Arun K. Varshneya

Gibbs-DiMarzio equation to describe the glass transition temperature trends in multicomponent chalcogenide glasses

Abstract High purity chalcogenide glasses incorporating Ge, Sb, Se, As and Te were prepared by vacuum melting of previously distilled 5 N to 6 N pure raw materials from which the surface oxide was also removed in some instances. Glass transition temperatures, Tg, of several chalcogenide glasses prepared by similar processing techniques were determined using differential scanning calorimetry (DSC). An emperical relationship between the glass transition temperature and average coordination number is proposed by modifying the Gibbs-DiMarzio equation for glass transition of a cross-linked polymer as a function of cross-link density. The results for the Tg of three ‘iso-structural’ systems are reported and discussed in light of presumed structural arrangements in these glasses.

Viscosity of chalcogenide glass-forming liquids: an anomaly in the ‘strong’ and ‘fragile’ classification

Abstract High purity chalcogenide glasses with an average coordination number, 〈r〉, between 2 and 2.8 were prepared by vacuum melting of pre-distilled samples in the GeSe binary and GeSbSe ternary systems. Viscometry and scanning calorimetry were performed to understand the nature of the property thresholds observed with respect to 〈r〉 in these glass-forming systems. There is a lack of correlation between viscosity-based and heat capacity-based classifications of strong and fragile liquids in the overconstrained region of these glass-forming systems. An argument based on Adam-Gibbs equation and mode decoupling is put forward for the observation of the anomaly.

Microhardness, surface toughness and average coordination number in chalcogenide glasses

Abstract Glasses covering the entire glass formation region in the GeSbSe chalcogenide system were prepared using 5- to 6-nines pure raw materials and careful processing techniques. Vickers microhardness and surface toughness of these, as well as several other isostructural glasses prepared similarly, were made using the microindentation technique. The microhardness of most of the glasses increased linearly with the average coordination number 〈 r 〉. The surface toughness showed a maximum at 〈r〉 ⋍ 2.6 . The empirical relationships between glass composition and the two properties so derived are examined, to some extent, in light of percolation concepts in these covalently bonded solids.

Microhardness and indentation toughness versus average coordination number in isostructural chalcogenide glass systems

Abstract Several high purity chalcogenide glasses were prepared by vacuum melting of previously distilled and/or surface oxide removed 5 to 6-nines pure Ge, Sb, Se, As and Te. Vickers hardness number and indentation toughness were measured using the microindentation technique. A previously proposed empirical relation between average coordination number and Vickers hardness number for chalcogen-rich chalcogenide glasses was refined and is discussed in relation to ‘rigidity percolation’ concepts.

Modified preparation procedure for laboratory melting of multicomponent chalcogenide glasses

High purity mid IR-transparent chalcogenide glasses are generally prepared by vacuum melting of 5–6 N pure raw materials such as Ge, Sb, Se, As and Te. For laboratory samples, a modified technique is proposed which involves a combination of surface oxide removal prior to, and distillation, melting and annealing after, batching the raw materials without breaking vacuum. Se, As and Te were distilled in sequence simply by increasing the temperature in steps. The need for porous quartz frit used by previous researchers during distillation was eliminated, thus reducing the possibility of impurity inclusions.

Measurement of progressive stress buildup during ion exchange in alkali aluminosilicate glass

Abstract A mixed-alkali aluminosilicate glass (glass transition temperature, Tg=460∘C) was ion exchanged at 375∘C in a fused silica cell in the hot-stage of a polarizing microscope. Buildup of stress-induced optical birefringence was measured as a function of the depth below the surface and time of exchange to 8 h. Surface compression increased steadily during this period. Upon rapidly cooling the specimen to room temperature, stresses decreased ∼20%, without a significant change in the thickness of the compression layer implying that the surface layer had a higher thermal contraction following the exchange. Differential thermal contraction of ion-exchanged layers could not have been the cause of either large differences between calculated stress magnitudes from those observed experimentally, or the observed “anomalous” tensile stress maximum. It is argued that the ion-exchanged layers are stuffed derivatives having physical properties different from those of the parent glass or the as-melted glasses of corresponding compositions.

Structure-property inter-relations in chalcogenide glasses and their practical implications

Abstract To understand the effects of structural features and to locate their signatures in the glassy and the liquid states, various properties were studied as a function of average coordination number, 〈r〉, in chalcogenide glasses and examined in relation to the chemically ordered covalent network model and constraint theory. The former predicts a threshold at 〈r〉 = 2.67 and the latter a threshold at 〈r〉 = 2.4. It is noted that most of the properties exhibit a threshold at 〈r〉 = 2.67 while only some show the threshold at 〈r〉 = 2.4. The reasons for the existence of the thresholds and various practical applications based on composition-property relationships are discussed.

Preliminary investigations of superconducting glass-ceramics in the Bi-Sr-Ca-Cu-B-O system

Abstract We have made glasses with starting batch stoichiometries (Bi-Sr-Ca-Cu) 4-3-3-4, 2-2-2-3 and 2-2-1-2 by a melt-quench process. Glasses thus formed superconduct upon suitable heat treatment due to devitrification of the Bi 2 Sr 2 Ca 1 Cu 2 O 8 phase. Upon addition of a glass-former (boric acid/silica), we were able to form stable glasses in a variety of batch compositions. Some of these compositions showed crystallization of the Bi 2 Sr 2 Ca 1 Cu 2 O 8 phase and were also superconducting in the 90 K range.

Comparison of AIPO4 and SiO2 glass structures using molecular dynamics

Abstract The structure of amorphous AlPO 4 was calculated using the method of molecular dynamics. Calculated radial distribution functions are in good agreement with those measured by Wignall et al. using X-ray diffraction. The structure, although similar to that of SiO 2 in many features, lacks full connectivity and does not appear as random. This may help explain the resistance of AlPO 4 to glass formation.

Stress-optic coefficient of GeAsSe chalcogenide glasses

Abstract Stress-induced optical retardation at λ = 1.8 μ m in GeAsSe chalcogenide glass disks subjected to a diametral compressive load was measured using a modified IR-polarizing microscope. The computed stress-optic coefficient, B , varied from −29.6 to +5.4 TPa −1 over the glass-forming region. Although the magnitude of B appeared to decrease with increasing Ge content, there appeared to be little direct correlation between B and the average coordination number, 〈(ifr)〉.