Unreacted Samples Research Articles

A modified Schlenk line for gas‐phase accelerating rate calorimetry capable of quantitatively sampling and safely handling hazardous gases

AbstractThis paper reports a new method using a modified Schlenk line for quantitative charging and safe disposal of gases in a gas‐phase accelerating rate calorimetry (ARC) test. The modified Schlenk line is capable of accurately measuring the mass of gas(s) charged to the ARC bomb. The method conveniently allows loading a single gas or multiple gases for reaction. It also provides dosing (or shot addition) capability to add a gaseous sample for when the gas is too reactive to be preloaded with other components. The modified Schlenk line enables safely handling toxic and extremely reactive gaseous chemicals, including collecting gases for further characterization, and disposing the gaseous products and unreacted samples into neutralizing scrubbers after the ARC run. The modified Schlenk line can also be utilized to test solid, liquid, or gas/liquid/solid mixed phases under specific atmospheres. Additional capabilities of the assembly line include quick leak checking and easily removing unwanted air/moisture in the headspace of the bomb or impurities (residual solvents) in the sample before an ARC run, resulting in more reliable data. The utility of this method is demonstrated in the thermal evaluation of the chlorination of toluene with chlorine gas and the thermal stability study of a halogenated ethane.

Thermal decomposition behaviour and chemical kinetics of tungsten based electrically controlled solid propellants

In recent times, there is a growing interest in the development of electrically controlled solid propellants (ECSPs) because they offer various advantages over conventional solid propellants. However, the fundamental understanding on the complex reaction mechanism of decomposition/combustion of these ECSPs is still limited. The present study attempts to expand the knowledge on thermal decomposition mechanisms of ECSPs based on lithium perchlorate (LP) oxidizer and poly vinyl alcohol (PVA) binder, and to further investigate the influence of tungsten (W) as a metal additive, using thermal analysis. The chemical kinetic parameters for different metallized samples are obtained and compared with the non-metallized baseline propellant. Decomposition of non-metallized sample occurs over three stages involving primary reactions between molten oxidizer and pyrolysis products of PVA, and secondary gas-phase reactions overlapping with pyrolysis of unreacted LP. However, metallized ECSPs exhibit only single step decomposition that incorporates metal oxidation as well. It is obtained that the initial decomposition temperature of non-metallized ECSP is 349 °C with a heat release from the overall decomposition reaction of 2043 J/g. Addition of W increases this overall heat release up to 43% for ECSP-M1 containing 5% W, whereas the decomposition temperature is reduced by ∼60 °C, when compared to the non-metallized case. This indicates that the thermal stability of ECSPs is lowered due to the inclusion of W particles. Further increase in W content diminishes the overall heat release by 10% and 13% with correspondingly more unreacted samples, and rises the effective activation energy by 36% and 60% for ECSP-M2 and ECSP-M3, respectively, relative to ECSP-M1. Based on the present findings, we propose the global reaction mechanism for both non-metallized and metallized ECSPs.

Assessment of local phase to mechanical response link: Application to the chemo-mechanical identification of rock phases subjected to reactive environments

An original approach to the problem of chemo-mechanical characterization of shale rocks is presented, which consists of combining grid nanoindentation technique, scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS), and multispectral image analysis. X-ray microanalysis of the major elements is performed over the indented region, resulting in high-resolution images of the spatial distribution of different elements over the indentation grid. The individual elemental maps of the image fields are mathematically merged to create a multispectral image proper for segmentation into distinct clustered phases and mapped on the indentation points. Unsupervised clustering analysis is performed on the multispectral image to determine the number of statistically definable mutually exclusive material phases.The method is illustrated through application to two types of shale rocks, demonstrating spatially varying microstructures as a result of exposure to reactive environments. Mancos shale subjected to water at ambient pressure and temperature, and Eagle Ford shale exposed to CO2-rich brine at high pressure and high temperature conditions. Clay-, carbonate-, and feldspar-rich phases in the Mancos shale show sensitivity to water exposure, demonstrating alterations in mechanical or compositional properties compared to similar phases in unreacted samples. Carbonate-rich phases in the Eagle Ford shale show three different regions of mechanical properties: dissolution region, followed by precipitation and no reaction regions. A discussion of chemical reactions responsible for mechanical variations in different material phases is included. The results show that the method is capable of successfully isolating different material phases at the indentation scale; thus, providing an efficient means to link chemical and microstructural properties to the mechanical response.

Effect of Maillard reaction in ammonia preserved natural rubber latex using reducing sugars

Investigation of Maillard reaction in natural rubber (NR) latex was carried out using two reducing sugars, glucose and ribose. Both reducing sugars together with a 1:1 mixture were added as 5 wt% solution to concentrated NR latex preserved with ammonia in a 1:1 ratio and preheated at 27, 70 and 90 °C for 2 h. The changes in the latex due to the reaction were characterised using pH and L values for 216 h (9 days). Cast latex films were then prepared and investigated for their swelling index (SI), gel content and tensile properties. Results obtained showed reduction in both pH and L values with time for the latexes with reducing sugars indicating progress of the Maillard reaction. For the Maillard reacted films, SI reduced whereas gel content, tensile stress and tensile strength increased. However, no significant changes were observed for the elongation at break of the reacted samples compared to the unreacted samples. Increasing the preheating temperature of the latex and reducing sugar mixtures to 70 and 90 °C from 27 °C increased the Maillard reaction which further improved the film properties. Although both reducing sugars improved the film properties, the improvements brought by reacting NR latex with ribose were more significant compared to reacting NR latex with glucose. From the current findings, it is evident that reducing sugars could be used to enhance the physical properties of natural rubber latex films. This technique will benefit the latex industry as it is environmentally friendly, easily applicable and cheaper compared to currently available techniques.

Chemical Speciation and Stability of Uranium in Unconventional Shales: Impact of Hydraulic Fracture Fluid.

Uranium and other radionuclides are prominent in many unconventional oil/gas shales and is a potential contaminant in flowback/produced waters due to the large volumes/types of chemicals injected into the subsurface during stimulation. To understand the stability of U before and after stimulation, a geochemical study of U speciation was carried out on three shales (Marcellus, Green River, and Barnett). Two types of samples for each shale were subjected to sequential chemical extractions: unreacted and shale-reacted with a synthetic hydraulic fracture fluid. A significant proportion of the total U (20-57%) was released from these three shales after reaction with fracture fluid, indicating that U is readily leachable. The total U released exceeds labile water-soluble and exchangeable fractions in unreacted samples, indicating that fluids leach more recalcitrant phases in the shale. Radiographic analysis of unreacted Marcellus shale thin sections shows U associated with detrital quartz and the clay matrix in the shale. Detrital zircon and TiO2 identified by an electron microprobe could account for the hot spots. This study shows that significant proportions of U in three shales are mobile upon stimulation. In addition, the extent of mobilization of U depends on the U species in these rocks.

Replacement reactions of copper sulphides at moderate temperature in acidic solutions

Understanding mineral transformations of composite copper sulphide ores and the associated reaction mechanism at low to medium temperatures in acidic solutions with oxidant restriction is important for in situ recovery operations because natural ore bodies typically comprise multiple sulphide species and their dissolution is oxidant consuming. Here we present experimental observations on mineral replacement reactions of coarse-grained natural bornite (Cu5FeS4)–chalcocite (Cu2S) samples in acidic solutions at 90 °C and in the absence of oxidant. Transformations were found to follow a three-stage reaction sequence for bornite and one stage for chalcocite. Bornite replacement commenced with the formation of digenite (Cu1.8S) along fractures and surfaces. Bornite–digenite interfaces are sharp and digenite rims contain nanochannels that connect to the original bornite. When copper dissolution exceeded 30%, copper-deficient lamellae exsolved from the bornite by solid-state diffusion; this texture appeared only in stronger acidic solutions with extended times and did not occur in the heating experiments without fluid. Chalcocite that was present together with bornite in the unreacted samples transformed into digenite characterized by laminar replacement textures. Finally, all phases were replaced by covellite (CuS) accompanied by the formation of porosity. These replacements showed the concomitant operation of fluid-induced solid-state diffusion and fluid-mediated mineral replacement by coupled dissolution–precipitation reactions. The textures found in the resulting covellite showed a promising increase in porosity that could be beneficial for hard-rock in situ recovery and suggests a pathway for refractory sulfide processing by acid pre-treatment.

Kinetic analysis of hydrothermal carbonization using high-pressure differential scanning calorimetry applied to biomass

Assessing kinetic parameters of hydrothermal carbonization still requires several tests and time-consuming analyses, usually relying on few data points. With a view to containing the experimental effort and testing the accuracy of models based on continuum curves, high-pressure differential scanning calorimetry was used for the first time to assess the heat release profile, the overall enthalpy change, and the kinetic parameters of a hydrothermal carbonization process in two substrates: digestate and sludge. The process-related thermal effect was established from the difference between the calorimetric curves for the unreacted samples during a first run and for the reacted samples obtained through a second run. The parameters of a kinetic model built on the Arrhenius reaction were adjusted on the basis of the calorimetric curves, adopting an nth-order and an autocatalytic reaction model. Activation energy values of 139.16 and 161.68 kJ/mol, pre-exponential factors around 2.15 × 1011 and 2.52 × 1014 s−1, reaction orders of about 2.68 and 2.46, and R2 values of 0.86 and 0.94 were obtained for digestate and sludge, respectively. Results were consistent with the available literature. Autocatalysis was negligible for both substrates, so the process could be modeled effectively as a single Arrhenius nth-order reaction with significant loss of precision. Coupled with more traditional approaches, this newly-proposed method may pave the way to describing the set of reactions taking place during hydrothermal carbonization by means of their enthalpy values, improving our knowledge of the process’s chemistry and kinetics.

Chromium isotope composition of organic-rich marine sediments and their mineral phases and implications for using black shales as a paleoredox archive

Chromium (Cr) stable isotopes in marine sediments, in particular iron formations, have been used to track changes in Earth’s surface redox conditions. In recent years several studies have been published that focus on shale-hosted Cr isotopes to further constrain the evolution of atmospheric oxygen levels. As shales are composed of a mixture of detrital and authigenic components, it is critical to know which phases host Cr and to determine any variations in Cr concentration and degree of fractionation between the different host phases. Here we assess whether shales have the potential to record Cr-isotope values that are meaningful to paleoenvironmental studies, focusing on representative examples of organic-rich marine sediments (black shale, phosphate and chert samples) from the Cambrian-aged Jiumenchong and Ediacaran-aged Liuchapo Formations (South China). We use a novel approach to isolate authigenic components which potentially record primary marine redox-fluctuations. Bulk sediment digests and sequential leaches (0.5 M HCL, 6 M HCl, HF and aqua regia), combined with X-ray diffraction and microbeam mineral mapping (Nanomin) characterisation of unreacted and leached sediment samples, allow us to identify potential Cr-hosting phases and their isotopic signatures. The black shale bulk sediment δ53Cr values vary between 0.02‰ ± 0.15‰ and 0.56‰ ± 0.10‰ and are thus slightly positively fractioned compared to the igneous Earth signature of −0.124 ± 0.101‰. The highest δ53Cr value was measured for a chert sample (0.98‰ ± 0.13‰). The leaching test results reveal that the majority of Cr (75–85%) is hosted in the silicate mineral fraction of the sediment, and is liberated during the 6 M HCl and HF acid leaching steps. Further, carbonates/phosphates and pyrite could be isolated as potential Cr hosts by our sequential leaching approach (liberated by the 0.5 M HCl and aqua regia leaching steps respectively). The δ53Cr values of each sequential leach step were generally within error of their respective bulk δ53Cr value. This is surprising as Cr derived from detrital phases is expected to have a different δ53Cr value from Cr in authigenic phases, which are thought to record δ53Cr of Cr in the water column, and suggests that the δ53Cr value of detrital minerals might deviate from the narrow range of values documented for igneous rocks. The similar δ53Cr values observed for the authigenic and detrital mineral phases suggest whole-rock diagenetic homogenisation, which complicates the application and interpretation of Cr stable isotopes in organic-rich black shales for paleoredox conditions.

Effect of bicarbonate and oxidizing conditions on U(IV) and U(VI) reactivity in mineralized deposits of New Mexico

We investigated the effect of bicarbonate and oxidizing agents on uranium (U) reactivity and subsequent dissolution of U(IV) and U(VI) mineral phases in the mineralized deposits from Jackpile mine, Laguna Pueblo, New Mexico, by integrating laboratory experiments with spectroscopy, microscopy and diffraction techniques. Uranium concentration in solid samples from mineralized deposit obtained for this study exceeded 7000 mg kg−1, as determined by X-ray fluorescence (XRF). Results from X-ray photoelectron spectroscopy (XPS) suggest the co-existence of U(VI) and U(IV) at a ratio of 19:1 at the near surface region of unreacted solid samples. Analyses made using X-ray diffraction (XRD) and electron microprobe detected the presence of coffinite (USiO4) and uranium-phosphorous‑potassium (U-P-K) mineral phases. Imaging, mapping and spectroscopy results from scanning transmission electron microscopy (STEM) indicate that the U-P-K phases were encapsulated by carbon. Despite exposing the solid samples to strong oxidizing conditions, the highest aqueous U concentrations were measured from samples reacted with 100% air saturated 10 mM NaHCO3 solution, at pH 7.5. Analyses using X-ray absorption spectroscopy (XAS) indicate that all the U(IV) in these solid samples were oxidized to U(VI) after reaction with dissolved oxygen and hypochlorite (OCl−) in the presence of bicarbonate (HCO3−). The reaction between these organic rich deposits, and 100% air saturated bicarbonate solution (containing dissolved oxygen), can result in considerable mobilization of U in water, which has relevance to the U concentrations observed at the Rio Paguate across the Jackpile mine. Results from this investigation provide insights on the reactivity of carbon encapsulated U-phases under mild and strong oxidizing conditions that have important implication in U recovery, remediation and risk exposure assessment of sites.

Organic Functional Group Chemistry in Mineralized Deposits Containing U(IV) and U(VI) from the Jackpile Mine in New Mexico.

We investigated the functional group chemistry of natural organic matter (NOM) associated with both U(IV) and U(VI) in solids from mineralized deposits exposed to oxidizing conditions from the Jackpile Mine, Laguna Pueblo, NM. The uranium (U) content in unreacted samples was 0.44-2.6% by weight determined by X-ray fluorescence. In spite of prolonged exposure to ambient oxidizing conditions, ≈49% of U(IV) and ≈51% of U(VI) were identified on U LIII edge extended X-ray absorption fine structure spectra. Loss on ignition and thermogravimetric analyses identified from 13% to 44% of NOM in the samples. Carbonyl, phenolic, and carboxylic functional groups in the unreacted samples were identified by fitting of high-resolution X-ray photoelectron spectroscopy (XPS) C 1s and O 1s spectra. Peaks corresponding to phenolic and carbonyl functional groups had intensities higher than those corresponding to carboxylic groups in samples from the supernatant from batch extractions conducted at pH 13, 7, and 2. U(IV) and U(VI) species were detected in the supernatant after batch extractions conducted under oxidizing conditions by fitting of high-resolution XPS U 4f spectra. The outcomes from this study highlight the importance of the influence of pH on the organic functional group chemistry and U speciation in mineralized deposits.

Cytotoxicity of Light-Cured Dental Materials according to Different Sample Preparation Methods.

Dental light-cured resins can undergo different degrees of polymerization when applied in vivo. When polymerization is incomplete, toxic monomers may be released into the oral cavity. The present study assessed the cytotoxicity of different materials, using sample preparation methods that mirror clinical conditions. Composite and bonding resins were used and divided into four groups according to sample preparation method: uncured; directly cured samples, which were cured after being placed on solidified agar; post-cured samples were polymerized before being placed on agar; and “removed unreacted layer” samples had their oxygen-inhibition layer removed after polymerization. Cytotoxicity was evaluated using an agar diffusion test, MTT assay, and confocal microscopy. Uncured samples were the most cytotoxic, while removed unreacted layer samples were the least cytotoxic (p < 0.05). In the MTT assay, cell viability increased significantly in every group as the concentration of the extracts decreased (p < 0.05). Extracts from post-cured and removed unreacted layer samples of bonding resin were less toxic than post-cured and removed unreacted layer samples of composite resin. Removal of the oxygen-inhibition layer resulted in the lowest cytotoxicity. Clinicians should remove unreacted monomers on the resin surface immediately after restoring teeth with light-curing resin to improve the restoration biocompatibility.

Chemical distribution of hazardous natural radionuclides during monazite mineral processing

It is very important to calculate the radioactivity concentration for low-grade monazite ore (50%) and different other materials produced as results of chemical processing stages to avoid the risk to workers. Chemical processing of low-grade monazite pass through different stages, washing by hydrochloric acid and digested with sulfuric acid and influence of pH on the precipitation of rare earth elements has been studied. The radioactivity concentrations of 238U(226Ra) and 232Th as well as 40K were calculated in crude low-grade ore and found to be 54,435 ± 3138, 442,105 ± 29,200 and 5841 ± 345 Bq/kg, respectively. These values are greatly higher than the exempt levels 25 Bq/kg. After chemical digestion of the ore, the results demonstrated that un-reacted material contains significant radioactivity reached to approximately 8, 13 and 23% for 238U, 232Th and 40K, respectively. The results show that 60% of 232Th are located in the digested white slurry with small portions of 238U and 40K. Most of 238U radioactivity is extracted in the green phosphoric acid which produced from conversion of P2O5 by H2SO4 into phosphoric acid. The average values of the Raeq for monazite ore, un-reacted black precipitate, white precipitate, brown precipitate and crystalline material samples were calculated and found to be 687,095 ± 44,921, 85,068 ± 5339, 388,381 ± 22,088, 313,046 ± 17,923 and 4531 ± 338 Bq/kg, respectively. The calculated values of Raeq are higher than the average world value (it must be less than 370 Bq/kg). Finally the external hazardous, internal hazardous and Iγr must be less than unity. This means that specific radiation protection program must be applied and implemented during monazite processing.

Alteration of Bakken reservoir rock during CO2-based fracturing—An autoclave reaction experiment

This study was conducted to document and assess the effects of fluid–rock interactions when CO2 is used to create the fractures necessary to produce hydrocarbons from low-permeability rocks. The primary objectives are to (1) identify and understand the geochemical reactions of CO2-based fracturing, and (2) assess potential changes in porosity and permeability of formation rock. Autoclave experiments were conducted at reservoir conditions exposing middle Bakken core fragments to CO2-saturated synthetic formation brine and to supercritical CO2 (sc-CO2) only. Ion-milled core samples were examined before and after the reaction experiments using scanning electron microscopy (SEM), which enabled us to image the reaction surface in extreme detail and unambiguously identify mineral dissolution and precipitation.The most significant change in the reacted samples exposed to the CO2-saturated brine is dissolution of the carbonate minerals, particularly calcite, which shows severe corrosion. Dolomite grains were corroded to a lesser degree. Quartz and feldspars remained intact, and some pyrite framboids underwent slight dissolution. Additionally, a small amount of calcite precipitation took place, as indicated by numerous small calcite crystals formed at the reaction surface and in the pores. The changes of aqueous chemical composition are consistent with the petrographic observations with an increase in Ca and Mg and associated minor elements, and a very slight increase in Fe and sulfate.When exposed to sc-CO2 only, changes observed include etching of the calcite grain surface and precipitation of salt crystals (halite and anhydrite) due to evaporation of residual pore water into the sc-CO2 phase. Dolomite and feldspars remained intact, and pyrite grains were slightly altered. Mercury intrusion capillary pressure (MICP) tests on reacted and unreacted samples show an increase in porosity when an aqueous phase is present but no overall porosity change with sc-CO2. The results also show an increase in permeability for brine-reacted samples.

Reactions of Al-PTFE under Impact and Quasi-Static Compression

Conventionally, the Al-PTFE is thought to be inert under quasi-static or static loads. However, here we reported an initiation phenomenon of Al-PTFE under quasi-static compression. Quasi-static tests suggest that reacted Al-PTFE samples had much higher toughness than unreacted samples. Dynamic test showed that the energy level needed to initiate the material was similar for quasi-static compression (88–100 J) and dynamic impact (77–91 J). The difference in density indicates that unreacted Al-PTFE has a higher crystallinity, which leads to the lower toughness. SEM images show numerous PTFE fibrils in unreacted composites which made the sample harder to crack and initiate.

The Green River Natural Analogue as A Field Laboratory To Study the Long-term Fate of CO2 in the subsurface

Understanding the long-term response of CO2 injected into porous reservoirs is one of the most important aspects to demonstrate safe and permanent storage. In order to provide quantitative constraints on the long-term impacts of CO2-charged fluids on the integrity of reservoir-caprock systems we recovered some 300m of core from a scientific drill hole through a natural CO2 reservoir, near Green River, Utah. We obtained geomechanical, mineralogical, geochemical, petrophysical and mineralogical laborator y data along the entire length of the core and from non CO2-charged control samples. Furthermore, we performed more detailed studies through portions of low permeability layers in direct contact with CO2-charged layers. This was done to constrain the nature and penetration depths of CO2-promoted fluid-mineral reaction fronts. The major reactions identified include the dissolution of diagenetic dolomite cements and hematite grain coatings, and the precipitation of ankerite and pyrite and have been used as input for geochemical 1D reactive transport modelling, to constrain the magnitude and velocity of the mineral-fluid reaction front. In addition, we compared geomechanical data from the CO2-exposed core and related unreacted control samples to assess the mechanical stability of reservoir and seal rocks in a CO2 storage complex following mineral dissolution and precipitation for thousands of years. The obtained mechanical parameters were coupled to mineralogy and porosity. Key aim of this work was to

Microstructure and porosity evolution during experimental carbonation of a natural peridotite

Carbonation experiments (T: 100–200°C; pCO2: 14.6–19.4MPa) were performed under closed system conditions on a natural peridotite from Almklovdalen (W-Norway) to investigate the microstructure and porosity evolution as a function of reaction progress. The unreacted samples (cubes of 2.5–3cm3) were composed mainly of forsteritic olivine (~80–90vol.%), clinochlore, orthopyroxene, amphibole and chromite. Initial porosities were on the order of 6–8% due to the presence of abundant inter- and intragranular microfractures. Microstructural observations after 7–42days of experimental treatment showed that olivine was by far the most reactive phase. After 7days at 100°C no carbonate products were formed, but longer reaction times produced small amounts of magnesite on the surface and within the fracture network of the peridotite cubes. Increasing temperatures led to faster reactions and pore space filling by magnesite. In addition, replacement rims (up to 50μm thick) composed of magnesite and amorphous silica formed around individual olivine grains. The replacement began with the formation of an amorphous silica layer that retained the shapes of the initial olivine grains. This layer remained permeable during the reaction as it allowed further dissolution of the underlying olivine. Increasing extents of carbonation were accompanied by decreasing porosities and enhanced cementation of grain boundaries. About 50% of the initial porosity was closed when carbonation reached about 10%. The reacted samples showed no evidence of reaction-induced cracking indicating that the carbonation reaction was self-limiting in our experiments. These results may have important implications for the efficiency of in situ CO2 sequestration in peridotites.

Redox Behavior of Uranium at the Nanoporous Aluminum Oxide-Water Interface: Implications for Uranium Remediation

Sorption-desorption experiments show that the majority (ca. 80-90%) of U(VI) presorbed to mesoporous and nanoporous alumina could not be released by extended (2 week) extraction with 50 mM NaHCO(3) in contrast with non-nanoporous α alumina. The extent of reduction of U(VI) presorbed to aluminum oxides was semiquantitatively estimated by comparing the percentages of uranium desorbed by anoxic sodium bicarbonate between AH(2)DS-reacted and unreacted control samples. X-ray absorption spectroscopy confirmed that U(VI) presorbed to non-nanoporous alumina was rapidly and completely reduced to nanoparticulate uraninite by AH(2)DS, whereas reduction of U(VI) presorbed to nanoporous alumina was slow and incomplete (<5% reduction after 1 week). The observed nanopore size-dependent redox behavior of U has important implications in developing efficient remediation techniques for the subsurface uranium contamination because the efficiency of in situ bioremediation depends on how effectively and rapidly U(VI) bound to sediment or soil can be converted to an immobile phase.

Effect of ball milling time on the substitution of carbon in glucose doped MgB 2 superconductors: Dispersion behavior of glucose

The effect of the ball milling time (BMT) on the substitution of the carbon in the glucose doped MgB 2 samples is investigated here. Using in situ solid state reaction, four different doped samples of Mg(B .98C .02) 2 were prepared by mixing powders of Mg, boron and glucose for 2 h, 4 h, 8 h and 12 h using planetary ball milling. A reference sample of un-doped MgB 2 was also prepared under same conditions. The particle size distribution of the un-reacted samples show a decrease in the particle size as the BMT is increased. Both the average particle size as well as the standard deviation show a substantial decrease with the increase in the milling time up to 8 h. After 8 h, the size reduction is rather insignificant. From the XRD data, the crystallite size of the doped MgB 2 computed using the Scherrer formula was found to decrease with the increasing BMT, showing a saturation level after 8 h of the milling time. TEM images also confirm the crystallite size obtained from the XRD data. The substitution of the C in the MgB 2 lattice, measured from the change in the c/a ratio, increases with increasing BMT. The maximum carbon substitution is achieved at approximately 8 h of BMT. Moreover, a systematic enhancement of the residual resistivity and a decrease in T C with an increasing BMT further confirms a progressive substitution of the carbon in the MgB 2. These results suggest that a minimum ball milling time is necessary to disperse the glucose uniformly for a maximum substitution of nano C in the B plane of MgB 2 lattice. The optimum BMT is found to be 8 h. Thus, the decrease in the particle size due to the ball milling enhances the dispersion of the constituent materials thereby favoring a greater substitution of the dopant in the MgB 2 during the solid-state reaction.

Fine Structure in XPS and XANES Spectra Acquired from a Series of Sphalerite Samples

The fine structure of high resolution XPS and XANES S L3,2-edge spectra collected from a series of high quality sphalerite samples are examined and discussed. Each of the sphalerite samples had a characteristic iron concentration and the range was from <0.01 wt % Fe to 14.79 wt% Fe. XPS spectra acquired from sphalerite surfaces leached in ferric sulphate-sulphuric acid media are presented and discussed. XANES near-edge spectra were evaluated by comparing S L3,2 -edge spectra acquired from unreacted sphalerite samples. The XPS data show that the sphalerite dissolution proceeds via the formation of a metal deficient sulphide that evolves into a surface dominated by polysulphide species. The XANES data show that S 2p electron transitions to empty S 3s and S 3d states occur across an energy range above the absorption edge. A chemical shift associated with iron to sulphur bonds is identified and a pre-edge feature associated with S - Fe bonding in sphalerite is identified.

Experimental determination of the effect of the ratio of B/Al on glass dissolution along the nepheline (NaAlSiO 4)–malinkoite (NaBSiO 4) join

The dissolution kinetics of five glasses along the NaAlSiO 4–NaBSiO 4 join were used to evaluate how the structural variations associated with boron–aluminum substitution affect the rate of dissolution. The composition of each glass varied inversely in mol% of Al 2O 3 (5–25 mol%) and B 2O 3 (20–0 mol%) with Na 2O (25 mol%) and SiO 2 (50 mol%) making up the remaining amount, in every case Na/(Al + B) = 1.0. Single-pass flow-through experiments (SPFT) were conducted under dilute conditions as a function of solution pH (from 7.0 to 12.0) and temperature (from 23 to 90 °C). Analysis of unreacted glass samples by 27Al and 29Si MAS-NMR suggests Al (∼98% [4]Al) and Si-atoms (∼100% [4]Si) occupy a tetrahedral coordination whereas, B-atoms occupy both tetrahedral ( [4]B) and trigonal ( [3]B) coordination. The distribution of [3]B fractionated between [3]B(ring) and [3]B(non-ring) moieties, with the [3]B(ring)/ [3]B(non-ring) ratio increases with an increase in the B/Al ratio. The MAS-NMR results also indicated an increase in the fraction of [4]B with an increase in the B/Al ratio. The 27Al peak maxima shift to lesser values with an increase in the B/Al ratio which suggests mixing between the [4]Al and [3]B sites, assuming avoidance between tetrahedral trivalent cations ( [4]Al–O– [4]B avoidance). Unlike the 27Al and 11B spectra, the 29Si spectra illustrate a subtle shift to more negative chemical shift (chemical shift range between −88 and −84 ppm) and increases in the spectral widths as the B/Al ratio increases. Raman spectroscopy of unreacted glass samples was also used to cross-check the results collected from MAS-NMR and suggested that NeB4 (the glass sample with the highest B content) may consist of B–Na enriched and Al–Si enriched micro-domains, which affected the measured dissolution rates. Results from SPFT experiments suggest a forward rate of reaction and pH power-law coefficients, η, that are independent of B/Al under these neutral to alkaline test conditions for all homogeneous glasses. The temperature dependence shows an order of magnitude increase in the dissolution rate with a 67 °C increase in temperature and suggests dissolution is controlled by a surface-mediated reaction, as indicated by the activation energy, E a , being between 44 ± 8 and 48 ± 7 kJ/mol. Forward dissolution rates, based on Na and Si release, for homogeneous glasses are independent of the B/Al ratio, whereas dissolution rates based on Al and B release are not. Normalized dissolution rates, based on B release, increase with the molar fraction of [3]B(ring). Finally in accord with previous studies, the data discussed in this manuscript suggest rupture of either the Al–O or Si–O bonds as the rate-limiting step controlling the dissolution of these glasses.