Amorphous Composition Research Articles

A three-dimensional nanoscale study in selected coal mine drainage

Sediments from coal mine drainages (CMDs) contain large quantities of suspended pollutants (possibly numerous chemical substances) along with sulfates and hazardous elements (e.g., chromium, zinc, copper, lead) that irreversibly accumulate in the water. As this accumulation can continue for decades after discontinuation of coal extraction, it is necessary to employ multidisciplinary approaches to control the threat in such zones. The quantity of amorphous material in some CMDs was evaluated by X-ray powder diffraction (XRD) using the Rietveld-based SIROQUANT software package. Modern Dual Beam Focused Ion Beam (FIB), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscope (H-TEM), and energy-dispersive X-ray spectrometer (EDS) were used to evaluate the occurrence and transformation of nanophases (NPs). FIB is used to determine the 3D distribution of different species (internal structure) within individual NPs, whereas EDS is used to observe NP features (e.g., shape, constituent, range, assembly, and form of polymerization). The mineralogy of the sediment from the Brazilian CMDs, including the proportions of quartz, clays, Al–Fe-oxides, and amorphous NPs, appears to be related to the nature of the mineral matter in the relevant coal cleaning rejects (CCRs). The sediments of CMDs from the Brazilian coal area derived at a lower-pH range have different amorphous compositions as compared to those derived at a higher pH range. These special amorphous compositions are shown to be related to several other sediment properties such as particle surface area. The information gleaned in this study will be useful for further geochemical evaluation of CMDs in other parts of the world.

First successful stabilization of consolidated amorphous calcium phosphate (ACP) by cold sintering: toward highly-resorbable reactive bioceramics.

In the field of bone regeneration, some clinical conditions require highly-resorbable, reactive bone substitutes to rapidly initiate tissue neo-formation. In this view, Amorphous Calcium Phosphates (ACP) appear as well suited bioceramics taking into account their high metastability. However, the metastability also leads to difficulties of sintering without transformation into crystalline compounds. In this work, various calcium phosphate samples (co)doped with carbonate (CO32-) and magnesium ions were synthesized by the double decomposition method in alkaline media using ammonium and potassium hydroxide solutions. The obtained amorphous powders possess an exceptionally-high carbonate content up to 18.3 wt%. Spark Plasma Sintering (SPS) at very low temperature (150 °C) was then utilized to consolidate initial powders with the view to preserve their amorphous character. The influence of the introduction of different apatite growth inhibitors such as carbonate (CO32-) and magnesium ions was studied. XRD and FTIR analyses revealed that sintered ceramics generally consisted in highly carbonated low-crystallinity apatites, which are expected to have higher solubility than conventional apatite-based systems. However, most interestingly, modulation of the doping conditions allowed us to retain, for the first time, the amorphous character of ACP powders after SPS. Such consolidated ACP compounds may now be considered as a new family of bioceramics with high metastability allowing the fast release of bioactive ions upon resorption.

Знесірчення промислових сульфідно-лужних розчинів сумішевими сорбентами

Modern methods of purification of hydrocarbons (oil, gas) from sulfur-containing compounds (H2S, RSH, RSSR, naphthenic acids, NaHS, Na2S) have been analyzed, which are based on their alkaline extraction followed by oxidation of air oxygen on homogeneous or heterogeneous catalysts. Despite the promising process of catalytic oxidation of sulfur-containing compounds in technical solutions, it has a number of significant drawbacks, primarily related to the activity of the catalyst and the ability to form a large number of intermediate products of the oxidation. Other known methods for the removal of sulfur-containing compounds from technical solutions are based on the chemical interaction of sulfide alkaline solutions with aqueous solutions of metal salts of III-V analytical groups or their interaction with elemental sulfur to form the corresponding sodium polysulfides. A number of studies have investigated the sorptive extraction of sulfur-containing compounds from technical aqueous solutions or commercial hydrocarbons/diesel fuels using various modifications of activated charcoal, natural sorbents, polymeric organic absorbers or mixed sorbents. However, despite the wide variety of sorptive materials studied, the problem of purification of industrial wastewater from sulfur-containing compounds is not completely solved and, in turn, depends on the chemical structure of the surface of the sorbent, as well as on the objects of purification, pH of media and several other factors. It has been developed an alternative method of purification of aqueous sulfide alkaline solutions/wastewater from sulfide and hydrosulfide ions by their adsorption on pre-regenerated mixed sorbents consisting of activated charcoal (AC) of brand Decolar A and kieselguhr (K) of brands Becogur 200 and Becogur 3500 with mass ratio of 4 : 6. It has been found that the degree of purification of the investigated solutions, which is strongly dependent on the initial concentrations of sulfide and hydrosulfide ions, is 95,7…96,6 % and indicates the high efficiency and possibility of using such technologies in industrial production. Surface study of mixed sorbents (AC+K) after purification of aqueous solutions was performed using X-ray phase analysis. The received data confirms the amorphous composition of the activated charcoal and the presence of the main SiO2 cristobalite, 4,02, on the left side of the diffractogram, as well as a number of peaks of various Na2S modifications.

First and second phase transition discovery in amorphous chalcogenide Se60Te30S10 composition

The Amorphous composition Se60Te30S10 was prepared by the quenching technique. XRD and IR results show an amorphous composition imbedded with Nano-crystallites, and hygroscopic nature of the composition. The (DTA) indicated existence of Tg at 319 K, and a complex peak consists of two overlapped crystallization peaks at 392 K and 403 K. Two endothermic minima at 481 K and at 738 K are observed by DTA, which ensures that the Se60Te30S10 composition is a solid solution. The crystallization activation energy is found equal to 0.77 eV. The (DSC) results list two endothermic peaks 1st one is a second order phase transition at T1 = 229 K, 2nd one is first order phase transition peak at T2 = 268 K. The dielectric properties results reveal the non-Debye relaxation character of present composition. A. C. conductivity analysis results by (SLPL) law reveal that, the (CBH) is the proper mechanism to explain the conductivity in the Se60Te30S10 composition.

Filament Extrusion and Its 3D Printing of Poly(Lactic Acid)/Poly(Styrene-co-Methyl Methacrylate) Blends

Herein, we report the melt blending of amorphous poly(lactide acid) (PLA) with poly(styrene-co-methyl methacrylate) (poly(S-co-MMA)). The PLAx/poly(S-co-MMA)y blends were made using amorphous PLA compositions from 50, 75, and 90wt.%, namely PLA50/poly(S-co-MMA)50, PLA75/poly(S-co-MMA)25, and PLA90/poly(S-co-MMA)10, respectively. The PLAx/poly(S-co-MMA)y blend pellets were extruded into filaments through a prototype extruder at 195 °C. The 3D printing was done via fused deposition modeling (FDM) at the same temperature and a 40 mm/s feed rate. Furthermore, thermogravimetric curves of the PLAx/poly(S-co-MMA)y blends showed slight thermal decomposition with less than 0.2% mass loss during filament extrusion and 3D printing. However, the thermal decomposition of the blends is lower when compared to amorphous PLA and poly(S-co-MMA). On the contrary, the PLAx/poly(S-co-MMA)y blend has a higher Young’s modulus (E) than amorphous PLA, and is closer to poly(S-co-MMA), in particular, PLA90/poly(S-co-MMA)10. The PLAx/poly(S-co-MMA)y blends proved improved properties concerning amorphous PLA through mechanical and rheological characterization.

Design of Cu-Zr-Al and Cu-Zr-Al-Sn bulk amorphous alloys with high glass-forming ability

The thermodynamic driving force is used to obtain the initial mixing ratio of binary eutectics to design Cu-based bulk metallic glasses. By adjusting approximately 5% proportionality coefficients from the primary mixing ratios of eutectic units, we directly locate amorphous compositions with critical casting diameter of 5 mm (Cu47.5Zr45.1Al7.4) and 7 mm (Cu47.3Zr45.8Al6.4Sn0.5) in Cu-Zr-Al and Cu-Zr-Al-Sn systems, respectively. Using the Cu-Zr-Al alloys as an example, the improvement of glass-forming ability (GFA) is systematically analyzed based on thermodynamics and atomic-scale structure. A relatively lower driving force for crystallization and larger populations of Cu-centered full icosahedra with the large tendency to form MROs jointly contribute to the stability of liquid. With regard to topology, the closer local atomic size ratio compared with the ideal value for icosahedral packing around the Cu atom may be the dominant factor for the higher number of Cu-centered full icosahedra and enhanced GFA.

Effects of HiPIMS pulse-length on plasma discharge and on the properties of WC-DLC coatings

High power impulse magnetron sputtering (HiPIMS) has been employed to deposit WC-DLC coatings on cemented carbide substrate from a tungsten target in acetylene (C2H2) reactive atmosphere. Pulse length of HiPIMS was varied from 50 μs to 200 μs at constant frequency to investigate its influences upon plasma discharge characteristics, target poisoning in the reactive HiPIMS process, and upon the chemical composition, microstructure, mechanical and tribological properties of the deposited coatings. Consequently an oscilloscope (OSC) was used to record the discharge current during the experiments. The results exhibit that the discharge current shows a significant drop at short pulses but a slow descent at long pulses during the deposition. The optical emission peak intensity of W (target material) monitored by optical emission spectroscopy (OES) displays the same tendency. The deposition rate and cross-sectional morphology of WC-DLC coatings were investigated by scanning electron microscope (SEM). Surface morphology and roughness were detected by atomic force microscopy (AFM). The chemical composition, crystal structure and carbon phase were analyzed by means of energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Raman spectroscopy respectively. The glow discharge optical emission spectroscopy (GDOES) was used to demonstrate the depth profiles of the WC-DLC coatings. The results verify that coatings of different depths deposited at longer pulse length exhibit better uniformity of elemental concentration. The hardness obtained by nano-indentation tester also shows significant changes from 35 GPa at 200 μs to 18 GPa at 50 μs pulse length of the deposited coatings. The friction coefficients and wear rates of WC-DLC coatings measured by ball-on-disk test and laser scanning confocal microscope (LSCM) show a strong dependency upon the amorphous carbon composition of regions close to surface, which is influenced by the pulse length of HiPIMS.

Investigation of amorphous alloys nanostructure and phase composition formed during integrated and split exposure of deformation and flash lamp annealing

Microstructure and phase composition of the Al85Ni7Fe4La4 amorphous alloy after separate and complex impacts of severe plastic deformation by shift under the pressure (SPD) with P=8 GPa, n=1-3 revolutions and flash lamp annealing (FLA) with the fluence F=10-25 J/cm2 were investigated. By X-ray structural analysis and transmission electronic microscopy (TEM) was shown, that in initially amorphous structure partial multiphase crystallization with formation of amorphousnanocrystalline composite (solid solution of crystalline Alcr and amorphous Alam and Al11La3, Al9(Fe, Ni)2 in aluminum) is developing during the SPD. FLA leads to partial nanocrystallization at F=10 J/cm2 and full crystallization after the FLA at 20-25 J/cm2. Phase transformation and static recrystallization processes are developed along with the crystallization in amorphous structural component at SPD and FLA integrated exposure.

Microcrystalline Apatite Minerals: Mechanochemical Activation for Agricultural Application

Phosphate minerals of apatite from three mines with different amounts of gangue minerals were activated by milling to increase their microcrystallinity and subsequent dissolution in a citric acid solution to serve as slow-release fertilizers for agricultural production. XRD (X-ray diffraction), FTIR (Fourier Transform Infrared), and SEM (Scanning electron microscope) were employed to characterize the properties of the prepared samples, such as changes in microcrystallinity, mineral composition, lattice structure, and granule morphology during the milling. With prolonged milling time, accompanied with the progress in microcrystallinity, the ratio of the formed amorphous compositions increased, resulting in higher dissolution in citric acid solution. In the case of carrot plants, the addition of the microcrystalline phosphate rock together with phosphorus bacteria allowed an efficient nutrient (P, K, and N) uptake as high as 77.0%, 36.7%, and 32.2%, which increased by 91.3%, 123.0%, and 105.2%, respectively, from the growth on an original soil without any addition, demonstrating clear contribution of the activated apatites in agricultural production.

Physical Stability and Viscoelastic Properties of Co-Amorphous Ezetimibe/Simvastatin System.

The purpose of this paper is to examine the physical stability as well as viscoelastic properties of the binary amorphous ezetimibe–simvastatin system. According to our knowledge, this is the first time that such an amorphous composition is prepared and investigated. The tendency toward re-crystallization of the amorphous ezetimibe–simvastatin system, at both standard storage and elevated temperature conditions, have been studied by means of X-ray diffraction (XRD). Our investigations have revealed that simvastatin remarkably improves the physical stability of ezetimibe, despite the fact that it works as a plasticizer. Pure amorphous ezetimibe, when stored at room temperature, begins to re-crystallize after 14 days after amorphization. On the other hand, the ezetimibe-simvastatin binary mixture (at the same storage conditions) is physically stable for at least 1 year. However, the devitrification of the binary amorphous composition was observed at elevated temperature conditions (T = 373 K). Therefore, we used a third compound to hinder the re-crystallization. Finally, both the physical stability as well as viscoelastic properties of the ternary systems containing different concentrations of the latter component have been thoroughly investigated.

Effect of Annealing on Magnetic and Mechanical Behaviors of NANOPERM-Type Alloys

Microstructure, ac magnetic, and mechanical properties of the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">76</sub> Mo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sub> alloy in the amorphous and nanocrystalline state were studied. Transmission electron microscopy was used to analyze the amorphous structure of as-quenched precursor and nanocrystalline phase of sample annealed at 783 K for 1800 s, i.e., the above primary crystallization temperature. Creation of α-Fe nanograins with an average grain size of 8.6 nm affects magnetic and mechanical properties. In order to understand the correlation between structural changes resulting in soft magnetic properties in the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">76</sub> Mo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sub> alloy frequency characteristics of complex permeability and core loss were drawn in the wide range of frequency and magnetic field. Moreover, from dependence of core losses versus maximum magnetic induction Steinmetz coefficients were also calculated for the as-quenched and annealed Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">76</sub> Mo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sub> alloy. Nanoindentation tests with Berkovich diamond indenter were performed for amorphous and nanocrystalline samples for analysis mechanical properties of pure amorphous matrix and nanocrystalline structure represented by α-Fe grains embedded in amorphous residue.

An electronic silicon-based memristor with a high switching uniformity

Amorphous insulators have localized wave functions that decay with the distance $r$ following exp($-r/\zeta$). Since nanoscale conduction is not excluded at $r<\zeta$, one may use amorphous insulators and take advantage of their size effect for nanoelectronic applications. Voltage-regulated nanoscale conductivity is already utilized in metal-insulator-metal devices known as memristors. But typically their tunable conductivity does not come from electrons but from migrating ions within a stoichastically formed filament, and as such their combined resistor-memory performance suffers. Here we demonstrate amorphous-silicon-based memristors can have coherent electron wave functions extending to the full device thickness, exceeding 15 nm. Remarkably, despite the large aspect ratio and very thin thickness of the device, its electrons still follow an isotropic, three-dimensional pathway, thus providing uniform conductivity at the nanometer scale. Such pathways in amorphous insulators are derived from overlapping gap states and regulated by trapped charge, which is stabilized by electron-lattice interaction; this makes the memristor exhibit pressure-triggered insulator$\rightarrow$metal transitions. Fast, uniform, durable, low-power and purely electronic memristors with none of the shortcomings of ion-migrating memristors have been fabricated from a variety of amorphous silicon compositions and can be readily integrated into silicon technology. Therefore, amorphous silicon may provide the ideal platform for building proximal memories, transistors and beyond.

Amorphous Calcium Salt Composition Bioavailability Evaluation in Chickens

Despite the market saturation with a wide range of calcium preparations in dietary supplements as well as in pharmaceuticals, calcium product safety and efficacy remain an item for further optimization. Bones and teeth mainly consist of calcium phosphate, but tablets and capsules are predominantly produced from calcium carbonate. On the other hand, in human food Ca2+ is bound with a lot of anions—carbonates, sulphates, chlorides, phosphates, citrates, oxalates etc. It seems that traditional calcium formulations due its low bioavailability cannot provide all spectra of biological calcium-dependant effects and dietary calcium might be safer and more effective than traditional calcium supplements and medicines. To increase calcium salt efficacy a new formulation of calcium salts has been developed. It is a mixture (water suspension) of amorphous calcium carbonate, citrate, hydrocarbonate, as well as magnesium hydrocarbonate and hydrocitrate as stabilizers. The capacity of a cockerel’s duodenal mucosa to absorb Ca2+ after peroral ingestion of the mentioned composition in vivo was as much as 126% higher in comparison with CaCO3 suspension intake. Vitamin D3 enhanced the amorphous antirachitic activity of calcium salts. The aim of the study was to compare original amorphous calcium salt composition bioavailability with Ca salts most often in food and medicines, as well as to evaluate the antirachitic activity of the mentioned composition in combination with vitamin D3 in chickens.

STRUCTURE FORMATION OF A COMPOSITE BASED ON NANOCARBON WITH THE ADDITION OF IRON-DOPED AMORPHOUS BORON IN CONDITIONS OF HIGH PRESSURES AND TEMPERATURES

Under conditions of intense high-temperature plastic deformation with the use of extracted fuller-ene soot, a new composite based on C-B-Fe heterophase amorphous nanostructured composition with high crack resistance and elasticity was obtained in two modifications: with a highly solid carbon "phase-base"−matrix filled with reinforcing nanocrystallites of carbides and borides of Fe and carboborides and especially with solid particles of the carbon phase distributed in the matrix with an amorphized surface.

States of thermochemically or electrochemically synthesized NaxPy compounds analyzed by solid state 23Na and 31P nuclear magnetic resonance with theoretical calculation

Phosphorus is a promising material for the electrode in sodium ion batteries (NIBs). In this study, the states of NaxPy compounds synthesized by thermochemical reaction and electrochemical sodiation were compared using solid state 23Na magic angle spinning (MAS) NMR, 23Na multiple quantum (MQ) MAS NMR, and 31P MAS NMR. The NMR signals in thermochemically synthesized NaxPy compounds (Na3P, NaP, Na3P7, Na3P11, NaP7) are assigned in reference to theoretical chemical shifts, based on first-principles calculations. Furthermore, the NMR signals in electrochemically prepared NaxPy compounds after two sodiation/desodiation cycles are ascribed to Na3P compounds and three amorphous compositions. The amorphous compounds ascribed to Na1−αP (0 < α < 1), Na2−βP (0 < β < 1), and Na3−γP (0 < γ < 1) are formed below 0.58 V in the charge and discharge process. These Na3P and the amorphous phases overlapped in the range from 0.20 V (sodiation process) to 0.58 V (desodiation process). 23Na and 31P NMR spectra reveal reversible sodiation and desodiation processes in the third cycle.

Amorphous copolyesters based on bibenzoic acids and neopentyl glycol

ABSTRACTHigh Tg amorphous copolyester thermoplastics were synthesized by incorporating 4,4′‐bibenzoate (4,4′BB) and 3,4′‐bibenzoate moieties into the polyester backbone via melt polycondensation. The high levels of crystallinity typically associated with 4,4′BB containing polyesters were suppressed through copolymerization of ethylene glycol, 1,4‐cyclohexane dimethanol, and neopentyl glycol (NPG) diols. NPG was shown to be highly effective in suppressing crystallization and was used to produce amorphous compositions with Tg’s as high as 129 °C. Diol ratios were determined by 1H NMR spectroscopy and molecular weights were assessed with inherent viscosity (ηinh). Thermogravimetric analysis showed single‐step weight losses in the range of 395–419 °C. Differential scanning calorimetry was used to determine melting points and glass transition temperatures over a wide range of copolyester compositions and identified amorphous compositions. Dynamic mechanical analysis confirmed Tg’s and was used to study β‐relaxations below the Tg. Rheological analysis revealed the effect of NPG structures on shear thinning and thermal stability. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 579–587

Optical characterization of nanostructured Ge1 − xSnxSe2.5 (x = 0, 0.3, 0.5) films

The paper reports the optical properties of thin films of nanostructured Ge1 − xSnxSe2.5 (x = 0, 0.3, 0.5) glassy alloys. The glassy alloys of Ge1 − xSnxSe2.5 (x = 0, 0.3, 0.5) were prepared using melt quenching method. Thin films of nanostructured Ge1 − xSnxSe2.5 (x = 0, 0.3, 0.5) glassy alloys were prepared using physical vapor deposition method. The films were characterized using XRD, EDX and TEM, which confirmed the amorphous nature, composition and formation of nanorods in the samples. Absorption and transmission spectra of thin films were recorded in the spectral range 400–2500 nm to obtain energy band gap, refractive index, extinction coefficient, dielectric constant etc. Results show that refractive index increases while band gap decreases on increase of Sn content in the Ge–Se system. This is due to the increase in density of defect states within band gap. The values of Urbach energy support the obtained results.

Functionalizing window coatings with luminescence centers by combinatorial sputtering of scatter-free amorphous SiAlON:Eu2+ thin film composition libraries

SiAlON window coatings are applied on an industrial scale to achieve e.g. scratch-resistance and anti-reflection. Doping these SiAlONs with rare-earths adds luminescent functionality, which could be applied in photovoltaics. By using a combinatorial reactive sputtering approach, an amorphous thin film composition library with a Si:Al ratio from 0.062:1 to 3.375:1 and a Eu doping from 4.8 at% to 26 at% is created. This library uniquely combines high absorption, strong emission and absence of light scattering. By combining position-dependent EDX measurements with transmission and emission spectra, properties like the index of refraction, absorption strength, emission wavelength and decay times of the library can directly be related to the composition. Throughout the library, an index of refraction of 1.63±0.03 is observed, typical for a film with low nitrogen content. The library also shows a large absorption coefficient of 1294±8cm−1at.%−1. Laser-excited emission spectra show that the library has a strong redshift from 500 nm to 550 nm with increasing Al concentration. An increase in Eu concentration also causes a shift of the emission to red. Decay spectra show that a high degree of Si greatly improves the luminescence intensity. These functionalized SiAlON coatings can be of great interest for transparent and scatter-free luminescent solar concentrators applied as windows.

Investigation of the nucleation delay time in Al-based metallic glasses by high rate calorimetry

During isothermal annealing at temperatures lower than the crystallization onset temperature, Tx, measured during continuous heating, the Al88Y7Fe5 metallic glass undergoes a period of delay time (τ) before the primary crystallization reaction. The delay time is an important parameter, which is closely related with the transport behavior and can be used to calculate the attachment frequency coefficient and steady state nucleation rate. Previously, τ was usually determined from the nanocrystal number density vs. annealing time plot through tedious TEM measurements. Now, a more effective approach to measure τ has been developed by analyzing the Tg shift resulting from annealing with high rate differential scanning calorimetry (FlashDSC). With the increase of annealing time (tannealing), Tg shifts to higher temperatures and in the Tg vs. tannealing plot, there is a break point in slope. Before this break point, no Al nanocrystals could be detected by transmission electron microscopy (TEM) but after this break point, Al nanocrystals were identified by TEM. Thus, the break point time corresponds to the onset of Al nanocrystal formation and the delay time. The underlying mechanism is unveiled through the matrix composition measurement by energy dispersive spectroscopy (EDS). The analysis reveals that before the break point the amorphous matrix composition is unchanged by annealing so that Tg shifts solely due to the increased glass stability through a relaxation process. After the break point, the precipitation of Al nanocrystals induces a composition change in the amorphous matrix so that Tg shifts to higher temperatures because of both the relaxation and the composition change effects.

Influence of processing temperature on the reaction product and strength gain in alkali-activated fly ash

The roles of temperature and curing periods on product formation and strength gain in alkali activation of low-calcium siliceous fly ash are evaluated. Higher temperature favors the kinetics of amorphous reaction product formation and inclusion of Si in the reaction product, which accompanies the densification of the microstructure. Following initial curing at 60 °C, the reaction product continues to form in the activated system on decreasing the temperature to 25 °C. Initial curing at 25 °C does not lead to formation of reaction product. Reaction product forms in the system for initial curing temperature of 40 °C and higher. The rate of the solid-based reaction, after hardening, is increased by temperature. The total amorphous reaction product content formed in the system depends on the water to solids ratio of the activated mix. The amorphous product composition, however, varies with temperature. With time there is an increase in the Si/Al ratio in the reaction product formed 40 °C and higher. For initial curing at 60 °C, there is no increase in the Si/Al ratio in the reaction product when the temperature is decreased to 25 °C. Reaction product with a higher Si/Al atomic ratio achieves a higher ultimate compressive strength. On increasing the Si/Al ratio in the reaction product from 2.5 to 3.3, the ultimate compressive strength achieved increased from 60 MPa to 74 MPa. Early curing, immediately after mixing, at a higher temperature is critical in achieving a higher Si/Al atomic ratio in the reaction product. Optimum initial curing time at high temperature is based on achieving a high Si/Al atomic ratio in the reaction product.