Constant Crystallization Temperature Research Articles

Study of melting and crystallization process of CsPbBr3 by differential thermal analysis

The crystalline CsPbBr3 was synthesized from CsBr (6N) and PbBr2 (5N) by the mechanochemical method with further fusion in quartz ampoule at 640-650 °С. After synthesis, the structure and chemical composition of the obtained material was confirmed by energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction analysis. The melting and crystallization of the obtained perovskite were investigated by the differential thermal analysis (DTA) with heating/cooling rates of 1, 5 and 10 °C/min. Measurements were carried on the self-constructed DTA-setup with S-type thermocouples in the range of 450-590 °C. Each cycle of heating/cooling was repeated three times to confirm the accuracy of the results obtained. A decrease in the melting point from ~ 568.1 °C to ~ 566.2 °C was demonstrated with an increase in the heating rate from 1 °C/min. up to 10 °C/min. respectively. Probably, it's due to the approach to equilibrium conditions of phase transformations at lower heating rates. We recorded an additional-endothermic effect during CsPbBr3 melting. This may indicate a complex process of melting the compound. The thesis of a two-stage melting mechanism of CsPbBr3 perovskite with an initial stage of fragmentation of the crystalline structure and subsequent dissolution of crystalline phase residues is proposed. It is reported that with increasing of the melt heating above a certain "critical" temperature (579-585 °C), its homogenization occurs, and the crystallization temperature is set at 540-550 °C for the heating/cooling rate of 1 ° C/min. and 538-543 °C for the rate of 5-10° C/min. All obtained data confirm the assumption of a two-step melting process of CsPbBr3 perovskite, and the relatively constant crystallization temperature after a critical point of overheating may also indicate a certain structure of the melt of the compound with short-range order in the arrangement of the structural units of the compound in the liquid phase.

Morphology Change of Nitratine (NaNO3) from Aqueous Solution, in the Presence of Li+ and K+ Ions

It is usually accepted that the nitratine (NaNO3) growth morphology is built by only the {10.4} form. In a previous paper, we noticed that it can be changed by addition of selected impurities. Accordingly, a deeper study was carried out to fix the conditions for nitratine morphology change in aqueous solutions by the addition of impurities. We used standard crystallization methodologies: constant crystallization temperature (from supersaturated solutions) and slow evaporation. Both K+ and Li+ ions can modify the nitratine growth shape. Additionally, in order to understand the shape change, we studied the crystal growth rate behavior of {10.4} faces in the presence of these ions and found that it shows a sudden decrease when an impurity is added. Finally, a simple model is proposed to describe how the morphology change takes place.

Pb-doped KCl: from adsorption to habit and crystal structure modification

Many papers dealing with alkali-halides crystal morphology have been published since the Sixties. The relationship between crystal growth or dissolution morphology and the presence of surface specific impurities is commonly recognized. Authors usually attributed morphological changes to the random adsorption of the impurity on specific crystal surfaces. The results of our research clearly show that several morphological changes can be correctly interpreted in terms of 2D-epitaxially absorbed phases [1], [2] leading both to morphological and structural changes. In the case of potassium chloride crystals, both in-situ and ex-situ observations on growing and etched crystals were performed [3] in order to demonstrate that the appearance of the octahedron in the crystal morphology is due to the step bunching effect ascribable to the precipitation of PbCl2 crystallites along the n steps on the {100} forms. Inspired by the Lian et al. [3] detailed “morphodrome” showing the crystal habit and surface modifications of KCl crystals grown in the presence of Pb as specific impurity, we performed a new series of growth and dissolution experiments in order to improve the old data. The impurity concentration ranges from 5 to 10000 ppm at constant crystallization temperature. Moreover we tried to enlarge the impurity concentration range, joining the molar ratio of the two known perovskite structures (KCl:PbCl2 and KCl:2PbCl2) in order to evaluate the continuity between morphological and structural changes from the cell distortion (anomalous mixed crystal formation due to impurity adsorption and 2D epitaxy between KCl and PbCl2) to the symmetry breakdown (from the anomalous mixed crystal at higher symmetry to the perovskite structure at lower symmetry).

Effect of High Intensity Ultrasound on Crystallization Behavior of Anhydrous Milk Fat

AbstractThe need to eliminate trans‐fatty acids from foods’ formulation resulted in the exploration of new lipid sources and alternative processing conditions that will improve the physicochemical characteristics and nutritional qualities of lipid‐based foods. In general, the physicochemical characteristics of lipid networks depend on the microstructure and crystallization behavior of the system. The objective of this work was to use high intensity ultrasound (HIU) as an additional processing condition to alter the crystallization behavior of a lipid model system (anhydrous milk fat). Results show that HIU application not only decreases the induction time of crystallization (faster crystallization) at a constant crystallization temperature, but also generates smaller crystals. In addition, higher viscosities are obtained when samples are crystallized after HIU application. The degree of supercooling, ultrasound application settings and a combination of both parameters influence the degree of ultrasound effect on the crystallization behavior.

Thickness-dependent Crystallization Behavior of Phase Change Materials

ABSTRACTWe have investigated the crystallization behavior of phase change materials as a function of their thickness. Thin films of variable thickness between 1 and 50nm of the phase change materials Ge2Sb2Te5 (GST), N-doped GST (N-GST), Ge15Sb85 (GeSb), Sb2Te, and Ag and In doped Sb2Te (AIST) were deposited by magnetron sputtering, and capped in situ by a 10nm thick Al2O3 film to prevent oxidation. The crystallization behavior of the films was studied using time-resolved X-ray diffraction. For each material we observed a constant crystallization temperature Tx that was comparable to bulk values for films thicker than 10 nm, and an increased Tx when the film thickness was reduced below 10 nm. The thinnest films that showed XRD peaks were 2 nm for GST and N-GST, 1.5 nm for Sb2Te and AgIn-Sb2Te, and 1.3 nm for GeSb. The observed increase in the phase transition temperature with reduced film thickness and the fact that very thin films still show clear phase change properties are indications that Phase Change Random Access Memory technology can be scaled down to several future technology nodes.

Isothermal Crystallization Kinetics of Poly(ε‐caprolactone) with Tetramethyl Polycarbonate and Poly(styrene‐co‐acrylonitrile) Blends Using Broadband Dielectric Spectroscopy

AbstractSummary: Phase behavior and isothermal crystallization kinetics of poly(ε‐caprolactone) (PCL) blends with tetramethyl polycarbonate (TMPC) and poly(styrene‐co‐acrylonitrile) with 27.5 wt.‐% acrylonitrile content have been investigated using broadband dielectric spectroscopy and differential scanning calorimeter. An LCST‐type phase diagram has been observed for PCL/SAN blend while all the different blend compositions of PCL/TMPC were optically clear without any phase separation structure even at high temperatures up to 300 °C. The composition dependence of Tgs for both blends has been well described by the Gordon‐Taylor equation. The phase diagram of PCL/SAN was theoretically calculated using the Flory‐Huggins equation considering that the interaction parameter is temperature and composition dependent. The equilibrium melting point of PCL depressed in the blend and the magnitude of the depression was found to be composition dependent. The interaction parameters of PCL with TMPC and SAN could not be calculated from the melting point depression based on Nishi‐Wang approach. The isothermal crystallization kinetics of PCL and in different blends was also investigated as a function of crystallization temperature using broadband dielectric spectroscopy. For pure PCL the rate of crystallization was found to be crystallization temperature (Tc) dependent, i.e., the higher the Tc, the lower the crystallization rate. The crystallization kinetics of PCL/TMPC blend was much slower than that of PCL/SAN at a constant crystallization temperature. This behavior was attributed to the fact that PCL is highly interacted with TMPC than SAN and consequently the stronger the interaction the higher the depression in the crystallization kinetics. It was also attributed to the different values of Tg of TMPC (191 °C) and SAN (100 °C); therefore, the tendency for crystallization decreases upon increasing the Tg of the amorphous component in the blend. The analysis of the isothermal crystallization kinetics was carried out using the theoretical approach of Avrami. The value of Avrami exponent was almost constant in the pure state and in the blends indicating that blending simply retarded the crystallization rate without affecting the crystallization mechanism.Dielectric constant, ε′, of pure PCL, blends of PCL/TMPC = 80/20 and PCL/SAN = 80/20 as a function of crystallization time at 47 °C and 1 kHz.magnified imageDielectric constant, ε′, of pure PCL, blends of PCL/TMPC = 80/20 and PCL/SAN = 80/20 as a function of crystallization time at 47 °C and 1 kHz.

Effects of reinforcement filler and temperature on the stability of β-crystal in glass bead filled polypropylene

The effects of crystallization temperature ( T c), glass bead content and its size on the formation of β-crystal and structural stability of originally formed β-crystal in glass bead filled polypropylene (PP) were examined. The differential scanning calorimetry (DSC) measurements indicated that the amount of β-phase in PP crystals was a function of the crystallization temperature and glass bead content. For a constant crystallization temperature, it was observed that the amount of β-crystal initially increased with increase in glass bead content up to 30 wt.%, and then decreased slightly with further increase in the filler content. From the DSC data, a disorder parameter ( S) was derived to define the structural stability of originally formed β-crystals. The structural stability of originally formed β-crystals was enhanced with increase in either the crystallization temperature or the glass bead content. Also, the influence of glass bead size (4–66 μm) on the formation and stability of β-crystals in PP/glass bead blends was studied. Large glass bead particles suppressed the formation and decreased the stability of β-crystals.

Soft Magnetic Properties of Nanocrystalline Fe–Co–B–Si–Nb–Cu Alloys in Ribbon and Bulk Forms

Ribbon and bulk nanocrystalline body-centered-cubic (bcc) (Fe,Co) alloys exhibiting good soft magnetic properties were synthesized in Fe71.5-xCoxB13.5Si10Nb4Cu1 system by the simple production processes of melt-spinning or casting and annealing. The glass-type alloys were formed in the Co content range below 30 at.%. These glassy alloys crystallized through two exothermic reactions. The first stage was due to the precipitation of nanoscale bcc-(Fe,Co) phase with a grain size of about 10 nm, and the second stage resulted from the decomposition of the remaining amorphous phase to α–(Fe,Co), (Fe,Co)2B, (Fe,Co)23B6, (Fe,Co)3Si, and (Fe,Co)2Nb phases. The glass transition temperature increased from 820 to 827 K with increasing Co content from 5 to 20 at.%, while the supercooled liquid region decreased slightly from 37 to 30 K because of the nearly constant crystallization temperature. By choosing the 10 at.% Co-containing alloy, we produced cylindrical glassy alloy rods 1.0 and 1.5 mm in diameter by copper mold casting. The subsequent annealing for 300 s at 883 K corresponding to the temperature just above the first exothermic peak caused the formation of nanoscale bcc-(Fe,Co) structure. The bcc-(Fe,Co) alloy rods exhibited good soft magnetic properties of 1.26 T for saturation magnetization and 5.0 A/m for coercive force, which were comparable to those for the corresponding bcc-(Fe,Co) alloy ribbon. The nanocrystalline alloy in a bulk form is encouraging for future use as a new type of soft magnetic material that requires three-dimensional shapes.

Crystallization of sunflower oil waxes

AbstractActivation free energies of nucleation (ΔGc) were calculated using induction times of crystallization measurements. Results showed that ΔGc decreased exponentially as wax concentration increased at a constant crystallization temperature (Tc). In contrast, for a constant supersaturation, ΔGc increased from 12 to 22°C but decreased between 22 and 35°C. Melting behavior of purified waxes and solutions of purified waxes in sunflower oil were studied by DSC after crystallization at fast and slow cooling rates (20 and 1°C/min, respectively). Low supercooling temperatures (Tc>65°C) showed an increase in the onset temperature (T0) as Tc increased for both fast and slow cooling rates. Broader peaks were obtained for samples crystallized at a slow cooling rate at the same Tc. Regarding the solutions of waxes in sunflower oil, the wax concentration (supersaturation of the system) controlled crystallization as well as Tc. As Tc increased, the enthalpy (ΔH) decreased at a constant wax concentration. When wax concentration decreased, ΔH decreased at a constant Tc. For a low driving force, a small shoulder was obtained in the DSC diagrams owing to some type of fractionation. These results showed that wax crystallization is affected by different experimental parameters, such as Tc and cooling rate, depending on the wax concentration of the sample.

Preparation of zinc cobalt oxide spinel single crystals by the chemical transport method

Single crystals of ZnxCo3−xO4 spinel representing well-shaped octahedra with a maximum size of about 4 mm have been obtained for the first time using chemical transport in a closed system with chlorine as a transporting agent. It is shown that the transport process takes place with a satisfactory rate at a temperature in the crystallization zone of about 820°C, crystals with a relatively high zinc concentration (x = 0.3−0.4) being obtained. It is established that this zinc content (x value) decreases with rising temperature of crystallization whereas at a constant crystallization temperature the zinc amount increases with the oxygen pressure in the ampoule. The increase of x in the crystals leads to an increase in lattice constant of the ZnxCo3−xO4 spinel.

The phase diagram and morphology of blends of poly(vinylidene fluoride) and poly(ethyl acrylate)

The phase diagram and crystallization behaviour of the polymer blend system consisting of poly(vinylidene fluoride) (PVF 2) and poly(ethyl acrylate) (PEA) have been examined. The melt exhibits phase separation upon heating to 10°C–50°C above the melting point of the PVF 2, depending on the composition. The cloud point and equilibrium melting point curve (for α-PVF 2) intersect at about 180°C and a composition of 50% (by weight) PVF 2. The polymer-polymer interaction parameter, χ, was calculated from the equilibrium melting point depression data and found to be −0.16 (at 170°C). Spherulite growth rate data have been measured as a function of composition and temperature. Assuming regime II crystallization a value of the product of the surface free energies of the α-PVF 2 crystals was calculated to be 4.4 × 10 −4J 2m −4. In blends crystallized from the one phase melt the texture of spherulites becomes more open and the spherulite extinction ring spacing (due to lamaller twist) becomes larger with increasing crystallization temperature. In addition the ring spacing increases with PEA content at constant crystallization temperature.

Influence of composition on the melt crystallization of isotactic random propylene/1‐butene copolymers

AbstractThe influence of composition on the overall rate of isothermal crystallization of isotactic random propylene/1‐butene copolymers is investigated. At a constant crystallization temperature a small fraction of 1‐butene co‐units inserted along a polypropylene chain causes a drastic reduction in the overall rate of crystallization. Further it is found that for a given rate of crystallization it is possible to crystallize the isotactic polypropylene at lower crystallization temperature by adding along the chain a small amount of 1‐butene as comonomer. The equilibrium melting temperature, the surface free energy of folding, the enthalpy and the entropy of fusion decrease with increasing the contents of 1‐butene in the copolymer samples.

The nucleation in oligomer melts

Abstract Impulse NMR has been used to study the crystallization conditions of oligoethylene- and oligo-butylene adipates as a function of the melt temperature at constant crystallization temperature. Residual crystal structures have been found in the melts above the melting temperature; these act as nuclei in the subsequent crystallization. The Avrami equation describing the crystallization kinetics from the melt has been modified on the basis of this result; in addition to the classical and homogeneous nucleation there is one in which the crystal nuclei are the residual crystal structures already existing in the melt.

Splitting of DTA‐peaks in polyethylene and its relation to the crystalline texture

AbstractLinear polyethylene crystallized from the melt or heat‐treated at temperature above 110°C. shows in the DTA thermogram an additional endothermal maximum Tm1 at a temperature lower by 10 to 20°C. compared with the normal maximum Tm0. The appearanee of Tm1 depends on the crystallization temperature, time, and temperature of heat‐treatment. For a constant crystallization time, the difference between Tm0 and Tm1 decreases with increasing of the crystallization temperature; at a constant crystallization temperature it increases with the crystallization time. By treating those specimen with two endothermal maxima at a temperature near that of Tm1, Tm1 splits into two peaks. This effect is interpreted by the assumption, that those crystalline regions which first correspond to the Tm1 maximum, partly form more stable crystals of greater thickness and higher melting point, and partly melt with subsequent formation of little crystallites of lower melting point during cooling. Under suitable conditions, the three endothermic peaks also can be observed with branched polyethylene.

Crystallization kinetics of poly‐3,3‐bis(chloromethyl) oxacyclobutane

AbstractThe crystallization kinetics of poly‐3,3‐bis(chloromethyl) oxacyclobutane has been investigated by the method of dilatometry, and at the same time the growth rate of spherulites was measured under the polarizing microscopy. Isothermal crystallizations of this polymer were carried out at constant crystallization temperature over the range of 150–160°C., and the rate constants were calculated by the Avrami equation. Crystallization proceeds through a mechanism of homogeneous nucleation and fibril growth. The growth rates of spherulites were measured at various constant crystallization temperatures of 75–140°C. Under a polarizing microscopy, the spherulites formed are seen to be of two morphological types; one is four‐cornered and another is petal‐shaped.

Crystallization of poly-[3,3-bis(chloromethyl)oxacyclobutane

The crystallization phenomena of poly[3,3-bis-(chloromethyl)oxacyclobutane] have been investigated by the methods of X-ray diffraction, dilatometry and infra-red spectroscopy. The rates of crystallization to the α-form of the polymers, having various values of intrinsic viscosity, were measured dilatometrically at the constant crystallization temperature over the range 140°–160°C and the results indicated that a maximum value might exist at a temperature lower than 140°C, probably close to 130°C. The experimental results were used to obtain the k and n values in Avrami's expression. It was found that n=2 (except at 145°C), and from Morgan and Keller's theory this would indicate that fibrillar growth follows nucleation which is sporadic in time. The fully extended specimen showed infra-red dichroism in the region 3000-500 cm −1. As the crystallization of polymers proceeded, the absorption band at 897 cm −1 increased markedly in intensity, whereas the intensity of the absorption band at 1027 cm −1 decreased, suggesting that the former is a crystalline and the latter an amorphous band. The rates of crystallization deduced from plots of the optical densities of these bands against temperature, were in good agreement with those obtained dilatometrically.