Kunihiko Nakashima

Phase separation in germanate glasses

Abstract Applying the hot-thermocouple method, the TTT diagrams were determined by observing the time change of the phase separation and the crystallization in the supercooled liquids of SrOGeO2, CaOGeO2 and PbOGeO2 systems as examples. The nucleation-growth type phase-separation with a binodal curve between the liquidus and crystallization temperature is observed. Moreover, the growth mechanism of the nucleation-growth type phase separation was discussed in terms of the occupied volume fraction of particles. The results obtained are summarized as follows below. Therefore, in the initial stage of growth, an average radius r of particles changes with the square root of time ( r ∝ t 1 2 ) in every composition. But in the later stage of growth, an average radius r of particles growth with the cubic root of time ( r ∝ t 1 3 ) corresponding to Ostwald ripening in the range of the small volume fraction within about 20%. On the other hand, in the range of the high volume fraction above about 20%, an average radius r of particles grows faster in proportion to their volume fraction by the process of connect and coalescence. The results of our experiment in the range of the high volume fraction can be represented by Furukawas theory for a binary fluid mixture. One may presume that the growth mechanism of particles is divided in the range of small volume fraction and in the range of high volume fraction.

Structural Role of Alkali Cations in Calcium Aluminosilicate Glasses as Examined Using Oxygen-17 Solid-State Nuclear Magnetic Resonance Spectroscopy

The structural roles of alkali and calcium cations are important for understanding the physical and chemical properties of aluminosilicate melts and glasses. Recently, oxygen-17 nuclear magnetic resonance (17O NMR) studies of calcium–sodium aluminosilicate glasses showed that these structural roles are not randomly given, but rather each cation has its own preferential role. However, the relationship between cation type and role preference in calcium aluminosilicate glass is not completely understood. In the present study, the structural roles of lithium, sodium, and potassium cations in selected calcium aluminosilicate glasses are investigated using 17O solid-state NMR experiments. Data from these experiments clearly show that potassium cations have a notably stronger tendency to act as charge compensators within the network structure, compared to sodium and lithium cations. The result of 17O NMR experiment also showed that sodium and lithium cations in part act as network modifier alongside with calcium cations.

Effect of additive oxides on the viscosities of CaO-SiO2-Al2O3 and CaO-Fe2O3 melts

The effects of MgO, TiO 2 or Fe 2 O 3 on the viscosity of 40CaO-40SiO 2 -20Al 2 O 3 /mass% slags have been measured by a rotating crucible viscometer. The viscosity of these quaternary slags decreased with increasing the content of additive oxide. At the same content of additive oxide, the viscosity decreases from MgO, TiO 2 to Fe 2 O 3 . In addition, the effects of SiO 2 , Al 2 O 3 or MgO on the viscosity of 26.1CaO-73.9Fe 2 O 3 / mass% (CF) and 14.9CaO-85.1Fe 2 O 3 / mass% (CF 2 ) slags have been measured. Viscosity of calcium ferrite slags increased with increasing SiO 2 , Al 2 O 3 or MgO content. Al 2 O 3 was found to be more effective for increasing the viscosity at the same content of additive oxide.

Oxygen Speciation in Multicomponent Silicate Glasses Using Through Bond Double Resonance NMR Spectroscopy

The description of the structure of aluminosilicate glasses is more often centered on its cationic constituents, and oxygen ions determine their connectivity, directly impacting the physical properties of those disordered materials. A very powerful approach to ascertain this short- to medium-range order is to use 17O NMR, but up to now the speciation of the chemical bonds was only ambiguously achieved for multicomponent glasses. Here, we propose to directly probe the very scarcely explored through-bond correlations using 17O{27Al} and 17O{23Na} solid-state nuclear magnetic resonance (NMR) double-resonance experiments. Our approach allows quantifying the strongly overlapping components of the 17O NMR spectra of a quaternary aluminosilicate glass. We observe a cooperative location of alkali and aluminum ions in the neighborhood of bridging oxygens, which is consistent with the modified random network model where the glass structure is composed of two regions: network structure and breakage region (i.e., channel).

Capacitance measurement of molten calcium silicate under shear stress field

Abstract The effect of shear stress on the crystallization behavior of molten 50CaO-50SiO2 (mol%) slag was investigated by in-situ measurements of its electrical capacitance in wide temperature range including supercooled region. It is well known that the electrical capacitance of liquids should be generally much higher than that of solids because of the differences in their respective polarization mechanisms. The difference was employed as a sensitive indicator of the crystallization of molten silicates in an experimental furnace equipped with an electrical-capacitance measurement system. The system comprised a Pt-based alloy crucible and a rotating rod that allow us to evaluate the effect of shear stress, both connected to a capacitance meter (LCR meter). As expected, at a particular temperature, the electrical capacitance of the molten calcium silicate abruptly decreased by roughly three orders of magnitude, which clearly indicated crystallization confirmed by corresponding microstructural analyses. It was also found that, for the rotating-rod measurements (with shear stress), the temperatures at which the capacitance abruptly dropped were higher than that without the shear stress. This suggests that the agitation effect by the rotating-rod accelerates the crystallization of molten calcium silicate.

Advances in Transient-Liquid-Phase Bonding of Ultra-high Temperature ZrC Ceramics

Abstract Full exploitation of the many attractive engineering properties of ultra-high temperature ceramics (UHTCs) requires that they can be joined. This paper explores progress in identifying joining strategies based on the use of transient liquid phases (TLPs). Wetting studies are used to explore the suitability of specific liquids for joining, while bonding studies provide the ultimate test. Sintering aids in the UHTC provide a major potential obstacle to successful joining, and dissolved impurities in the TLP can also complicate the joining process. Nonetheless, we show that well-bonding interfaces can be achieved when ZrC ceramics are bonded at 1673 K using a Ni/Nb/Ni multilayer interlayer.

Viscosity Estimation of Spherical Particles Dispersed Suspension

The viscosity measurement has been made for suspensions consisting of polyethylene beads and silicone oil matrix as a function of mean diameter of beads and shear rate. The relative viscosity is found to increase with increasing the volume fraction of beads and decrease with the mean diameter of beads and shear rate. We also tested several viscosity equations to reproduce the viscosity values of spherical particle dispersed suspension. However, any equation could not reproduce well the experimental viscosity values of suspension measured under various conditions. Some modification is needed by considering some experimental conditions (volume fraction, mean diameter of beads and shear rate). Then, the modified Einstein–Roscoe equation is recommended for reproducing the viscosity values of suspension under various conditions.

Wettability of Cu-Based Alloys on Alumina and Joining of Alumina with Microdesigned Nickel-Chromium Alloy Interlayer

Monolithic ceramic materials are being widely developed for a variety of applications ranging from use as structural components to high-performance electronic substrates. Recently, however, major efforts have been made to use ceramics in structural applications under severe operating conditions. Joining of ceramics to ceramics and to metals is considered one of the key technologies that will either expand or restrict the potential use of ceramic materials in a wide range of applications, such as heat engines, heat exchangers and recuperators.

Rate of Oxidation of Pb, Ni in Liquid Copper by Molten Slags

Removal behavior of Pb, Ni from liquid copper by Cu2O-Fe2O3 based slag with one or two additive oxides among CaO, B2O3, SiO2, P2O5 and TiO2 was investigated in the temperature range from 1,200 to 1,300 °C. The results obtained are as follows,1) The Pb removal was largely increased by the addition of acidic oxide, but the Ni removal was at most 50 % for all slag compositions investigated.2) The addition of P2O5 and TiO2 was obtained nearly same good results as the addition of B2O3. Thus, P2O5 and TiO2 was found to be practically used as a substitute for B2O3.3) The Ni removal was enhanced by increasing temperature, which could be explained to the decrease of activity coefficient of NiO. However, The Pb removal was retarded by increasing temperature.4) The kinetic behavior of Pb and Ni removal by oxidizing slags could be well simulated by the reaction model based on the two film theory.

TTT (TIME-TEMPERATURE-TRANSFORMATION) DIAGRAM OF THE SrO-GeO//2 SYSTEM.

TTT diagram is necessary in glass forming systems to understand various phenomena such as phase separation and crystallization in supercooled liquid. A hot-thermocouple technique to establish the TTT diagram of glass forming systems has been proposed, which enables heating, temperature measurement, DTA and observation the state of the sample. Some TTT diagrams in the SrO-GeO2 system have been determined by this method. In this system phase separation of nucleation-growth type was observed at high temperature in supercooled state. From the TTT diagram various cooling conditions can be determined for obtaining transparent glass and phase separated glasses of nucleation-growth, spinodal decomposition and crystallization types. It is concluded that the TTT diagram is important to develop “new glassy materials”.