Solution-mediated polymorphic transformation of stearic acid

Polymorphic transformations of stearic acid in their crystallization solutions have been studied. When stearic acid crystals were left in the ethanolic saturated mother liquor, the B-form modification was obtained throughout the whole process. In n-hexane and benzene during the first 2–3 hours, the C-polymorph was the predominant form, later transformed progressively into the B-form, so that towards the end of the experiment the whole precipitate was in the B-form. The proposed explanation is that the transformation is due to dissolution caused by thermal fluctuation in the bath, and subsequent precipitation. The dissolution and reprecipitation was followed up by micro photographs. In the presence of Span 60, only C form was obtained in all the solvents. – As a result of this study it is suggested to pay attention to possible solution-mediated polymorphic transformations of the crystallized material which may cause a misleading interpretation of the factors affecting the crystallization of the fatty acid.     

Isothermal nanocrystallization kinetics during polymorphic transformation

The experimental value of Avrami exponent during polymorphic nanocrystallization deviates from the theoretical prediction of the Kolmogorov-Johnson-Mehl-Avrami (KJMA) isothermal kinetics. The assumption of negligible nuclei volume, implicit in KJMA, is violated for all such transformations involving nano-sized grains. A discrete adaptation of the KJMA model is used to study the effect of the non-zero nuclei volume on nanocrystallization kinetics. Transformations initiated by homogeneous and heterogeneous nucleation are analyzed under constant nucleation and growth rate conditions. For homogeneous nucleation, a universal scaling relation with respect to finite nuclei size is found. In contrast to this, for the heterogeneous case a series of linear scaling relations are observed with respect to the nucleation conditions.     

The effect of dynamic recrystallization on the growth velocity of new phase crystals at polymorphic transformation of tin and thallium

The analysis of dynamic recrystallization influencing the crystal growth of the new phase in high purity tin and thallium (99.9995%) is carried out. It is shown that in tin, in which the β → α transition proceeds on the dislocation or the normal mechanisms (depending upon a supercooling degree, ΔT) a superposition of dynamic recrystallization results in an increase of the growth velocity of α-Sn crystals. The most distinct of acceleration is observed at −21 ÷ − 23 °C, when the growth velocity of recrystallization centers and the maximal growth velocity, V_max, of the crystals are comparable values. The dynamic recrystallization superposition in thallium offers to transform the transformation mechanism. Thereby an erase of the features of the martensite α ⇄ β transition mechanism and an appearance of the features of the normal crystal growth mechanism of the new phase are observed.     

Polymorphous transformation of quartz induced by mechanical treatment

Because of its strongly damaged structure (partially crystalline) mechanically activated α-quartz is not transformed to β-quartz. Rather, mechanically activated α-quartz is rapidly transformed to the cristobalite modification even at temperatures as low as 1200 °C. The acceleration and temperature shift of the quartz-cristobalite transformation is caused by mechanically induced defects. The activation energy of the transformation is thus reduced by about 50%.     

Defect generation on mechanical surface defects before and after the oxidation (I). TEM investigations on the nucleation of oxidation stacking faults at mechanically induced weak surface defects

In order to obtain information on the nucleation mechanism of oxidation stacking faults, mechanically induced weak surface defects as simulated by low-load scratches were investigated with TEM before and after a short-time wet oxidation at 1050 °C. The experimental results could not confirm the existing models which explain stacking fault nucleation by the splitting of complete dislocations. Rather it was found that clusters of silicon interstitials are generated due to strong local compression during scratching, which then act as nuclei for stacking faults in the subsequent oxydation process.     

Time and temperature induced phase transformation in L‐isoleucine hydrochloride monohydrated crystal

Semi organic crystals have been intensively studied aiming applications in nonlinear optical (NLO). Such applicability requires crystals with both high quality and thermal stability, which make the full characterization of this material a topic issue. In this paper, single crystals of L‐isoleucine hydrochloride monohydrated (L‐Ile.H2O.HCl) was synthesized by slow evaporation technique and characterized by thermal analysis and X‐ray diffraction (XRD) measurements. XRD results at 298 K showed that the sample crystallize with the orthorhombic structure, and the lattice parameters obtained by Rietveld refinement were a = 5.873(3) Å, b = 24.814(4) Å, and c = 6.873(5) Å. Thermal analysis measurements shows four decomposition stages between 328 ‐480 K which were associated with loss of water by desolvation, loss crystallization water and decomposition of L‐Ile.HCl. XRD measurements as a function of temperature up to 428 K show an irreversible phase transformation. This transformation was obtained after 32 hours keeping the L‐Ile.HCl.H2O sample at 413 K. The analysis shows that phase transformation occurs due to water and chlorine losses without destroy the amino acid carbon chain and in the end of transformation only the precursor amino acid remains. L‐Ile.HCl.H2O present low thermal stability and the phase transformation is time and temperature dependent.     

Torsion and tension stress induced transformation of surface magnetic structure in Co-rich amorphous microwires

The effect of torsion and tensile stress on the surface magnetic properties in Co-rich glass covered amorphous microwires has been studied. The magnetization reversal in microwires has been studied by magneto-optical Kerr effect. Calculations of hysteresis loop was performed taking into account the existence of helical magnetic anisotropy in the outer shell of the microwire. The comparative analysis of the results of the experiment and the calculation demonstrates that the observed transformation of the surface hysteresis is related to the variation of the angle of the helical surface structure.     

Investigations of the Thermoluminescence and EPR of mechanically activated and irradiated quartz

Thermoluminescence (TL) measurements are useful for a characterization of the state of order in mechanically activated quartz after X-ray and γ-irradiation. As the luminescence at room temperature (RT) the TL also decreases with decreasing crystallinity of the quartz. On the contrary, the concentrations of the EPR centers in quartz (E′, oxygen defect electron (ODE)) created by mechanical activation as well as irradiation increase with decreasing crystallinity. It is shown, that the electronic defects, which cause the TL, are different from the EPR centers.     

Effect of γ-irradiation on mechanically activated and on non-activated LiF

By means of EPR spectroscopy lattice defects have been investigated, which had been produced in OH⁻ containing LiF by mechanical activation and subsequent irradiation (mainly γ-irradiation). The structure of irradiation-induced hydrogen centers shows characteristic differences dependent on the kind of LiF: H_s⁰ (hydrogen atoms in anion vacancies) are created in mechanically activated LiF powders, whereas H_i⁰ (hydrogen atoms in interstitial positions) in LiF single crystals. In both the cases also F centers are produced besides other paramagnetic defects. The observed defects can be used for an estimation of the degree of distortion in mechanically activated LiF.     

Effects of the rate of supersaturation generation on polymorphic crystallization of m-hydroxybenzoic acid and o-aminobenzoic acid

Effects of the rate of supersaturation generation on polymorphic crystallization have been investigated through evaporation and cooling crystallization experiments of m-hydroxybenzoic acid (m-HBA) in methanol, acetone and ethyl acetate, and o-aminobenzoic acid (o-ABA) in ethanol. The rate of supersaturation generation has been altered by systematically changing either the cooling rate or the evaporation rate of solvent using a jacketed crystallizer and a microfluidic evaporation device, respectively. The results have revealed that the rate of supersaturation generation and the tendency of the formation of the less stable polymorph are positively correlated. Kinetic effects are dominant when the rate of supersaturation generation is high, thereby producing the metastable polymorphs (orthorhombic m-HBA; Form II of o-ABA); on the contrary, more stable polymorphs (monoclinic m-HBA; Forms III and I of o-ABA) are formed when the rate of supersaturation generation is low and the thermodynamic effects are prevailing.     

Defect generation on mechanically surface defects before and after the oxidation (II). Dislocation generation at cracks

The generation of crystal defects at residual cracks due to a wet oxidation of chemically mechanically polished silicon wafers at 1100 °C was investigated. It was found that the nucleation of oxidation stacking faults at these elastic cracks is impeded as compared to the regions between the cracks. However, extended dislocations, dislocation dipoles, certain stacking faults, and precipitates are generated at such cracks. These crystal defects and the crack itself impede the nucleation of oxidation stacking faults at the crack, as they locally reduce the excess interstitials at the oxidation front.     

Order parameter in the quasicrystal-crystal transformation

Some aspects of the relationship between quasicrystal and crystal structures not considered earlier are discussed. It is shown that the atomic positions in a quasicrystal lattice are intermediate between the positions of periodic incommensurate crystal lattice. For the octagonal symmetry, two such generating lattices can be square and equal to one another. Then any quasicrystal position lies exactly between two crystal positions in different lattices. The classical (phason) mechanism of the quasicrystal-crystal phase transition is interpreted as a relative homogeneous deformation of generating crystal lattices leading to the commensurability of these lattices. The order parameter of such a transition is defined.     

Effect of preparation conditions on the phase transformation of mesoporous alumina

A post-hydrolysis method is proposed for the easy-and-fast preparation of mesoporous γ-alumina (MA) at room temperature using an alkyl carboxylate as a chemical template. Water was used as an initiating solvent in the hydrolysis of aluminum alkoxide in the terminal reaction procedure. Hydrolysis and condensation reactions were simultaneously induced. The initial phase of as-made MA was bayerite and/or (pseudo)boehmite, and was transformed into active alumina at 250 °C, which remained thermally stable at temperatures up to 600 °C. The phase transformation of alumina was analyzed with DTA, XRD and ²⁷Al MAS NMR. Based on an ²⁷Al MAS NMR analysis, calcined alumina prepared via a hydrothermal method contained a high ratio of Al^VI to Al^IV (∼4.8), which is not fully transformed into active alumina, and aluminum hydroxide (AlOOH or Al(OH)₃) was detected. These results suggest that high temperature is required to obtain an active form of alumina, when the hydrothermal method is used. Another factor is the ratio of Al^VI to Al^IV in the MA, which is also dependent on the pH of the reactant. Pore size, surface area, and pore structure could be controlled by adjusting the molar ratio of water to aluminum precursor, and the preparation temperature. The pore size of MA was adjustable from 2 to 7 nm, and the MA indicated a large surface area of 300 ∼ 500 m²/g.     

On the phase diagram of polymorphic ethanol: Thermodynamic and structural studies

It is well known that ethanol exhibits a very interesting polymorphism presenting different solid phases: a fully-ordered (monoclinic) crystal, a (bcc) plastic crystal, which by quenching becomes an orientationally-disordered crystal with glassy properties (hence sometimes named ‘glassy crystal’), and the ordinary amorphous glass. We have carried out calorimetric, X-ray diffraction, and Brillouin-scattering experiments above liquid-nitrogen temperatures and have found several new features that shed more light on the rich and interesting phase diagram of ethanol. Firstly, we have identified up to four different varieties of the monoclinic crystalline phase depending on the thermal history. We also present new specific-heat data of these glassy and crystalline phases below the glass transition temperature up to the melting temperature. Furthermore, we have unexpectedly found that the amorphous phase can also be obtained by the unusual route of a very slow cooling of the liquid in some particular experimental set-ups, evidencing the heterogeneous character of the crystallization kinetics of these molecular glass-formers.     

Influence of minor addition of Pd, Ag or Au on the microstructural evolution of mechanically alloyed Zr-Ni alloys

In this work, the influence of minor addition of Pd, Ag or Au (1 at.%) on the microstructural evolution of Zr-Ni alloys during mechanical alloying has been investigated using X-ray diffraction and differential scanning calorimetry. The experimental results show that the minor addition of Pd, Ag or Au has no significant influence on the amorphization process and glass forming ability of Zr-Ni amorphous alloys. In comparison with Ag and Pd, the minor addition of Au can enhance the mechanical and thermal stability of the mechanically alloyed Zr-Ni alloy. Discussion has been made based upon the evolution of the nearest-neighbor distance of atoms with increasing milling time.     

Influence of polyvinylpyrrolidone on the precipitation of calcium carbonate and on the transformation of vaterite to calcite

The precipitation process of calcium carbonate (CaCO₃) in the absence and presence of poly (N-vinyl-2-pyrrolidone) (PVP) was investigated using scanning electron microscopy, powder X-ray diffraction, and Fourier transform infrared spectroscopy methods at room temperature. The results indicate that PVP does not affect the polymorphy, but has influence on their morphology and size of CaCO₃ crystals. With the addition of PVP, the amorphous CaCO₃ could aggregate into bigger amorphous spherulites before transforming to crystalline state. Also, the transformation from the thermodynamically unstable vaterite to the stable calcite was investigated in the presence of PVP. As a particle-stabilizing agent, PVP molecules inhibit the formation of vaterite, however, promote the formation of calcite as well as the rate of the solvent-mediated transformation from vaterite to calcite.     

Crystallography of the Martensitic transformation in Indium-Thallium Alloys

On cooling a single crystal of Indium containing 20.75 atomic per cent Thallium may transform martensitically by the migration of a single interface, covering the whole cross-section, from the high temperature face-centred cubic β-structure to the facecentred tetragonal α-structure. Applied external mechanical stress is able to reverse the direction of interface motion during transformation. The α-martensite consists of a stack of fine parallel twins which are suficiently wide to be visible under an optical microscope. There are no longrange stresses, provided the twins have mean thickness ratio 1.6:1. By extension or compression the whole specimen may undergo a conversion into one or the other orientation by movement of the twin boundaries, leading to ferroplasticity or ferroelasticity. The geometry and crystallography of and the effects of external stresses on the β → α transformation have been calculated on the basis of the phenomenological crystallographic martensite theory.     

Crystal and molecular structure of 0,0,0-propan-1,2,3-trithionobenzoate (3TBZ): Implication for thermally induced solid-state transformation of 3TBZ

The crystal and molecular structure of 0,0,0-propan-1,2,3-trithiobenzoate (3TBZ) has been determined by direct methods. Crystals are monoclinic,a=6.810(2),b=35.950(2),c=9.415(2) Å,=94.78(2)°, space group P2₁/c,Z=4. The structure was refined using full-matrix least-squares toR=0.078,R _w=0.064, 1957 reflections CuK radiation, =1.5418 Å. The molecule has one relatively short S---C (methylene) intramolecular distance, S(2)---C(38)=3.440(4)Å, approximately 1.5 Å less than the shortest S---C distance observed in related crystal structures. An hypothesis related to this short contact is proposed in order to account for the unusual pyrolitic behavior of this compound in the solid state.