Observed Melting Point Research Articles

Simulation of the Melting Behavior of the UO2-Zircaloy Fuel Cladding System by Laser Heating

The current research focuses on laser melting and successive analysis of laboratory-scale uranium dioxide nuclear fuel samples in direct contact with Zircaloy-4 cladding. The goal was to characterize the melted and refrozen interfaces, in particular, observing local changes of the melting point and interdiffusion of fuel and cladding materials under inert gas (Ar), in the presence of hydrogen (Ar + 6% H2) or in air. Results obtained by laser heating UO2 pellets clad in a Zircaloy ring were interpreted in light of reference tests performed on pellets in which UO2 and zirconium were blended in a series of given compositions. The sample composition was analyzed by scanning electron microscopy to verify the occurrence of diffusion and segregation phenomena during the laser-heating cycles. Laser-melting experiments were performed on pellets of uranium dioxide clad in Zircaloy-4 rings to simulate the configuration of a light water reactor fuel rod. Under inert gas, the material interdiffusion resulted in consistent melting point depression (of up to 200 K below the melting point of pure UO2) at the interface between the fuel and the cladding. Experiments carried out in the presence of H2 displayed a more limited effect on the melting temperature, but they resulted in a remarkable embrittlement of the whole structure, with large fragmentation of the Zircaloy cladding. This was probably due to the formation of brittle and highly volatile Zr hydrides. The observed melting point decrease was even more pronounced (up to over 400 K) under air in uranium-rich samples, due to the change in the stoichiometry of UO2 in UO2+x.

Melting Point and Viscosity Measurement of Continuous Casting Mold Flux with Volatiles

The composition of fluoride‐containing mold fluxes changes when their melting point or viscosity are measured because the fluoride in the mold fluxes evaporates at high temperatures. Therefore, the measured melting points and viscosities are not those of the original fluxes, and thus, the results are not accurate. Therefore, evaluating the effects of evaporation is essential to accurately investigate the relevant properties. Herein, the chemical composition of the slag is determined using thermogravimetric analysis (TGA), roasting tests, and evaporation mechanism after the physical properties of the slag are measured using conventional methods. The measured melting point and viscosity are then matched with the modified slag composition instead of the original composition. This method is applied to different fluoride‐containing mold fluxes. The weight of the mold fluxes is reduced by more than 10% during the measurements, indicating that a large amount of the volatiles react and evaporate. The actual flux compositions corresponding to the observed melting point or viscosity of the mold fluxes are obtained through roasting test and TGA; therefore, one can easily match the measured mold flux properties to the flux composition. This study will help in determining other physical properties of slag and other melts containing volatiles.

Does the trihydrate of atorvastatin calcium possess a melting point?

To decide whether an active pharmaceutical ingredient can be used in its amorphous form in drug formulations, often the glass transition is studied in relation to the melting point of the pharmaceutical. If the glass transition temperature is high enough and found relatively close to the melting point, the pharmaceutical is considered to be a good glass former. However, it is obviously important that the observed melting point and glass transition involve exactly the same system, otherwise the two temperatures cannot be compared. Although this may seem trivial, in the case of hydrates, where water may leave the system on heating, the composition of the system may not be evident. Atorvastatin calcium is a case in point, where confusing terminology, absence of a proper anhydrate form, and loss of water on heating lead to several doubtful conclusions in the literature. However, considering that no anhydrate crystal has ever been observed and that the glass transition of the anhydrous system is found at 144 °C, it can be concluded that if the system is kept isolated from water, the chances that atorvastatin calcium crystallises at room temperature is negligible. The paper discusses the various thermal effects of atorvastatin calcium on heating and proposes a tentative binary phase diagram with water.

Melt-memory of polyethylenes with halogen substitution: Random vs. precise placement

Polyethylenes with Br, Cl, or F atoms placed at an equal distance of 21 or 15 backbone carbons are known to crystallize as homopolymers, accommodating the halogen in layered crystallites. In contrast, analogs with a random distribution display a crystallization path dominated by sequence-length selection. A consequence of the sequence selection of random copolymers is a constrained interlamellar region and broader melting peaks displaced at higher temperatures than analog systems with the precise placement. Precision and random ethylene-vinyl halides are excellent models to substantiate as a general behavior the strong melt-memory observed in random ethylene 1-alkene copolymers, which contrasts with a much weaker or lack of melt-memory seen in linear polyethylene. While precision polyethylenes with Br, Cl, or F placed on each 21st or 15th backbone carbon show negligible deviation in crystallization rate above the observed melting, the increase in crystallization rate of analogs with the random distribution is observed even from melts 60° above the observed melting point. These data give further evidence of the sharp difference of melt-memory behavior between homopolymers and random copolymers, regardless of whether the co-unit participates in the crystalline regions. Differences in melt-annealing behavior, and a recrystallization of precision polyethylenes that depends on the polymorph that melts prior a subsequent cooling, are also highlighted.

Comprehensive analysis of field-electron emission properties of nanosized silicon blade-type and needle-type field emitters

The reproducibility of complementary metal-oxide-semiconductor (CMOS) technology makes it very promising for creating commercially available vacuum emission micro/nanoelectronic devices. However, there are a number of challenges that occur with CMOS, including current hysteresis, transition to the generation of self-sustained plasma, and thermal melting of the cathode. These issues affect the process of field-electron emission and lead to instability and subsequent degradation of field-emission cathodes. More detailed study is needed in order to address these negative effects. In this study, an array of nanoscale silicon needle-type cathodes and a single blade-type cathode were placed in vacuum to characterize their field-emission properties. The hysteresis nature of the field-emission current and the smooth transition from field emission to the generation of self-sustained plasma in the interelectrode space were simultaneously observed. Based on these experimental results, the authors propose the possible origins and mechanisms underlying these two phenomena. It was theoretically found that at field-emission currents corresponding to the observed melting point of the silicon nanocathodes, the melting point of silicon is not reached, which indicates the need to take into account additional effects of field emission, such as sputtering of the anode material. The results are useful for developing field-emission nanodevices based on silicon CMOS technology.

Comparison of the observed size-dependent melting point of CdSe nanocrystals to theoretical predictions

Cadmium selenide nanocrystals were observed to have a size-dependent melting point which was depressed relative to the bulk melting temperature. The observed size-dependent melting point ranged from 500-1478 K, while a model based on the surface area to volume ratio predicted that is should range between 774-1250 K. The nanocrystals were heated in situ in the electron microscope, and the melting point was almost immediately followed by the vaporization of the CdSe nanocrystals, allowing for straightforward determination of the melting temperature. The differences between the observed melting point of CdSe nanocrystals and the values predicted by the surface area to volume ratio model indicates that additional factors are involved in the melting point depression of nanocrystals.

Impact of Dimerization on Phase Separation and Crystallinity in Bulk Heterojunction Films Containing Non-Fullerene Acceptors

We report the synthesis of monomeric and dimeric dicyanovinylbenzothiadiazole-based non-fullerene acceptors and investigate the phase behavior of blend systems comprising poly(3-n-hexylthiophene) (P3HT) and the new materials. Differential scanning calorimetry (DSC) thermograms of blends with different compositions revealed that no eutectic composition existed for the blends, in contrast to previously reported P3HT/non-fullerene acceptor combinations. Analysis of the DSC data shows that the thermal properties of the P3HT and the dimeric non-fullerene acceptor are almost independent of blend composition. However, the thermal properties of blends containing the monomeric acceptor were found to depend strongly on the blend composition, with increasing concentrations of the monomeric acceptor depressing the observed melting point of the polymer. These results are complemented by specular X-ray diffraction data, which reveals stark differences in the crystallinity of the two blend systems. In the case of the di...

Investigation on drug solubility enhancement using deep eutectic solvents and their derivatives

Deep eutectic solvent (DES) is a room temperature liquid typically formed by mixing two solid compounds, such as a quaternary ammonium salt (QAS) (e.g. choline chloride) and a hydrogen bond donor (HBD) (e.g. urea or a carboxylic acid) at their eutectic composition. Very often, a range of room temperature liquids can also be obtained near the eutectic composition. Hence, it is more convenient to introduce a more general term deep eutectic solvent derivatives (DESDs) to describe a wide range of DES-like derivatives including those derived from ternary mixtures. The melting point of the mixture is lowered because the hydrogen bonding between DESD components reduces the lattice energy of components of the eutectic system. Based on the analysis of available data for 22 such choline chloride-based DES pairs, we found that the observed melting point depression can be statistically correlated with the difference between the hydrogen bonding contribution (δh) and the polar contribution (δp) to the solubility parameter of the hydrogen bond donor (HBD) component. The correlation was validated with a new DESD based on glycolic acid and choline chloride, which form DESDs at a molar ratio between 1:1 and 1:4 with DES-like properties. As a room temperature liquid, this DESD exhibits a wide range of solubility enhancement on several weakly basic poorly water-soluble drugs. For example, the solubility of itraconazole, piroxicam, lidocaine, and posaconazole has been observed to increase by 6700, 430, 28, and 6400-fold, respectively as compared to their aqueous solubility at room temperature. Furthermore, another new ternary DESD based on choline chloride, glycolic acid, and oxalic acid at a molar ratio of 1:1.6:0.4 is shown to further increase the solubility of itraconazole to a remarkable level of 5.36mg/mL (a 53,600-fold increase!). Because the components of such DESDs can include those biodegradable ones that had previously been used in formulated human products, the potential applicability of suitable DESDs to drug delivery, especially in enhancing drug solubility for topical formulations could be very attractive.

Annealing of poly (ethylene terephthalate)

Poly (ethylene terephthalate), PET, crystallized isothermally at 234°C has been annealed above the crystallization temperature in regimes II, below the onset of melting, and in regime III, in the range of observable melting [1,2]. The mechanism has been analyzed by differential scanning calorimetry and FTIR spectroscopy and shown to be one of the limited recrystallization nucleated by crystalline regions which survived melting followed by a process of lamellae thickening.Annealing above the crystallization temperature in regime II where melting and recrystallization did not occur involved lamellae thickening by growth of the lamellae along the chain direction. This was a diffusion controlled mechanism involving either diffusion of non-crystallizable impurities or chain entanglements. The process of stem thickening was the same as that of secondary crystallization previously observed in isothermal crystallization [3] and which is responsible for the observed melting point of the polymer being dependent on the annealing time as well as the temperature.In regime III melting of the polymer occurred initially followed by recrystallization on the residual lamellae acting as seeds by one dimensional growth along the original growth direction. This was later accompanied by thickening of the lamellae with the same characteristic time dependence of diffusion control as observed in regime II and with secondary crystallization. This involves reptation of the molecular chain from the entangled melt onto the growth face.

Crystallization of novel poly(ε-caprolactone)-block-poly(propylene adipate) copolymers

Poly(e-caprolactone)-block-poly(propylene adipate) (PCL-block-PPAd) copolymers were prepared using a combination of polycondensation and ring opening polymerization of e-CL. 1H-NMR and 13C-NMR spectroscopy showed that the prepared copolymers were block. Also, the copolymer composition was calculated from NMR spectra and was found similar to the feeding ratio. The copolymers formed PCL crystals as was proved by WAXD. The crystallization rates and degree of crystallinity, measured from DSC crystallization experiments, decreased with PPAd content. The equilibrium melting points of PCL were estimated applying the Hoffmann–Weeks method and the observed melting point depression was analyzed using the Nishi–Wang equation which showed that there is some miscibility of the copolymer segments. Isothermal crystallization experiments after self-nucleation were performed to distinguish the nucleation and crystal growth stages during isothermal crystallization. The secondary nucleation theory was then used and the obtained data for crystallization rates, estimated from the inverse of the crystallization half-times, were analyzed. The resulting values for nucleation constant K g, and also for the surface free energies and work of chain folding, increased with PPAd content due to topological restrictions.

From Salts to Ionic Liquids by Systematic Structural Modifications: A Rational Approach Towards the Efficient Modular Synthesis of Enantiopure Imidazolium Salts

This paper reports a simple and robust modular synthetic strategy that leads to a large variety of configurationally and structurally diverse imidazole-based chiral ionic liquids (CILs) by lipase-catalyzed resolution. The intimate microscopic interactions of the supramolecular ionic network of these imidazolium chiral salts at the molecular level are investigated both spectroscopically (NMR, FT-IR-ATR) and theoretically, and a topological analysis of the experimental electron densities obtained by X-ray diffraction of single crystals is performed. Our results support the key role played by the relative configuration of the -OR group on the hydrogen-bonding pattern and its strong influence on the final physical properties of the imidazolium salt. We also obtained a reasonable correlation between the observed melting point and the non-covalent interactions. The spectroscopic data and the topological analysis reflect the key role played by hydrogen bonds between the OH and imidazolium C2H groups in both cation-anion and cation-cation interactions, with the presence of an OH group leading to an additional inter-cation interaction. This interaction significantly affects the properties of stereoisomeric salts. Even more interestingly, we also studied the effect of the chirality by comparing enantiopure CILs with their racemic mixtures and found that, with the exception of trans-Cy6-OH-Im-Bn-Br, the melting points of the racemic mixtures are higher than those of the corresponding enantiomerically pure forms. For stereoisomeric examples, we have successfully explained the differences in melting temperatures in light of the corresponding structural data. Chirality should therefore be taken into account as a highly attractive design vector in the preparation of ILs with specifically desired properties.

Anomalous melting behavior of cyclohexane and cyclooctane in poly(dimethyl siloxane) precursors and model networks

AbstractBuilding on previous observations of anomalous melting behavior of solvents in polyisoprene, we have expanded our insight into the melting behavior of organic solvents in polymers and polymer networks through a calorimetric investigation of cylcohexane and cyclooctane in poly(dimethyl siloxane) (PDMS) precursors and model networks. The results are contrary to general expectations. Besides deviations between the predictions of the Flory‐Huggins model and observed melting point depression of the small molecule organics, it is found that the melting point depression of cyclohexane in model networks is lower than that in the uncrosslinked precursors and unaffected by the molecular weight between crosslinks, which is not consistent with the general observation that higher crosslinking density leads to greater melting point depression. We interpret the observed phenomenon in terms of phase separation. In the case of cyclooctane, it exhibits a double melting peak in the model networks with high molecular weight between crosslinks. Furthermore, the heats of fusion of both cyclohexane and cyclooctane decrease with increasing polymer volume fraction which violates the underlying assumption that the heat of fusion of solvent in the polymer is the same as that in the bulk for both the Flory‐Huggins model and the Gibbs‐Thomson equation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2779–2791, 2008

Temperature dependence vapour pressure measurements of Mg(tmhd) 2(tmeda) [(tmhd = 2,2,6,6-tetramethyl-3,5-heptanedione, tmeda = N, N, N′, N′-tetramethylethylenediamine

The reaction between the magnesium β-diketonate complex Mg(tmhd) 2(H 2O) 2 and 1 equiv. of N, N, N′, N′-tetramethylethylenediamine (tmeda = Me 2NCH 2CH 2NMe 2) in hexane at room temperature yielded Mg(tmhd) 2(tmeda). The standard enthalpy of sublimation (83.2 ± 2.3 kJ mol −1) and entropy of sublimation (263 ± 6.3 J mol −1 K −1) of Mg(tmhd) 2(tmeda) were obtained from the temperature dependence vapour pressure, determined by adopting a horizontal dual arm single furnace thermogravimetric analyser as a transpiration apparatus. From the observed melting point depression DTA, the standard enthalpy of fusion (58.3 ± 5.2 kJ mol −1) was evaluated, using the ideal eutectic behaviour of Mg(tmhd) 2(tmeda) as a solvent with bis(2,4-pentanedionato)magnesium(II), Mg(acac) 2 as a non-volatile solute.

A Nitration Reaction Puzzle for the Organic Chemistry Laboratory

Treatment of phenylacetic acid with 90% HNO3 yields a product, I, whose observed melting point is 175–179 °C and whose equivalent weight is approximately 226 grams. Treatment of phenylacetic acid with 70% HNO3 yields a product, II, whose observed melting point is 106–111 °C and whose equivalent weight is approximately 181 grams. Based on these data and 1H NMR and infrared spectral data, students are asked to deduce the structures of nitration products I and II. The above nitration experiment illustrates fundamental electrophilic aromatic substitution concepts while posing an interesting structure elucidation puzzle for organic chemistry students. The nitration of phenylacetic acid requires students to ponder the directing ability of the carboxymethyl group. Given that nitric acid is a powerful oxidizing agent, students must also ponder whether the carboxymethyl group is subject to oxidation under the reaction conditions employed. Lastly, students must determine what impact, if any, the concentration of the nitric acid used will have on the structures of the nitration products obtained.

The melting behavior of aluminum nanoparticles

The melting behavior of aluminum nanoparticles having an oxide passivation layer is examined using a differential scanning calorimetry (DSC). Both broad and narrow size-distributed particles are studied, and the weight-average particle radius ranges from 8 to 50 nm. With decreasing particle size, the melting response moves towards lower temperatures and the heat of fusion decreases. The effect of the oxide coating on the particles is to apply a compressive force to the aluminum core, thereby increasing the observed melting point and the heat of fusion. The melting point depression, both corrected and uncorrected for the effects of the oxide shell, is linear with the reciprocal of particle radius, as predicted by Gibbs–Thomson equation, although only the corrected data give a value of the solid–liquid interfacial tension comparable to those reported in the literature. The size-dependent heat of fusion is significantly smaller than that predicted by the effects of the surface tension indicating that the solid nanoparticle is at a higher energy than expected, presumably due to the presence of defects or irregularities in the crystal structure at or emanating from the surface. This hypothesis is tested using our data, as well as using data in the literature for tin nanoparticles.

Analysis of pore size distribution by 2H NMR

The pore size distributions of four controlled pore glasses with mean diameters ranging from ca. 7.9 to 23.9 nm were analysed by measuring the (2)H NMR signals from the liquid fraction of confined benzene-d(6) and cyclohexane-d(12) as a function of temperature, in steps of ca. 0.1-1 K. The liquid and solid components of the adsorbates were distinguished, on the basis of the spin-spin relaxation time T(2), by employing a spin-echo sequence. The experimental intensity curves of the non-frozen liquids are well represented by a sum of two error functions. The observed melting point depressions are well represented by the simplified Gibbs-Thompson equation DeltaT = k(p)/R where R is the pore radius and k(p) is a characteristic property of the adsorbate. The k(p) value mainly affects the position of the pore size distribution curve, i.e., the mean pore radius, while the slope of the intensity curve determines the width of the distribution curve. In practice, the NMR method can only be used to determine pore sizes with reasonable accuracy in the mesoporous range unless liquids undergoing larger melting point depressions than the ones investigated so far can be found.

Crystallinity and Miscibility of Poly(vinyl isobutyl ether)/Poly(ε-L-lysine) Blends by Solid State 13C NMR Study

The relation between crystallinity and miscibility of semicrystalline polymer blends of poly(vinyl isobutyl ether) (PVIBE) and poly(e-L-lysine) (e-PL) is investigated by 13C cross-polarization with magic-angle-spinning (CPMAS) NMR (nuclear magnetic resonance) and DSC measurements. Analysis of the 1H spin-lattice relaxation curves observed from both PVIBE and e-PL signals, which are indirectly obtained from well-resolved 13C CPMAS NMR spectra, suggested that the domains of e-PL in the blends are in the range of 50–100 nm scale. Two-spin or three-spin models were used to simulate the 1H relaxation curves taking into account the contribution of a 1H spin-diffusion rate between PVIBE and e-PL. Furthermore, it was found that blending e-PL largely influences the crystallinity of PVIBE but that of e-PL is little affected by blending PVIBE except for 10/1 composition. The observed melting point of e-PL shifted towards lower temperature accompanied by PVIBE content. This shift was explained by the Gibbs-Thomson effect that the decrease of thickness of crystalline phase causes the depression of the melting point.

Determination of pressure effect on the melting point elevation of Al nanoparticles encapsulated inAl2O3without epitaxial interface

We report a quantitative measurement of the pressure effect on melting point elevation of encapsulated nanoparticles without epitaxial interfaces. By means of in situ x-ray diffraction, a substantial melting point elevation of about 15 K in encapsulated Al nanoparticles in Al2O3 shells without epitaxial interfaces was observed. Meanwhile, a pressure buildup of about 0.25 GPa on the Al nanoparticles was determined due to constraint of the rigid Al2O3 shell. The correlation between the measured pressure and melting point elevation in the present system verified that the observed melting point elevation is a pressure-induced phenomenon and the pressure effect on melting point for encapsulated nanoparticles follows the rule for bulk solids.

Structural Analysis of Crystalline Nylon 6,12 Exposed to Gamma Radiation

Characterisation of crystalline poly(amide 6,12)fibers subjected to various γ-irradiation doses is presented. The specimens were analysed through their thermal, vibrational and morphological properties. All the experimental evidence points to the conclusion that chain scission may have occurred, forming low molecular weight species (oligomers) due to the degradation of the neat polymer chain. The thermal behaviour (fusion point and crystallinity) provides evidence that the oligomers formed act as plasticifing agents. The observed melting point depression of the irradiated poly(amide 6,12) samples is explained by Flory's theory [1]. The vibrational properties studied by Infrared spectroscopy (FT-IR) show the increase in crystallinity; the images obtained by atomic force microscopy (AFM) are also an evidence of the surface pattern modification by γ-irradiation.

Melting Point Range and Phase Diagrams- Confusing Laboratory Textbook Descriptions

This paper proposes use of a modified phase diagram as a better tool for students to understand the relationship of observed melting point behavior and a physically correct phase diagram.