Kinetic study of isothermal crystallization process of Gd2Ti2O7 precursor's powder prepared through the Pechini synthetic approach

Crystallization process of Gd₂Ti₂O₇ precursor's powder prepared by Pechini-type polymerized complex route has been studied under isothermal experimental conditions in an air atmosphere. It was found that the crystallization proceeds through two-parameter Šesták–Berggren (SB) autocatalytic model, in the operating temperature range of 550 °C≤T≤750 °C. Based on the behavior of SB parameters (M, N), it was found that in the lower operating temperature range, the crystallites with relatively low compactness exist, which probably disclosed low dimensionality of crystal growth from numerous nucleation sites, where the amorphous solid is produced. In the higher operating temperature region (above 750 °C), it was established that a morphological well-defined and high-dimensional particles of the formed pyrochlore phase can be expected. It was found that at T=850 °C, there is a change in the rate-determining reaction step, from autocatalytic into the contracting volume mechanism.     

First-principles study of ground- and metastable-state properties of XO (X = Be,Mg, Ca,Sr, Ba,Zn and Cd)

The evolution of the local atomic order of an amorphous Ni₄₆Ti₅₄ alloy produced by mechanical alloying as a function of temperature was studied by synchrotron X-ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques. XRD measurements at several temperatures (25 °C, 350 °C, 412 °C, 430 °C, 450 °C and 515 °C) were performed and analyzed using the reverse Monte Carlo (RMC) simulations method or the Rietveld refinement procedure. The experimental total structure factor for samples at 25 °C and 350 °C, which are amorphous in nature, were simulated by using the RMC method, and the local structures of the alloy at both temperatures were determined, indicating a decrease in its density as the temperature increases. At 412 °C, the XRD pattern shows a partially crystalline sample, indicating that the crystallization process is in progress. At 430 °C, 450 °C and 515 °C, the XRD measurements indicate the presence of two crystalline phases, NiTi and NiTi₂, whose structural parameters (lattice parameters, coherently diffracting domains (CDD) sizes, microstrains and relative amount of phases) were determined using the Rietveld refinement procedure. DSC measurements at different heating rates furnished the crystallization temperature, enthalpy and activation energy of the crystallization process, and these values are similar to those found in other amorphous alloys of the Ni-Ti system. They also showed the existence of a second exothermic process, which was related to diffusive processes in the crystalline phases, which could be associated with the changes in the CDD sizes happening from 450 °C to 515 °C.     

Messung der spezifischen Wärme und der Fehlordnungsenergie von amorphen Blei-Wismut-Schichten

Amorphous Pb-Films with a 30 atomic-% Bi-content are produced by evaporation onto a cooled Be-foil at 20 °K in high vacuum. The specific heat of these films in the temperature range from 5–8 °K is found to be 1.6 times larger than after crystallization, which takes place during warming up between 30 and 40 °K. The stored energy released during this crystallization amounts to 1.5 kcal/mol.     

Shape-memorizable styrene-butadiene block copolymer. I. Thermal and mechanical behaviors and structural change with deformation

Thermal properties in the range from room temperature to 150°C, mechanical properties from room temperature to 80°C, and structural changes by drawing and contraction at 80°C followed by crystallization have been studied in a crystalline styrene-butadiene block copolymer, which has the property of shape memory, using differential scanning calorimetry (DSC), mechanical analysis, wide-angle x-ray diffraction (WAXD), and smallangle x-ray scattering (SAXS). This copolymer has the crystal transformation temperature, the melting temperature of the trans- 1,4-polybutadiene domains, and the higher glass transition temperature of the polystyrene domains. When a high strain is adopted for the deformation at 80°C (i.e., between the melting temperature of the polybutadiene [PB] domains and the glass transition temperature of the polystyrene regions) and crystallization conditions with fixed ends are employed, a fibrillar structure with a better regularity of long spacings and a high orientation of crystals forms. When the drawn sample is allowed to contract at 80°C, the high contraction or the shape recovery appears. Nevertheless, crystallization after contraction presents essentially the same supermolecular structure as that before contraction. It is suggested that the molecular chains of polybutadiene were inhibited from flowing freely by the glassy polystyrene molecules and that there must be some structural units separated by amorphous domains that contribute to the elongation and contraction at the high temperature.     

Surface characterization and H2S-sensing potential of iron molybdate particles produced by supercritical solvothermal method and subsequent oxidation

The mostly crystalline polymorph β-FeMoO₄ was prepared by solvothermal synthesis from organic precursors, followed by high temperature supercritical drying in an autoclave. Crystallization of the synthesized particles occurred during subsequent heat treatment at 350 °C, confirmed by X-ray diffraction pattern analysis. The presence of Fe³⁺ ions in the powder, both well-crystallized and amorphous after heat treatment at 500 °C, was confirmed by room temperature Mössbauer spectrum. Thick-film gas sensors were prepared by conventional hand coating of a paste, the Fe₂(MoO₄)₃ powder mixed with an α-terpineol-based solvent, over the Au electrodes. The response of the prepared sensors to H₂S gas in the low concentration range 1–10 ppm in air was investigated. Moderately fast response and recovery times were observed. The iron molybdate, produced at low temperature, may be successfully used in the preparation of a H₂S gas sensor.     

In-situ and ex-situ study of crystallization behaviour of magnetron sputtered Ni63Zr37 amorphous thin film

ABSTRACT

The stages of crystallization of magnetron sputter-deposited Ni₆₃Zr₃₇ film with mostly amorphous structure have been investigated by differential scanning calorimetry (DSC) and in-situ annealing at 300°C by use of heating stage on a high-resolution transmission electron microscope (HRTEM). These results have been further confirmed by grazing incidence X-ray diffraction analyses of thin film specimens annealed ex-situ at 300°C for various durations. The temperature for crystallization found by DSC has been found to increase from 371°C to 434°C with an increase in heating rate from 3°C/min to 10°C/min, and the apparent activation energy for amorphous to crystalline transformation has been found as ～260.2?kJ/mol from the Kissinger plot. Studies on HRTEM using in-situ heating stage have shown the crystallization to occur on annealing at 300°C for ～10?min. Crystallization at a temperature lower than that found by DSC is attributed to structural relaxation with reduction of free volume due to thermal activation. It has been observed that Ni₃Zr forms first due to its large negative enthalpy of formation, and is followed by the formation of Ni-rich solid solution (Ni_ss) grains. HRTEM studies have shown grain rotation with the formation of partial dislocations at Ni₃Zr-Ni_ss interfaces as well as twinning followed by detwinning with dislocation formation in the Ni_ss matrix possibly to reduce the interfacial energy.     

Low temperature synthesis of iron containing carbon nanoparticles in critical carbon dioxide

We develop a low temperature, organic solvent-free method of producing iron containing carbon (Fe@C) nanoparticles. We show that Fe@C nanoparticles are self-assembled by mixing ferrocene with sub-critical (25.0 °C), near-critical (31.0 °C) and super-critical (41.0 °C) carbon dioxide and irradiating the solutions with UV laser of 266-nm wavelength. The diameter of the iron particles varies from 1 to 100 nm, whereas that of Fe@C particles ranges from 200 nm to 1 μm. Bamboo-shaped structures are also formed by iron particles and carbon layers. There is no appreciable effect of the temperature on the quantity and diameter distributions of the particles produced. The Fe@C nanoparticles show soft ferromagnetic characteristics. Iron particles are crystallised, composed of bcc and fcc lattice structures, and the carbon shells are graphitised after irradiation of electron beams.     

pH-Dependent shape changes of water-soluble CdS nanoparticles

Annealing of silicon-carbon nanoparticles was performed in argon at atmospheric pressure to enable formation of silicon carbide nanomaterials and/or carbon structures. Three precursor powders with increasing crystallinity and annealing temperatures from 1,900 to 2,600 °C were used to gain information about the effect of precursor properties (e.g. amorphous vs. nanocrystalline, carbon content) and annealing temperature on the produced materials. Three structures were found after annealing, i.e. silicon carbide crystals, carbon sheets and spherical carbon particles. The produced SiC crystals consisted of several polytypes. Low annealing temperature and increasing crystallinity of the precursor promoted the formation of the 3C-SiC polytype. Raman analysis indicated the presence of single-layer, undoped graphene in the sheets. The spherical carbon particles consisted of curved carbon layers growing from the amorphous Si–C core and forming a ‘nanoflower’ with a diameter below 60 nm. To our knowledge, the formation of this kind of structures has not been reported previously. The core was visible in transmission electron microscopy analysis at the annealing temperature of 1,900 °C, decreased in size with increasing temperature and disappeared above an annealing temperature of 2,200 °C. With increasing crystallinity of the precursor material, fewer layers (~5 with the most crystalline precursor) were detected in the carbon nanoflowers. The method presented opens up the possibility to produce new carbon nanostructures whose properties can be controlled by changing the properties of the precursor material or by adjusting an annealing temperature.     

Isothermal Crystallization and Melting Behavior of Composites Composed of Poly(L-lactic Acid) and Poly(glycolic Acid) Fibers

Several composites of poly (L-lactic acid) (PLLA) with poly (glycolic acid) (PGA) fibers were prepared. The isothermal crystallization kinetics and melting behavior of PLLA and all of the composites were characterized by using differential scanning calorimetry. The experimental data were processed by using the Avrami equation. The relative parameters, such as the Avrami exponent and half-time crystallization, revealed that PGA fibers had positive effects on the crystallization of PLLA, but these effects had only a minimal dependence on the PGA fiber content. Moreover, at low isothermal crystallization temperatures (85°C～110°C), recrystallization during the heating scan was observed, which could lower the melting point of the samples to a certain extent.     

Studies on crystalline forms of Nylon 6. III. Crystallization from the glassy state

The relative amounts of the α- and γ-crystalline forms of nylon 6 obtained from the glassy state under different crystallization conditions have been studied by the X-ray diffraction procedure described in the previous paper. The weight fraction of the γ-form decreases with increasing crystallization temperature above 160°C and that of the α-form increases. Growth of the γ-form is predominant in crystallization at 100°C and of the α-form at 200°C. Differential scanning calorimetry and density data are presented. A large amount of a thermally unstable form is included in the γ-form crystallized from the glassy state at lower crystallization temperatures and is transformed into the a-form with annealing at 200γC. Thermal stability of the γ-form obtained from the glassy state was quantitatively investigated and discussed.     

Polytherms of the Wetting Angles of Pb-Ni (0.3 at %) Melt on Nickel Substrates

Polytherms of the wetting angle of Pb-Ni (0.3 at %) melt on pressed nickel substrates and substrates cut from a plate of nickel of grade NP-2 are studied in the temperature range from the melting point to 850°C by the sessile drop method in a vacuum chamber with a residual pressure of 10^-2 Pa. The wettability thresholds are found. The wetting of both pressed substrates and NP-2 nickel begins at a temperature of 500°C or higher. After crystallization, the morphology of the droplet surface and the zones near it is studied using scanning electron microscopy. Pb _n Ni _m intermetallides of pyramidal form are detected, and zones where the melt spreads along the grain boundaries followed by the crystallization and formation of fibrous structures are found.     

High-Speed Melt Spinning of Polylactide/Poly(Butyleneterephthalate) Bicomponent Fibers: Mechanism of Fiber Structure Development and Dyeing Behavior

Bicomponent fibers consisting of polylactide (PLA) as the sheath and poly(butylene terephthalate) (PBT) as the core were produced by high-speed spinning to obtain materials suitable for medical clothing. The higher-order structure of the PLA fiber component appeared to exhibit simple, alternately stacked, uniaxially oriented amorphous and crystalline regions. Therefore, fairly large tanδ peaks were observed for single-component PLA fibers, even when the orientation-induced crystallization was achieved by high-speed spinning. By conjugating PLA with PBT, although limited mutual interference with the crystallization of each component occurred, both the PLA (Mw?=?170,000, L-lactide content?=?98.7%) and PBT (intrinsic viscosity?=?0.835-0.865 dL/g) could crystallize on a high-speed spinning line, and the proposed formation of a shish-kebab-like structure in the PBT component enhanced the thermal stability of the bicomponent fibers, particularly resulting in shrink-proof properties. The bicomponent fibers developed herein could be deeply dyed at 98?°C, with results comparable to those of industrial polyester, and peeling of the PLA skin layer was rarely observed, even when the dyed fibers were flattened by a rubbing force.     

In situ transmission electron microscopy observations of?the?crystallization of?amorphous?Ge?films

The crystallization of amorphous Ge films has been studied as a function of annealing temperature between 400 and 700°C by in situ transmission electron microscopy (TEM). It is found that crystallization does not occur until the annealing temperature reaches 650°C, which is nearly 250°C higher than the crystallization temperature in previous reports. The high crystallization temperature and average crystal size obtained by in situ TEM are in agreement with those from Raman spectroscopy and X-ray diffraction measurement. The kinetics analysis indicates that homogeneous nucleation is the dominant crystallization mode and the activation energy is up to about 3.1 eV.     

Dielectric study of natural inyoite in the frequency range from 10−1 to 107 Hz and the temperature range from −50 to 140°C

For the first time, the method of dielectric dispersion in the 10^?1–10⁷ frequency range is applied to study temperature the dependences of permittivity, conductivity, and dielectric modulus of natural inyoite in the temperature range from ?50 to 140°C. An anomalous increase in the parameters under consideration is observed at temperatures between 87 and 98°C. According to thermal gravimetrical measurements, this range is characterized by an anomaly resulting from a partial loss of crystallization water.     

A COMPARISON OF THE HOT-COMPACTION BEHAVIOR OF ORIENTED,HIGH-MODULUS,POLYETHYLENE FIBERS AND TAPES*

The purpose of this article is twofold. First, there is an account of the hot-compaction behavior of a new, highly oriented, high-modulus polyethylene (PE) tape with the trade name of Tensylon® (manufactured by Synthetic Industries, USA). This tape, produced by a melt spinning route, has mechanical properties comparable to those of commercially available gel-spun fibers. Unidirectional samples were produced for a range of compaction temperatures to determine the optimum compaction conditions to obtain the best mechanical properties of the resulting compacted sheets. Second, the mechanical properties of the best Tensylon sample, manufactured at a compaction temperature of 153°C, was compared with three other hot-compacted, highly oriented PE materials, based on Certran®, Dyneema®, and Spectra® commercial PE fibers. The results showed that the optimum compaction temperature was in most cases about 1°C below the point at which substantial crystalline melting occurred. At this optimum temperature, differential scanning calorimetry (DSC) melting studies showed that approximately 30% of the original oriented phase had been lost to bond the structure together. In the case of Dyneema, the properties of the fiber were not translated into the properties of a compacted sheet, and morphological studies showed that this was because melting did not occur on the fiber surfaces, but rather in the interior of the fiber due to a skin structure. The properties of the compacted Tensylon tapes were found to be exceptional, combining very high modulus and strength with interlayer bonding and good creep resistance. Moreover, the optimum temperature appeared to be about 2°C below the point at which complete melting occurred, giving a wider processing window for this material.

     

The Preparation and Study on Ultrahigh Molecular Weight Polypropylene Gel‐spun Fibers

A novel polypropylene (PP) fiber was prepared by using gel spinning/crystallization from dilute solutions of ultrahigh molecular weight isotactic polypropylene (i‐UHMWPP), and subsequently drawing at various temperatures. The influence of drawing temperature on the properties of the resulted fibers was investigated. We found that the draw‐ability and mechanical as well as crystallization properties of the fibers obtained were dramatically improved with increasing drawing temperature. When the drawing temperature is below the α‐crystal relaxation temperature of PP, which was measured by wide‐angle X‐ray diffraction (WAXD) analysis as 100–120°C, the fibers are characterized by lower crystallinity and smaller crystals with less perfection, resulting in brittle fracture and subsequently poor mechanical durability. With drawing at temperatures above the α‐crystal relaxation temperature of PP, a novel UHMWPP fiber with Young's modulus of 27 GPa and tensile strength of 1.3 GPa was obtained. Higher crystallinity and larger crystals with better perfection and orientation were observed in this fiber.     

Crystallization of polycarbonate from the glassy state. part I. Thin films cast from solution

The morphology of glassy amorphous thin polycarbonate film cast from solution is affected by thermal treatments. Annealing above 80° C and below T_g results in an increase in the size of the ordered regions, nodules, up to several hundred Angströms. The crystallization process from the glass, taking place at 145° C, is divided into three major steps. At first the nodules merge into patches which aggregate to form lamellar planar structures. In some cases the planar structures are well-formed single crystals. Following this, spherulitic arms develop from the planar structures as centers. These arms at first consist of aggregates of large nodules which recrystallize to form lamellae; the final morphology is spherulitic in nature. The effect of film thickness and of several substrates on the morphology has been observed. Applying stress at room temperature to the crystalline film results in a breaking up of the lamellae into small blocks.     

Multiple Melting Behavior of High-Speed Melt Spun Polylactide Fibers

High-speed melt spinning of polylactide (PLA) was conducted and the structure and multiple melting behavior of the as-spun fibers were investigated. In the analysis of temperature modulated differential scanning calorimetry (TMDSC) thermograms for the as-spun PLA fibers taken-up at 1 and 6 km/min, the peaks around the melting temperature region in the reversing heat flow (RHF) and nonreversing heat flow (NRHF) curves were mainly separated into (1) a pair of an endothermic peak (Peak L) in RHF and an exothermic peak (Peak R) in NRHF in a low temperature region, (2) an endothermic peak (Peak M) both in RHF and NRHF (only in RHF for PLA fiber spun at the low-speed) in an intermediate temperature region, and (3) an endothermic peak (Peak H) both in RHF and NRHF in a higher temperature region. Wide-angle X-ray diffraction (WAXD) measurements were conducted during the heating process of the as-spun fibers cut into powders. In the case of fibers obtained at 1 km/min, disordered crystals, i.e. α′-form crystals, were formed through cold crystallization followed by a disorder-to-order phase transition, i.e. α′ to α crystalline modification, with partial melting of the α′ crystals around 148.5°C in the temperature range of Peaks R and L. Finally, the α form crystals melted above 169.4°C, in the temperature range of Peak H. On the other hand, the PLA crystals generated by the orientation-induced crystallization during the spinning process at a spinning velocity of 6 km/min did not show a WAXD profile of perfect α form crystals but showed an intermediate structure having lattice spacings between the α′ and α forms. Such intermediate crystals did not transformed into α form crystals during the heating process.     

Structure and properties of polybutylene crystallized from the glassy state. I. X-ray scattering,DSC, and torsion pendulum

Isotactic polybutene (PB) can be quenched into a completely glassy state by quenching molten films into a solid-liquid mixture of isopentane, Freon, or ethanol. The crystallization of PB from the glass form was studied by x-ray scattering, differential scanning calorimetry (DSC), and dynamic mechanical spectroscopy (torsion pendulum). As for crystallization from the melt, PB crystallizes from the glass into a tetragonal crystal structure (Form II) at ca. 0°C, depending on sample thickness, and then transforms to the twinned hexagonal structure (Form I) upon aging at room temperature. In the presence of isopentane, PB crystallizes partially from the glass into the untwinned hexagonal (Form 1′) structure at ca. -70°C; the rest of the sample starts to transform to tetragonal structure at ca. -30°C and nearly completes crystallization at ca. 0°C. The exact temperatures of both transformations depend on the amount of isopentane present and sample thickness. Upon aging at room temperature the tetragonal structure converts to the twinned hexagonal structure even faster than in the absence of isopentane. Dynamic mechanical experiments show the presence of two relaxation-like peaks for the ultraquenched samples: T_r (L) = -27°C and T_r (U) = -15°C. X-ray diffraction, DSC, and torsion pendulum experiments show that PB crystallizes from the glass at T_r (U).     

Unusual structural phase transition in nanocrystalline zirconia

The present work is the first example demonstrating that a hydrous zirconia formed by precipitation can yield a nearly pure nanocrystalline monoclinic zirconia at a temperature as low as 320 °C. The X-ray diffraction pattern of the hydrous zirconia heated to 310 °C shows that diffraction peaks begin to emerge and reveals a just crystallized mixture of predominantly monoclinic zirconia (70%) with some tetragonal zirconia(30%). In other words, the hydrous zirconia formed in the present work yields the predominantly monoclinic structure coexisting with the tetragonal one as soon as crystallization starts at low temperature (310 °C). This is an important exception to the general principle that amorphous zirconia precursors first convert to the tetragonal structure of zirconia with increasing calcination temperature and then transform to the monoclinic one at a higher temperature (∼600 °C). At the crystallization temperature (310 °C), the monoclinic crystallite size is about 17 nm and the tetragonal one 28 nm. The monoclinic crystallite is much smaller than the tetragonal one with which it co-exists. This result is also not consistent with the traditional view that a critical particle size effect is responsible for the stability of the tetragonal and monoclinic structures. When the temperature (310 °C) is slightly raised to 320 °C, the XRD pattern shows a nearly pure monoclinic zirconia. The crystallite size of the monoclinic zirconia is around 15 nm, and it does not change appreciably as calcination temperature is increased from 320 to or above 400 °C. The unusual structural phase transition has been investigated by several complementary experimental tools: X-raydiffraction and surface analyses, and infrared and Raman spectroscopies. PACS 81.07.-b; 64.70.Nd; 82.80.-d; 78.67.-n; 81.05.Je