Valence state change and refractive index change induced by femtosecond laser irradiation in Sm3+ doped fluoroaluminate glass

We report femtosecond laser induced valence state and refractive index change in transparent Sm³⁺-doped fluoroaluminate glass. The effect of annealing on the induced changes was studied and the thermal stability of these changes was discussed. The results show that the femtosecond laser induced valence state change is more stable than the induced refractive index change. The observed phenomenon could be applied to design the thermally erasable or stable storage medium.     

The effect of annealing on the mechanical properties of a ZrAlNiCu metallic glass

In this paper, the effects of annealing and nanocrystallization on the mechanical properties of a Zr₅₅Al₁₀Ni₅Cu₃₀ metallic glass have been studied. It has been shown that the high volume fractioned nanocrystals facilitate the formation of shear bands and thus decrease the yield stress. At the same time, the nanocrystals also facilitate the formation of interfacial voids during compression, resulting in substantial decrease in the plasticity of the metallic glass.     

Optical and mechanical properties of far infrared transmitting glass–ceramics

We have studied the mechanical properties of a new family of infrared transmitting glass–ceramics based on the GeS₂–Sb₂S₃–CsCl system. Cs-based crystals have been generated inside the glass matrix and the crystal size is controlled to be less than 100 nm in order to keep the transparency of the composite materials. The presence of crystals leads to significant increase of the fracture toughness and the critical load for crack initiation. The increases depend on the type and on the size of crystals. Other mechanical properties such as Young’s modulus, Shear modulus are much less sensitive to these crystals.     

Molecular dynamics simulation of high-pressure densification of fluorozirconate glass

Molecular dynamics simulations have been performed for a fluorozirconate glass and its model crystals in order to investigate the mechanism of the peculiar high-pressure densification phenomenon of the glass. All the polymorphs and the pressure-induced phase-transition of the model crystals were satisfactorily reproduced. The changes in the density, the F⁻ coordination number and the connectivity of ZrF_n polyhedra during the compression-decompression process are investigated under the glassy state. The density increases under high pressure, accompanying the increases in the coordination number and the connectivity. The effect of annealing treatment near T_g is also investigated. The effect was significant around 20 GPa, around which a maximum was found in the treatment-pressure dependence of density after decompression, only for the samples with the annealing treatment before decompression.     

Growth of single crystals of the high-pressure ɛ-FeOOH phase

Single crystals of the high-pressure ?-FeOOH phase are grown from an aqueous solution at p = 7 GPa in the temperature range 580–350°C. Dark brown crystals of prismatic habit are obtained that have characteristic sizes of 0.1 × 0.1 × 0.7 mm. A region of the t-x phase diagram of the H₂O-Fe₂O₃ system at p = 7 GPa is constructed on the basis of the data obtained.     

Effects of Zn on the glass forming ability and mechanical properties of MgLi-based bulk metallic glasses

Through the addition of Zn element, Mg-Li-Cu-Zn-(Y, Gd) bulk metallic glasses (BMGs) with diameter of 2 mm have been successfully fabricated by conventional copper mold injection casting method. The X-ray diffractometer (XRD) patterns and differential scanning calorimeter (DSC) traces demonstrated that the as-cast Mg₆₀Li₅Cu₂₀Zn₅(Y, Gd)₁₀ BMGs were fully amorphous phase. By the minor addition of 5 at.% Zn, the supercooled liquid regions (SLR) of Mg₆₀Li₅Cu₂₀Zn₅(Y, Gd)₁₀ BMGs increased 6 K and 3 K respectively compared with that of Zn-free samples, which implied that their glass forming ability (GFA) and thermal stability were improved. It was also proved that the addition of Zn effectively enhanced the alloys’ fracture strength (453 MPa and 456 MPa) and elastic strain (0.75% and 0.92%), which were much higher than that of Mg₆₀Li₅Cu₂₅Y₁₀ BMG (403 MPa, 0.62%) and Mg₆₀Li₅Cu₂₅Gd₁₀ BMG (412 MPa, 0.82%).     

A study of mechanical homogeneity in as-cast bulk metallic glass by nanoindentation

A surface softening effect induced during copper-mould suction casting of bulk metallic glass is investigated as a function of rod diameter and glass fragility index, m, by nanoindentation. A reduction in hardness and reduced modulus at the rod surface is found to be favoured in small diameter castings and in fragile systems, respectively resulting from limited in-situ annealing and from a greater diversity of metastable atomic environments in the potential energy landscape of fragile glasses. Enhanced propensity for shear transformation zone nucleation in the low moduli surface is explained in terms of reduced atomic connectivity arising from a reduction in local co-ordination number and a lowering of the shear modulus. Finally, the structure and mechanical diversity that is possible in as-cast bulk metallic glass rods is explored through a relative quantification of shear modulus and plastic zone size across the whole as-cast state and in a single rod. These findings illustrate the sensitivity of bulk metallic glass to preparation, especially in respect of thermal history, potentially making replication of mechanical data between researchers problematic.     

Amorphization from the quenched high-pressure phase of silicon and germanium

The phase transitions from the quenched high-pressure phase, including amorphous state, have been investigated for crystalline silicon and germanium at various pressures. X-ray diffraction patterns have been measured at pressures up to 15 GPa and temperatures down to 90 K by an energy-dispersive method using synchrotron radiation and a diamond anvil cell. The quenched β-Sn phase undergoes an amorphous phase transition when heated at 1.5 GPa for c-Si and 2.0 GPa for c-Ge. On the other hand, the quenched β-Sn phase transforms into a metastable crystalline phase when heated at higher pressures. The phase behavior is discussed in relation to the pressure dependence of the height of potential barrier between the β-Sb and amorphous phases and that between the β-Sn and metastable phases. The coordination number for the pressure-induced amorphous germanium, obtained through amorphization from the quenched high-pressure phase, is estimated to be about 4.     

A glass with high crack initiation load: Role of fictive temperature-independent mechanical properties

The crack initiation load of a series of calcium aluminosilicate glasses and selected commercial glasses were evaluated using Vickers indentation. The results showed that a calcium aluminosilicate glass containing 80 mol% SiO₂, 10 mol% Al₂O₃ and 10 mol% CaO exhibited a high crack initiation load comparable to that of the less-brittle glass (LB glass) developed by Asahi Glass Co., Ltd. It has previously been determined that glasses experience a fictive temperature increase by indentation. The indented region of a glass, therefore, acquires, in general, different mechanical properties, such as hardness and elastic moduli, from the original, unindented glass. The extent of these mechanical property changes depends upon the glass composition and a certain glass composition with fictive temperature-independent mechanical properties can have the deformed region with matching mechanical properties to those of the undeformed region of the glass. It was found that the calcium aluminosilicate glass having no fictive temperature dependence on elastic moduli gave the highest crack initiation load. However, this composition did not coincide with fictive temperature-independence of hardness or density.     

Consolidation and mechanical properties of Cu46Zr42Al7Y5 metallic glass by spark plasma sintering

Cu₄₆Zr₄₂Al₇Y₅ metallic glass with nearly 100% relative density was obtained by spark plasma sintering (SPS) with a diameter of 15 mm, which was larger than the largest size of 10 mm for the as-cast specimen. The fracture strength of the sintered specimen reached 2044 MPa, which was 15% higher than that of the as-cast Cu₄₆Zr₄₂Al₇Y₅ glassy specimen. The densification and compressive properties of the sintered specimens were related to sintering temperature. Structural changes of the specimens sintered at various sintering temperatures resulted in the difference of macro-mechanical properties.     

Comparison of glass formation by mechanical alloying and solid-state interdiffusion in Ni---Zr composites

The amorphization process in mechanically alloyed Ni---Zr powders has been investigated by optical microscopy, scanning and transmission electron microscopy, X-ray diffraction, differential scanning calorimetry and saturation magnetization measurements. Starting from elemental crystalline Ni and Zr powders, ball milling first produces a characteristically layered microstructure. Further milling leads to an ultrafine composite in which amorphization by solid-state reaction sets in between 4 and 16 h milling time. Longer milling results in fully homogeneous amorphous material. The obtained results corroborate the similarity of the amorphization process during mechanical alloying with the solid-state interdiffusion reaction in artificially modulated multilayer composites. In particular, mechanical alloying prevents intermetallic phase formation during the interdiffusion reaction because of extremely thin starting layers, thus resulting in a wider glass-forming range than obtained by other preparation techniques based on solid-state interdiffusion.     

Polymorphous transformation of quartz induced by mechanical treatment

Because of its strongly damaged structure (partially crystalline) mechanically activated α-quartz is not transformed to β-quartz. Rather, mechanically activated α-quartz is rapidly transformed to the cristobalite modification even at temperatures as low as 1200 °C. The acceleration and temperature shift of the quartz-cristobalite transformation is caused by mechanically induced defects. The activation energy of the transformation is thus reduced by about 50%.     

Micro-photoluminescence study on defects induced by ion microbeam in silica glass

We have evaluated the irradiation effects by ion microbeam on silica glass for various ion species by means of a micro-photoluminescence technique. Defect generation and refractive index change were observed for silica at the area of 10 μm × 50 μm scanned by ion microbeam of H⁺, He⁺, N⁴⁺, C⁴⁺, O⁴⁺, and Si⁵⁺ with energy from 1.7 to 18 MeV. The μ-PL spectroscopy measurements were performed along the side surface perpendicular to the microbeam irradiated surface. Based on the comparison with a result of SRIM (stopping and range of ions in matter) simulation, the defect generation mechanism was discussed in terms of the energy deposition processes due to electronic and nuclear stopping powers. We conclude that the electronic stopping power is responsible for the defect generation at the track of ions. The effect of the nuclear stopping power is also not negligibly small at the end of range.     

Crystallization of CoFeSiB metallic glass induced by long-time ball milling

Milling up to 800 h causes amorphous Co_70.3Fe_4.7Si₁₀B₁₅ alloy, prepared in the form of thin ribbon, to partially crystallize thus forming a powder material consisting of an amorphous phase and fcc-Co nanocrystals with an average grain size of about 10 nm. A gradual increase of the nanocrystalline fcc-Co fraction, produced by ball milling, was detected. Prolonged milling results in destabilization of the fcc-Co phase and oxidation of the powder material (presence of CoO phase after 1500 h of milling). The thermal stability studies of as-quenched and milled Co_70.3Fe_4.7Si₁₀B₁₅ alloy emphasized a two step crystallization behavior. During the first crystallization event, cobalt rich phases, i.e., fcc-Co and hcp-Co crystallize, whereas after the second crystallization event, Co₂B and Co₂Si are formed.     

Elemental redistribution in glass induced by a 250-kHz femtosecond laser

We report the elemental redistribution in a Bi-doped oxyfluoride glass induced by a 250-kHz femtosecond laser. Our results show that the relative concentration of the network modifier Bi ions in the modified region is higher than that in the unmodified regions. However, the relative concentration of the network modifier Ca ions is opposed to that of Bi ions. This is the first time to observe the differential distribution of relative concentration between the network modifiers. These results are important to practical application in the fabrication of waveguide lasers and amplifiers in active ion-doped glasses.     

Changes in surface morphology of silicate glass induced by fast electron irradiation

Binary potassium-silicate glass was irradiated with a defocused electron beam. During the irradiation the alkali ions migrate from the surface into the depth and alkali ions depleted layer is created near the surface. Such changes in the chemical composition are also accompanied with changes of the glass structure and finally result in the volume changes of the irradiated glass. This was directly studied using atomic force microscope (AFM). A series of exposures with energy of electrons of 7–50 keV and with different doses were performed. For low doses the irradiated glass is continuously depressed with the increasing dose, indicating this way the structural changes leading to the volume compaction. It is suggested the compaction is caused by the relaxation processes of the silica subnetwork. A further increase of the electron dose causes a formation of the small bump inside the center of the depression. The bump arises with the dose and finally exceeds the surrounding surface. It is suggested that the expansion is connected with the migration of alkali ions and the formation of Si–O–O–Si bonds which result in the formation of new rings with new space requirements.     

Amorphization induced by mechanical alloying and cold rolling

Solid state amorphous phase change is studied in the Ni---Ti system. It is found that the crystalline to amorphous transition can be obtained by the mechanical alloying, or by annealing of the multilayers produced by mechanical alloying and/or cold rolling. Different products are found after the annealing of the multilayers. The activation energies and interdiffusion coefficients are obtained for the solid state amorphous reaction.     

Appearance of fast ionic conduction in AgI-doped chalcogenide glass powders prepared by mechanical milling

(AgI)_x(As₂Se₃)_1−x glass powders were successfully obtained up to 70 mol% AgI content by the mechanical milling method. Electrical conductivities, which show the exponential increase with addition of AgI, are comparable to the values of the corresponding melt-quenched glasses. The conductivity of 60 mol% AgI-doped glass shows great increase at the early period of the milling. A DSC signal for the β–α phases transition of crystalline AgI becomes weak at the medium stage of the milling, whereas glass transition appears clearly. EXAFS measurements at Ag, I, As, and Se K-edges have been carried out for (AgI)_0.6(As₂Se₃)_0.4 glasses with different period of the milling. The amorphization process of (AgI)_0.6(As₂Se₃)_0.4 system was accompanied by a decrease of the nearest neighboring coordination number of Ag at the medium stage of the milling. These results suggest that the structural disordering of AgI units in the glass matrix would be strongly related to the appearance of fast ionic conduction.     

Elastic anomalous behavior of silica glass under high-pressure: In-situ Raman study

We study the changes in the Raman optical vibrations of pure silica glass under high-pressure up to 4.3 GPa and room temperature, namely in the elastic domain. Several mechanical anomalies, as the decrease of bulk modulus between 0 and 2 GPa, have been revealed many years ago (P.W. Bridgmann, Am. J. Sci 10 (1925) 359), but no physical experiments have explained what happens at the atomic scale. Our experiments show that gradual structural reversible rearrangement from 0 to 2 GPa leads to a more flexible material, in good agreement with molecular dynamics (MD) simulations (L. Huang, J. Kieffer, Phys. Rev. B 69 (2004) 224203). Above 2 GPa, a fast homogenization occurs.     

Amorphous phase separation and crystallization in a lithium silicate glass prepared by the sol-gel method

A 10Li₂O---90SiO₂ (mol%) gel-glass has been prepared by using tetramethyl orthosilicate and lithium iso-propoxide as starting materials. The phase separation and crystallization behaviour was compared with the corresponding conventionally melted glass using DTA, X-ray diffraction and TEM. The same crystallization phase was found in both the gel glass and melted glass upon heating above 650°C. However, the rate of crystallization in the gel-glass was higher than in the melted glass. TEM revealed amorphous phase separation in the gel glass and melted glass. However, the morphologies were different, an interconnected microstructure being observed in the gel glass and a droplet structure in the melted glass. These differences can be partly attributed to differences in OH content. Other potential influencing factors are also considered. After 650°C for 2 h lithium disilicate crystals were observed in the volume of the gel glass by TEM. As the crystals grew they absorbed Li₂O from the surrounding lithia-rich amorphous phase so that silica-rich (lithia depleted) diffusion zones formed around them.