First Stage Of Crystallization Research Articles

Fluid inclusions in apatite from Jacupiranga calcite carbonatites: Evidence for a fluid-stratified carbonatite magma chamber

Carbonatites of the Jacupiranga alkaline–carbonatite complex in São Paulo State, Brazil, were used to investigate mineral–fluid interaction in a carbonatite magma chamber because apatite showed a marked discontinuity between primary fluid inclusion-rich cores and fluid inclusion-poor rims. Sylvite and burbankite, apatite, pyrite, chalcopyrite and ilmenite are the common phases occurring as trapped solids within primary fluid inclusions and reflect the general assemblage of the carbonatite. The apatite cores had higher Sr and REE concentrations than apatite rims, due to the presence of fluid inclusions into which these elements partitioned. A positive cerium anomaly was observed in both the core and rim of apatite crystals because oxidised Ce 4+ partitioned into the magma. The combined evidence from apatite chemistry, fluid inclusion distribution and fluid composition was used to test the hypotheses that the limit of fluid inclusion occurrence within apatite crystals arises from: (1) generation of a separate fluid phase; (2) utilization of all available fluid during the first stage of crystallization; (3) removal of crystals from fluid-rich magma to fluid-poor magma; (4) an increase in the growth rate of apatite; or (5) escape of the fluids from the rim of the apatite after crystallization. The findings are consistent with fractionation and crystal settling of a carbonatite assemblage in a fluid-stratified magma chamber. Secondary fluid inclusions were trapped during a hydrothermal event that precipitated an assemblage of anhedral crystals: strontianite, carbocernaite, barytocalcite, barite and norsethite, pyrophanite, magnesian siderite and baddeleyite, ancylite-(Ce), monazite-(Ce) and allanite. The Sr- and REE-rich nature of the secondary assemblage, and lack of a positive cerium anomaly indicate that hydrothermal fluids have a similar source to the primary magma and are related to a later carbonatite intrusion.

Nanostructure development in wet amorphous amylopectin as revealed by in situ X‐ray scattering methods

AbstractCompletely amorphous, transparent bars of amylopectin were prepared by injection molding of pure native semicrystalline samples (Waxy Maize). The formation of a semicrystalline morphology was studied during annealing treatment at various temperatures in a wet atmosphere, using simultaneous WAXS and SAXS. Amylopectin samples crystallized during 20 days in a humid atmosphere at room temperature and subjected to a successive melting process were also studied by X‐ray scattering. Results indicate that individual molecules crystallize independently from each other, similar to the case of native amylopectin grains. During the first stages of crystallization, the changes in the SAXS pattern suggest that uncorrelated crystal blocks are formed, which may arrange to lamellae (if a secondary network of double‐helix net‐points is hydrothermally dissociated). At the beginning of the crystallization process, only few amylopectin molecules (about 10%) are incorporated into the nanostructure. A shell‐like structure of semicrystalline layers, comparable to that of a native grain, develops. When crystallization proceeds further, the initially thin shell layers thicken. This causes the amorphous interlayers to be subjected to inner tensions, leading to a decrease in the melt temperature. After a storage time of 20 days in a humid atmosphere, amylopectin reaches a crystallinity level of 54%, only slightly lower than that of the initial native grains. Upon heating the retrograded amylopectin, immediately before complete melting, the long‐period shows a value of 15 nm with a crystal thickness, derived from WAXS, of only 4 nm. Such a structure, which has not been reported before, is due to the relaxations of the inner tensions during melting, which lead to a disappearance of inserted interlamellar crystals. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3832‐3839, 2006

Atomistic simulations of spinodal phase separation preceding polymer crystallization

Many polymeric materials crystallize when cooled below their melting temperature. Although progress has been made in our understanding of the crystallization process through both experimental and theoretical efforts, these studies have focused mainly on the crystal nucleation and growth mechanism, where critical nuclei are formed from a metastable state during the first stages of crystallization, leading ultimately to the growth of crystal domains. Attention has also been given to the structure during the precrystallization (induction period). A pretransition state occurring before crystallization has been characterized as an unstable phase separation initiated by density and orientational fluctuations. These fluctuations are caused by an increase in the average length of rigid trans segments along the polymer backbone during the induction period. These observations are consistent with the theory proposed in ref. 14 on the isotropic-to-nematic transition of polymer liquid crystals, that is, the parallel ordering of polymers is caused by an increase in chain rigidity. Here we use large-scale computer simulations to investigate melts of polymers in the early ordering stages (induction period) before crystallization. In the ordered domains we identify growing dense regions similar to smectic liquid crystals. Our simulations reveal a 'coexistence period' in the ordering before crystallization, where nucleation and growth mechanisms coexist with a phase-separation mechanism.

In situ microscope and impedance study of polymer electrolytes

Simultaneous impedance measurements and optical observations of polymer electrolytes were conducted in a computer-controlled setup, which combined a dielectric analyzer, a polarizing microscope with a heating stage and a digital camera. The polymer film was placed between glasses with ITO conductive layers in a cell preserving argon atmosphere. The setup allowed automated recording of impedance spectra and microscope images consecutively at a set of stabilized temperatures between −20 and 100 °C. Measurements were repeated at a constant temperature when a drift of impedance was detected. The influence of melting and crystallization phenomena on electrical properties of high molecular weight PEO and PEO:LiTFSI system (50:1 molar ratio) was investigated. The evolution of impedance spectra was correlated with changes of polymer morphology during various heating and cooling runs. Impedance spectra were analyzed by fitting an equivalent circuit, which modeled: the ionic conductivity, the dielectric constant, the electric polarization due to inhomogeneity in the bulk of sample and the electrode polarization. The results indicate that the large decrease of ionic conductivity is related to the closing of amorphous conductivity pathways by the growing spherulites in the first stage of crystallization, and to the densification of the structure and interfacial phenomena in the second stage.

Comment on the Structure of Amorphous Starch as Derived from Precursors of Crystallization: The Role of the Entanglement Network

A model based on data about the critical entanglement molecular weight is proposed to derive the statistical chain segment length, ls, of amylose. This value is somewhat larger than one turn of the single helix molecule. This implies a high mobility of the amylose molecule, leading to a statistical shape in which the V‐helix of the amylose molecule is subjected to a local varying extension. On the one hand, the model proposed explains the structure of the amorphous state; on the other it allows the understanding of the formation mechanism of a double helix from two single helices. Discussion of results assumes the formation of chain entanglements through rapprochement of statistical single helix molecules. The direct distance ξ=15 nm between entanglements in the amorphous phase can be calculated from the ls value. It is shown that ξ can be determined from X‐ray scattering data that characterize the early state of crystallization of amylose. Crystals are formed within the amorphous matrix in the entanglement‐free regions. Since some of the first crystals are organized in lamellar stacks, as evidenced by small angle X‐ray scattering (SAXS) results, their long period, L, provides a first indication about the average distance between entanglements. The crystals appearing during the first stages of crystallization, giving rise to the SAXS maximum, model, therefore, the distance between entanglements, which are segregated into the amorphous layers. Previous X‐ray experiments performed on injection‐molded potato and pea starch yield a characteristic value of L=15–16 nm during the early state of crystallization of amylose, in accordance with the above‐mentioned ξ‐calculated value (The crystalline fraction remains within the range of 7–12%). Results indicate that the entanglement network of an amorphous starch melt is transferred into the solid state. The critical temperature is about 120–130°C. The melt entanglement network, thus, is a precursor of the amylose network of the solid state. A second component of the amylose network, which leads to a densification of the entanglement network, is also discussed. This secondary network develops from net points of double helices at lower temperature (70°C) and defines the characteristic temperature of gelatinization of native starch in water.

Effect of Fe addition on the crystallization behaviour and Curie temperature of CoCrSiB-based amorphous alloys

The crystallization behaviour and Curie temperature of melt-spun Co71− x Fe x Cr7Si8B14 (x = 0, 2, 3.2, 4, 6, 8 and 12 at.%) amorphous alloys have been studied. Differential scanning calorimetry (DSC) showed two stages of crystallization. The first stage of crystallization (T X1) in the alloy with 6 at.% Fe was the highest and it had the highest activation energy. X-ray diffraction studies revealed that the primary crystalline phase is hcp-(CoCr)2Si for Fe-free alloy, whereas (CoFeCr)2Si and (CoFeCr)3Si phases were formed with the addition of Fe. hcp-Co was also formed along with these phases. The secondary crystalline phases were fcc-Co and various boron-rich phases. The Curie temperature of the alloys also changed with the addition of Fe to the system. Like the primary crystallization temperature, the Curie temperature of the alloys did not vary systematically with the Fe content. The addition of Fe to the Co-based system changes the nearest-neighbour interaction. This changes the exchange interaction between the transition metal elements. Due to the asymmetry in the Bethe–Slater curve, a systematic variation with Fe addition was not observed in the Curie temperature measurement.

SAXS Study of the Nucleation of Glycine Crystals from a Supersaturated Solution

Nucleation of crystalline solids, the first stage of crystallization from solution, is not yet fully understood. This is true for both small molecules of low molecular weight and more complicated large molecules. To obtain direct structural information about the process of nucleation and crystallization of small molecules, small-angle X-ray scattering (SAXS) has been used to study the crystallization of the amino acid glycine from its supersaturated aqueous solution. The scattering data was analyzed using the unified fit model, which is well-suited for studying complex systems that may contain multiple levels of related structural features. The results suggest that glycine molecules exist as dimers in supersaturated solution. The system obeys power-law behavior that indicates the presence of fractals in the solution. A transformation from mass fractal structure to surface fractal structure is observed during the crystallization process, which could be the signature of a two-step nucleation process.

Solvent-Induced Crystallization in Poly(ethylene terephthalate) during Mass Transport: Mechanism and Boundary Condition

The mechanism of mass transport and the related crystallization in poly(ethylene terephthalate) (PET) were studied. Solvent-induced crystallization can occur during the transport process in PET at low temperature. The important effect of changing the surrounding medium to solvent molecules is to reduce the glass transition temperature. This phenomenon is called “plasticization”. The extent of plasticization relies on the amount of solvent around the polymer molecules, i.e., the concentration of solvent which will depend on the mass transport before saturation. Evidences for the transport mechanism in the first stage of crystallization were revealed. The distinct diffusion front was determined from the measurements of optical microscope and microhardness. The differential scanning calorimeter (DSC) curves displayed crystallization exothermic peaks whose areas decreased with the amounts of amorphous regions, representing the solvent-induced crystallization (SINC) process. The observed phenomenon of multiple-stage crystallizations is associated with the variation of boundary conditions during mass transport, which was ignored in most theoretical analyses.

Influence of Nb on the first stage of crystallization in Fe86−xNbxB14 amorphous alloys

Abstract The kinetics of the first stage of crystallization (nanocrystallization) in the Fe 86− x Nb x B 14 amorphous alloys was studied by applying differential scanning calorimetry and magnetic balance techniques. It was shown that the nanocrystallization temperature increases with increasing Nb content by more than 80 K. Activation enthalpy of the nanocrystallization process increases from 2 eV for 2 at.% of Nb to 4 eV for 5–8 at.% of Nb. The results obtained by the mentioned experimental techniques are also discussed from the methodological point of view.

Amorphous to icosahedral phase transformation in rapidly quenched Al–Cu–V and Al–Cu–Ti alloys

A comparative investigation of stabilization of amorphous phase and its transformation behaviour in Al75Cu15V10, Al65Cu25V5Ti5 and Al50Cu45Ti5 alloys has been carried out employing X-ray diffraction (XRD) and transmission electron microscopy (TEM) and supported by differential scanning calorimetry (DSC). The DSC curve corresponding to amorphous alloy Al75Cu15V10 shows two stages of crystallization with peak positions at 306 °C and 420 °C. XRD and TEM investigations reveal that the first stage of crystallization corresponds to nanosized crystal formation of Al2Cu phase, which is dispersed in the amorphous matrix. The second stage of crystallization corresponds to growth of icosahedral phase. Partial substitution of Ti in place of V in Al65Cu25V5Ti5 alloy stabilizes amorphous phase, which after subsequent annealing at 400 °C for 2 h transforms to a crystalline phase. Amongst several compositions in the Al–Cu–(Ti,V) alloy system prepared during present investigation, amorphous-to-icosahedral phase transformation has only been observed in Al50Cu45Ti5 alloy after isothermal annealing at 300 °C for 2 h. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magma mixing in the Sithonia Plutonic Complex, Greece: evidence from mafic microgranular enclaves

¶Mafic microgranular enclaves occur in most calc-alkaline granitoids, and it is widely accepted that they represent the remnants of basic magmas that interacted with more acid magmas. In this work we present new data on mafic microgranular enclaves occurring in the granodiorites of the Sithonia Plutonic Complex (Northern Greece). Enclave properties have been studied using different methods. Quantitative textural analysis has been carried out in order to decipher the crystallization history of enclaves once they have been entrained in the more acid and cooler host magma. In particular, the nucleation density (C), the mode (M) and the crystal index (n) of enclaves has been measured. Along with textural analysis, the size of enclaves has also been estimated using a method that, based on two-dimensional sections of enclaves, allows the estimation of volume of enclaves. Geochemical analyses have been performed to investigate the degree of chemical interaction that enclaves suffered from the host acid magma. The different data sets have been utilized to furnish a general evolutionary model of the magmatic interaction process between the basic and the acid magmas that led to the formation of the granodioritic host rock and related mafic microgranular enclaves. It is concluded that, as the magmatic interaction process proceeded, the crystallization of enclaves involved the nucleation of apatite and epidote (first stage of crystallization) followed by biotite, ± hornblende, plagioclase, and titanite (second stage of crystallization); the last minerals that nucleate were quartz and K-feldspar. During crystallization enclaves underwent contamination by the host acid magma through flow channels opened during the transfer of mineral phases from the host magma to the enclaves. When the two magmas attained similar rheological behaviour a two-end member mixing process was favoured inducing progressively more vigorous mixing dynamics. Volumetric analysis of enclaves indicates that the smaller ones suffered a more intense geochemical interaction compared to the larger ones. We interpret this evidence as being strictly related to the kinematics of the mixing process, the latter governed by chaotic dynamics. Enclaves are viewed as portions of the basic magma that did not mix completely with the acid host magma and survived the mixing process. Host rocks are considered as volumes of the magmatic system where the more efficient mixing dynamics produced different, generally higher, degrees of hybridisation.

Magnetic and structural behaviours of nanocrystalline Fe 70.8 Nb 3.7 Cu 1 Al 2.7 Mn 0.7 Si 13.5 B 7.6 alloy

The crystallization behaviour of Fe 70.8 Nb 3.7 Cu 1 Al 2.7 Mn 0.7 Si 13.5 B 7.6 alloy prepared in the form of amorphous ribbons by melt-spinning technique was studied using differential scanning calorimetry and the temperature variation in resistivity. An X-ray diffaction and transmission electron microscopy study showed the formation of f-Fe(Si, Al) and/or Fe 3 (Si, Al) nanoparticles after the first stage of crystallization. The activation energy for this nanophase formation was 68 kcal mol m1 . The brittleness of the alloy increased with the formation of nanoparticles after heat treatment. Superior soft magnetic properties were achieved when the material was heat treated at 790 K for 15 min. The particle size at the optimum heat treatment condition for superior soft magnetic properties was found to be 6.0 -0.5 nm which was less compared than for the Fe-Nb-Cu-Si-B system. The observed coercivity value at the optimum heat treatment condition was found to be 0.32 A m m1 (approximately 4 mOe). The presence of Al in the alloy reduced the particle size and the magnetic anisotropy energy of the system, which resulted in superior soft magnetic properties of the heat-treated materials.

Vibrational Spectroscopy and Analytical Electron Microscopy Studies of Fe-V-O and In-V-O Thin Films

Orthovanadate (M 3+VO4; M = Fe, In) and vanadate (Fe2V4O13) thin films were prepared using sol-gel synthesis and dip coating deposition. Using analytical electron microscopy (AEM), the chemical composition and the degree of crystallization of the phases present in the thin Fe-V-O films were investigated. TEM samples were prepared in both orientations: parallel (plan view) and perpendicular (cross section) to the substrate. In the first stages of crystallization, when the particle sizes were in the nanometer range, the classical identification of phases using electron diffraction was not possible. Instead of measuring d values, experimentally selected area electron diffraction (SAED) patterns were compared to calculated (simulated) patterns in order to determine the phase composition. The problems of evaluating the ratio of amorphous and crystalline phases in thin films are reported.

Positron states in hydrogenated amorphous silicon

Low-hydrogen-concentration a-Si:H grown by HW-CVD forms a continuous random network with no detectable free volume in the form of microvoids, and no evidence of a microcrystalline phase. Over a wide temperature range we observe a single positron state in the amorphous network with a temperature-independent lifetime of 322 ps. From the temperature dependence of the positron diffusion we show that this is a localized state and present direct observation of hopping diffusion of positrons. On annealing up to 400 °C the amorphous network is seen to relax and the first stages of crystallization occur. There is also evidence of vacancy clustering to form a low concentration of microvoids. The structural relaxation has a very low-activation energy, around 0.1 eV, and is interpreted in terms of a reconfiguration of the fundamental defect identifed by positron annihilation.

Magnetic behavior of nanocrystalline FeNbCrBCu alloys at elevated temperatures

The magnetic behavior of nanocrystalline Fe73.5Nb4.5Cr5B16Cu1 alloys is investigated in a series of specimens with different volume fractions of crystalline phase. It is shown that the Curie temperature of amorphous phase firstly decreases after structural relaxation in amorphous state and then rapidly increases during the first stages of crystallization. The strikingly different behavior of coercivity at elevated temperatures is observed for the samples with low and high volume fractions of nanocrystalline particles.

Short-range order of Zr 62− xTi xAl 10Cu 20Ni 8 bulk metallic glasses

The short-range order and crystallization behavior of slowly cooled Zr 62− x Ti x Al 10Cu 20Ni 8 bulk metallic glasses have been investigated in terms of the atomic pair correlation function as a function of Ti content x (2≤ x≤7.5). The structural parameters point to the presence of chemical short-range order in these bulk glasses. An enhanced local excess free volume around the Ti atoms is concluded from density measurements. The first stage of crystallization in Zr 62− x Ti x Al 10Cu 20Ni 8 bulk glasses is related to changes in the medium-range order while the first neighborhood is retained. The atomic pair correlation functions of the first crystallization products are similar for all titanium contents. There is no indication of any special atomic arrangement for the particular alloy forming quasicrystals upon heating ( x=3). In case of Zr 54.5Ti 7.5Al 10Cu 20Ni 8 an ultrafine microstructure consisting of clusters of 2 nm in size is formed as the first step of crystallization.

Inherent submicroporosity and crystallization of amorphous alloys

The effect of inherent submicroporosity (regions with an excess free volume) in amorphous alloys on the regularities of their crystallization is investigated. Inherent submicroporosity arises in amorphous alloys prepared by quenching under different conditions. It is proved that this effect should be taken into account in the first stage of crystallization of amorphous alloys.

Nanocrystallization of bulk ZrCuTi metallic glass

The phase transformations during heating of Zr 30Cu 60Ti 10 bulk metallic glass were studied by differential scanning calorimetry, dilatometry, X-ray diffraction, transmission and high resolution electron microscopy. The crystallization of bulk metallic glass was found to occur in several stages. According to X-ray diffraction studies the structure was closely similar to amorphous structure when the sample was heated to the end of the first DSC sub-peak (781 K). However, the volume effect of the first stage of transformation is 1.6%; it forms about 80% of total volume effect of the crystallization (2%). A change of the shape of the diffraction maxima has been analyzed for the cases of structure relaxation and nanocrystalline structure formation. The nanocrystalline structure with very fine grain size was found to form during first stage of crystallization. The grain size of the nanocrystals was 1–5 nm.

As-cast quasicrystalline phase in a Zr-based multicomponent bulk alloy

An icosahedral quasicrystalline phase is obtained directly from the melt by copper mold casting of a Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 alloy. On the other hand, rapid quenching of the alloy leads to an amorphous phase. Upon annealing, the amorphous structure precipitates quasicrystals in the first stage of crystallization. The microstructure of the quasicrystalline state is quite different for the two preparation routes, which is correlated with the asymmetry of the nucleation and growth rate upon cooling or heating. The quasicrystals formed upon slow cooling from the melt have a size of about 1 μm. In contrast, the quasicrystals formed by annealing do not exceed a size of 5–10 nm.

Huge magnetic hardening ascribed to metastable crystallites during first stages of devitrification of amorphous FeSiBNbSn alloys

A huge magnetic hardening (i.e., increase of coercivity from 5 A/m up to 4.4 kA/m) was reported for Fe75Si11B10Nb3Sn alloy ribbons during their very first stage of crystallization from an initial amorphous state. In contrast, Fe78Si11B10Sn1 showed no such hardening, while a moderate hardening was observed for Fe76Si11B10Nb3. This outstanding change was ascribed to the generation of metastable nanocrystallites that disappear upon heating at higher temperatures. A noticeable softening was then recovered (with a decrease of coercivity down to 0.8 kA/m) once the optimum homogeneous conventional nanocrystalline phase was achieved. Both structure and magnetic evolutions are followed by different techniques.