The crystallization mechanism of high-cordierite glass

The crystallization mechanism of high-cordierite (2MgO·2Al₂O₃·5SiO₂) from a bulkglass containing B₂O₃ and P₂O₅ as nucleating agents was studied using X-ray diffraction, differential thermal analysis and a polarizing microscope. Thin glass specimens, with a mirror surface, were heated rapidly in an electric furnace in the temperature range 888 to 1363° C for a desired time and then rapidly quenched to room temperature. The normal rate of growth of precipitated cordierite crystals was measured and their morphological change was observed photographically. The growth rate increased with temperature, and the maximum rate occurred at about 1250° C. The crystal morphology was hexagonalprismatic, elongated along thec-axis. Faceted interface morphology was observed when the range of undercooling was from 7 to 174° C. Judging from the relationship between the reduced growth rate and the degree of undercooling, the crystallization mechanism in the range of lower undercooling was governed by a layer growth depending on the surface nucleation mechanism. In the range of higher undercooling, continuous growth was seen and at intermediate undercooling a transition range from a layer growth to a continuous growth was evident.     

Solubility in the ternary system fullerenol-d-uranyl sulfate-water at 25°C

The solubility in ternary systems fullerenol-d-UO₂SO₄-H₂O at 25°C was studied by the method of isothermal saturation in ampules. The diagram consists of two branches corresponding to crystallization of fullerenol-d crystal hydrate and uranyl sulfate trihydrate and contains one nonvariant point of eutonic type, corresponding to joint saturation with both crystal hydrates. The eutonic is very close to the figurative point corresponding to the binary system UO₂SO₄-H₂O at 25°C. Strongly pronounced salting-out effect is observed in the crystallization branch of fullerenol-d crystal hydrate.     

Effect of sol–gel modifications on formation and morphology of nanocrystalline lanthanum aluminate

LaAlO₃ powders are prepared by sol–gel method. The effect of preparation conditions on morphological properties and crystal formations are investigated. iso-Propanol/tert-butanol and ethyl acetoacetate/ethylene glycol monomethylether are used solvents and complexing agents, respectively. Samples are dried with conventional and freeze-drying methods and calcined between 600 and 1000 °C. TGA, DTA, XRD, SEM and TEM methods are used for characterization. It is observed that freeze-dried sample prepared with tert-butanol has the lowest LaAlO₃ formation temperature and uniform rhombohedral crystals. But conventionally dried sample, prepared with iso-propanol has smallest agglomerates at 1000 °C and does not show clear crystallization temperature in DTA analysis. The XRD peaks of LaAlO₃ crystal are observed at 600 °C for all samples prepared by various sol–gel conditions.     

Effect of three-stage isothermal annealing on the nucleation process in GeSe2 glasses

We studied the mechanism of crystallization of GeSe₂ bulk glass into the so-called low-temperature GeSe₂ crystalline phase. Initial isothermal annealing at a temperature T₁ below 400 °C and subsequent isothermal crystallization at 440 °C were carried out in a differential scanning calorimeter (DSC). The crystallization time (t_c), defined by the interval from the onset of annealing at 440 °C to the observation of a DSC peak, became shorter with increasing T₁. Nucleation occurs during this initial annealing. Next, T₁ was fixed at 330 °C and specimens were subsequently annealed at T₂<T₁. It was found that t_c increased as T₂ decreased from 330 °C. The nuclei generated at T₁ were reconverted to the glassy state.     

Crystallization process and electro-optical properties of In2O3 and ITO thin films

Amorphous indium oxide (In₂O₃) and 10-wt% SnO₂ doped In₂O₃ (ITO) thin films were prepared by pulsed-laser deposition. These films were crystallized upon heating in vacuum at an effective heating rate of 0.00847 °C/s, while the evolution of the structure was observed by in situ X-ray diffraction measurements. Fast crystallization of the films is observed in the temperature ranges 165–210 °C and 185–230 °C for the In₂O₃ and ITO films, respectively. The crystallization kinetics is described by a reaction equation, with activation energies of 2.31 ± 0.06 eV and 2.41 eV and order of reactions of 0.75 ± 0.07 and 0.75 for the In₂O₃ and ITO films, respectively. The structures of the films observed here during heating are compared with those obtained upon film growth at different temperatures. The resistivity of the films depends on the evolution of the structure, the oxygen content and the activation of tin dopants in the films. A low resistivity of 5.5 × 10⁻⁴ Ω cm was obtained for the In₂O₃ and ITO films at room temperature, after annealing to 250 °C the resistivity of the ITO film reduces to 1.2 × 10⁻⁴ Ω cm.     

Kinetics of calcium molybdate crystallization

Kinetics of crystallization of calcium molybdate from unstirred molten solutions of lithium chloride of low to medium supersaturation in platinum crucibles by the process of continuous cooling at 5° C hr⁻¹ from temperaturesT ₀=700 and 750°C are investigated. The crystal size measured by optical microscopy for different crystallization periods reveals that both crystal length and width generally increase with cooling period. The degree of crystallizationα _t,also increases with cooling period, attaining a maximum of 0·90. The diffusion rate constants,K _Dlat 700 and 750°C are 0·0776 and 0·1138 respectively. The effect of variation of the crystallization temperature on the crystal size and their number is also studied.     

Growth behavior of interfacial Cu–Sn intermetallic compounds of Sn/Cu reaction couples during dip soldering and aging

The formation and growth of intermetallic compound (IMC) layer at the interface between pure Sn and a Cu substrate during dip soldering and aging were studied. The soldering was conducted at 250 °C using dipping method, followed by aging treatment at 150 °C for up to 10 days. The results showed that the IMC layer flattened with aging duration because the grooves in scallop-like IMC provide a more convenient access for Cu atoms to dissolve and react with solders and previous IMCs. And when isothermal aging was subjected, the growth rate of Cu₆Sn₅ was lower than that of Cu₃Sn due to Cu₃Sn growing rapidly with aging time by consuming Cu₆Sn₅ at the interface of Cu₃Sn/Cu₆Sn₅. Kirkendall voids were observed at Cu₃Sn/Cu interface as well as inside the Cu₃Sn layer as the Sn/Cu couple was aged at 150 °C for prolonged time, with which the Cu₃Sn IMC dominates the interfacial IMCs growth. During solid-state aging, the mean diameter (d) of interfacial Cu₆Sn₅ grains increased dramatically with the increasing time (t). The relationships between d and t were given to be d = 1.22 t ^0.291 for samples formed at 250 °C for 1 min and d = 1.53 t ^0.259 for samples formed at 250 °C for 5 min, respectively.     

The glass to NZP crystal transformation in a mixed alkali germano-phosphate glass matrix

A mixed alkali germano-phosphate glass composition (Li_0.5Na_0.5Ge₂(PO₄)₃) has been prepared and characterized. The crystallization mechanism of a NZP-type phase, the nucleation rate and the non-isothermal kinetic parameters have been studied by differential thermal analysis (DTA).The plot of the DTA peak height versus the glass particle size has shown a great tendency for surface crystallization. The nucleation rate type curves have given a maximum at 465 ± 2 °C and confirmed that nucleation was instantaneous at this temperature. Furthermore, the average value of the Avrami exponent (5.4 ± 0.4) has revealed a tri-dimensional growth controlled by the development of the glass–crystal interface with a decreasing nucleation rate in the temperature range explored. From non-isothermal experiments, the apparent activation energy for the overall crystallization phenomenon has been founded in the range 260–273 kJ/mol whereas the activation energy for crystal growth was 488 ± 6 kJ/mol.     

Magnetic and structural properties of NdFeB thin film prepared by step annealing

The crystallization of the amorphous phase into the tetragonal Nd₂Fe₁₄B (Φ) phase and the corresponding changes in magnetic properties have been examined by step annealing experiment using a 2 μm thick NdFeB film sample. Microstructural and magnetic analysis indicate that the film was magnetically soft as deposited with the coercivity H_ci⊥ <16 kA m⁻¹ and the remnant magnetization 4πM_r⊥ <0.02 T. Annealing at a temperature of 550 °C, a coercivity value around 784 kA m⁻¹ was developed and diffraction analysis showed evidence of Φ phase 002l peaks being aligned perpendicular to the film plane. At an optimum annealing temperature of 575 °C, the remnant magnetization of this anisotropic thin film is around 0.60 T with intrinsic coercivity of ∼1340 kA m⁻¹. Annealing the film sample at 200 °C≤T_ann≤750 °C showed variations in magnetic properties that were mostly due to the change in the perpendicular anisotropy. Based on 4πM_s⊥ values plotted against T_ann, a dip in 4πM_s⊥ values was observed as T_ann increased in the soft-to-hard magnetic characteristics transition region and rose as the hard crystalline phase started to form. The results show that the magnetic properties of the NdFeB film were slightly influenced by the presence of NdO, film surface roughening and the small increase in crystal size as a consequence of repeated heat treatment. At T_ann ∼300 °C, the nominal saturation magnetization indicated a certain degree of weak perpendicular magnetic anisotropy in the film sample considered to be essential in the enhancement of coercivity in crystallized films.     

Solid-phase crystallization of amorphous SiGe films deposited by LPCVD on SiO2 and glass

The crystallization kinetics and film microstructure of poly-SiGe layers obtained by solid-phase crystallization (SPC) of amorphous SiGe with Ge fractions (x) in the 0 to 0.42 range have been studied in detail. Amorphous SiGe layers 100 nm thick were deposited by LPCVD at 450°C on thermally oxidized Si wafers and 7059 Corning glasses, using Si₂H₆ and GeH₄ as gas sources. The samples were crystallized at 550°C and low pressure (below 9 Pa). The evolution of the crystallization and the resulting film microstructure were characterized by X-ray diffractometry and transmission electron microscopy. The experimental results on growth kinetics fit the Avrami’s model. The characteristic crystallization time decreases with x, slowly for x<0.3 and more abruptly for higher values of x. The transient time depends exponentially on x in all the intervals. The crystallized films have a (111) preferred orientation for low values of x and evolve to a randomly oriented polycrystal as x increases. The grain size in the fully crystallized layers decreases with increasing x. The results are similar for the films deposited on silicon dioxide and glass.     

On the development of two characteristically different crystal morphology in SiO2–MgO–Al2O3–K2O–B2O3–F glass-ceramic system

The present work demonstrates how crystals with two different characteristic morphologies can be formed in SiO₂–MgO–Al₂O₃–K₂O–B₂O₃–F glass-ceramic system by adopting two sets of heat treatment experiments. In our study, single stage heat treatment experiments were performed at 1,000°C for varying holding time of 8–24 h with 4 h time interval and as a function of temperature in the range of 1,000–1,120°C with 40°C temperature interval. The constant heating rate of 10°C/min was employed for both sets of experiments. The microstructural changes were investigated using Fourier transformed infrared spectroscopy (FT-IR), SEM-EDS and XRD. For temperature variation batches, the microstructure is characterized by interlocked, randomly oriented mica plates (‘house-of-cards’ morphology). An important and new observation of complex crystal morphology is made in the samples heat treated at 1,000°C for varying holding times. Such morphology appears to be the results of composite spherulitic-dendritic like growth of mica rods radiating from a central nucleus. The possible mechanism for such characteristic crystal growth morphology is discussed with reference to a nucleation-growth kinetics based model. The activation energy for crystal nucleation and Avrami index are computed to be 388 kJ/mol and 1.3 respectively, assuming Johnson–Mehl–Avrami model of crystallization. Another important result is that a maximum of around 70% of spherulitic-dendritic like crystal morphology can be obtained after heat treatment at 1,000°C for 24 h, while a lower amount (~58%) of interlocked plate like mica crystals is formed after heat treatment at 1,040°C for 4 h.     

Effects of post-annealing on the microstructure and mechanical properties of friction stir processed Al–Mg–TiO2 nanocomposites

Aluminum matrix nanocomposites were fabricated via friction stir processing of an Al–Mg alloy with pre-inserted TiO₂ nanoparticles at different volume fractions of 3%, 5% and 6%. The nanocomposites were annealed at 300–500 °C for 1–5 h in air to study the effect of annealing on the microstructural changes and mechanical properties. Microstructural studies by scanning and transmission electron microscopy showed that new phases were formed during friction stir processing due to chemical reactions at the interface of TiO₂ with the aluminum matrix alloy. Reactive annealing completed the solid-state reactions, which led to a significant improvement in the ductility of the nanocomposites (more than three times) without deteriorating their tensile strength and hardness. Evaluation of the grain structure revealed that the presence of TiO₂ nanoparticles refined the grains during friction stir processing while the in situ formed nanoparticles hindered the grain growth upon the post-annealing treatment. Abnormal grain growth was observed after a prolonged annealing at 500 °C. The highest strength and ductility were obtained for the nanocomposites annealed at 400 °C for 3 h.     

Microstructure and dielectric properties of CaCu3Ti4O12 ceramic

CaCu₃Ti₄O₁₂ (CCTO) was prepared by the solid state technique. The sample was calcined at 900 °C/12 h and sintered at 1050 °C/24 h, then subjected to XRD to ensure CCTO formation. The microstructure was observed by SEM. XRD results identified both samples as single phase CCTO, whereas the microstructure shows abnormal grain growth and large pores. Sintering was studied in the temperature range of 950–1050 °C for 3–12 h. Increasing sintering temperature enhances the density and secondary formation of Cu₂O. A clear grain boundary and dense microstructure were observed. The results show that the sample sintered at 1040 °C/10 h yields a clearly uniform grain size with the highest ε_r (33,210).     

Structure and ac properties of electron-beam evaporated zirconia thin films

The structure and ac properties of ZrO₂ thin film capacitors were studied as a function of deposition temperature. Zirconia films deposited on molybdenum electrodes were polycrystalline. The monoclinic phase was the major phase detected in the films deposited at temperatures between 300—600 °C. At lower deposition temperatures (190–200 °C), the cubic phase was the major crystalline phase formed. A major (111&#x0304;) and a minor (111) fiber axis were observed for the monoclinic phase. Average crystallite size increased from 118 to 484 Å for the (111&#x0304;) and 175 to 265 Å for the (111) crystallites as deposition temperature increased from 200 to 600 °C. Ac conduction in Mo-ZrO₂-Mo structures at fields < 0.05 MV/cm was dominated by electron hop with a frequency independent loss behavior between 25 and 150 °C. Between 150 and 325 °C, a σ_d ∝ f^y dependence was observed where y depended on measurement and deposition temperatures. The remanence of interfacial polarization was suggested as an additional loss mechanism in this temperature range and for frequencies of 60-10⁵ Hz.     

Investigation of the crystallization process and crystal structure of Si-incorporated GeSb phase-change films

The effect of Si incorporation on the crystallization process and crystal structure of Te-free Sb-rich GeSb was investigated in this study. Si concentrations were controlled to 0, 5.1, 9.3, and 12.8 at.% by controlling the sputtering power of the GeSb alloy target (20:80 at.% for Ge:Sb) and Si target. After film deposition, the crystallization process and crystal structure were investigated. Crystallization temperature increased from 320 to 400 °C and the overall crystallinity was decreased with increasing Si concentration. These were analyzed by sheet resistance measurements after thermal annealing and optical contrast measurements by optical static testing. Glass transition temperatures were calculated and increased from 240 to 285 °C with increasing Si concentration. Considering the proportional relation between the glass transition temperature and crystallization temperature, it is thought that more energy is required for crystallization with increased Si concentration. A study of the crystallization process kinetics was conducted by applying the Johnson–Mehl–Avrami model to the optical static test results, which were carried out under a pseudo-isothermal process. The Avrami coefficient was 4.10 and decreased to 3.18 when the crystallization was generated with increased Si concentration from 0 to 12.8 at.%. Therefore, crystallization speed was thought to decrease with increased Si concentration. Based on the results of crystal structure analysis by XRD and HRTEM, the crystal structure of our Sb-rich GeSb PCM was revealed to be a typical Sb structure, i.e., an A7 hexagonal structure with lattice parameters of a = 4.26 Å and c = 11.45 Å. No crystal phase of Ge or Si was observed and no evidence of the structure change in Sb crystals due to Ge or Si incorporation was observed.     

Segregation study and segregation modeling of Ti in Pb[(Mg1/3Nb2/3)0.60Ti0.40]O3 single crystal grown by Bridgman method

Relaxor ferroelectric 0.60Pb(Mg_1/3Nb_2/3)O₃–0.40PbTiO₃ single crystal was synthesized by Bridgman method. The segregation along the length of this crystal was observed by various physico-chemical characterization techniques. First, we note the evolution of the crystalline structure along the length of the as-grown crystals. The tetragonality factor c/a increases from 1.013 to 1.020. This evolution has been correlated with the Ti content determination by flame atomic absorption spectrometry, and it is shown that this is due to the evolution of Ti concentration along the crucible. Furthermore, a continuous increase of the fraction of tetragonal phase with increasing Ti content was observed. Hence, we could compare the experimental data with a standard segregation model and therefore calculate the effective segregation coefficient k_eff for this composition. This coefficient was found to be equal to 0.849. The variation of the titanium content along the ingot causes an inevitable fluctuation of the dielectric and piezoelectric properties of the single crystal. Due to the increase of Ti concentration, the Curie point was 186 °C at the bottom of the ingot and 222 °C at the top. The piezoelectric coefficient of oriented and poled <0 0 1> samples varies from 1500 to 250 pC/N.     

Investigation of germanium films and GeSi interface structure by transmission electron microscopy

The structure of germanium films (d≌0.3 μm) evaporated onto a silicon substrate and the GeSi interface have been investigated by transmission electron microscopy (TEM). Ge was evaporated in a vacuum of approximately 10^?6 Torr onto (111) Si. Epitaxial growth was observed at substrate temperatures T_s>500°C. The films grown at T_s = 520°–600°C (low temperature epitaxy) were characterized by a high density of stacking faults (SF), microtwin lamellae and dislocations which lay normal to the film surface. In this case the interface dislocations were regular and the dislocation density was equal to (3–4)×10¹⁰ cm^?2. At T_s = 700°–850°C (high temperature epitaxy) few stacking faults were discovered. Dislocations in the interior of the film formed three-dimensional networks as a result of interactions of different slip systems and dislocation climb. The formation of misfit dislocations was apparent at the interface. The region T_s = 630°–700°C is an intermediate region.     

Crystal growth in a reactive atmosphere

Concepts of reactive atmosphere processing (RAP) are applied to crystal growth. Steady state pyrolysis of CBr₄ shows the material to be applicable to RAP growth at <600°C and, therefore, inapplicable to the growth of KBr (mp = 730°C). Steady state pyrolysis of CCl₄ shows the material to be applicable to RAP growth up to 900°C and, therefore, useful to metal chlorides in general.     

Interfacial SiC formation in polysiloxane-derived Si–O–C ceramics

Poly(methylsilsesquioxane) (CH₃SiO_1.5)_n (PMS) loaded with 40 vol.% Si-filler powder was pyrolyzed in inert atmosphere up to 1400 °C to fabricate Si–O–C composite ceramics. The evolution of the interface microstructure between the filler and the matrix was studied by high resolution electron microscopy (HREM), energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy. While below pyrolysis temperatures of 1000 °C no filler reaction was observed (inert filler regime), a porous interface layer of nanosized ß-SiC was formed at reaction temperatures above 1200 °C. Due to a high fraction of open porosity of 13% (1000 °C) to 19% (1400 °C) in the polymer-derived Si–O–C matrix, gas-phase transport and reaction processes involving CO and SiO as the dominant species are likely to occur at the interface boundary layer.     

Sensitivity of damage to microstructure evolution occurring during long-term high-temperature annealing in a semi-crystalline polymer

This study aimed to highlight the role of microstructure evolutions induced by high temperature annealing on damage in a semi-crystalline polymer during long-term applications. It was based on a polar form of poly(vinylidene fluoride) (T _g = −40 °C; T _m = 170 °C) and the long term annealing context dealt with burst resistance tests performed for 1000 h in a temperature range from 95 °C up to 140 °C. A secondary crystallization was observed after annealing by Differential Scanning Calorimetry and a consistent phenomenology was evidenced in Dynamic Mechanical Analysis. A decrease of the amorphous phase mobility and a weak reorganization of primary crystals were observed at the same time. Tensile tests on annealed specimens pointed out modulus and yield stress reinforcement, partial disentanglement in the amorphous phase and a raise of the volume strain. Thermally-induced microstructure evolutions were shown to enhance cavitation and slow down crack opening displacement kinetics. This last effect would result from both a raise of the yield stress in primary crystals and secondary crystallization.