Reinhard Conradt

Isotropic shrinkage of platelet containing glass powder compacts during isothermal sintering

Abstract The effect of rigid, i.e., non-densifying, inclusions on the densification kinetics during sintering of composite glass powder compacts was investigated experimentally. A soda-lime glass containing Al 2 O 3 -platelets as a rigid inclusion phase was considered. Data on axial and radial shrinkage during sintering were obtained by using heating microscopy, which allows in situ monitoring of the densification process. The presence of rigid platelets retarded the densification of the compacts. The sintering behavior of all samples was isotropic. By comparison with previous results on anisotropic shrinkage of glass composite powder compacts, it is suggested that shrinkage isotropy in the present samples is a result of the isotropic microstructure of the green bodies, which were prepared by isostatic compaction.

Investigations on the role of surface layers in HLW glass leaching

A corrosion test series was performed to clarify the role of reaction product layers on the corrosion of a simulated HLW borosilicate glass in a salt brine under hydrothermal conditions. The layers were unprotective at 200/sup 0/C. At 120/sup 0/C, slight protective effects occurred when the leachant contained dissolved reaction products. The consequences for the long term behavior between 120 and 200/sup 0/C is a constant glass dissolution rate.

Thermodynamics of Glass Melting

First, a model based on linear algebra is described by which the thermodynamic properties of industrial multi-component glasses and glass melts can be accurately predicted from their chemical composition. The model is applied to calculate the heat content of glass melts at high temperatures, the standard heat of formation of glasses from the elements, and the vapor pressures of individual oxides above the melt. An E-fiber glass composition is depicted as an example. Second, the role of individual raw materials in the melting process of E-glass is addressed, with a special focus on the decomposition kinetics and energetic situation of alkaline earth carriers. Finally, the heat of the batch-to-melt conversion is calculated. A simplified reaction path model comprising heat turnover, content of residual solid matter, and an approach to batch viscosity is outlined.

Determination of the Corrosion Mechanisms of High-Level Waste Containing Glass

The German concept of high level waste final storage provides the use of certain glasses containing radioelement oxides as glass components. These waste forms are to be stored in rock salt formations in order to isolate the waste from the biosphere. The efficiency of this isolation is a most important question. The aim is to achieve a high safety standard that remains valid under extreme conditions such as the uncontrolled water entrance to the deposit.

Experimental and Theoretical Investigation of the Elastic Moduli of Silicate Glasses and Crystals

A combined quantum-mechanical and thermodynamic approach to the mechanical properties of multicomponent silicate glasses is presented. Quantum chemical calculations based on density-functional theory (DFT) on various silicate systems were performed to explore the crystalline polymorphs existing for a given chemical composition. These calculations reproduced the properties of known polymorphs even in systems with extensive polymorphism, like MgSiO3. Properties resting on the atomic and electronic structure, i.e., molar volumes (densities) and bulk moduli were predicted correctly. The theoretical data (molar equilibrium volumes, bulk moduli) were then used to complement the available experimental data. In a phenomenological evaluation, experimental data of bulk moduli, a macroscopic property resting on phononic structure, were found to linearly scale with the ratios of atomic space demand to actual molar volume in a universal way. Silicates ranging from high-pressure polymorphs to glasses were represented by a single master line. This suggests that above the Debye limit (in practice: above room temperature), the elastic waves probe the short range order coordination polyhedra and their next-neighbor linkage only, while the presence or absence of an extended translational symmetry is irrelevant. As a result, glasses can be treated – with respect to the properties investigated – as commensurable members of polymorphic series. Binary glasses fit the very same line as their one-component end-members, again both in the crystalline and glassy state. Finally, it is shown that the macroscopic properties of multicomponent glasses also are linear superpositions of the properties of their constitutional phases (as determined from phase diagrams or by thermochemical calculations) taken in their respective glassy states. This is verified experimentally for heat capacities and Young’s moduli of industrial glass compositions. It can be concluded, that the combined quantum mechanical and thermochemical approach is a truly quantitative approach for the design of glasses with desired mechanical properties, e.g., for the development of high-modulus glasses.

Chemical-Mechanical Polishing of Optical Glasses

This paper presents the Chemical-Mechanical Polishing of optical and fine-optical glasses, which is employed to fulfill the optical requirements for the surfaces of optical lenses. We present the effect of chemical interactions in the polishing process of optical lenses and show how these interactions can be influenced by the additions of certain chemicals. Furthermore, we present a thermodynamic simulation tool, by which these interactions can be modeled. Thus it is possible to understand the chemistry in the suspension and to simulate recent polishing processes in advance, with new additives or with various intrinsic glass ions from different glasses.

Partition Function and Configurational Entropy in Non-Equilibrium States: A New Theoretical Model

A new model of non-equilibrium thermodynamic states has been investigated on the basis of the fact that all thermodynamic variables can be derived from partition functions. We have thus attempted to define partition functions for non-equilibrium conditions by introducing the concept of pseudo-temperature distributions. These pseudo-temperatures are configurational in origin and distinct from kinetic (phonon) temperatures because they refer to the particular fragments of the system with specific energies. This definition allows thermodynamic states to be described either for equilibrium or non-equilibrium conditions. In addition; a new formulation of an extended canonical partition function; internal energy and entropy are derived from this new temperature definition. With this new model; computational experiments are performed on simple non-interacting systems to investigate cooling and two distinct relaxational effects in terms of the time profiles of the partition function; internal energy and configurational entropy.

Selective laser melting of glass powders

Glass material is extensively utilized in various industrial fields due to its unique properties such as high melting temperature, transparency, as well as high compression strength; however, conventional glass production processes are not optimum for freeform fabrication applications. As such, fabrication of complex geometries in a time and cost effective manner is not currently realizable. Nevertheless, existing fabrication methods and corresponding constraints may be augmented by Additive Manufacturing (AM) techniques in which free form geometries are fabricated based on Computer Aided Design (CAD) data in a layer-wise manner. With respect to this, this study investigates the feasibility of Selective Laser Melting (SLM) of soda-lime and borosilicate glass powder followed by fabrication of test geometries for industrial applications ranging from micrometer to decimeter in size. Initially, SLM process parameter and scan strategy investigations and optimization for different powder particles sizes of soda-lime glass are carried out. SLM fabricated parts are analyzed regarding their bulk density and surface roughness. Furthermore, systematic SLM process parameter optimization both analytically and experimentally, powder rheology, and analysis of the sintering behavior of the borosilicate glass under various atmospheric conditions with the aim of evaluating the density and surface roughness of the SLM processed parts are among the main objectives of this study. Regarding this, process parameter optimization as well as scan strategy adaptation are carried out using different powder particles. Furthermore, powder rheology is analyzed using Hausner Ratio (HR) and dynamic angle of repose (avalanche angle) measurements. With respect to this, bulk and tapped densities as well as the avalanche angle of powders with different particle sizes, distributions, and shapes are studied and classified regarding their powder flow regimes based on the obtained experimental layer-wise lamination results. Moreover, using Hot Stage Microscopy (HSM) viscosity points of different powder particles sizes and distributions are studied and applied for developing an analytical model which is validated by the systematic process parameter SLM study as well as on-site thermal process monitoring. Furthermore, SLM optimum process parameters are applied under different working atmospheres with the aim of studying the surface morphology of the SLM fabricated parts. Finally powder bed’s bulk density is optimized by polydisperse powder mixture preparation while keeping the powders’ lamination quality in the desirable flow regime. Results have shown, although developed optimum scan strategy can be applied for a wide range of powder particle sizes, process parameters need to be optimized for each powder individually. The SLM fabricated parts led to an amorphous structure (within the conducted measurement range of 20 ° ≤ 2θ ≤ 160 ° using CrKα radiation) regardless of the powder particle size. SLM fabricated samples from soda-lime glass powders yield a maximum density of 2.43 g/cm3 and a surface roughness (Ra) of 0.88 μm using a 60 W laser power, 0.067 m/s scan speed, 50 μm hatch spacing and a 150 μm layer thickness. Regarding SLM of borosilicate material, optimum flow lamination quality is achieved by creating angular shape monodisperse powders of 200 μm – 212 μm with a HR of 1.21 and an average avalanche angle of approximately 39 °. However, comparison of the flow dynamics of the rounded edge particles to the angular shape particles concluded that the SLM lamination quality could be further improved by using spherical powders. Using optimum process parameters in which the powder bed reaches the viscosity of the half ball point temperature, parts with a bulk density of 1.86 g/cm3 and a surface roughness (Ra) of 10.75 μm are fabricated. Final part’s bulk density of 2.13 g/cm3 is achieved under synthetic air using the volumetric mixed bidisperse particles of 63 μm – 90 μm and 200 μm – 212 μm powders in a ratio of 42 % and 58 % respectively. The surface roughness (Ra) of the fabricated part is improved from 2 μm to 0.9 μm using the CO2 laser polishing process. For demonstration purposes, complex glass geometries have been manufactured, promoting the possibilities of manufacturing custom, complex components via the SLM process.

Barium-Free Glass-Ceramic Sealants from the System CaO-MgO-B2O3-Al2O3-SiO2 for Application in the SOFC

The planar solid oxide fuel cell (p-SOFC) is a promising configuration of a high-T fuel cell. Barium alumosilicate glass ceramics are suggested to use as sealants by many authors since these materials seem to meet the requirements to establish a hermetic and electrically insulating seal between the steel components of the SOFC. However, in long-term application, the formation of BaCrO4 may degrade the interfacial strength between glass-ceramics and metallic interconnect and disrupt the cell components. In this work, a series of barium-free glass-ceramics in the system of CaO-MgO-B2O3-Al2O3-SiO2 were prepared. The selected compositions were located within the constitutional range of åkermanite – forsterite –anorthite. The thermal properties including glass transition temperature (Tg) and crystallization temperature (Tc) of the produced glasses were measured. The effect of boron oxide additions was studied in order to optimize the softening and flow behavior. For the experiments, a hot – stage microscope was used. Some glass compositions were mixed with high – CTE akermanite powder in order to increase their CTE after sintering. The CTE of the investigated materials after sintering at 900 °C, 2 h, ranged from 10.5 to 11.8 10−6 K−1.

Condensation during Flue Gas Cleaning

Nowadays, it is an operational routine to clean the flue gases from glass furnaces to remove noxious air pollutant substances. The established processes are highly effective, however there are failures caused by the condensation of aggressive fluxes inside the large and expensive flue gas filters. To this day, the process control of the flue gas purification equipment is based on empirical data and on experience. Currently available data do not allow to base process design and control on a solid thermodynamic and kinetic foundation. It is the aim of this work to find out under which conditions condensation really takes place. Therefore, first results of collected kinetic data on the condensation from the system Na2O-H2O-SO2-O2-CO2 are shown to estimate the dew points of liquid and solid condensates as a function of flue gas composition and temperature. The measurements are performed in a self-constructed condensation tube. Investigations are particularly focused on the formation of pyrosulfates and bisulfates.