Primary Crystallizations Research Articles

PHASE EQUILIBRIA IN THE Tl4P2Se6 - TlSbP2Se6 SYSTEM

Analysis of the literature and previous studies of the authors showed that the section Tl4P2Se6 - TlSbP2Se6 is quasi-binary, and given the closeness of the structures of the ternary and quaternary phases, there was a probability of mutual solubility of the initial components. All syntheses were carried out in vacuumed up to 0.13 Pa quartz ampoules. The synthesis of the samples in the system was carried out from the previously synthesized Tl4P2Se6 and TlSbP2Se6 at 850 K (48 hours). Annealing was carried out at 573 K (240 h), followed by quenching into ice water. The starting components were synthesized from pre-synthesized Tl2Se and elemental components taken in stoichiometric ratios. 11 samples were synthesized within the system (10 and 5 mol% at the starting compounds).The obtained samples were investigated by DTA methods (vacuum quartz vessels, chromel-aluminum thermocouple, heating rate 6°C/min) and XRD (DRON 4, Cu Kα radiation). On the diffraction patterns of the samples obtained, two reflex systems corresponding to the original compounds were observed. The Tl4P2Se6 – TlSbP2Se6 phase diagram is based on the DTA and XRD results with areas of solubility less than 5 mol% based on the initial components. Three polymorphic modifications of Tl4P2Se6 cause four lines of primary crystallization: three based on low-temperature (α-phase), medium-temperature (α/ -phase) and high-temperature (α// - phase) modifications of Tl4P2Se6, as well as TlSbP2Se6. The eutectic process temperature (678 K) is lower than the polymorphic transformation temperature. The lines of the primary crystallizations of the α and β phases intersect at the non-variant eutectic point with the coordinates: 678 K, 55 mol% TlSbP2Se6. Metatectical processes involving high-temperature (α// - phase) and medium-temperature (α/ - phase) modifications of Tl4P2Se6 are realized at temperatures of 746 K (α// ↔L + α/) and 708 K (α/ ↔L + α).

PHASE EQUILIBRIUM IN SYSTEM (LiF)2 – Li2CO3 – Li2SO4

The liquidus surface of the system (LiF)2-Li2SO4-Li2CO3 was studied by the calculated and differential thermal method of physicochemical analysis. An analysis of the systems of lower dimensionality of the facets of the investigated object showed that the most informative, for the experimental study, is the sections located in the crystallization field of lithium fluoride. A study of the DTA of a number of compositions located at the initially chosen polythermal section in the lithium fluoride crystallization field, the ratios of lithium sulfate and carbonate in the eutectic are determined. The composition of the triple eutectic was revealed by studying a non-invariant cut from the vertex of the triangle (LiF), through a point showing a constant ratio of sulfate and lithium carbonate in the eutectic, to the fusion of the thermal effects of the primary and tertiary crystallizations. The complexity of the study was that lithium carbonate is the most fusible component in the system, and according to the literature, after the melting of lithium carbonate, decomposition begins, which greatly complicates the interpretation of research results. In order to avoid the decomposition of lithium carbonate, each experimentally studied composition was heated to the melting temperature of lithium carbonate and kept in isothermal mode, at a temperature below its melting. Thus, the theoretical melting calculations and the region of location of the non-invariant composition have been extrapolated, allowing to limit the number of experimentally studied samples, and the subsequent experimental investigation of DTA of two polythermal sections revealed a eutectic composition crystallizing at 476 ° C and containing LiF-20 eq.%, Li2SO4 - 51 eq.%, Li2CO3 – 29 eq.%. The discrepancies between theoretical calculations and experimental studies are 8.3% in temperature and 5.05% in composition.

Thermodynamic analysis of binary Fe85B15 to quinary Fe85Si2B8P4Cu1 alloys for primary crystallizations of α-Fe in nanocrystalline soft magnetic alloys

Fe-based Fe85B15, Fe84B15Cu1, Fe82Si2B15Cu1, Fe85Si2B12Cu1, and Fe85Si2B8P4Cu1 (NANOMET®) alloys were experimental and computational analyzed to clarify the features of NANOMET that exhibits high saturation magnetic flux density (Bs) nearly 1.9 T and low core loss than conventional nanocrystalline soft magnetic alloys. The X-ray diffraction analysis for ribbon specimens produced experimentally by melt spinning from melts revealed that the samples were almost formed into an amorphous single phase. Then, the as-quenched samples were analyzed with differential scanning calorimeter (DSC) experimentally for exothermic enthalpies of the primary and secondary crystallizations (ΔHx1 and ΔHx2) and their crystallization temperatures (Tx1 and Tx2), respectively. The ratio ΔHx1/ΔHx2 measured by DSC experimentally tended to be extremely high for the Fe85Si2B8P4Cu1 alloy, and this tendency was reproduced by the analysis with commercial software, Thermo-Calc, with database for Fe-based alloys, TCFE7 for Gibbs free energy (G) assessments. The calculations exhibit that a volume fraction (Vf) of α-Fe tends to increase from 0.56 for the Fe85B15 to 0.75 for the Fe85Si2B8P4Cu1 alloy. The computational analysis of the alloys for G of α-Fe and amorphous phases (Gα-Fe and Gamor) shows that a relationship Gα-Fe ∼ Gamor holds for the Fe85Si2B12Cu1, whereas Gα-Fe < Gamor for the Fe85Si2B8P4Cu1 alloy at Tx1 and that an extremely high Vf = 0.75 was achieved for the Fe85Si2B8P4Cu1 alloy by including 2.8 at. % Si and 4.5 at. % P into α-Fe. These computational results indicate that the Fe85Si2B8P4Cu1 alloy barely forms amorphous phase, which, in turn, leads to high Vf and resultant high Bs.

Low temperature synthesis of NaA zeolite membranes: The effect of primary and secondary crystallizations

NaA zeolite membranes were prepared on porous α-alumina support tubes (inner side) by primary (in-situ) and secondary (ex-situ) crystallization processes at 65°C for 6h, 12h and 24h each. For secondary crystallization process, poly(ethyleneimine) (PEI) was used as a buffer layer for proper attachment of the NaA seed crystals (prepared at 65°C/2h) onto the alumina support surface. The membranes prepared by primary crystallization process showed thicknesses of about 10μm, 20μm and 30μm for growth times of 6h, 12h and 24h, respectively, while the membranes prepared by secondary crystallization process rendered the corresponding thicknesses of 25μm, 35μm and 70μm, respectively. PEI coating rendered good quality of membrane for secondary crystallization process with lower growth time (6h). The better quality of membrane which had interlocked dense structure with minimum defects was obtained for primary crystallization at 65°C for 12h growth time. Pervaporation study for ethanol/water mixture was performed with the prepared membranes.

A kinetic model that fits nicely isothermal and non-isothermal bulk crystallizations of polymers from the melt

A model describing the low temperature solid state phase transformation kinetics observed in a metal organic framework by differential scanning calorimetry (DSC) at several cooling rates is modified so that the reaction rate is now expressed as a function of time and temperature. Thus, when applied to ramp data, the new model exactly matches the former one but, additionally, it allows to explain isothermal data. The new model is tested for primary crystallizations of two polymers from the molten state, using DSC data, cooling ramp experiments at several cooling rates and isothermal experiments. Good fittings were obtained at all the varied experimental conditions with both polymers. The model makes use of three fitting parameters with physical meaning: an upper critical temperature, Tc, an energy barrier, and a reaction-order, n+1. Additionally, and previously to perform the kinetic fitting, the dependence of the time to the maximum crystallization rate peak on the isothermal temperature was investigated. That dependence was found to follow a simple model which makes use of two parameters related to the limits of the temperature range in which the crystallization may occur. The polymers used in this work were a commercial extruded polyamide and pristine syndiotactic polypropylene.

High-temperature deformation and crystallization behavior of a Cu36Zr48Al8Ag8 bulk metallic glass

The influence of annealing on the crystallization behavior of a Cu36Zr48Al8Ag8 (at.%) bulk metallic glass (BMG) was investigated. In both isochronal and isothermal annealing processes, the effective activation energies of the primary crystallizations were obtained as 295.8 ± 13.4 and 302.7 ± 14.5 kJ/mol by applying the Kissinger and Ozawa methods, respectively. Using the isothermal transformation kinetics described by the Johnson–Mehl–Avrami model, the Avrami exponent n was found to range between 2.56 and 3.25, which indicates that the primary crystallization behavior was three-dimensional diffusion-controlled growth with an increasing nucleation rate. The high-temperature deformation behavior of a Cu36Zr48Al8Ag8 BMG was then investigated by performing a series of compression tests after rapid heating within a supercooled liquid region. It was found that at least 14–17 dense randomly packed atoms are necessary to produce a unit local flow when the present BMG is subjected to non-Newtonian homogeneous deformation, as described by the transition state equation. Deformation and processing maps were also constructed based on the dynamic materials model to predict optimum bulk formability in a Cu36Zr48Al8Ag8 BMG taking warm deformation-induced crystallization within a supercooled liquid into account.

Melting temperature evolution of non-reorganized crystals. Poly(3-hydroxybutyrate)

In the present study the correlation between the melting behaviour of poly(3-hydroxybutyrate) (PHB) original, non-reorganized crystals and the crystallinity increase during isothermal crystallization is presented and discussed. Since the reorganization processes modify the melting curve of original crystals, it is necessary to prevent and hinder all the processes that influence and increase the lamellar thickness. PHB exhibits melting/recrystallization on heating, the occurring of lamellar thickening in the solid state being excluded. The first step of the study was the identification of the scanning rate which inhibits PHB recrystallization at sufficiently high T c . For the extrapolated onset and peak temperatures of the main melting endotherm, which is connected to fusion of dominant lamellae, a double dependence on the crystallization time was found. The crystallization time at which T onset and T peak change their trends was found to correspond to the spherulite impingement time, so that the two different dependencies were put in relation with primary and secondary crystallizations respectively. The increase of both T onset and T peak at high crystallization times after spherulite impingement was considered an effect due to crystal superheating and an indication of a stabilization process of the crystalline phase. Such stabilization, which produces an increase of the melting temperature, is probably connected with the volume filling that occurs after spherulite impingement.

Crystallization kinetics and crystalline structure of biodegradable Poly(ethylene adipate)

The crystallization kinetics, morphology and crystalline structure of biodegradable poly(ethylene adipate) (PEA) with high and low molecular weights (HMW, LMW) were systematically investigated. The crystallization kinetics obtained from differential scanning calorimetry analysis indicated that both the primary and secondary crystallizations occur during the isothermal and non-isothermal crystallization processes for HMW and LMW PEA samples. Under the same crystallization condition, LMW PEA was found to crystallize faster than HMW PEA. The PEA sample forming the ring-band spherulite presents a higher Avrami exponent value ( n) than that without the ring-band spherulite, due in part to the different crystal growth mechanism. From the results of wide angle X-ray diffraction and Fourier transform infrared spectroscopy, it was confirmed that the crystalline structure of PEA is not dependent on the crystallization temperature ( T c) and MW. From the in-situ FTIR investigation of the crystallization kinetics, it is concluded that, during the isothermal crystallization process at 42 °C, the structural rearrangement of the ester group precedes that of the C C backbone.

Nonisothermal crystallization kinetics of linear metallocene polyethylenes

AbstractThe effect of weight‐average molecular weight (Mw) on the nonisothermal crystallization kinetics of linear metallocene polyethylene (m‐PE) was studied with modulated differential scanning calorimetry. Six linear m‐PEs of molecular weights in the range 122–934 kg/mol were prepared by gas‐phase polymerization. The cooling rate (R) was varied in the range 2–20°C/min, and it significantly affected the crystallization behavior. Mw had a weak influence on both the peak crystallization temperature and the crystallization onset temperature. All m‐PEs showed primary and secondary crystallizations. At both low and high R's, the crystallinity showed a significant drop (∼ 30%) when Mw was increased from 122 to 934 kg/mol. At low R's (< 10°C/min), the rate parameters in the modified Avrami method [primary rate constant (kR)] and Mo method [F(T)] of analyses agreed in suggesting that an increased Mw slowed the rate of crystallization. The Mw dependency of kR followed the Arrhenius type (kR = kRoe281/Mw, where kRo is a rate‐dependent constant). However, at higher R's, kR approached a constant value. The order parameters obtained by the different methods of analysis (n and α) were independent of Mw, which suggests that the crystal type remained the same. Hoffman–Lauritzen theory was used for data analysis, and activation energy per segment showed a significant decrease, from 225.0 to 11.8 kJ/mol, when Mw was increased from 152 to 934 kg/mol. Finally, all methods of analysis suggested a significant effect of Mw on slowing the overall crystallization process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Effects of branching characteristics and copolymer composition distribution on non-isothermal crystallization kinetics of metallocene LLDPEs

The effects of branch content (BC) and copolymer composition distribution (CCD) on the non-isothermal crystallization kinetics of metallocene m-LLDPEs were studied using modified Avrami analysis, modulated differential scanning calorimetry (MDSC), and Crystaf. Several m-LLDPEs and an m-HDPE – all having comparable M w and PDI – were experimented. In addition, a ZN-LLDPE was used for comparison purposes. The branch content, unlike the used cooling rates (2–6 °C/min), significantly affected the crystallization behavior. Crystallization peak temperature, T c peak , decreased linearly with increasing BC. All the m-LLDPEs showed primary and secondary crystallizations. The secondary crystallization showed to be more pronounced at high BC. The primary crystallization Avrami parameter n for m-HDPE ranged between 3.72 and 4.50, indicating spherulitic crystal growth whereas that for the m-LLDPEs, varied from 2.02 to 5.70. The ZN-LLDPE (having broader composition distribution) offered higher values of T c onset and T c peak than the m-LLDPEs with similar BC, M w, and PDI. It, unlike the m-LLDPEs and m-HDPE, fairly agreed with the crystallization kinetic model proposed by Liu et al. The lamella thickness of the m-LLDPEs, L, calculated as per Gibbs–Thomson equation, showed to be in the range 2–16 nm, depending on BC and it decreased approximately following the relationship: L (nm) = 15.0 e (−0.0498BC).

Glass forming ranges of Al–rare earth metal alloys: thermodynamic and kinetic analysis

Using CALPHAD databases, chemical short-range ordering is analyzed and related to metastability of undercooled melts of Al–rare earth metals (including Ce, Gd, Nd and Y). A quasi-kinetic analysis of the primary crystallizations predicts the best glass forming ranges for the quenched alloys, in agreement with those experimentally determined.

Synthesis and non-isothermal crystallization behavior of poly(ethylene-co-1,4-butylene terephthalate)s

A series of random poly(ethylene-co-1,4-butylene terephthalate)s (PEBTs), as well as poly(ethylene terephthalate) (PET) and poly(1,4-butylene terephthalate) (PBT), were synthesized by the bulk polycondensation. Their composition, molecular weight, and thermal properties were determined. All the copolymers are crystallizable, regardless of the compositions, which may originate from both even-atomic-numbered ethylene terephthalate and butylenes terephthalate units that undergo inherently crystallization. Non-isothermal crystallization exotherms were measured over the cooling rate of 2.5–20.0 K/min by calorimetry and then analyzed reasonably by the modified Avrami method rather than the Ozawa method. The results suggest that the primary crystallizations in the copolymers and the homopolymers follow a heterogeneous nucleation and spherulitic growth mechanism. However, when the cooling rate increases and the content of comonomer unit (ethylene glycol or 1,4-butylene glycol) increases, the crystallization behavior still becomes deviated slightly from the prediction of the modified Avrami analysis, which is due to the involvement of secondary crystallization and the formation of relatively low crystallinity. Overall, the crystallization rate is accelerated by increasing cooling rate but still depended on the composition. In addition, the activation energy in the non-isothermal crystallization was estimated.

Microstructural evaluation of primary crystallization with diffusion-controlled grain growth

A model has been developed for evaluating grain size distributions in primary crystallizations where the grain growth is diffusion controlled. The body of the model is grounded in a recently presented mean-field integration of the nucleation and growth kinetic equations, modified conveniently in order to take into account a radius-dependent growth rate, as occurs in diffusion-controlled growth. The classical diffusion theory is considered, and a modification of this is proposed to take into account interference of the diffusion profiles between neighbor grains. The potentiality of the mean-field model to give detailed information on the grain size distribution and transformed volume fraction for transformations driven by nucleation and either interface- or diffusion-controlled growth processes is demonstrated. The model is evaluated for the primary crystallization of an amorphous alloy, giving an excellent agreement with experimental data. Grain size distributions are computed, and their properties are discussed.

Refinement of Size Distributions for Primary Crystallizations

ABSTRACTThe microstructure developed in primary crystallizations is studied under realistic conditions. The primary crystallization of an amorphous alloy is modeled by considering the thermodynamics of a metastable phase transition and the kinetics of nucleation and crystal growth under isothermal annealing. A realistic growth rate, including an interface controlled growth at the beginning of the growth of each single grain and diffusion controlled growth process with soft impingement afterwards is considered. The reduction in the nucleation rate due to the compositional change in the remaining amorphous matrix is also taken into account. The microstructures developed during the transformation are obtained by using the Populational KJMA method, from the above thermodynamic and kinetic factors. Experimental data of transformed fraction, grain density, average grain size, grain size distribution and other related parameters obtained from annealed metallic glasses are modeled.

Transport Properties of Poly(ethylene terephthalate) Crystallized from the Glassy State

Poly(ethylene terephthalate) was crystallized from the glassy state at 120 and 200°C. The structural organization of samples, after the primary and secondary crystallizations, was analyzed by density, X-rays, infrared and transport properties of dichloromethane vapor. The values of crystallinity derived with different methods do not agree, indicating that the crystallized samples cannot be considered simply biphasic. Since the fraction of the impermeable phase is much higher than the fraction of the crystalline phase, it suggested that the presence of mesomorphic form, impermeable to the vapors at low activity. With this hypothesis, the complete composition of the crystalline samples, in terms of fraction of crystalline, amorphous and mesomorphic form was derived. A value of density of the mesomorphic form of 1.39 g/cm3 was also derived.

Transport Properties of Poly(ethylene terephthalate) Crystallized from the Glassy State

Poly(ethylene terephthalate) was crystallized from the glassy state at 120 and 200°C. The structural organization of samples, after the primary and secondary crystallizations, was analyzed by density, X-rays, infrared and transport properties of dichloromethane vapor. The values of crystallinity derived with different methods do not agree, indicating that the crystallized samples cannot be considered simply biphasic. Since the fraction of the impermeable phase is much higher than the fraction of the crystalline phase, it suggested that the presence of mesomorphic form, impermeable to the vapors at low activity. With this hypothesis, the complete composition of the crystalline samples, in terms of fraction of crystalline, amorphous and mesomorphic form was derived. A value of density of the mesomorphic form of 1.39 g/cm3 was also derived.

Crystallization of block copolymers: 3. Crystallization behaviour of an ε-caprolactone-butadiene diblock copolymer

The crystallization behaviour of an ε-caprolactone- block-butadiene diblock copolymer was observed by small-angle X-ray scattering employing synchrotron radiation. The processes of primary and secondary crystallizations were separately analysed by procedures usually used for homopolymer crystallization, and were compared with the case of a poly(ε-caprolactone) homopolymer (PCL). The Avrami analysis at the primary crystallization showed an exponent n ranging from 2 to 3. This value is comparable to n evaluated for PCL and also n widely reported for crystalline homopolymers, indicating that the crystallization of the PCL block drives the primary stage without the influence of the existing microphase structure of the block copolymer. In the secondary crystallization, on the other hand, the rate was significantly retarded when a microphase structure existed in the system.