation of the behavior of one-dimensional metal nanocrystals near the melting point. For this purpose, the arrays of In, Sn, and Zn nanowires with different diameters have been electrochemically grown in the pores of anodic aluminum oxide (AAO), which is confirmed by the results of the microscopy and the phase X-ray diffraction analysis. The melting of nanowire arrays with different diameters has been investigated by means of differential scanning calorimetry (DSC). Aside from the expected melting temperature decrease, with decreasing the diameter of nanowires, it has been established that the melting peaks of nanostructure arrays have a complex shape that requires detailed elaboration in order to more accurately define the melting temperature. It is shown that the signal waveform while melting depends on geometric parameters of the structure, and the peak being mapped onto the DSC curve is the result of superposition of the melting peaks of nanowires with several characteristic dimensions. For the arrays of In, Sn, and Zn nanowires in AAO, there have been defined the melting temperature values according to the methodology offered, and there has been presented the dependence of the melting temperature decrease on the nanowires' diameter.

Raft crystals of poly(isoprene)-block-poly(ferrocenyldimethylsilane) and their surface wetting behavior during melting as observed by AFM and NanoTA

We report on the morphology evolution during heating and melting of lamellar poly(isoprene)-block-poly(ferrocenyldimethylsilane) (PI76-b-PFDMS76) raft crystals deposited at the native oxide surface of silicon (SiO2) or at a highly ordered pyrolytic graphite (HOPG) surface, studied by in situ temperature controlled atomic force microscopy. Crystals deposited on hydrophilic SiO2 surfaces revealed an irreversible decrease in length at temperatures of up to tens of degrees above their expected melting temperature, while maintaining their platelet-like structure. Crystals deposited on hydrophobic HOPG surfaces initially decreased in length below their expected melting temperature, while at 120 °C and above a typical molten morphology was observed. In addition, the irreversible formation of a PI76-b-PFDMS76 wetting layer around the crystals was observed upon increasing the temperature. These observations in the morphological behavior upon heating emphasize the role of interfacial energy between a surface deposited block copolymer based macromolecular nanostructure and its supporting substrate.

The Cysteine-rich Region of Type VII Collagen Is a Cystine Knot with a New Topology

Collagens are a group of extracellular matrix proteins with essential functions for skin integrity. Anchoring fibrils are made of type VII collagen (Col7) and link different skin layers together: the basal lamina and the underlying connective tissue. Col7 has a central collagenous domain and two noncollagenous domains located at the N and C terminus (NC1 and NC2), respectively. A cysteine-rich region of hitherto unknown function is located at the transition of the NC1 domain to the collagenous domain. A synthetic model peptide of this region was investigated by CD and NMR spectroscopy. The peptide folds into a collagen triple helix, and the cysteine residues form disulfide bridges between the different strands. The eight cystine knot topologies that are characterized by exclusively intermolecular disulfide bridges have been analyzed by molecular modeling. Two cystine knots are energetically preferred; however, all eight disulfide bridge arrangements are essentially possible. This novel cystine knot is present in type IX collagen, too. The conserved motif of the cystine knot is CX3CP. The cystine knot is N-terminal to the collagen triple helix in both collagens and therefore probably impedes unfolding of the collagen triple helix from the N terminus.

High temperature in-situ observations of multi-segmented metal nanowires encapsulated within carbon nanotubes by in-situ filling technique

Multi-segmented one-dimensional metal nanowires were encapsulated within carbon nanotubes (CNTs) through in-situ filling technique during plasma-enhanced chemical vapor deposition process. Transmission electron microscopy (TEM) and environmental TEM were employed to characterize the as-prepared sample at room temperature and high temperature. The selected area electron diffractions revealed that the Pd4Si nanowire and face-centered-cubic Co nanowire on top of the Pd nanowire were encapsulated within the bottom and tip parts of the multiwall CNT, respectively. Although the strain-induced deformation of graphite walls was observed, the solid-state phases of Pd4Si and Co-Pd remain even at above their expected melting temperatures and up to 1,550 ± 50°C. Finally, the encapsulated metals were melted and flowed out from the tip of the CNT after 2 h at the same temperature due to the increase of internal pressure of the CNT.

Molecular Weight and Branching Distribution of a High Performance Metallocene Ethylene 1‐Hexene Copolymer Film‐Grade Resin

AbstractSummary: The bivariate, or cross branching distribution of a gas‐phase produced, film‐grade ethylene 1‐hexene copolymer with enhanced Elmendorf tear in machine direction, MD, and in transverse direction, TD, (> 400 g/mil) and high dart impact has been characterized through the analysis of fractions obtained by molecular weight and 1‐hexene composition. The molecular weight fractions, obtained by a solvent‐non‐solvent fractionation technique, are each mixtures of molecules with at least two different 1‐hexene compositions, one component with a constant relatively high density (∼1 mol% hexene) and a second of a lower density broadly distributed along the molecular weight fractions. The content of the low density component increases with increasing molecular weight of the fraction while the level of 1‐hexene decreases. The mixed compositional character of these fractions is easily inferred by their high crystallization rates and both high melting and crystallization temperatures compared to the values of model random ethylene copolymers. The set of compositional fractions obtained by TREF display an increasing 1‐hexene concentration with increasing molecular weight, and except for the highest molecular weight components (Mw > 150,000 g/mol) their melting and crystallization behavior followed the random pattern. Higher than expected melting temperatures and a constancy of the high melting temperature peak with increasing crystallization temperature, suggests that the intra‐chain 1‐hexene distribution of the highly branched, high molecular weight fraction deviates strongly from the random behavior. These structural features and the bimodal character of the composition distribution of this resin, that contains high molecular weight chains with both low and high 1‐hexene contents, are correlated with the enhanced key film properties.

Phenomenological description of dispersion of 8-60-nm-thick silicon thin films into drops on Al2O3 inert surface

The dispersion of 8-60-nm-thick amorphous silicon thin films on a surface of aluminum oxide was studied using scanning electron microscopy and measuring the current through a thin film during vacuum heating. The temperature of the observed process of silicon thin film dispersion does not correspond to its expected melting temperature, which suggests a dispersion, rather than melting, process, since the thin amorphous silicon film already has a yield property. It was found that the dispersion temperature increases as the silicon thickness is reduced. It is assumed that this phenomenon is caused by the structure and energy effect of the Al2O3 substrate, which propagates into silicon to a certain depth.

Specific Detection of Cytokeratin 20-Positive Cells in Blood of Colorectal and Breast Cancer Patients by a High Sensitivity Real-Time Reverse Transcriptase-Polymerase Chain Reaction Method

A real-time reverse transcriptase-polymerase chain reaction (RT-PCR) method for detection of cytokeratin 20-positive cells in blood characterized by two novel features was developed and tested on 99 patients with colorectal cancer, 110 with breast cancer, and 150 healthy subjects. To optimize the specificity and sensitivity of the method, two novel features were used. First, a primer overlapping two adjacent exons was generated to inhibit nonspecific amplification both in healthy donors and cancer patients; second, a non-end-point first-round amplification was used to increase sensitivity. The number of first-round cycles was chosen to reach the highest level of sensitivity while conserving quantitative characteristics. PCR efficiency increased from 88.9% in single-round RT-PCR to 99.0% in nested real-time RT-PCR. To establish sensitivity and specificity of the method, HT29 cells were serially diluted with normal blood. Detection limit improved from 100 HT29 cells (single-round RT-PCR) to 1 to 10 cells (nested real-time RT-PCR) per 3 ml of whole blood. None of the healthy subjects was positive, whereas 22 and 29% of all colorectal and breast cancer patients, respectively, had cytokeratin 20 cell equivalents in blood. The association between cytokeratin 20 cell equivalents and metastasis was statistically significant for breast (P = 0.026) but not colorectal cancer patients (P = 0.361). Negativity of all 150 healthy controls examined confers diagnostic potential to the method.

A simple method for calibrating the temperature in dynamic mechanical analysers and thermal mechanical analysers

An intrinsic requirement in reliable dynamic mechanical analysis (DMA) or thermal mechanical analysis (TMA) is the precise determination of the sample temperature. In this work a straightforward method is proposed and implemented that allows correction of the average sample temperature for a variety of probe geometries (flexural, tensile and compression). Hydrophilic polymeric-based matrices, with geometries similar to the sample to be tested, are swelled with a pure liquid (in this case water) and placed in the correct testing place of the equipment; a static force is applied at low temperature and the probe position is monitored in a temperature scan, where the melting of the standard is clearly detected and compared with the expected melting temperature. This allows the average sample temperature to be obtained, using a sample that exhibits similar thermal resistance to the sample to be tested, instead of probing specific locations inside the furnace, as it is usually proposed in calibration routines with pure metals. For the specific configurations analysed, a strong and similar dependence of the thermal lag on the scanning rate was found for both flexural and compression modes that also depends weakly on the sample dimensions. More complicated dependence of the thermal lag on the scanning rate is found for the tensile mode, probably due to the complex thermal environment existing inside the furnace with this arrangement. However, the results indicate that there is a significant contribution from the thermal resistance of the sample in the temperature correction. It is suggested that the proposed method may be employed in other situations such as isothermal and non-isothermal creep/stress relaxation experiments or in rheometers.

Failure to genotype herpes simplex virus by real-time PCR assay and melting curve analysis due to sequence variation within probe binding sites.

Real-time PCR with melting curve analysis of PCR products is a rapid procedure for detecting and genotyping herpes simplex virus (HSV). When testing mucocutaneous samples for HSV by a real-time PCR assay targeting the DNA polymerase gene, we found that some PCR products had atypical melting curves that did not conform to the expected melting temperatures for HSV type 1 or 2. Sequence analysis showed that these strains had base-pair mismatches over the probe binding sites. An alternative assay is required to type such atypical isolates.

Melting Temperature of Simulated High-Burnup Mixed Oxide Fuels for Fast Reactors

The melting (solidus) temperatures were measured for fuel simulating burnups of 50, 90, 130, 170, 210 and 250 GWd/t (SIMFUEL) which were added non-radioactive soluble fission products (FPs) to unirradiated fast reactor MOX fuel. The melting temperatures for fuels of 250 GWd/t which were blended non-radioactive soluble FPs and irradiated fuels of 110.6 and 119.0 GWd/t were also compared to the SIMFUEL of 250 GWd/t. The melting temperature decrease of the SIMFUEL tended to saturate with increasing burnup and the melting temperatures appeared virtually constant above 170 GWd/t. An equation for melting temperature was obtained from the proposed equation in the previous report by revising the coefficient of the fifth term: where is the expected melting temperature (K), X 1 the plutonium fraction (Pu/(Pu+U)), X 2 the americium fraction (Am/(Pu+U+Am)), and X 3 the burnup (GWd/t). After the temperature measurement, the radial distribution of eight FP oxide additives in SIMFUEL of 250 GWd/t was measured by X-ray microanalysis. Relative characteristic X-ray intensities of Zr, Ce, La and Sr had the same radial profiles, whilst they did not correspond to the X-ray intensity profiles of Nd and Y.

Melting Temperature of Simulated High-Burnup Mixed Oxide Fuels for Fast Reactors.

The melting (solidus) temperatures were measured for fuel simulating burnups of 50, 90, 130, 170, 210 and 250 GWd/t (SIMFUEL) which were added non-radioactive soluble fission products (FPs) to unirradiated fast reactor MOX fuel. The melting temperatures for fuels of 250 GWd/t which were blended non-radioactive soluble FPs and irradiated fuels of 110.6 and 119.0 GWd/t were also compared to the SIMFUEL of 250 GWd/t. The melting temperature decrease of the SIMFUEL tended to saturate with increasing burnup and the melting temperatures appeared virtually constant above 170 GWd/t. An equation for melting temperature was obtained from the proposed equation in the previous report by revising the coefficient of the fifth term: where is the expected melting temperature (K), X 1 the plutonium fraction (Pu/(Pu+U)), X 2 the americium fraction (Am/(Pu+U+Am)), and X 3 the burnup (GWd/t). After the temperature measurement, the radial distribution of eight FP oxide additives in SIMFUEL of 250 GWd/t was measured by X-ray microanalysis. Relative characteristic X-ray intensities of Zr, Ce, La and Sr had the same radial profiles, whilst they did not correspond to the X-ray intensity profiles of Nd and Y.

Melting Temperature of Irradiated Fast Reactor Mixed Oxide Fuels

The melting (solidus) temperatures of irradiated mixed oxide fuels were measured and the compositions of the fuels on the temperature measurement date were calculated. The fuels contained about 29wt% Pu initially and were irradiated up to 124GWd/t in the experimental fast reactor JOYO. A melting temperature correlation was obtained by an experimental regression analysis using 21 measurements: where [Tcirc]m is the expected melting temperature (K), X 1 the plutonium fraction (Pu/(Pu+U)) and X 2 the americium fraction (Am/(Pu+U+Am)) at the measurement, and X 3 the burnup (GWd/t). The equation shows that the melting temperature decreases by 5, 4, and 3K per 10GWd/t at 50, 100 and 150GWd/t, respectively. The effects of actinides, such as Pu and Am, on melting temperature could be explained by an ideal solution model. However, the decreases caused by soluble fission products could not be well explained in terms of the ideal solution model.

Melting Temperature of Irradiated Fast Reactor Mixed Oxide Fuels.

The melting (solidus) temperatures of irradiated mixed oxide fuels were measured and the compositions of the fuels on the temperature measurement date were calculated. The fuels contained about 29wt% Pu initially and were irradiated up to 124GWd/t in the experimental fast reactor JOYO. A melting temperature correlation was obtained by an experimental regression analysis using 21 measurements:where [Tcirc]m is the expected melting temperature (K), X1 the plutonium fraction (Pu/(Pu+U)) and X2 the americium fraction (Am/(Pu+U+Am)) at the measurement, and X3 the burnup (GWd/t). The equation shows that the melting temperature decreases by 5, 4, and 3K per 10GWd/t at 50, 100 and 150GWd/t, respectively. The effects of actinides, such as Pu and Am, on melting temperature could be explained by an ideal solution model. However, the decreases caused by soluble fission products could not be well explained in terms of the ideal solution model.

Rapid renaturation of complementary DNA strands mediated by cationic detergents: a role for high-probability binding domains in enhancing the kinetics of molecular assembly processes.

The rate of renaturation for complementary DNA strands can be enhanced greater than 10(4)-fold by the addition of simple cationic detergents, and the reaction is qualitatively and quantitatively very similar to that found with purified heterogeneous nuclear ribonucleoprotein A1 protein. Under optimal conditions, renaturation rates are greater than 2000-fold faster than reactions run in 1 M NaCl at 68 degrees C. The reaction is second-order with respect to DNA concentration, and reaction rates approach or equal the rate with which complementary strands are expected to encounter each other in solution. Renaturation can even be observed well above the expected melting temperature of the duplex DNA, demonstrating that some cationic detergents have DNA double-helix-stabilizing properties. The reaction is also extremely rapid in the presence of up to a 10(6)-fold excess of noncomplementary sequences, establishing that renaturation is specific and relatively independent of heterologous DNA. This finding also implies that up to several thousand potential target sequences can be sampled per strand per second. Such reagents may be useful for procedures that require rapid nucleic acid renaturation, and these results suggest ways to identify and design other compounds that increase the kinetics of association reactions. Moreover, this work provides further support for a model relating the existence of flexible, weakly interacting, repeating domains to their function in rapid molecular assembly processes in vitro and in vivo.

On the thermal characterization of the system sulfur-tetracyanoethylene

A DSC characterization of the sulfur-tetracyanoethylene (TCE) system was performed, and the thermal traces obtained quantitatively analyzed. It was shown that (a) some type of solid state interaction takes place between sulfur and TCE, which forces part of the sulfur to be “frozen” into its rhombic form; (b) variable amounts of sulfur and TCE fail to melt at their expected melting temperatures; and (c) a much higher than expected enthalpy change is associated with sulfur polymerization. A phenomenological model is proposed according to which the sulfur and TCE that fail to melt at their melting temperatures undergo fusion in the wake of the liquid-phase sulfur polymerization process. It is shown that the model proposed is valid in the entire composition range studied.