Aggregates Of Crystallites Research Articles

Homogenization of Polycrystalline¶Aggregates

The effective elastic properties of a polycrystalline material depend on the single crystal elastic constants of the crystallites comprising the polycrystal and on the manner in which the crystallites are arranged. In this paper we apply the techniques of homogenization to put the problem of determining effective elastic constants in a precise mathematical framework that permits us to derive an expression for the effective elasticity tensor. We also study how the homogenized elasticity tensor changes as the probability characterizing the ensemble changes. Under the assumption that the field of orientations of the crystallographic axes of the crystallites is an independent random field, we show that our theory is compatible with the formulation used in texture analysis. In particular, we are able to prove that the physical assumption made by [10] in his study of weakly-textured polycrystals holds true. In addition, we establish some elementary bounds on the material constants that characterize the effective elasticity tensor of weakly-textured orthorhombic aggregates of cubic crystallites.

塑性滑りモデルを用いた多結晶集合体による幅拘束１軸延伸シンジオタクチックポリプロピレンフィルム結晶各回折面法線の配向分布関数評価

塑性滑りモデルを用いた多結晶集合体による幅拘束１軸延伸シンジオタクチックポリプロピレンフィルム結晶各回折面法線の配向分布関数評価

Optimal bounds on texture coefficients

Let w be the orientation distribution function of a polycrystalline aggregate of crystallites with symmetry Gcr and with group of texture symmetry Gtex. In this paper we obtain a “recipe” on how to derive optimal bounds on the texture coefficients Wlmn associated with w. In particular, we find explicit bounds in the case in which Gtex is a group with orthorhombic symmetry and Gcr is either a group with cubic symmetry or a group with hexagonal symmetry.

Nucleation-limited aggregation of crystallites in fractal growth

We report in this article a crystallization behavior of NH 4Cl in a gel system, which leads to a new type of fractal aggregate. In situ optical microscopy and atomic force microscopy are used to study the growth process and the aggregate morphology. Microscopically the crystallites in the aggregate branches have remarkable correlations with their crystallographic orientations, which introduces a strong effective anisotropy to the fractal growth. Macroscopically, the fractal pattern has a scaling property different from the fractals generated by diffusion-limited aggregation and that generated by homogeneous nucleation-limited aggregation. It seems that the fractal patterns do not arise exclusively in the far-from-equilibrium situations; nor do they need the anisotropy for the fractal aggregation to be very low.

KrF laser CVD of titanium oxide from titanium tetraisopropoxide

Titanium oxide films were prepared by KrF laser CVD from gaseous titanium tetraisopropoxide in an atmosphere of helium gas, and also of oxygen-containing helium gas as well. In both cases, the deposition rate was found to increase approximately linearly with increasing laser fluence, though the rate observed in the presence of oxygen was much greater than the rate found in the other case. The oxide deposits formed in the absence of oxygen consisted of TiO2 (anatase) and Ti3O5, while the deposits formed in the presence of oxygen consisted of anatase with small amounts of rutile. In the latter case, the deposits exhibited a strong preferred orientation of crystal planes, with (112) and (220) planes being oriented parallel to the substrate surface, depending on the magnitude of laser fluence. when viewed from a morphological standpoint, the oxide film formed in the absence of oxygen consisted of somewhat loose aggregated crystallites of anatase and Ti3O5, while the film formed in the presence of oxygen consisted of compact aggregates of titanium dioxide crystallites. Moreover, a possible cause of formation of Ti3O5 was suspected.

Investigations on sol-gel derived lanthanum doped lead titanate (PLT) films

Pb1.05−xLax(Ti1−x/4)O3 (x = 0.00, 0.05, 0.1, 0.15, 0.20, 0.25 and 0.30) thin films were deposited on sapphire (0001) substrates by an acetic acid modified sol-gel technique. The films were characterized, using various techniques, in terms of their phase formation behavior, composition, microstructure and electrical characteristics. It was observed that the sol-gel derived films crystallize into the perovskite phase without any paraelectric cubic pyrochlore phase formation. For less than 15 at% lanthanum composition, the films show a tetragonal phase which transforms into a cubic phase for higher lanthanum content. The observed reduction of the tetragonal distortion with La addition is due to the pronounced shrinkage of the ‘c’ axis. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition. The XPS results are in excellent agreement with the nominal composition of the films. All films are extremely smooth, crack- and pinhole-free, and have a dense uniform microstructure. The grains are not single crystalline, instead each grain is the aggregate of numerous tiny perovskite crystallites. The apparent grain size increases with La addition. The values of dielectric constant, remnant polarization, saturation polarization and coercive field are comparable with the best reported values of sol-gel derived PLT thin films.

Synthesis of large single crystals of the large-pore aluminophosphate molecular sieve VPI-5

The synthesis of VPI-5 has been optimized for the production of large single crystals with dimensions up to 400×35×35 μm3. All syntheses produced aggregates of needle-like crystallites. In most cases these were highly intergrown, but under the optimised conditions, individual hexagonal needles radiating from a central point were formed. The synthesis procedure of Jahn et al., which was known to produce a highly crystalline material and which is unusual in that polyphosphoric acid rather than orthophosphoric acid is used as the P source, was taken as a starting point. Optimization was done in three stages: (1) using polyphosphoric acid with an average chain length of 8 (nav=8) and di-n-propylamine as the organic additive; (2) using a shorter, commercially available polyphosphoric acid (nav=4) and a variety of organic additives; and (3) using polyphosphoric acid with nav=8 and di-n-butylamine (DBH), which was found to be the most suitable organic additive in the second stage. Key parameters were found to be the use of the longer chain polyphosphoric acid, the water content of the synthesis mixture, the ultrasonic treatment of the gel, the selection of the organic additive and the reaction temperature. With a gel composition of Al2O3:P2O5:95.4H2O:DBA and a reaction temperature 123°C, large single crystals could be grown.

Vitrification of radioactive waste by reaction sintering under pressure

Silicate nuclear waste glasses were synthesized by reaction sintering of powdered precursors under pressure. The glass samples contained a glass matrix phase with embedded zirconia (baddeleyite) particles. A waste composition with 38 wt% of ZrO 2 was prepared with a waste loading of 30–50 wt% at 800°C and 28 MPa, by hot isostatic pressing. The glass former was commercial amorphous silica powder to which simulated waste was added as calcined oxides. Phase compositions and microstructure of the sintered glass samples were characterized using scanning and analytical electron microscopy. The results show that extensive sintering took place and that a continuous glass phase was formed, particularly at higher waste loading. Waste components such as Na 2O, CaO, MnO 2, La 2O 3, Fe 2O 3, Cr 2O 3, and P 2O 5 dissolved completely in the glass phase. ZrO 2 was also dissolved but recrystallized from the glass as aggregates of baddeleyite crystallites surrounding the original silica particles. MCC-1 type chemical durability tests showed that the glasses are durable with dissolution rates similar to or lower than that of the highly durable French R7T7 borosilicate glass. This glass contains 13 wt% high-level radioactive waste from light water reactor fuel reprocessing and has a melting temperature of 1150°C. The long-term chemical durability of our sintered glasses is expected to be as high as that of rhyolitic glasses, based on hydration energies of 3.7 and 3.3 kJ/mole, respectively. Rhyolitic glasses show little alteration over geological periods of time with a typical corrosion rate of 1 μm/1000 yr.

Annealing effects on phase transformation and powder microstructure of nanocrystalline zirconia polymorphs

Nanocrystalline zirconia powders in pure form and doped with yttria and calcia were prepared by the precipitation method. In the as-prepared condition, all the doped samples show only monoclinic phase, independent of the dopants and dopant concentration. On annealing the powders at 400 °C and above, in the case of 3 and 6 mol% Y2O3 stabilized ZrO2 (3YSZ and 6YSZ), the monoclinic phase transforms to tetragonal and cubic phases, respectively, whereas in 3 and 6 mol% CaO stabilized ZrO2 (3CSZ and 6CSZ), the volume percentage of the monoclinic phase gradually decreases up to the annealing temperature of about 1000 °C and then increases for higher annealing temperatures. The presence of monoclinic phase in the as-prepared samples of doped zirconia has been attributed to the lattice strain effect which results in the less symmetric lattice. For the annealing temperatures below 1000 °C, the phenomenon of partial stabilization of the tetragonal phase in 3CSZ and 6CSZ can be explained in terms of the grain size effect. High resolution transmission electron microscopy (HRTEM) observations reveal the lattice strain structure in the as-prepared materials. The particles are found to be a tightly bound aggregate of small crystallites with average size of 10 nm. The morphology of the particles is observed to be dependent on the dopants and dopant concentration.

Use of silicate crystallite mesoporous material as catalyst support for Fischer–Tropsch reaction

Novel uniform mesoporous materials (silicate crystallite mesoporous material, SCMM) were synthesized by hydrothermal treatment of Si, Mg and/or Al containing hydroxide precipitates, along with quaternary ammonium salt, and were applied as catalyst support for Fischer–Tropsch reaction. SCMM is composed of aggregates of homogeneous layer silicate crystallites, as identified by X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) photographs which are structurally analogous to smectite clay. The mesopore of SCMM corresponds to interparticle space of disk-shaped crystallites (6–25 nm in diameter) aggregated by edge-to-face bonding. Two kinds of SCMM with variable negative charge locations were used as support materials of Co-catalysts for hydrogenation of CO. Conversion of CO was high on SCMM compared with that of silica gel. Furthermore, the main products obtained were hydrocarbons. In case of SCMM type with negative charge near the surface, olefins and branched hydrocarbons were efficiently produced in CO conversion reaction; where as another SCMM type having negative charge at the middle of crystallite platelets produced mainly normal hydrocarbons for the same reaction. This may be due to the role of the negative charge of SCMM which affects the chemical state of Co-catalyst supported on SCMM. The function of SCMM as compared to other mesoporous materials is better in terms of homogeneous and site-specific distribution of negative charge, which is further helpful in controlling the surface phenomenon as confirmed by the existence of linear CO species adsorbed on Co.

Nucleation and Growth for Synthesis of Nanometric Zirconia Particles by Forced Hydrolysis

Monodispersed nanoscale metal oxide powders are important precursors for the preparation of advanced ceramics with uniform nanostructures and properties. Hydrous zirconia particles in nanometer dimensions were synthesized via forced hydrolysis, that is, homogeneous hydrolysis and controlled hydrothermal polymerization/condensation of zirconium tetramers in aqueous solutions of zirconyl inorganic salt. This thermohydrolytic route uses inexpensive starting chemicals—that is, inorganic metal salt, which makes the forced-hydrolysis approach more competitive than other sol–gel routes using metal alkoxides. Hydrothermal treatment of zirconyl salt solutions (0.01 to 0.075 M) produced nanocrystalline monoclinic ZrO2powder with a narrow size distribution. The hydrous zirconium oxide particles are somewhat porous, cube-shaped aggregates of small crystallites (< 5 nm). The nucleation and nanoparticle growth in zirconyl chloride aqueous solutions (100°C) was successfully monitored with a custom-designed, low-power dynamic laser light-scattering spectrophotometer. Extensive experimental evidence strongly supports the position that particle growth is mainly via an aggregation mechanism. On the other hand, the growth kinetics are controlled by the coupled events involving polymerization/condensation and colloidal coagulation. Both the controlled-reaction and the controlled-aggregation approaches were studied to reduce the induction period for nucleation as well as to enhance particle growth kinetics. The present study of the synthesis process and the characterization of nanosize powders constitute prerequisite steps for fabrication of dense, nanophase zirconia materials (with grain size on the order of 1 to 100 nm) that are expected to have improved mechanical and thermomechanical stability at elevated temperatures.

The influence of silicic acid on aluminium hydroxide precipitation and flocculation by aluminium salts

Silic acid is always present in natural waters and interactions between silicic acid and aluminium have been reported over wide concentration ranges. Since aluminium salts are commonly used as coagulants in water treatment, it is possible that their effects may be modified by the presence of silicic acid. In many cases flocculation of particles in water is dependent on the formation of an amorphous precipitate of aluminium hydroxide and silicic acid may influence the precipitation process. Using a simple continuous optical technique, the influence of silicic acid on the precipitation process and on the flocculation of clay suspensions by aluminium sulphate has been examined. It has been found that silicic acid can promote or prevent the aggregation of primary aluminium crystallites depending on solution pH and the concentration of silicic acid. These effects are closely matched by the effect of silicic acid on the flocculation process. At pH 7, with about 0.3 mM of silicic acid, the growth of precipitate and flocculation are enhanced; but with higher silicic acid concentrations these processes are hindered and eventually prevented altogether (at around 1.5 mM). The effects of silicic acid are more pronounced at pH 8, where less than 0.1 mM is sufficient to enhance precipitation and flocculation, and at 0.25 mM they are completely inhibited. These trends have been confirmed by determining the residual turbidity of suspensions after a standard stirring and sedimentation procedure and by electrophoretic mobility measurements on particles shortly after dosing of coagulant. It appears that the effects are closely connected with silicic acid–aluminium interactions, especially with regard to modifications in the charge of precipitated species.

二軸延伸ポリ（ｐ‐フェニレンスルフィド）フィルムの延伸に伴う各結晶面法線の配向分布関数の塑性滑りによるシミュレーション 第１報 多結晶集合体を用いた塑性滑りによる結晶各面法線の配向分布関数シミュレーション法の基礎モデル

二軸延伸ポリ（ｐ‐フェニレンスルフィド）フィルムの延伸に伴う各結晶面法線の配向分布関数の塑性滑りによるシミュレーション 第１報 多結晶集合体を用いた塑性滑りによる結晶各面法線の配向分布関数シミュレーション法の基礎モデル

Structural study of mesoporous titania and titanium–stearic acid complex prepared from titanium alkoxide

A mesoporous titania was prepared from titanium tetraisopropoxide and stearic acid (STA). The pore size of the titania increases linearly from ca. 5 to 15 nm with increase in the STA/Ti ratio (X) for X 1.5. A mol equivalent of STA coordinates with Ti atom forming a bidentate carboxy group and excess STA forms propyl stearate. Upon calcination, the propyl stearate is removed at a lower temperature by vaporization, and does not affect the pore structure of the titania. The Ti–STA precursor has the layered structure of the Ti–STA complex, whose interplanar distance increases with increasing STA content, while the mesoporous titania is composed of aggregates of crystallites of anatase. During calcination of the precursor, decomposition of the complex and crystallization of anatase occur concurrently. The crystallization process is considered to be affected by the interplanar distance of the layered structure in the precursor, so that both the pore size and the pore volume increase with increase in the STA/Ti ratio.

Effect of N-Sulfonation on the Colloidal and Liquid Crystal Behavior of Chitin Crystallites

Chitin crystallites were heterogeneously N-sulfonated in an aqueous medium using triethylamine/sulfur trioxide (TEA/SO3) or pyridine/sulfur trioxide. The extent of N-sulfonation of the crystallites has been controlled by the amount of TEA/SO3added in the reaction. The concentration of sulfur in the crystallites after N-sulfonation was quantified using conductimetric titration and elemental analysis. The ratio of N-sulfonated amino groups to amino groups (S/N) was calculated based on the titration data. The presence of N-S bonds assumed to be at crystallite surfaces was demonstrated by X-ray photoelectron spectroscopy (XPS). After N-sulfonation, the crystallites have two ionizable groups at their surface: —NH3+and —NHSO3H(Na). The former is pH dependent. The colloidal properties of the N-sulfonated crystallites having different S/N were investigated by plotting the zeta potential as a function of the pH of the suspension. The isoelectric point was found to change with the level of N-sulfonation. Transmission electron microscopy shows that the aggregation of crystallites depends strongly on the extent of N-sulfonation. Above a certain concentration, the original chitin crystallites form tactoids (chiral nematic domains) in an aqueous medium. This phenomenon was not observed for the crystallites with a low extent of surface N-sulfonation (below 70%). At about 80% N-sulfonation, the formation of tactoids was once again observed.

Titanium disulphide thin film prepared by plasma-CVD for lithium secondary battery

Titanium disulphide fine power and preferred oriented thin film were prepared by plasma-CVD. The fine powder product was an aggregate of plate-like crystallites of some 10 nm in width, and less than 10 nm in thickness. Crystalline orientation of TiS 2 in the film was investigated in relation to deposition rate and film thickness. The preferred orientation of the TiS 2 basal plane perpendicular to the substrate was obtained on films with a thickness of more than ~ 10 μm at a deposition rate of ~ 4 × 10 −3 g/cm 2 · h. Both the fine powder and the preferred orientation resulted in large discharge capacities in a lithium battery cathode application.

Evidence for Aggregation in Oxalate Stone Formation: Atomic Force and Low Voltage Scanning Electron Microscopy

Purpose The aim of this investigation was to differentiate between aggregation and crystal growth by studying the structure of oxalate stones at high spatial resolution using recently developed microscopy techniques. Materials and Methods Sections from 6 complete human oxalate stones and 4 stone fragments were prepared by ultramicrotomy and examined by both low voltage scanning electron microscopy and atomic force microscopy. Results The scanning electron microscopy showed lamellar structures up to 10 micrometer in size, consistent with previous results, and provided evidence that these structures were composed of smaller particles. The atomic force microscopy clearly showed arrays of the small particles, whose size varied between 500 Angstrom and 2800 Angstrom. Conclusion Our images suggest that an ordered aggregation of small crystallites is responsible for oxalate stone formation.

On the separation of stress-induced and texture-induced birefringence in acoustoelasticity

In this paper we develop a simple micromechanical model of a prestressed polycrystalline aggregate, in which the texture-induced and stress-induced anisotropies of the aggregate are precisely defined; here the word ‘texture’ always refers to the texture of the aggregate at the given prestressed configuration, not to that of a perhaps fictitious natural state of the aggregate. We use this model to derive, for a prestressed orthotropic aggregate of cubic crystallites, a birefringence formula which shows explicitly the effects of the orthotropic texture on the acoustoelastic coefficients. From this formula we observe that, generally speaking, we cannot separate the total birefringence into two distinct parts, one reflecting purely the influence of stress on the birefringence, and the other encompassing all the effects of texture. The same formula, on the other hand, provides for each material specific quantitative criteria under which the ‘separation of stress-induced and texture-induced birefringence’ would become meaningful in an approximate sense.

Poly(ethyleneimine) Inhibition Effect on Calcium Oxalate Crystallization

It is shown that in contrast to poly(N,N'-dimethylaminoethyl methacrylate), polyethyleneimine has the most significant inhibitory effect on the calcium oxalate (CO) crystallization among the polymer inhibitors tested up to now. This inhibition effect is complex: on the CO nucleation, crystal growth rate and on the crystallite aggregation

Quasicrystals, crystals and multiple twins in rapidly solidified Al[sbnd]Cr[sbnd]Si, Al[sbnd]Mn[sbnd]Si and Al[sbnd]Mn[sbnd]Cr[sbnd]Si alloys

The coexistence of quasicrystals and rational approximant structures (RAS) has been observed in melt-spun Al80Cr14Si6, Al80Mn14Si6 and Al75Mn10Cr5Si10 alloys. The presence of a b.c.c. α-AlMnSi phase in AlMnSi and α-AlMnSi(Cr) phase in AlMnCrSi has been seen. A multiple twinning around an irrational axis of the RAS has been reported in an aggregate of fine size cubic crystallites in all three alloys. Selected area diffraction patterns show that the crystalline aggregate symmetry is linked to the icosahedral point group symmetry (m35). Various ways of expressing the twin relationship in the cubic crystalline aggregates have been discussed. The thermal stability of the icosahedral phase at high temperatures reveals that the icosahedral phase in AlMnSi and AlMnCrSi alloys transforms to α-AlMnSi at temperatures of 690 and 670 K, respectively. In AlCrSi alloy, heating to a high temperature (615 K) leads to the transformation of the icosahedral phase into a new metastable phase having an ordered cubic structure equivalent to α-AlMnSi. The occurrence of multiple twinning leading to icosahedral symmetry in the as-spun AlCrSi alloy is presumably due to this metastable phase.