Die Fließgrenze bei behinderter Formänderung

Ohne Zusammenfassung Mitteilung aus der Staatlichen Materialprüfungsanstalt an der Techn. Hochschule Darmstadt Der Notgemeinschaft der Deutschen Wissenschaft sei an dieser Stelle für die Unterstützung der Versuche gedankt.     

High-Resolution Electron Microscopy and Convergent-Beam Electron Diffraction of Sintered Undoped Hydroxyapatite

Transmission electron microscopy (TEM) was used to characterize the microstructure of sintered undoped hydroxyapatite (OHAp). Conventional TEM observations were accom-panied by high-resolution electron microscopy (HREM) and convergent-beam electron diffraction (CBED) studies. CBED analysis enabled the determination of the space group of the OHAp, P6₃/m. Densification of the material was performed by pressureless sintering at 1250°C for 30 min. The undoped sample was comprised of apatite as the only crystalline phase, in addition to a small volume fraction (4%) of closed porosity. In general, the observed microstructure was homogeneous and consisted of equiaxed apatite grains with an average grain diameter of }1–2μm. No indication of a residual amorphous phase, which may have formed during sintering, was observed at multigrain junctions. HREM studies on grain boundaries also revealed that no intergranular glass film was present along two-grain junctions, indicating that densification proceeded without a liquid phase. A slightly disordered region at the interfaces was observed, suggesting an extended grain-boundary structure.     

Architecture Concepts for Multimedia Signal Processing

Architectural concepts are presented aimed at future multimedia processing schemes. Starting from an analysis of current and future multimedia applications, specific computational requirements are derived. It will be shown that multimedia applications benefit from an exhaustive and flexible exploitation of parallelism. Three architectural concepts—reconfigurable computing, simultaneous multithreading, and associative controlling—are presented, and their potential to increase further the performance on future multimedia applications is investigated.     

Functionalized gold nanoparticles: a detailed in vivo multimodal microscopic brain distribution study

In the present study, the in vivo distribution of polyelectrolyte multilayer coated gold nanoparticles is shown, starting from the living animal down to cellular level. The coating was designed with functional moieties to serve as a potential nano drug for prion disease. With near infrared time-domain imaging we followed the biodistribution in mice up to 7 days after intravenous injection of the nanoparticles. The peak concentration in the head of mice was detected between 19 and 24 h. The precise particle distribution in the brain was studied ex vivo by X-ray microtomography, confocal laser and fluorescence microscopy. We found that the particles mainly accumulate in the hippocampus, thalamus, hypothalamus, and the cerebral cortex.     

Inhibition of structural carbohydrate fermentation by forage phenolics

The benzoic and cinnamic acids found in forages were tested for their inhibitory effects on digestion of cellulose and hemicellulose by mixed cultures of rumen microorganisms in vitro. Rate and extent of cellulose digestion with the addition of cinnamic acids were determined. The benzoic acids and aldehydes were also compared for their inhibitory potential. Of the benzoic acid derivatives, only the parent compound (benzoic acid) inhibited in vitro dry matter disappearance (IVDMD) of cellulose after 48 h, whereas most of the cinnamic acids were inhibitory. None of these compounds depressed IVDMD of hemicellulose. Extent of cellulose digestion was unaffected by cinnamic acids, but rate of digestion was significantly reduced. The ranking of compounds for inhibitory potential was essentially reversed between 48 and 168 h of fermentation as compared with the ranking for 0 to 48 h of fermentation. There were also shifts in the molar proportions of volatile fatty acids produced between these times. The rapid reduction in concentration of cinnamic acids by microbial fermentation may explain these shifts. Benzoic aldehydes were more inhibitory of structural carbohydrate fermentation than were acids. No clear relationship existed between addition of hydroxyl or methoxyl groups to benzoic or cinnamic acids and their depressing effects on IVDMD. Cinnamic acids, which are more abundant in forages, were more inhibitory than the benzoic acids; but for all compounds, it appeared that hemicellulolytic bacteria were generally more tolerant of these compounds than the cellulolytic bacteria.     

Phase separation of a hafnium alkoxide-modified polysilazane upon polymer-to-ceramic transformation—A case study

The polymer-to-ceramic transformation of a hafnium alkoxide-modified polysilazane was investigated via thermogravimetric analysis coupled with in situ mass spectrometry (TG/MS), nuclear magnetic resonance (MAS NMR) and transmission electron microscopy (TEM). The results indicate that the structural evolution of the polysilazane upon ceramization is strongly affected by the modification with hafnium alkoxide. Thus, the content of carbon in the ceramic backbone was relatively low, whereas a large amount of SiN₄ sites and a segregated carbon phase was present in the sample. Furthermore, this study revealed the formation of a SiHfCNO amorphous single phase ceramic via pyrolysis of the polymer at 700 °C, whereas at higher pyrolysis temperatures precipitation of hafnia was observed, leading to an amorphous hafnia/silicon carbonitride ceramic nanocomposite. The precipitation of hafnia was shown to not rely on decomposition processes, but to be a result of rearrangement reactions occurring within the ceramic material.     

Kinetics and dynamics of thermally-induced shape-memory behavior of crosslinked short-chain branched polyethylenes

Systematic investigation of the relation between shape-memory (SM) behavior and characteristics of the covalent network and the crystalline domains of a crosslinked polymer, i.e., crosslink density and crystallinity, respectively, was performed using homogeneous ethylene-1-octenecopolymers (EOC) as model polymers. The EOCs have been crosslinked by 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (DHBP) decomposition. Two EOCs with a degree of branching of 30 and 60 hexyl side chains per 1000C atoms with each four different crosslink densities were employed. The investigated EOCs differ significantly in crystallinity, melting temperature (T_m) and crosslink density. The crosslinked EOC undergone the programming at a strain of 100% showed high strain fixity ratio (R_f) and strain recovery ratio (R_r). The R_f and R_r values increase with increasing crystallinity and crosslink density, respectively, and decline only slightly in a subsequent SM cycle. The switching temperature (T_sw) is strictly related to T_m and decreases with increasing degree of branching as well as crosslink density in the temperature range of 101–63 °C. T_sw remains nearly unchanged when the programming temperature (T_pr) or the load during SM recovery is varied. The kinetics of SM recovery as characterized by the temperature dependence of recovery rate is controlled by the melting behavior. The specific work generated by the programmed specimen during thermally-induced recovery under constant load, gains with increasing crosslink density, and is proposed as dynamical characteristic of practical relevance. The opportunity of tailoring T_sw by variation of the degree of branching and crosslink density makes such polymers attractive candidates for applications requiring T_sw temperatures in the range from 60 to 100 °C.     

Kinetics of nucleation and crystallization in poly(?-caprolactone) (PCL)

The recently developed differential fast scanning calorimetry (DFSC) is used for a new look at the crystal growth of poly(?-caprolactone) (PCL) from 185 K, below the glass transition temperature, to 330 K, close to the equilibrium melting temperature. The DFSC allows temperature control of the sample and determination of its heat capacity using heating rates from 50 to 50,000 K/s. The crystal nucleation and crystallization halftimes were determined simultaneously. The obtained halftimes cover a range from 3 × 10⁻² s (nucleation at 215 K) to 3 × 10⁹ s (crystallization at 185 K). After attempting to analyze the experiments with the classical nucleation and growth model, developed for systems consisting of small molecules, a new methodology is described which addresses the specific problems of crystallization of flexible linear macromolecules. The key problems which are attempted to be resolved concern the differences between the structures of the various entities identified and their specific role in the mechanism of growth. The structures range from configurations having practically unmeasurable latent heats of ordering (nuclei) to being clearly-recognizable, ordered species with rather sharp disordering endotherms in the temperature range from the glass transition to equilibrium melting for increasingly perfect and larger crystals. The mechanisms and kinetics of growth involve also a detailed understanding of the interaction with the surrounding rigid-amorphous fraction (RAF) in dependence of crystal size and perfection.