Study of hydration of two cements of different strengths

Main hydration products of two cement pastes, i.e. CSH-gel, portlandite (P) (and specific surface S) were studied by static heating, and by SEM, TEM and XRD, as a function of cement strength (C-33 and C-43) hydration time (th) and subsequent hydration in water vapour.Total change in mass on hydration and air drying, M_o, increased with strength of cement paste and with hydration time. Content of water escaping at 110 to 220°C, defined as water bound with low energy, mainly interlayer and hydrate water, was independent on cement strength but its content increased with (th). Content of chemically bound (zeolitic) water in CSH-gel, escaping at 220-400°C, was slightly dependent on strength and increased with (th). It was possibly derived from the dehydroxylation of CSH-gel and AFm phase. Portlandite water, escaping at 400-500°C, was independent on cement strength and was higher on longer hydration. Large P crystals were formed in the weaker cement paste C-33. Smaller crystals were formed in C-43 but they increased with (th). Carbonate formated on contact with air (calcite, vaterite and aragonite), decomposed in cement at 600-700^oC. It was high in pastes C-33(1 month) and C-43(1 month), i.e. 5.7 and 3.3%, respectively; it was less than 1% after 6 hydration months (low sensitivity to carbonation) in agreement with the XRD study showing carbonates in the air dry paste (1month), and its absence on prolonged hydration (6 months) and on acetone treatment. Water vapour treatment of (6 months) pastes or wetting-drying increased this sensitivity.Nanosized P-crystals, detected by TEM, could contribute to the cement strength; carbonate was observed on the rims of gel clusters.This revised version was published online in November 2005 with corrections to the Cover Date.     

Electrical and structural properties of Pd/V/n‐type InP (111) Schottky structure as a function of annealing temperature

Palladium/Vanadium (Pd/V) Schottky structures are fabricated on n‐type InP (100) and the electrical, structural and surface morphological characteristics have been studied at different annealing temperatures. The extracted barrier height of as‐deposited Pd/V/n‐InP Schottky diode is 0.59 eV (I–V) and 0.79 eV (C–V), respectively. However, the Schottky barrier height of the Pd/V Schottky contact slightly increases to 0.61 eV (I–V) and 0.84 eV (C–V) when the contact is annealed at 200 °C for 1 min. It is observed that the Schottky barrier height of the contact slightly decreases after annealing at 300, 400 and 500 °C for 1 min in N₂ atmosphere. From the above observations, it is clear that the electrical characteristics of Pd/V Schottky contacts improve after annealing at 200 °C. This indicates that the optimum annealing temperature for the Pd/V Schottky contact is 200 °C. Basing on the auger electron spectroscopy and X‐ray diffraction results, the formation of Pd‐In intermetallic compound at the interface may be the reason for the increase of barrier height upon annealing at 200 °C. The formation of phosphide phases at the Pd/V/n‐InP interface could be the reason for the degradation in the barrier heights after annealing at 300, 400 and 500 °C. From the AFM results, it is evident that the overall surface morphology of the Pd/V Schottky contacts is fairly smooth. Copyright © 2011 John Wiley & Sons, Ltd.     

Calcite,vaterite and aragonite forming on cement hydration from liquid and gaseous phase

Cement hydration products were studied as influenced by the hydration conditions (hydration time in liquid phase; relative humidity, RH, in gaseous phase). The formation of calcium hydroxide (portlandite, P) and its transformation to calcium carbonates is mainly discussed here. More hydration products, including P, were formed in liquid phase (paste) than in water vapor (powder), due to the higher availability of water molecules. Full hydration was observed only in the paste hydrated for 6 month, otherwise the P content, estimated from its water escape, DM(400-800°C), increased after storage in water vapor of the prehydrated paste. All the three polymorphs of CaCO₃ (calcite, vaterite and aragonite) were found on prolonged contact with air of the hydrated powder (XRD, HRTEM). Their content was dependent on sequence of RH conditions on hydration: higher after water retention, WR, on lowering RH=1.0→0.95→0.5, than after water sorption, WS, on increasing RH in the inverse order. It increased also on wetting and drying, both of hydrated powder and paste. Ca was found to accumulate on the micro-surfaces of WR samples (SEM, TEM), whereas more Al was observed on WS samples and the crystallinity of hydration products was here higher (ED). Dissolution-diffusion-recrystallization was possible: small Al-ions concentrated at one end and the bigger Ca ions - at the other end of some needles (TEM). At 400-500°C the P in cement transforms in air into CaCO₃, which decomposes at 600-700°C. Thus the sensitivity to carbonation was estimated from ΔM(600-800°C). This value was similar in pastes hydrated for 1 month and in powder (WR). It was lower in powder WS and much lower in the paste (6 months). It increased pronouncedly when the prehydrated paste was stored in water vapor in WS. The nanocrystals of portlandite, vaterite and aragonite, embedded in the amorphous matrix, were observed by HRTEM in the hydrated powder. They may contribute to the cement strength. This revised version was published online in July 2006 with corrections to the Cover Date.     

Surface Dynamics of A Vanadyl Pyrophosphate Catalyst for n‐Butane Oxidation to Maleic Anhydride: An In Situ Raman and Reactivity Study of the Effect of the P/V Atomic Ratio

This work focused on investigating the effect of the P/V atomic ratio in vanadyl pyrophosphate, catalyst for n‐butane oxidation to maleic anhydride, on the nature of the catalytically active phase. Structural transformations occurring on the catalyst surface were investigated by means of in situ Raman spectroscopy in a non‐reactive atmosphere, as well as by means of steady‐state and non‐steady‐state reactivity tests, in response to changes in the reaction temperature. It was found that the nature of the catalyst surface is affected by the P/V atomic ratio even in the case of small changes in this parameter. With the catalyst having P/V equal to the stoichiometric value, a surface layer made of α_I‐VOPO₄ developed in the temperature interval 340–400 °C in the presence of air; this catalyst gave a very low selectivity to maleic anhydride in the intermediate T range (340–400 °C). However, at 400–440 °C δ‐VOPO₄ overlayers formed; at these conditions, the catalyst was moderately active but selective to maleic anhydride. With the catalyst containing a slight excess of P, the ratio offering the optimal catalytic performance, δ‐VOPO₄ was the prevailing species over the entire temperature range investigated (340–440 °C). Analogies and differences between the two samples were also confirmed by reactivity tests carried out after in situ removal and reintegration of P. These facts explain why the industrial catalyst for n‐butane oxidation holds a slight excess of P; they also explain discrepancies registered in the literature about the nature of the active layer in vanadyl pyrophosphate.     

Carborane polymers. IV. Polysiloxanes

Carboranes attached to silicon through straight-chain alkyl groups were prepared and characterized for thermal stability by TGA and molecular weight change on heating. The monomers for these polymers were prepared generally by platinum-catalyzed addition of a silylhydride to an alkenyl or dialkenyl carborane. Polymerization was effected by hydrolysis-condensation of chlorosilanes, ring opening of cyclosiloxanes, and condensation of alkoxy and chlorosilanes. Two types of polymer structures were prepared, one contained m-carborane in the chain backbone, the other contained o-carborane as pendant alkylcarborane groups. Both types were obtained as elastomers; however, higher proportions of carborane in the polymers reduced elasticity and finally resulted in nonelastomers. TGA of the backbone carborane siloxane polymer indicated degradation at 370°C. in nitrogen and at 235°C. in air. Chain scission, as determined by molecular weight decrease, was observed on heating in nitrogen at 350°C. TGA of the pendant carborane siloxane polymer indicated that degradation in nitrogen and in air occurred at greater than 400°C. However, chain scission, as determined by molecular weight decrease, was observed upon heating at 300°C. in nitrogen.     

Thermal stability and nonisothermal crystallization of short fiber-reinforced poly(ether ether ketone) composites

The thermal stability of a short carbon-fiber-reinforced PEEK composite was assessed by thermogravimetry and by a Rheometrics dynamic analyzer. The results indicated that holding for 10 min at 380°C was a suitable melting condition to avoid the thermooxidative degradation under air. After proving that the heating rate of 50°C/min can be used to evaluate the crystallinity, a heating stage was used to prepare nonisothermally crystallized specimens using cooling rates from 1 to 100°C/min after melting at 400°C for 3 or 15 min. The degree of crystallinity and the melting behavior of these specimens were investigated by DSC at a heating rate of 50°C/min. The presence of three or four regions indicated that the upper melting temperature, T_m, changed with the crystallization temperature. The first region with the highest T_m, which corresponded to the cooling rate of 1°C/min, can be associated with the crystallization in regime II. There was a second region where T_m decreased as the amount of crystals formed in regime II decreased with increasing cooling rate from 5 to 20°C/min. The third region, a plateau region, corresponded to regime III condition in which the crystals were imperfect. In the fourth region, the cooling was so fast that crystallization was incomplete during the cooling for the melting condition of 400°C for 15 min. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2225–2235, 1998     

The effect of temperature on the sorption of U(VI) onto ZrP2O7 in the presence of oxalic acid

The sorption of uranium onto ZrP₂O₇ in the presence of oxalic acid has been investigated as a function of temperature (20, 40 and 60 °C). Using several complementary analytical methods to characterize the solid surface, it has been shown that the oxalic acid interact with the zirconium diphosphate affecting its surface reactivity. A significant influence of temperature on the sorption reaction has been revealed in the batch experiments. Temperature dependence sorption data and microcalorimetric measurements have been used to determine enthalpy change associated to the sorption reaction. The results have shown that oxalic acid has an important effect on uranium sorption, which is more evident at 60 °C.     

Effect of heat treatment on the morphology and structure of crystals,powders, and fibers of polyacrylonitrile and saran

As part of a study of chemical and physical changes accompanying the formation of carbons by the pyrolysis of polymers, conventional electron microscopy, electron diffraction, and scanning electron microscopy techniques have been used to examine structural and morphological features of polyacrylonitrile (PAN) crystals, powder, and fibers, and of Saran and poly(vinylidene chloride) (PVDC) powder. Changes accompanying the heating of these polymers in air and in nitrogen have been investigated. PAN crystals grown from propylene carbonate were similar to those obtained by Klement and Geil. When heated in air at 220°C they retained their morphology, and electron diffraction gave the same reflections as PAN. On further heating to 400°C in nitrogen the morphology was retained, but the diffraction was lost. Crystals treated in nitrogen alone at 200°C showed morphology similar to that of the polymer. PAN powders and fibers retained discernable external features of their morphology on heating to 800°C. These results are discussed with reference to changes which take place when poly(vinylidene chloride) and Saran are heated in the range 150–180°C, which results in the loss of one hydrogen chloride per monomer unit, and are subsequently carbonized at 800°C. The development of pore structure and the adsorptive properties of Saran carbons are also discussed.     

The pyrolytic behavior of ethylenetetracarboxylic acid

The controlled pyrolysis of ethylenetetracarboxyclic acid at 220°C followed by methylation of the pyrolyzate results in the formation of methyl propiolate as the major product along with lesser amounts of methyl fumarate. Evidence is presented which indicates that the methyl fumarate probably arises from some precursor in the pyrolyzate during the methylation reaction. Extended heating of the tetra acid at its melting point produces, in addition to the substances cited above, ethylenetetracarboxylic dianhydride and dicarboxymaleic anhydride. The high-temperature (300–400°C) pyrolysis of the tetraacid gives l0–20% yields of dimethylmaleic anhydride.     

How xerogel carbonization conditions affect the reactivity of highly disperse SiO<Subscript>2</Subscript>–C composites in the sol–gel synthesis of nanocrystalline silicon carbide

A transparent silicon polymer gel was prepared by sol–gel technology to serve as the base in the preparation of highly disperse SiO₂–C composites at various temperatures (400, 600, 800, and 1000°C) and various exposure times (1, 3, and 6 h) via pyrolysis under a dynamic vacuum (at residual pressures of ~1 × 10^–1 to 1 × 10^–2 mmHg). These composites were X-ray amorphous; their thermal behavior in flowing air in the range 20–1200°C was studied. The encapsulation of nascent carbon, which kept it from oxidizing in air and reduced the reactivity of the system in SiC synthesis, was enhanced as the carbonization temperature and exposure time increased. How xerogel carbonization conditions affect the micro- and mesostructure of the xerogel was studied by ultra-small-angle neutron scattering (USANS). Both the carbonization temperature and the exposure time were found to considerably influence structure formation in highly disperse SiO₂–C composites. Dynamic DSC/DTA/TG experiments in an inert gas flow showed that the increasing xerogel pyrolysis temperatures significantly reduced silicon carbide yields upon subsequent heating of SiO₂–C systems to 1500°C, from 35–39 (400°C) to 10–21% (1000°C).     

Synthesis of Ultrafine Cobalt Ferrite by Thermal Decomposition of Citrate Precursor

Ultrafine particle of CoFe2O4 has been synthesized using citrate precursor technique. Thermal decomposition of the citrate precursor, Co3Fe6O4(C6H6O7)8·6H2O, was investigated by TG, DTA and DTG techniques, gas and chemical analyses and was found to decompose in one or two major steps depending on the heating rate in static/flowing air atmosphere. In the lower heating rate (5°C min-1), metastable acetonedicarboxylate complex was isolated with the evolution of CO gas and coordinated water molecule in the temperature range of 120-220°C. Complete decomposition of the citrate network occurs between 220-330°C with the simultaneous evolution of CO2 and acetone. However both these steps appeared as simultaneous and/or single step process (between 120-160°C), when the heating rate is high (10°C min-1 and above). The ultrafine CoFe2O4 particles are observed as the aggregates having surface area 133.8 m2 g-1 composed of 4.8 nm crystallites. The citrate precursor and the decomposed products were characterized by IR, NMR, XRD, SEM and surface area measurements. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal decomposition of the 1:1 bismuth molybdate and its implications for catalytic oxidation

To study the stability of the 1:1 bismuth molybdate, thin films (300–500 Å) have been prepared by thermal evaporation of the powder samples followed by heating in air or oxygen. Electron diffraction revealed that in an oxidizing atmosphere and at temperatures higher than 400°C or in vacuum and at temperatures higher than about 350°C the 1:1 compound decomposes into the 2:1 bismuth molybdate and MoO₂. These results suggest that during catalytic oxidation and (at least) for temperatures greater than 400°C the 2:1 compound forms near the surface of the 1:1 phase, being responsible for the selective catalytic oxidation.     

Angle-dependent XPS Analysis of (NH4)2Sx-treated InP(001) Surfaces

Angle-dependent high-resolution XPS spectra of S 2p, In 3d_5/2 and P 2p have been measured on the InP(001) sample etched chemically, treated with (NH₄)₂S _x at room temperature (RT), exposed to air at RT and annealed at 400°C in a vacuum. Three kinds (S-I, S-II and S-III) of chemical states of sulphur on the (NH₄)₂S _x -treated InP(001) surface at RT are found. It is suggested that S-I, S-II and S-III correspond to sulphur in the bulk, sulphur bridge-bonded to indium on the surface and elemental sulphur, respectively. Chemical state of S-III is decreased for the treated sample exposed to air at RT for 1 month. It is removed upon annealing the sample at 400°C in a vacuum, while S-I and S-II remain on the surface. The thickness of the sulphide layer on the annealed surface is estimated to be about one monolayer. Angle-dependent XPS spectra of S 2p and In 3d_5/2 are discussed.     

Studies of the Polydiacetylene Bis-p-chlorocinnamate of 10,12-Docosadiyn-1,22-diol

The bis-p-chlorocinnamate of 10,12-docosadiyn-1,22-diol polymerized with ⁶⁰Co gamma radiation to give a soluble polydiacetylene (PDA), PDA-ClCIN-22. We have studied PDA-ClCIN-22 in both crystallographic powder and solution coated films. Thin films of solution coated PDA-ClCIN-22 exhibit an absorption maximum of 544 nm, similar to the concentrated solution in chlorobenzene. Irradiation of these films with 254 nm UV light in air leads to loss of intensity in the visible spectrum. The thin films of PDA-ClCIN-22 are amorphous by X-ray powder diffraction. Irradiation of both PDA-ClCIN-22 powder and film with 254 nm light did not result in photochemical reactivity of the chlorocinnamate group as recorded by FTIR spectra. Differential scanning calorimetry (DSC) studies of PDA-ClCIN-22 reveal that presence of monomer and endothermic transitions near 116°C and 134°C on a first heating. On a second heating of a sample of PDA-ClCIN-22 taken to 150°C on first heating, the 116°C and 134°C endotherms are absent. These endotherms are also lost by monomer extraction with boiling cyclohexane. One possible source of the endotherms would be conformational transitions in the side chain.     

Thermal behaviour of synthetic pyroaurite-like anionic clay

Thermal behaviour of synthetic pyroaurite-like anionic clay with molar ratio Mg/Fe=2 was studied in the range of 60-1100°C during heating in air. TG/DTA coupled with evolved gas analysis, emanation thermal analysis (ETA), surface area measurements, XRD, IR and Mössbauer spectroscopy were used. Microstructure changes characterized by ETA were in a good agreement with the results of surface area measurements and other methods. After the thermal decomposition of the pyroaurite-like anionic clay, which took place mainly up to 400°C, a predominantly amorphous mixture of oxides is formed. A gradual crystallization of MgO (periclase) and Fe₂O₃ (maghemite) was observed at 400-700°C by XRD. The MgFe₂O₄ spinel and periclase were detected at 800-1100°C. The spinel formation was also confirmed by Mössbauer spectroscopy.     

Thermal analysis of sulfurization of polyacrylonitrile with elemental sulfur

Thermal analysis of sulfurization of polyacrylonitrile (PAN) with elemental sulfur was investigated by thermogravimetry and differential thermal analysis of the mixture of polyacrylonitrile and elemental sulfur up to 600°C. Due to the volatilization of sulfur, the different heating rate (10 and 20 K min⁻¹) and different mixture proportion of polyacrylonitrile and elemental sulfur were adopted to run the analysis. The different heating rates make the DSC curves of sulfur different, but make the DSC curves of PAN similar. In the DSC curve of sulfur for the heating rate of 20 K min⁻¹ around 400°C, a small exothermic peak occurs at 400°C in the wide endothermic peak around 380∼420°C, indicative of that there is an exothermic reaction around 400°C. In the DSC curves of the mixture, the peaks around 320°C are exothermic as the content of sulfur is below 3.5:1 and endothermic as the content of sulfur is over 4:1, indicating that one of the reactions between PAN and sulfur takes place around 320°C. In the TG curves of the mixture, the mass losses begin at 220°C, and sharply drop down from 280°C. The curves for the low sulfur content obviously show two steps of mass loss, and curves for the high sulfur content show only one step of mass loss, indicative of more sulfur is benefit for the complete sulfurization of PAN. This study demonstrates that the TG/DSC analysis can give the parameter for the sulfurization, even if the starting mixture contains the volatile sulfur.     

Mild pre-heating of organic cation-exchanged clays enhances their interactions with nitrobenzene in aqueous environment

Aqueous sorption kinetics and equilibrium isotherms of nitrobenzene were studied on two series of sorbents that were prepared by (i) replacing inorganic exchangeable cations in Wyoming bentonite with tetraethylammonium (TEA) and benzyltrimethylammonium (BTMA) and (ii) heating synthesized complexes in air at different temperatures (between 150 and 420°C). The aim of this work was to examine recently observed enhancement of aqueous sorption of a probe organic sorbate on organoclays after mild thermal pre-treatment of sorbents. Thermal pre-treatment of TEA- and BTMA-clays at 150°C results in the maximal enhancement of nitrobenzene–sorbent interactions as compared with treatment of original bentonite and its exchange complexes formed with long-chain quaternary ammonium (n-hexadecyltrimethylammonium, HDTMA). Based on C, N content data and FTIR spectra of TEA- and BTMA-clay complexes, no indications of decomposition of organic matter were found in organoclays heated at 250°C (and below this temperature). Suppressed hydration of pre-heated sorbents resulting in a lessening of water–organic sorbate competition for sorption sites is considered to be responsible for thermally induced enhancement of nitrobenzene–sorbent interactions. In the HDTMA-based organoclays, the long-chain aliphatic groups of the quaternary ammonium can additionally interact with clay surface thus competing with organic sorbate–sorbent surface interactions and, in this way, mitigating the enhancement of nitrobenzene sorption on thermally treated sorbents.     

Evolved gas analysis at thermal treatment of some solid fossil fuels

Coupled TG-FTIR technique was used for identification of gaseous compounds evolved at thermal treatment of six coal samples from different deposits (Bulgaria, Russia, Ukraine). The experiments were carried out under dynamic heating conditions up to 900°C at heating rates of 5, 10 or 50 K min^–1 in a stream of dry air. The emission of CO₂, H₂O, CO, SO₂, COS, methane, methanol, formic acid, formaldehyde, acetaldehyde, chlorobenzene was clearly identified in FTIR spectra of the samples studied. The formation of ethanol, ethane, ethylene and p-xylene, at least on the level of traces, was also identified. At the heating rate of 5°C min^–1 the temperature of maximum intensities of the characteristic peaks of COS was 270°C, of formaldehyde, formic acid, ethane and methanol 330°C, of SO₂, CO, acetic acid, ethylene and p-xylene 400°C and of chlorobenzene 500°C. At 10°C min^–1 and 50°C min^–1 these temperatures were shifted, respectively, by 70–300°C and 150–450°C towards higher temperatures and the respective absorption bands in FTIR spectra were, as a rule, more intensive.     

Formation of Al‐enriched surface layers through reaction at the Mg‐substrate/Al‐powder interface

Al‐enriched surface layers containing a Mg₁₇Al₁₂ intermetallic phase and a solid solution of Al in Mg were fabricated by heating Mg specimens in contact with Al powder in a vacuum furnace. The layer formation process proceeded through partial melting at the Mg‐substrate/Al‐powder interface. The test results suggest that a good contact between the Al powder and the Mg substrate is required during heat treatment. In this study, a pressure of 1 MPa was applied to improve the contact of the Al powder with the Mg specimen. When the powder was pressed down during heating, it was possible to reduce the process temperature from 450 °C to 440 °C. The layers produced at 440 °C in a short heating time (40 min) were thick, continuous and uniform. The microhardness of the Al‐enriched layers was much higher than that of the Mg substrate. The results of the electrochemical corrosion tests indicated that the Mg specimens with an Al‐enriched surface layer had better corrosion resistance than the bare Mg. Copyright © 2014 John Wiley & Sons, Ltd.     

Surface characterization and electrical property of carbon fibers modified by air oxidation

Polyacrylonitrile‐based carbon fibers were modified by oxidation in air, and a systematic study of surface groups and surface resistance at different treated temperatures was made. Progressive fiber weight loss occurred with increasing extents of air oxidation, and it was approximately proportional to the extent of air oxidation from the onset of oxidation up to 400 °C. At this point 4.4% of the initial fiber weight had been lost. A faster loss of weight occurred as the extent of air oxidation increased from 400 °C to 700 °C. X‐ray photoelectron spectroscopy studies (C 1s and O 1s) indicated that the oxygen/carbon atomic ratio rose rapidly from 2.64% (as‐received carbon fiber) to 42.83% as the oxidation temperature was increased to 400 °C. Fourier transform infrared spectra showed the relative intensity of the peaks at about 3440 cm^?1 from ―OH stretching vibrations and at 1634 cm^?1 from ―C?O stretching vibrations increased significantly at 400 °C. FESEM micrographs showed that as‐received fibers show relatively smooth surface. With oxidation temperature increasing, the fiber surface was rougher. The surface resistance of treated carbon fibers decreased obviously with increasing oxidation temperatures. The most decrease was about 100% at 400 °C. Copyright © 2014 John Wiley & Sons, Ltd.