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Eingeladener Vortrag
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COMPOSITE MATERIALS OF A 99.7 % ALPHA-ALUMINA MATRIX WITH PLATELETS OF SILICON CARBIDE (SICP) HAVE BEEN PREPARED BY GELCASTING. DENSIFICATION WAS CARRIED OUT BY GAS-PRESSURE SINTERING WITHOUT FURTHER SINTERING AIDS. FIVE DIFFERENT COMPOSITIONS WITH 0, 5, 10, 15, AND 20 VOL.% SICP RESP. HAVE BEEN INVESTIGATED AND TRIBOLOGICAL PROPERTIES WERE CHARACTERIZED. INCREASING AMOUNT OF SICP AS RIGID PARTICLES IN THE FINE GRAINED MATRIX COMPLICATE THE DENSIFICATION PROCESS. WITH THE APPLICATION OF 50 BAR AR PRESSURE DURING THE FINAL SINTERING STEP AT A TEMPERATURE OF 1700 DEGREE C THE DENSITY OF THE SAMPLES COULD BE INCREASED BY 3-5%. INVESTIGATION OF THE TRIBOLOGICAL PROPERTIES AGAINST STEEL AS A COUNTERBODY UNDER UNLUBRICATED OSCILLATING SLIDING CONDITIONS SHOWED A HIGH COEFFICIENT OF FRICTION AND AN INCREASING WEAR RATE UP TO A SICP CONTENT OF 10 VOL. % . AT 15 AND 20 VOL. % SICP RESP. BOTH THE COEFFICIENT OF FRICTION AND THE COEFFICIENT OF WEAR DROP DOWN.
Influence of Slip Viscosity on the Mechanical Properties of High Purity Alumina made by Gelcasting
(1997)
The gelcasting process was used for manufacturing porous ceramic bodies from nano sized powders. 3 different powders with specific surface areas between 12 and 30 m²/g were used. Solid content of the corresponding slurries was dependent on the milling procedure and the use of a suitable dispersant. For bimodal GradeA powder solid content was as high as 58 vol%. For sintering temperatures between 1250 and 1600°C the porosity varied between 45 and 20%, whereas the pore radii varied between 50 and 91nm. The grain size of crystallites in the sintered microstructures was about 1um. The reflection ratio for visible light increased in comparison to a reference sample made from conventional powder from 0.8 to 0.95 in the wavelenght range around 400nm.
Keramik-Keramik Verbindungen mit hoher Festigkeit durch Verschweissen von Al2O3 Grünkörperteilen
(2005)
The infrared-optical properties and particularly the emissivity of sintered ceramics depend on the material's bulk properties and structural properties such as porosity, pore size distribution and pore diameter. The infrared-optical properties can be varied across a wide range by specifically selecting the material's bulk and structural properties. A major concern of the present study is the development of a ceramic coating material with a total emissivity below 0.2 at a temperature of 800 °C. For this purpose, various samples of a TiO2-Y2O3 (TY) ceramic composite were produced with different properties. On variation in the Y2O3 content from 5 to 60 wt%, the porosity of the ceramic composites varies from 3 % to 47 % with the mean pore diameter lying in a range from 0.3 µm to 1.7 µm. The addition of graphitic pore builders increases the porosity up to 60 % and the mean pore diameter to a maximum of 15 µm. By optimizing the total porosity and the pore size of the TY ceramics, a total emissivity as low as 0.17 at 800 °C could be achieved.
Die Dispergiereigenschaften von verschiedenen nanoskaligen Pulvern mit unterschiedlicher Kornfraktion im Bereich von 10 bis 250 nm werden in verdünnten und konzentrierten keramischen Schlickern mit der Ultraschallspektroskopie untersucht. Alle stabilen Suspensionen weisen im entsprechenden pH-Bereich eine monomodale Partikelgrößenverteilung auf. Durch die Charakterisierung der Suspensionen mit der Ultraschallspektroskopie kann eine verbesserte Bewertung hinsichtlich Homogenität und Agglomeratbildung auch für nanoskalige Pulver erhalten werden. Dies ermöglicht eine bessere Qualitätskontrolle prozessrelevanter Schritte bei der Verarbeitung.
Fünf verschiedene Messverfahren wurden zur Partikelgrößenanalyse von nanodispersen ZrO₂-Pulver verglichen. Mit der Laserstreulichtanalyse, der dynamischen Lichtstreuung (heterodyne DLS und homodyne DLS-PCS), der Sedimentationsanalyse im Zentrifugalfeld und der Ultraschallspektrometrie wurden wässrige ZrO₂-Suspensionen mit verschiedenen Feststoffkonzentrationen hinsichtlich ihrer Partikelgrößenverteilung analysiert. Als Referenz diente die REM-Analyse zur Ermittlung der Primärpartikelgröße (ca. 40 nm). Mit den hier vorgestellten Messverfahren konnten in den entsprechenden Suspensionen nur Sekundärpartikel im Bereich von 105 nm bis 224 nm detektiert werden, die somit auf das Vorhandensein von Aggregaten bzw. harten Agglomeraten hinweisen.
Chemical admixtures like superplasticisers or stabilising agents are of ever increasing importance for modern concrete technology. They liberate the workability of concrete from its dependency on water content, and thus, open the gate towards innovative and future oriented concrete technologies such as selfcompacting concrete. Meanwhile admixtures have become common practice in concrete technology, but the understanding of these highly complex polymers in the entire concrete system lags far behind their application. Due to its complex time-dependent, multi-phase and multi-scale behaviour, flowable concrete systems are highly complicated and cannot be described comprehensively by simple models. It is therefore extremely challenging to identify the relevant parameters that predominantly control flow phenomena on different size scales, since these may occur on any scale between the nano scale (e.g. superplasticizer adsorption) and macro scale (e.g. grading of the aggregates). The present study discusses fundamental mechanisms at the interface between particle or hydrate surfaces and the fluid phase at a very early stage of concrete formation, and links these effects to macroscopic flow phenomena. Methods are discussed that appear promising interdisciplinary tools for enhancement of the understanding of the relevant interactions that are responsible for the macroscopic flow of flowable concrete.
The tribological behavior of different low infrared (IR) emission ceramic materials from the titanate group has been investigated at room temperature under gross slip fretting conditions against 100Cr6 ball bearing steel and α-alumina as the counterface materials. For all material pairs, friction and wear depended largely on the relative humidity of the environment. When paired with steel the low IR-emission ceramic disks showed higher wear resistance. Under dry conditions, almost no wear was found on the low IR-emission ceramic specimen. The high wear on the steel counter body is caused not only by tribo-oxidation, which is the main wear mechanism, but also by abrasion. In dry conditions a COF of 0.6 and in humid conditions a COF of 0.2 have been measured. When paired with α-alumina the results on wear are just opposite to those with steel counterbody and the low IR-emission ceramic materials show much lower wear resistance under all conditions. This is explained by the predominance of abrasion and the relatively large difference in hardness between the low IR-emission ceramic and α-alumina. In high humidity environments the results point to the formation of stable reaction layers causing lower friction and wear.
It has been successfully demonstrated that ceramic materials can be joined in the green
state without a second phase by using low pressure injection molded parts. The investigation of the
joining interface revealed that a high quality interface can be achieved by carefully adjusting the
different manufacturing steps. The use of monomodal particle size distribution in the used powder
CT3000SG is inferior to a broader particle size distribution obtained by replacing 33% of the finer
alumina powder by coarser CT1200SG. In this way the dewaxing process is significantly improved
when the wall thickness of the part exceeds 3 mm. The investigation of the mechanical properties of
the joined and sintered parts revealed, that the bending strength of the joined specimens achieved
about 80 % of the unjoined, monolithic specimens.
Aiming at ceramic materials of low thermal radiation emittance titania yttrium oxide ceramics have been prepared by sintering in air at 1,100 and 1,400 °C, respectively, and characterised. Due to its high infrared refractive index, average pore sizes up to 1 µm for optimal Mie scattering cross sections and over 35% porosity the material is a promising candidate for elevated temperature insulations. The essentially in large pore growth is achieved by graphitic nano and micro particle additives.
The microstructure parameters of porosity, specific surface area, median and average pore sizes are determined by mercury intrusion porosimetry. Beyond this classical technique supplementary measurements by X-ray Absorption and X-ray refractometry reveal considerably larger average pore sizes due to closed pores not accessible for mercury. An additional advantage of the X-ray techniques is their non-destructive application, which allows other treatments afterwards. The combined application of both methods provides additional information and reliability in case of complex ceramic microstructures and may serve for improved developments of porous ceramics with optimised thermal scattering for energy savings in high temperature applications.
Resolution of special capacitive sensors can be improved enormously by replacement of the gaseous material between the capacitor plates by a ceramic dispersion with high permittivity. High dielectric liquid dispersions of ceramic submicron and nano barium titanate powders should be qualified as dielectric fluid with long-term shelf life for this use.
Investigations for the production of stabilized barium titanate dispersions in aqueous and organic liquids by use of different dispersant aids were performed. Characterization of dispersed particles and agglomerates was done by use of zeta potential measurements for aqueous dispersions as well as particle sizing by laser diffraction, ultrasound spectroscopy and dynamic light scattering for aqueous and organic dispersions. Optimization of stability was evaluated by accelerated sedimentation measurements with an optical centrifuge. Best results were achieved for barium titanate dispersions in silicone oil.
Hard materials consist of a hard phase embedded in a metallic binder. In order to achieve high toughness and strength, it is necessary to have a perfect mixing of hard phase and binder, which is mainly achieved by ball milling. Niobium carbide (NbC) has a high potential to substitute tungsten carbide as hard material.
The publication presents the development of stable homogeneous and de-agglomerated NbC-dispersions. To prevent agglomeration of the powder, stable suspensions were achieved by surface treatments with the dispersants (PD and HD), which resulted in a charge reversal from a negative to a positive zeta potential. This surface-modified powder guaranteed a stable re-dispersion in the binder suspension. Nickel powder was added as metallic binder. This suspension was suited for 3D-printing. The green samples could be sintered in vacuum or Argon atmosphere.
Improved wear resistant ceramics of metal incorporated NbC processed with gas pressure sintering
(2017)
Tungsten carbide (WC) dominates wear protection and machining since more than 90 years due to its hardness with an associated good toughness and high melting point.
It was recently demonstrated that Niobium Carbides are wear resistant and show a better performing under cutting than WC grades. In order to profit of the good properties of Niobium Carbide, specific processing techniques need to be developed. The density of NbC is half of WC and offers colloidal processing for perfectly mixing NbC with metallic binder (up to 15 vol.-% Ni) and secondary carbide-particles. The used NbC-powder has an average particle size d50 of 3 micron measured by laser granulometry analysis. The true density was 7.68 g/cm3. An Oxygen content of the used NbC of 0.4 % is caused by residual slag content.
It is possible to increase the stability of the NbC-suspensions and to reduce their viscosity by adding a specific dispersing agent.
This surface-modified NbC powders (containing Ni)were uniaxially pressed and then gas pressure sintered in vacuum or Argon atmosphere. All samples reached more then 98 % of there theoretical density. The micrographs showed a homogenius distribution of the Ni-binder in the product. The tribological behaviour under dry sliding and oscillating up to 600 °C and the mechanical properties were studied.
NM 105, Ti02 (P25) could not be ignited as dust layer and dispersed in air as dust/air-mixture as well. This dust is not dust explosible and the burning behaviour corresponds to Burning Class 1 (no Ignition). The results have shown that the tested sample is thus not combustible at all, because it is already oxidized completely.