On the method for determining the true strength of inorganic glasses

The possibility of determining the true structural strength of glass by bending of glass fibers with a defect-free surface is considered. Two methods are compared, viz., the method of transverse three-point bending in which the breaking stress (strength) is determined, and the method of two-point bending in which the breaking strain is determined. In the latter case, the dependence of the elastic modulus on strain is required for determining the breaking stress (strength). The strength measured in three-point bending is compared with the strength calculated from the breaking strain measured in two-point bending. It is shown that the measurements based on these two methods give close values of strength for defect-free silica fibers used as optical waveguides. The observed difference of ～12% in the values of strength is explained by the difference in the loading rates obtained using these two methods. The advantages and disadvantages of these two techniques are analyzed.     

Charging of inorganic glasses during irradiation and the energetics of their structure

Conclusions The energetics of the glass structure glasses characterizing the magnitude of the cation-oxygen bond constitutes the key factor which determines the quantity of charge stored. In the successive replacement of cation-modifiers, most charge is stored in those glasses in which the lowest concentration of hole color centers is produced. In the event of a percentage change in the content of the cation-modifier in the glass structure, most charge is stored in those glasses that lie closest to the metaphosphate line.A law of charge storage in phosphate glasses has been established. The kinetics of charge storage is more complex: a rapid increase in the quantity of charge stored is observed in the first stage of storage, whereas in the second stage the quantity of charge stored is observed to decrease slowly at large doses. Analysis of the curves indicates two processes occurring in the glasses during irradiation.The mechanism of neutralization of electrically active centers consists in a neutral P₂O₇ complex being formed from the charged radicals PO ₃ ^– and PO ₄ .Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 60–65, July, 1977.     

Atomic structures of Zr-based metallic glasses

The atomic structures of Zr-Ni and Zr-Ti-Al-Cu-Ni metallic glasses were investigated by using classical molecular dynamic (MD), reverse Monte Carlo (RMC), ab initio MD (AIMD) simulations and high resolution transmission electron microscopy (HRTEM) techniques. We focused on the short-range order (SRO) and medium-range order (MRO) in the glassy structure. It is shown that there are icosahedral, FCC-and BCC-type SROs in the Zr-based metallic glasses. A structural model, characterized by imperfect ordered packing (IOP), was proposed based on the MD simulation and confirmed by the HRTEM observation. Furthermore, the evolution from IOP to nanocrystal during the crystallization of metallic glasses was also explored. It is found that the growth from IOP to nanocrystal proceeds through three distinct stages: the formation of quasi-ordered structure with one-dimensional (1D) periodicity, then 2D periodicity, and finally the formation of 3D nanocrystals. It is also noted that these three growth steps are crosslinked. Supported by the National Natural Science Foundation of China (Grant Nos. 50431030 and 50471097), the National Basic Research Program of China (Grant No. 2007CB613901), and the Programme of Introducing Talents of Discipline to Universities (Grant No. B07003)     

Atomic Bose and Anderson glasses in optical lattices

An ultracold atomic Bose gas in an optical lattice is shown to provide an ideal system for the controlled analysis of disordered Bose lattice gases. This goal may be easily achieved under the current experimental conditions by introducing a pseudorandom potential created by a second additional lattice or, alternatively, by placing a speckle pattern on the main lattice. We show that, for a noncommensurable filling factor, in the strong-interaction limit, a controlled growing of the disorder drives a dynamical transition from superfluid to Bose-glass phase. Similarly, in the weak interaction limit, a dynamical transition from superfluid to Anderson-glass phase may be observed. In both regimes, we show that even very low-intensity disorder-inducing lasers cause large modifications of the superfluid fraction of the system.     

超高强块体非晶合金的研究进展

研制具有极限力学性能的金属材料一直是材料研究人员的梦想.超高强块体非晶合金是一类具有极高断裂强度(4 GPa)、高热稳定性(玻璃化转变温度通常高于800 K)和高硬度(通常高于12 GPa)的新型先进金属材料,其代表合金材料Co-Ta-B的断裂强度可达6 GPa,为目前公开报道的块体金属材料的强度记录值.本文系统地综述了该类超高强度块体非晶合金的组分、热学性能、弹性模量及力学性能,阐述了该类材料的研发历程;以弹性模量为联系桥梁,阐明了该类超高强块体非晶合金材料各物理性能的关联性,并揭示了其高强度、高硬度的价键本质.相关内容对于材料工作者了解该类超高强度金属材料的性能和特点,并推进该类材料在航空航天先进制造、超持久部件、机械加工等领域的实际应用有着重要意义.     

Magnetism and structure of metallic glasses

Hyperfine Interactions - Metallic glasses are a rather new class of materials. They offer exciting possibilities for finding new materials with combinations of properties unknown for crystalline...     

Pulse EPR spectroscopy of Cu2+-doped inorganic glasses

Two-frequency continuous-wave and pulse EPR (electron paramagnetic resonance) spectroscopical techniques are applied to determine static and dynamic EPR parameters of Cu²⁺ ions in oxide and fluoride glasses. The investigations are focussed on the analysis of strain effects in the glassy matrices, the identification of the magnetic nuclei in the vicinity of Cu²⁺ ions as well as the determination of the dependence of the phase memory timeT _M on temperature and resonance field. The results obtained by X-band continuous-wave EPR, X- and S-band echo-detected EPR, and X- and S-band electron spin echo envelope modulation studies of Cu²⁺-doped inorganic glasses yield information on the local symmetry of the Cu²⁺ coordination polyhedra, the chemical nature of the atoms in the second and higher coordination spheres, the distribution of the parameters of the static spin Hamiltonian and the low-temperature motions of the dopant-containing structural units. Special techniques like 2-D Mims ENDOR (electron nuclear double resonance) and hyperfine-correlated ENDOR are applied for the first time to doped inorganic glasses. From the spin relaxation measurements a stronger tendency of the Cu²⁺ ions to aggregate is found for fluoride glasses in comparison to aluminosilicate and phosphate glasses.     

Surface and near-surface structure of silicate glasses

This paper presents analytic results, derived by diffractional low and large angle X-ray scattering methods, currently applied in our laboratory, as well as novel methods of electron emission microscopy and interference-phase optical microscopy for studying the internal structure, and that of surface and near-surface layers of raw and stepwise-etched, composite oxide silicate glasses of the type SiO₂-Na₂O-CaO-MgO. The structural and topographic studies are supplemented with quantitative analysis of chemical composition on a micro-scale and in micro-regions by electron microprobe and microscopic measurements of the temperature characteristics in a series of the above glasses. Moreover, a discussion is given of chemical corrosion, surface strcuture and topography, and the distribution of micro and macro-inhomogeneity regions in surface layers. Structural parameters are obtained which described the surface and internal structure of silicate glasses well adapted to the construction of an approximate aperiodic nodal model. The applications of the latter and the accuracy of its parameters are discussed.     

Origin of temperature-sensitive lines in the Raman spectra of inorganic glasses

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 51, No. 4, pp. 628–634, October, 1989.     

Atomic scale fluctuations govern brittle fracture and cavitation behavior in metallic glasses

We perform atomistic simulations on the fracture behavior of two typical metallic glasses, one brittle (FeP) and the other ductile (CuZr), and show that brittle fracture in the FeP glass is governed by an intrinsic cavitation mechanism near crack tips in contrast to extensive shear banding in the ductile CuZr glass. We show that a high degree of atomic scale spatial fluctuations in the local properties is the main reason for the observed cavitation behavior in the brittle metallic glass. Our study corroborates with recent experimental observations of nanoscale cavity nucleation found on the brittle fracture surfaces of metallic glasses and provides important insights into the root cause of the ductile versus brittle behavior in such materials.     

Urbach tails and the structure of chalcogenide glasses

A novel analysis of optical absorption tails of inorganic network glasses is shown to provide important information on the structure of the glass. The anomalous composition and temperature dependence of absorption tails in Ge_xSe_1?x and As_xSe_1?x systems indicate that these glasses retain locally layered structures at particular stoichiometries corresponding to GeSe₂ and As₂Se₃, and a reversible structural change is taking place well below the glass transition temperature. A phenomenological model for the absorption tail slope of glasses is proposed, analogous to the Urbach rule for crystalline materials.     

High-zirconium bulk metallic glasses with high strength and large ductility

In this paper,high-zirconium Zr66+2xAl9-x(Ni1/3Cu2/3) 25-x(x=0,1,2 at.%) bulk metallic glasses with high strength and large ductility were fabricated by copper mould casting.The effects of zirconium content on the glass-forming ability(GFA),thermal properties and mechanical properties were investigated using X-ray diffractometer(XRD),differential scanning calorimeter(DSC),and mechanical testing system in compressive and three-point bending modes,respectively.The high-zirconium BMGs show the critical diameters of 3-5 mm,the supercooled liquid region ranging from 70 K to 99 K,and the yield strength of over 1700 MPa.The Zr 70 Al 7(Ni1/3Cu2/3) 23 BMG exhibits a large compressive plastic strain up to 21% and a high notch toughness value of 60.6 MPa m 1/2.The increase in Zr content results in the decrease in GFA and thermostability,and in the improvement of plasticity under compressive and three-point bending conditions.The superior plasticity of high-zirconium BMGs is attributed to their high Poisson’s ratio and small elastic modulus ratio /B.     

Atomic structure in liquid selenium

Comparison of radial distribution functions of liquid selenium, amorphous selenium and crystalline selenium indicates a gradual loss of parallel chain-like configurations in the liquid at increasing temperatures.     

Atomic mechanism of the heating-induced phase transitions of the simple monatomic glasses

Atomic mechanism of the heating-induced phase transitions of the monatomic Lennard-Jones (LJ) glass has been studied via molecular dynamics (MD) simulations. Monatomic LJ glass was heated up at two different heating rates, crystallization occurs at the lowest one and further heating leads to the melting of LJ crystal. Thermodynamics of the phase transitions and corresponding evolution of structural properties upon heating have been analyzed in details. Atomic mechanism of a crystallization of the glassy state was monitored via spatio-temporal arrangements of the atoms involved in the 1421 bond-pair of the fcc crystalline structure. The 1421 bond-pair was detected via the Honeycutt-Andersen analysis [J.D. Honeycutt, H.C. Andersen, J. Phys. Chem. 91 (1987) 4950]. We found that crystallization of the monatomic LJ glass occurs via homogeneous local rearrangements of atoms in the glassy matrix and we found an important role of the liquid-like atoms (existed in the glassy state) in crystallization of the system. In addition, spatio-temporal arrangements of the liquid-like atoms in the system upon further heating were shown in order to clarify the atomic mechanism of a melting of the obtained LJ crystal. Liquid-like atoms were defined by the Lindemann melting criterion. Our results provide previously un-reported data and give deeper understanding of the heating-induced phase transitions in the less stable metallic glasses, which have been observed in practice.     

Atomic tunneling states with dissipation in glasses: temperature-dependent two-level systems

By taking account of the influence of dissipation on atomic tunneling states in glasses, we introduce a density of states of two-level systems which is non-linearly temperature dependent. Based on this model, we can explain the “excess T³” anomaly in the specific heat and the “plateau” in the thermal conductivity of glasses.