Solid-state Transformation Mechanism Research Articles

Controlled Growth of Silicon Oxide Nanowires from a Patterned Reagent

A new process to fabricate silicon oxide nanowires (NWs) from a patterned reagent is reported. Arrays of NWs grow on patterned nanodots containing exposed hydrogen silsesquioxane (HSQ)/Fe−SiO2 nanocomposites during annealing at 900 °C. The NWs were seeded by metallic iron nanoparticles, and the resulting microstructure and morphology of the NWs is directly related to the size of the individual iron nanoparticles. The growth process could be dominated by a solid-state transformation mechanism in which iron nanoparticles, originally embedded in a SiO2 matrix, diffuse to the surface and act as nucleation sites, the exposed HSQ being the source for the growing NWs.

Host rock solid-state transformation in a shock-induced melt vein of Tenham L6 chondrite

The host-rock fragments entrained in a 580-μm-wide melt vein of the Tenham L6 chondrite were investigated using field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) to better understand the solid-state transformation mechanisms and the shock conditions. The melt vein consists of a matrix of silicate plus metal-sulfide that crystallized from immiscible melts, and sub-rounded host-rock fragments that have been entrained in the melt and transformed to polycrystalline high-pressure silicates by solid-state transformation mechanisms. These high-pressure phases include ringwoodite, low-Ca majorite, clinoenstatite, hollandite-structured plagioclase and Ca-rich majorite. The Ca-rich majorite occurs as a symplectitic intergrowth with a Ca-poor amorphous silicate phase in a 200 μm-diameter chondrule in the vein. This intergrowth seems to be the result of a disproportionate breakdown of a Ca-rich clinopyroxene precursor into Ca-rich majorite and (FeMg)SiO 3 perovskite, which subsequently vitrified upon pressure release. The TEM observations suggest that most solid-state transformations in the Tenham are reconstructive. The transformation of olivine to polycrystalline ringwoodite appears to involve incoherent intracrystalline nucleation and interface-controlled growth. Lamellae in partially transformed olivine are not continuous coherent lamellae, but rather lamellae of polycrystalline ringwoodite, which is inconsistent with a coherent lamellar transformation mechanism. Growth rate calculations based on published kinetic data suggest that the time required to grow 1 μm ringwoodite crystal is ∼ 100 ms at 1600 K, suggesting that the minimum shock pulse of approximately 100 ms.

Pyroxene polymorphs in melt veins of the heavily shocked Sixiangkou L6 chondrite

Sixiangkou is a heavily shocked L6 chondrite that contains numerous shock-induced melt veins with varying widths. This paper focuses on pyroxene polymorphs in shock-induced melt veins in the meteorite and discusses their implication to the high pressure and temperature conditions during impact metamorphism. The major high-pressure polymorphs are majorite and majorite-pyrope solid solution. Diopside, akimotoite and jadeite were also observed in melt veins of the Sixiangkou chondrite. Diopside coexists with ringwoodite (+ akimotoite) in coarse-grained fragments of the melt veins. Two mechanisms were proposed for the lack of phase transformation of diopside: a) low local temperature and b) a very sluggish transformation rate. Majorite and majorite-pyrope occur as coarse-grained polycrystalline and fine-grained solid solution. They were formed through solid state transformation and crystallized from a melt under pressures, respectively. Akimotoite coexists with low-Ca pyroxene in coarse-grained fragments of melt veins. They have nearly identical chemical compositions to the host low-Ca pyroxene, implying a solid state transformation mechanism. Jadeite appeared as fine-grained and was originated through retrograde transformation of lingunite under moderate post-shock pressure and high temperature conditions. The coexistence of different high-pressure pyroxene polymorphs reflects an inhomogeneous distribution of temperature in the shock-induced melt veins and a kinetic effect of phase transformations of minerals during shock metamorphism.

In situ TEM examinations of octacalcium phosphate to hydroxyapatite transformation

The transformation of octacalcium phosphate (OCP) to hydroxyapatite (HA) was induced by electron beam irradiation in transmission electron microscopy (TEM). The transformation process and crystal structure changes were examined via bright-field images, electron diffraction, high-resolution transmission electron microscopy (HRTEM) and the fast Fourier transformation pattern of HRTEM images. The results indicate a mechanism of solid-state transformation occurring in the OCP crystals. The TEM examinations and crystal structure analysis also indicate the crystallographic orientation of OCP (0 1 0)//HA (0 1¯ 0) and OCP (0 0 1)//HA (0 0 1¯), which differs from the previously proposed orientation of HA growth on an OCP apatite layer.

Dependence of the mechanism of phase transformation of Fe(III) hydroxide on pH

The phase transformation from Fe(OH) 3 gel to α-Fe 2O 3 particles at different initial pHs at about 100 °C was studied. The time necessary for completing the above process was determined. The results showed that the time of phase transformation from Fe(OH) 3 gel to α-Fe 2O 3 particles shortened with the increase of initial pH at pH < 4.5. In this pH range, β-FeOOH, as an intermediate product, was obtained and hematite was formed by dissolution/reprecipitation mechanism. However, in the pH range from 4.5 to 9.0, the transformation time prolonged with increasing pH. In this pH range, no intermediate product was found. From Fe(OH) 3 gel to hematite, there are two transformation pathways. One is the dissolution/reprecipitation mechanism and the other is the solid state transformation mechanism. With the pH close to the point of zero charge (pzc) of Fe(OH) 3 gel, the later mechanism gradually predominated.

Mechanism of the Solid-State Transformation of a Calcium-Deficient Hydroxyapatite (CDHA) into Biphasic Calcium Phosphate (BCP) at Elevated Temperatures

Three types of poorly crystallized calcium-deficient hydroxyapatites (CDHA) with Ca/P molar ratios 1.50, 1.58, and 1.67 were prepared from CaHPO4·2H2O and KOH. These were sintered at 1050 °C for 4 h. Well-crystallized β-tricalcium phosphate (β-TCP), biphasic calcium phosphate (BCP), and stoichiometric hydroxyapatite (HA) were produced, respectively. The sintered and unsintered calcium phosphates were studied by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. In addition, Ca/P molar ratios were determined by a chemical analysis. Thermogravimetric analysis (TGA) revealed a loss of water for the period of sintering. On the basis of the experimental data, a structure of BCP was suggested. After applying the numerical values of ionic (OH-, Ca2+, and PO43-) diffusion coefficients at 1000 °C, the solid-state transformation mechanism of CDHA into BCP was proposed.

Comparison of Structural Models of Mixed-Layer Illite/Smectite and Reaction Mechanisms of Smectite Illitization

AbstractThis paper compares mechanisms of the reaction of smectite to illite, in light of structural models for interstratified illite/smectite (I/S). The crystal structure of I/S has been described previously by a nonpolar and polar 2:1 layer model. In a nonpolar model, individual 2:1 layers are chemically homogeneous, whereas a polar model assumes a 2:1 layer can have a smectite charge on one side and an illite charge on the other side. Several kinds of data support the polar model; however, more determinations of the negative charge of expandable sites in I/S are needed to confirm such a model.Assuming a polar 2:1 layer model for I/S, we compare the mineralogical and geochemical consequences of several reaction mechanisms for smectite illitization: 1) solid-state transformation (SST), 2) dissolution and crystallization (DC) and 3) Ostwald ripening (OR). Features of an SST model are the replacement of smectite interlayers by illite interlayers, resulting in gradual changes in interlayer ordering, polytype, chemical and isotopic composition and crystal size and shape. Several SST models are possible depending on the nature of the reaction site (framework cations, polyhedra or interlayers). In contrast, DC models allow for abrupt changes in the structure, composition and texture of I/S as illitization proceeds. Several DC models are possible depending on the nature of the rate-controlling step, for example, diffusional transport or surface reactions during crystal growth. The OR model represents the coarsening of a single mineral where the smallest crystals dissolve and nucleate onto existing larger crystals, allowing for evolution in the overgrowth but not in the template crystal.An SST mechanism, involving either reacting polyhedra or reacting interlayers, seems to best model illitization in rock-dominated systems such as bentonite. A DC mechanism seems to best model illitization in fluid-dominated systems such as sandstone and hydrothermal environments. Both DC and SST mechanisms can occur in shale. Differences in reaction mechanism may be related to permeability. An OR model poorly describes illitization because of the progressive mineralogical and chemical changes involved. For many geologic environments, it is important to consider alternate origins for I/S such as kaolinite illitization and detrital. Further work is needed to clarify the DC crystal growth process in terms of a structural model of I/S and to determine which specific SST or DC model best characterizes illitization in geologic systems.

Solid-state transformation mechanisms of associated minerals to aluminosilicates

Thermochemical and mechanochemical transformation mechanisms of associated minerals in clay (quartz, calcite, Al and Fe oxyhydroxides) are comparatively examined. The phase transformation sequence resulting from heating and, on the other hand from dry grinding of each of the associated minerals, depends on the structure and physicochemical properties of the starting materials.Grinding treatments may cause local heating and local pressure effects in some selective metal oxides and in calcite, respectively, accounting for their structural conversions.

Niobium Carbide Synthesis from Niobium Oxide: Study of the Synthesis Conditions, Kinetics, and Solid-State Transformation Mechanism

The carburization of B-niobium oxide (B-Nb2O5) to niobium carbide (NbC) in 20% (v/v) CH4/H2was studied at temperature-programmed conditions. The reaction required high temperatures, greater than 1370 K, and variations of heating rate (0.04–0.17 K s−1) and molar space velocity (400–1600 h−1) had only a minor effect on the product specific surface area (Sg). In the course of the transformationSgincreased from 1 m2g−1to about 20 m2g−1, and scanning electron microscopy showed the development of macropores of about 100 nm. The progress of the reaction was followed by mass spectroscopic analysis of the gaseous products, which identified two distinct stages. X-ray diffraction analysis of reaction intermediates showed that in the first stage B-Nb2O5was reduced to NbO2, and in the second stage NbO2was simultaneously reduced and carburized to NbC. The first reduction occurred by a nucleation mechanism with an activation energy of 100 kJ mol−1. Independent experiments with NbO indicated that it was not involved in the reaction pathway. However, X-ray photoelectron spectroscopy revealed the presence of an oxycarbide phase which was probably the intermediate in the final transformation. Overall the reaction took place by the following steps:[formula]The oxycarbide phase transformed rapidly to the product NbC and was not observable as a bulk phase by XRD.

95/05773 13C n.m.r. analysis of catalyst carbon deposits simulated by model compounds

The transformation processes from Al-doped ferrihydrites prepared by two procedures (subsequent precipitation (SP) and co-precipitation (CP)) to hematite in the presence or absence of trace Fe(II) have been investigated. The products have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS), as well as by a kinetic dissolution process in HCl solution. It was found that Al-doped ferrihydrite prepared by the SP method could be transformed into hematite more easily than that prepared by the CP method in the absence of Fe(II), whereas the opposite was true in the presence of trace Fe(II). The transformation process without Fe(II) was completed through a solid-state transformation mechanism. However, in the transformation process with trace Fe(II), both the dissolution re-precipitation mechanism and the solid-state transformation mechanism operate simultaneously, with the former predominating, especially at a high nAl(III)/nFe(III) molar ratio. In addition, the present investigation has provided a pathway for the synthesis of discoid Al-doped hematite particles, and the effect of Al3+ ions on the formation of discoid hematite particles is also discussed.

Solidification and solid-state transformation mechanisms in Si alloyed high-chromium white cast irons

Chromium white cast irons are widely used in environments where severe abrasion resistance is a dominant requirement. To improve the wear resistance of these commercially important irons, the United States Bureau of Mines and CSIRO Australia are studying their solidification and solid-state transformation kinetics. A ternary Fe-Cr-C iron with 17.8 wt pct (pct) Cr and 3.0 pct C was compared with commercially available irons of similar Cr and C contents with Si contents between 1.6 and 2.2 pct. The irons were solidified and cooled at rates of 0.03 and 0.17 K · s-1 to 873 K. Differential thermal analysis (DTA) showed that Si depresses the eutectic reaction temperature and suggests that is has no effect upon the volume of eutectic carbides formed during solidification. Microprobe analysis revealed that austenite dendrites within the Si alloyed irons cooled at 0.03 and 0.17 K·s-1 had C and Cr contents that were lower than those of dendrites within the ternary alloy cooled at the same cooling rate and a Si alloyed iron that was water quenched from the eutectic temperature. These lower values were shown by image analysis to be the result of both solid-state growth (coarsening) of the eutectic carbides and some secondary carbide formation. Hardness measurements in the as-cast condition and after soaking in liquid nitrogen suggest an increase in the martensite start temperature as the Si content was increased. It is concluded that Si’s effect on increasing the size and volume fraction of eutectic carbides and increasing the matrix hardness should lead to improved wear resistance over regular high-chromium white cast irons.

Characterization of Illitization of Smectite in Bentonite Beds at Kinnekulle, Sweden

AbstractStructure, morphology, and chemical composition of illite/smectite (I/S) containing 30–50% smectite layers (% S) from Kinnekulle bentonites, Sweden, of diagenetic origin were examined using X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). Interlayer arrangements of I/S changed from random interstratification to short-range ordered at about 40% S. The transition from random to ordered structure proceeded continuously as reflected by the gradual decrease in probability of two smectite neighbors (Pss) towards zero. TEM observations of water-dispersed samples that had not been cation-exchanged showed that the I/S consisted dominantly of flakes coexisting with laths having a length/width ratio of about 4, regardless of % S. The thickness of the I/S particles ranged from 30 to 100 Å, and no systematic variation in thickness was detected with decreasing % S. The chemical composition of the I/S also changed continuously with decreasing % S. These observations suggest no dissolution of smectite layers and no recrystallization of illite layers during the formation of the I/S in these bentonites; rather, cationic substitutions occurred within a smectite precursor (termed a solid-state transformation mechanism). A comparison of interlayer order, particle texture, and chemistry of the I/S from various types of rocks suggests that the mechanism of smectite-to-illite conversion in the range 100% S-30% S was related to the porosity and permeability of original rocks. The solid-state transformation mechanism appears to have predominated in rocks of low porosity and permeability.

Planar and line defects in the sapphirine polytypes

Transmission electron microscopy of various natural and synthetic sapphirines shows the existence of extensive polytypic variation in some samples. In addition to the previously described 1Tc and 2M polytypes, 3Tc, 4M and 5Tc structures are observed. The structural principles underlying the polytypism and the associated diffraction behaviour are reviewed, and the correlation between the nature of the observed stacking faults and their origin during crystal growth, in deformation or as a result of solid-state transformation is discussed. Pairs of c/2 partial dislocations observed in synthetic material provide a likely solid-state transformation mechanism, although the evidence suggests that most observed stacking variation arises during growth. A new type of non-conservative planar defect has been found in sapphirine from Finero, Italy, suggestive of unit-cell scale intergrowth with a surinamite- or pyroxene-like structure. A polysomatic model of the sapphirine and surinamite structures as stackings of pyroxene 〈P〉 and spinel 〈S〉 modules is proposed.

7, 10 and 14 Å mixed-layer phyllosilicates studied structurally by TEM in pelitic rocks of the Piemontese zone (Venezuela)

Transmission electron microscopy has been used to study the interstratification of 14 Å chlorite with a 1:1 7 Å phyllosilicate (serpentine type structure) and of 14 Å chlorite with a 2:1 10 Å phyllosilicate (mica type structure). Samples were from pelitic rocks of low grade metamorphism in the Piemontese zone (Venezuela). The 7 Å phase was directly observed as thick packets possibly exhibiting different stacking sequences and, more frequently, as isolated layers intercalated within 14 Å chlorite layers. Such a 7 Å phase is probably a precursor of the 14 Å chlorite. Various interlayered mica- 14 Å chlorite structures were also observed as intermediate states of mica chloritization. In particular, a solid state transformation mechanism, such as a mica layer laterally changing into a chlorite layer (a 1:1 reaction) with increasing volume, as proposed by Olives et al. (1983), was evidenced. Lastly during this study, particular attention was paid to contrast details of TEM images which are very sensitive to experimental parameters (defocus, specimen thickness and orientation,...). Even the use of simple lattice fringe images is to be done with great care, specially for a 7-Å/14-Å mixed multiple layers study. Examples show that, as soon as optimum imaging conditions are left, one-dimensional image analysis of 7 Å and 14 Å phases may be erroneous or ambiguous and such phases confused.

Layer-By-Layer Mechanism of Smectite Illitization and Application to a New Rate Law

AbstractA layer-by-layer mechanism explains important features of mixed-layer clay minerals formed during the illitization of smectite, including the occurrence of randomly interstratified illite/smectite, the transition to ordered interstratifications, and the development of long-range ordering. A variety of solid-state transformation mechanisms were tested with a stochastic model, which accounts for interactions among clay layers. The model produces most successful results when the reaction of smectite layers with one illite nearest neighbor is favored over smectites with no illite neighbors by a factor of about two, and over those with two illite neighbors by a factor of ten or more. Synthetic X-ray powder diffraction patterns calculated from model results compare well with those of illite/smectite minerals. These results suggest a new kinetic rate law. Solutions to this rate law for reaction within sediments undergoing burial give mineralogical profiles with depth similar to those observed in subsiding sedimentary basins.