Dieter Hofmann

Knowledge‐Based Approach towards Hydrolytic Degradation of Polymer‐Based Biomaterials

The concept of hydrolytically degradable biomaterials was developed to enable the design of temporary implants that substitute or fulfill a certain function as long as required to support (wound) healing processes or to control the release of drugs. Examples are surgical implants, e.g., sutures, or implantable drug depots for treatment of cancer. In both cases degradability can help to avoid a second surgical procedure for explanation. Although degradable surgical sutures are established in the clinical practice for more than 30 years, still more than 40% of surgical sutures applied in clinics today are nondegradable.1 A major limitation of the established degradable suture materials is the fact that their degradation behavior cannot reliably be predicted by applying existing experimental methodologies. Similar concerns also apply to other degradable implants. Therefore, a knowledge-based approach is clearly needed to overcome the described problems and to enable the tailored design of biodegradable polymer materials. In this Progress Report we describe two methods (as examples for tools for this fundamental approach): molecular modeling combining atomistic bulk interface models with quantum chemical studies and experimental investigations of macromolecule degradation in monolayers on Langmuir-Blodgett (LB) troughs. Finally, an outlook on related future research strategies is provided.

On the sublimation growth of SiC bulk crystals: development of a numerical process model

Abstract The development of a numerical process model to simulate the sublimation growth of SiC bulk crystals is discussed. Radiation, conduction and convection are considered as heat transfer mechanisms. Mass transport by diffusion and convection is taken into account. First results on the simulation of heat and mass transfer in a 2 inch SiC growth set-up show a negligible effect of convection on process conditions. Chemical reactions in the SiC-graphite system have also been implemented into our model. Preliminary analysis on the dependence of concentration fields and growth velocity on possible chemical reaction mechanisms, e.g. graphitization, reveal that the incorporation of chemical processes into modelling is very important for an accurate description of SiC sublimation growth.

SiC-bulk growth by physical-vapor transport and its global modelling

4H- and 6H-SiC bulk crystals have been prepared by physical vapor transport (PVT) both in resistively and inductively heated growth reactors. Epitaxial SiC layers were grown on the wafers by chemical vapor deposition. Structural and electrical material properties of the 1–1.4 inch boules and epitaxial layers were investigated by defect etching and optical microscopy, stress birefringence and Hall effect. Single crystalline material exhibits a low micropipe density MPD ≈ 70 cm−2 and stress level. Blocking characteristics of the epitaxial layers have been determined electrically revealing high breakdown fields of 1.8–1.9 MV/cm. Finally simulation results applying a process model of SiC PVT crystallization including heat and mass transfer and chemical reactions are presented.

Global numerical simulation of heat and mass transfer for SiC bulk crystal growth by PVT

Abstract A modeling approach for the numerical simulation of heat and mass transfer during SiC sublimation growth in inductively heated physical vapor transport (PVT) reactors is introduced. The physical model is based on the two-dimensional solution of the coupled differential equations describing mass conservation, momentum conservation, conjugate heat transfer including surface to surface radiation, multicomponent chemical species mass transfer and advective flow. The model also includes the Joule volume heat sources induced by the electromagnetic field. The evolution of the temperature profiles inside the crucible and of the crystallization front is studied. The radial temperature gradient at the crystal/gas interface causes strong radial non-uniformity of the growth rate and, in turn, influences the shape of the growing crystal. Results of calculations are compared to experimental observations to analyse the validity of the modeling approach. Both the computed growth rates, their temporal evolution and the shape of the growing crystal agree with experimental data.

Analysis on defect generation during the SiC bulk growth process

Abstract SiC crystals (1.2–1.5′′ diameter) were grown by the modified Lely technique on seeds with different micropipe densities in order to study the defect generation during seeding and subsequent bulk growth. The micropipe generation is found to be strongly correlated with the occurrence of second phases in SiC like carbon inclusion formation. Model approaches for stable SiC growth conditions, i.e. without inclusions, are discussed. Numerical modeling was performed to reveal the radial and axial temperature gradients of our crucible set-up. Stress formation and micropipe generation are determined to be enhanced in the presence of a large axial temperature gradient.

Determination of charge carrier concentration in n- and p-doped SiC based on optical absorption measurements

We have investigated the effect of doping on absorption for various SiC polytypes, i.e., n-type (N) 6H–SiC, 4H–SiC, and 15R–SiC, p-type (Al) 6H–SiC, and 4H–SiC, and p-type (B) 6H–SiC. For these polytypes the band-gap narrowing with higher doping concentration is observed. In addition, for n-type doping below band-gap absorption bands at 464 nm for 4H–SiC, at 623 nm for 6H–SiC, and at 422 and 734 nm for 15R–SiC are observed. The peak intensities of these absorption bands show a linear relation to the charge carrier concentration obtained from Hall measurements. The corresponding calibration factors are given. As an application a purely optical wafer mapping of the spatial variation of the charge carrier concentration is demonstrated.

On the mechanisms of micropipe and macrodefect transformation in SiC during liquid phase treatment

The evolution of hollow-core defects present in vapor phase grown SiC bulk crystals during subsequent liquid phase epitaxial treatment has been investigated in a wide range of supersaturation conditions. Hollow macrodefects were found to decompose into a number of micropipes (MP) already at supersaturations close to zero. The elimination of pure screw-dislocation based MP requires a higher supersaturation. Micropipes were observed to dissociate into individually acting non-hollow core dislocations. After decomposition the activity of growth center based on a MP is usually reduced and a new center may dominate the growing surface. A model for the mechanism of MP transformation is proposed which is based on BCF theory and Chernovs theory of morphological stability.

Growth of 2 inch Ge: Ga crystals by the dynamical vertical gradient freeze process and its numerical modelling including transient segregation

Abstract A process model of the dynamical vertical gradient freeze technique for heat transfer and segregation is introduced. Model predictions for favorable growth conditions are verified in a new specially designed multizone furnace by 2 inch Ge: Ga crystal growth experiments. The controlled establishment of a convex solid-liquid interface shape is demonstrated. Measured dopant distribution shows a good correlation with the results of the segregation model.

Knowledge-Based Tailoring of Gelatin-Based Materials by Functionalization with Tyrosine-Derived Groups

Molecular models of gelatin-based materials formed the basis for the knowledge-based design of a physically cross-linked polymer system. The computational models with 25 wt.-% water content were validated by comparison of the calculated structural properties with experimental data and were then used as predictive tools to study chain organization, cross-link formation, and estimation of mechanical properties. The introduced tyrosine-derived side groups, desaminotyrosine (DAT) and desaminotyrosyl tyrosine (DATT), led to the reduction of the residual helical conformation and to the formation of physical net-points by π-π interactions and hydrogen bonds. At 25 wt.-% water content, the simulated and experimentally determined mechanical properties were in the same order of magnitude. The degree of swelling in water decreased with increasing the number of inserted aromatic functions, while Youngs modulus, elongation at break, and maximum tensile strength increased.

Global modeling of the SiC sublimation growth process: prediction of thermoelastic stress and control of growth conditions

Abstract The current status of the mathematical model for heat and mass transfer during SiC bulk crystal growth from the vapor phase in inductively heated reactors is reviewed. Results on the simulation of thermoelastic stresses during the growth process are presented. Stresses have been analyzed to exceed considerably the critical resolved shear stress σ CRS =1 MPa which is generally assumed to be the indicator for the onset of dislocation formation in SiC. It is shown that the conditions for stress formation at fixed positions in the crystal vary considerably during growth and that geometric modifications can contribute significantly to a reduction of the stress level. The possible impact of semitransparency of SiC on additional stress generation is discussed. As effective tool for process control and optimization an inverse modeling procedure is introduced.