Mechanism Of Glass Formation Research Articles

Glass formation from low molecular weight organic melts

Glass formation from the melt of organic monomers was studied for a variety of different organic molecular structures with Tg near ambient temperature. Crystallization is suppressed by one or more of the molecular properties, hydrogen bonding, interlocking, dipolar, and hydrogen bonding, combined with hindered rotational isomerism. Examples of materials in each category are presented for illustration. The viscosity of undercooled liquids was characterized by the Vogel-Tammon-Fulcher (VTF) equation, η = A cxp[DT0/(T - T0)], where A, D, and T0 are experimentally determined parameters. Our experimental D values are discussed in relation to the molecular structure and glass formation mechanism. The insight provided by our interpretation is intended to assist in the design of new molecular structures with controlled viscosity-temperature characteristics, as well as glass-forming ability by cooling from melts.

The partitioning of iron during the combustion of pulverized coal

The partitioning of iron during pulverized coal combustion was investigated both experimentally and theoretically. Emphasis was on determining how coal variables and combustion conditions influenced the formation of slagging precursors. Experimental work consisted of burning a suite of six well-characterized coals in an aerodynamically well-defined 17 kW downflow combustor. Speciation of iron in collected ash samples was determined by Mössbauer Spectroscopy. A model was developed to predict the partitioning of iron for the coals and experimental conditions examined. The model was based on a competition between pyrite oxidation and iron capture by silicates and required as input, CCSEM data on the initial distribution of mineral matter in the coal. It used literature-derived mechanisms and kinetics to calculate pyrite oxidation and char combustion rates, together with a new glass formation mechanism, coupled with current knowledge of sintering. Requiring only two unknown parameters to be fitted, the model agreed well with 26 sets of experimental data, which covered a wide range of iron partitioning in the fly ash. Both experimental and theoretical results suggest that removal from the parent coal, of extraneous iron alone, may not be very effective in eliminating slag formation in boilers.

Formation of a metallic glass by thermal decomposition of Fe(CO)5.

Iron pentacarbonyl has been thermally decomposed in an organic liquid. M\ossbauer spectroscopy and x-ray diffraction studies show that the sample contains small particles of a metallic glass. Annealing of the particles at 523 K results in crystallization of the particles into a mixture of \ensuremath{\alpha}-Fe and \ensuremath{\chi}-${\mathrm{Fe}}_{5}$${\mathrm{C}}_{2}$. The mechanism of glass formation is discussed.

Halide glasses

An increasing interest is devoted to fluoride, and more generally, to halide glasses because of their potential use for making infra-red optical components and ultra-low-loss optical fibres. This paper presents the main fluoride systems based on ZrF 4, HfF 4, AlF 3, ZnF 2, on transition metals and on actinides and lanthanides, all of which act as glass progenitors. The main chloride glasses based on ZnCl 2, BiCl 3, ThCl 4 and CdCl 2 are also described. The aspects of glass synthesis reviewed include fluorination by NH 4HF 2 and reactive atmosphere processing, which are currently used to prepare and refine fluoride glasses. The manufacturing of glass samples and the drawing of fibres are discussed with the problems of nucleation and crystallization in mind. The characteristic temperatures for the major families of halide glasses are detailed in the text, but, generally speaking, the value of T G lies between 200 and 500° C for fluoride glasses and under 200° C for chloride and mixed halide glasses. The optical transmission range normally encompasses the U.V., visible and medium I.R. spectra. The position of the multiphonon absorption edge depends closely on the glass composition. The main optical characteristics are given and their potential use for making infra-red optical components, low-loss optical fibres, electrochemical barriers and laser hosts are presented and discussed. The occurrence of numerous new halide glasses provides experimental evidence for understanding the glass formation mechanism in non-oxide systems.

Mechanism of glass formation in portland cement clinker

In the system C 3AC 4AFSiO 2MgO, the glass formation region that is closely similar in composition to the interstitial material of portland cement clinker has been determined. The interstitial melt that exists in equilibrium with the silicates at the clinkering temperature contains a little silica. This forms clusters, which increase the viscosity of the melt and cause the latter to supercool appreciably. Crystallization of C 3A and C 4AF increases the concentration of clusters, which progressively interlock in the residual liquid, thus giving a glass on quenching. The presence of SiO 2 in the interstitial melt also enables the metastable formation of the orthorhombic aluminate and the oriented overgrowth of the ferrite on the aluminate to occur through encouraging supercooling of the melt.

Polymer solidification as a dissipative process

AbstractBecause their conformational entropy is a nonlinear function of size, polymer molecules can only solidify under conditions far from equilibrium with the formation of dissipative structures. A dissipative structure is nonhomogeneous, it evolves with time from an initially space‐dependent fluctuation, and has a characteristic size that depends upon the initial perturbation and the conditions that apply during its entire history. The glass transition is envisioned as the formation of the ultimate dissipative structure where all of the latent heat of crystallization is dissipated within the structure. The mechanism of glass formation can be described as the spinodal decomposition of a homogeneous liquid into two interpenetrating networks of high‐ and low‐energy solids with a characteristic distance and a gradient of density and/or composition extending through both high‐ and low‐energy regions.

Properties and structure of AsTe glasses: (I). Glass-forming ability and related properties

AsTe glasses have been investigated for determining the molecular mechanisms of glass formation and of crystallization. Stable and homogeneous bulk glasses are prepared throughout the range of concentration from 20 to 65 at % As, by using an improved quenching method. GFA (glass-forming ability) is determined with respect to the rate of cooling. Density, microhardness, T g and ΔC p at T g are measured in the whole range of glass formation. Finally a quantitative analysis of the kinetics of crystallization is carried out. It appears that most properties which involve molecular motions exhibit anomalies for As 40 Te 60 composition (As 2Te 3 is the unique definite compound of the system). So GFA shows a sharp depression for this critical composition; similarly enthalpy and kinetics of crystallization are drastically modified at As 40Te 60. For Te-rich glasses, crystallization occurs at low temperature by homogeneous nucleation with a small energy of activation. For As-rich glasses, crystallization occurs at much higher temperatures. Above 200°C it is controlled by growth with an high activation energy which indicates deep rearrangements of the relative positions of the atoms by chemical diffusion. A coherent analysis of all the data is developed which shows that important differences of local order exist between amorphous and crystalline states and also between the two well-distinct types of glasses we are dealing with. The aim of paper II is to determine these differences from diffraction data.

The mechanism of glass formation

The mechanism of glass formation