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1.
Martin Michálek Monika Michálková Gurdial Blugan Jakob Kuebler 《Ceramics International》2018,44(3):3255-3260
Up to now, commercially available alumina ceramics were claimed to have strength between 400 and 550 MPa. However, our study shows strength ~ 2 times higher for commercially available alumina than commonly believed. The average and characteristic strength, measured on 31 pure alumina ceramic discs by ball on three balls (B3B) test, were 1205 ± 93 MPa and 1257 MPa, respectively, with a Weibull modulus of m = 11.8. Tested specimens were in form of discs with a diameter of 5 mm and thickness 0.5 mm. The grain size distribution of the alumina is bimodal with an average grain size of ~ 850 nm measured at the surface. The fracture reveals a mixed transgranular / intergranular failure mode. To avoid incorporation of additional flaws, the discs were tested as sintered. The characteristic flexural strength measured in B3B was recalculated according to Weibull theory for standard 4-point bending bars of size 3 × 4 × 45 mm as bend 856 MPa. The measured strength of nearly 900 MPa shows the potential of strength for high purity alumina ceramics. 相似文献
2.
Basic information on modeling and digital computer simulation of variable-reluctance (VR) step motors is provided. The nonlinear equations of the system are derived, and a method for implementing them on a digital computer is presented. As an example, a typical printer with a step motor is designed with this approach. The problem is defined by giving the desired system performance criteria and the step motor parameters and characteristics. The specification of the control schemes determined the design. Computer simulation runs are made to determine the optimal control parameters of the system. The case study demonstrates the advantages of the computer-aided design, before the physical construction of the experimental system. 相似文献
3.
Gurdial Blugan Mousab Hadad Jolanta Janczak-Rusch Jakob Kuebler Thomas Graule 《Journal of the American Ceramic Society》2005,88(4):926-933
Commercial-grade Si3 N4 –TiN composites with 0, 10, 20, and 30 wt% TiN content have been characterized. Submicrometer grain-size Si3 N4 was reinforced with fine TiN grains. Density, Young's modulus, coefficient of thermal expansion, and fracture toughness increased linearly with TiN content. Increased strength was observed in the Si3 N4 +20 wt% TiN, and Si3 N4 +30 wt% TiN composites. Fractography was used to characterize the different types of fracture origins. Improvements in toughness and strength are due to residual stresses in the Si3 N4 matrix and the TiN particles. A threefold improvement in dry wear resistance of the Si3 N4 +30 wt% TiN composite over the Si3 N4 matrix was observed. 相似文献
4.
Gabriel Adusei Sanjukta Deb John W. Nicholson Liyuan Mou Gurdial Singh 《应用聚合物科学杂志》2003,88(2):565-569
Vibrational overtone activated polymerization of acrylonitrile (AN) has been demonstrated using two initiators, benzoyl peroxide (BP), and 2,2′‐(azobis)isobutyronitrile (AIBN). Excitation of the fifth vibrational overtone of the CH stretch of AIBN at 627 nm and BP at 604 nm initiates the reaction. Monomer conversions were monitored by a gravimetric method. In both cases, the wavelength selectivity was investigated by irradiating the monomer/initiator mixtures at the absorption maximum of the initiator, the absorption maximum of the monomer, and at a wavelength (580 nm for AN/AIBN and 625 nm for AN/BP) where neither the initiator nor the monomer absorbs light. For the AN/AIBN mixture, after 48 h the monomer conversion for the irradiation on the peak absorption of AIBN (627 nm) is about twice as large as the irradiation at 580 nm. For the AN/BP mixture, after 48 h the monomer conversion for the irradiation on the peak absorption of BP (604 nm) is about a factor of 2.2 larger than for the irradiation at 625 nm. The overall quantum yields of both polymerizations were estimated. After 48 h the overall quantum yield for the AN/AIBN mixture irradiated at the initiator absorption (Φ627 = 21083) is about 10 times larger than for the sample irradiated at the pure monomer absorption (Φ595 = 1942). For the AN/BP mixture, the 604 nm quantum yield (Φ604 = 1096) is about 2.4 times larger than the 595 nm quantum yield (Φ595 = 448). The influence of the initiator concentration is also presented. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 565–571, 2003 相似文献
5.
ABSTRACTThe aim of the present work was changing the surface properties of alumina ceramic via sintering of samples in spinel MgAl2O4 and magnesia MgO powder bed. During sintering nitrogen and air atmosphere were used; and the reference material was sintered at the same conditions but in an alumina powder bed. All samples were sintered to a density higher than 99.2% of theoretical density of alumina. In order to examine samples SEM, EDX and XRD (X-ray diffraction from 18 to 70 °2Theta) analysis were performed together with B3B flexural strength measurement. Results revealed that by sintering of alumina samples in a spinel powder bed pore-free surfaces were prepared, which had a favourable effect on B3B flexural strength. This was ascribed to the high solubility of Al3+ in spinel at sintering temperatures, which increases the sintering driving force. A further positive effect on B3B flexural strength was observed when samples were sintered in nitrogen instead of air. 相似文献
6.
The design, construction, and operation of experimental models of a dc-type hybrid step motor are discussed. This form of hybrid motor consists of a variable-reluctance step motor, the rotor of which is appropriately wound as a multipole dc motor. A computer-aided design procedure is discussed that is used to obtain the optimum parameters of the dc portion of the hybrid motor. The results obtained with two experimental models of this type of hybrid motor are presented. Both prototypes show a marked improvement in the running torque characteristics due to the dc windings. Torque-speed curves indicate load torques of 400 oz-in available at speeds up to several thousand revolutions per minute with one of the proto-types. This type of hybrid motor may be controlled in one of three different ways: a pure step motor, a pure dc motor, or a dc-aided step motor. The relative advantages, disadvantages, and application of each control mode are discussed. Several control strategies-open and closed loop-are presented for speed control as well as point-to-point type control of position. The practical feedback feasibility of this concept for high-power applications in the integral horsepower range is discussed. 相似文献
7.
Si3N4-TiN based multi-layer laminates exhibit differences in residual stress between individual layers due to a variation of the thermal expansion coefficient between the layers. The residual stress distribution in these multi-layer laminates is known to improve the apparent macroscopic fracture toughness. In this work, the tribological behaviour of bulk, composites and multi-layers laminates are investigated. Si3N4 bulk, Si3N4 based composites with 10, 20 and 30 wt% TiN and different multi-layer laminates have been tested under dry conditions with reciprocal movement using a ball-on-block configuration. In particular, the influence of sliding directions with respect to the layer orientations has been investigated.The experimental results show that wear resistance increased with increasing TiN content in Si3N4-TiN composites. However, multi-layer laminates exhibit an up to three times higher apparent fracture toughness, but do not show an improvement of wear resistance compared to composites. 相似文献
8.
Martin Michálek Monika Michálková Gurdial Blugan Jakob Kuebler 《Journal of the European Ceramic Society》2018,38(1):193-199
The present work aimed with the carbon contamination in alumina ceramics and its influence on sinterability of alumina in low vacuum and atmospheres of argon and nitrogen. The commercially available alumina was coated with carbon and sintered at different atmospheres to investigate the effect of carbon presence on alumina sintering behaviour. The sintering conditions were: heating/cooling rates 5 °C/min and 1.7 °C/min until the maximum temperature of 1400 °C and a dwell time of 2 h. The microstructure of the samples was investigated from fracture and surface, prior to polishing, chemical or thermal etching. The non-densified (porous) surface layer was found in the samples sintered in nitrogen and vacuum, however, sintering in argon atmosphere showed a negligible effect on the surface. The core of investigated specimens exposes a transgranular/intergranular fracture mode and is dense in all cases. In the case of vacuum sintering, the strong carbon diffusivity was also noticeable by the dark grey color of the samples. Interestingly, the formation of aluminium nitride took place during sintering of carbon coated alumina samples in a nitrogen atmosphere at 1400 °C. The thickness of the reactive porous layer was approximately 15 μm beneath the surface. Such a porous layer is inappropriate to the desired features of final ceramic products. Presented results lead to better understanding of the sintering behaviour of ceramic and to suitable selecting of the set-up by densification conditions. 相似文献
9.
Jongmoon Jang Pradeep Vallachira Warriam Sasikumar Fatemeh Navaee Lorenz Hagelüken Gurdial Blugan Juergen Brugger 《Ceramics International》2021,47(6):7593-7601
In an implantable electrode, such as a pacemaker electrode, fibrotic tissue formation due to a foreign body reaction is an important challenge affecting the efficiency to transmit the electrical signal of the device. The chemical inertness, biocompatibility, and electrical conductivity of polymer-derived ceramics (PDCs) are promising features in terms of overcoming this challenge. Here, the electrochemical behavior of polymer-derived silicon oxycarbide (SiOC) and titanium-doped SiOC (SiTiOC) ceramic electrodes for use as pacemaker electrodes is investigated by measuring impedance spectroscopy and cyclic voltammetry. In addition, typical stimulation electrodes such as iridium oxide, titanium nitride, platinum, and glassy carbon were prepared and loaded simultaneously into a custom-made electrochemical testing platform for comparison with SiOC and SiTiOC electrodes under identical conditions. The SiOC and SiTiOC electrodes shows a wide electrochemical stability window in the range of ?0.9 to 1.2 V with a double layer capacitance as the charge injection mechanism at the electrode/phosphate-buffered saline interface. Also, analyzing the voltage transient shows that the maximum charge injection of the SiTiOC electrode was about 28 μC/cm2. The results of the electrochemical evaluation and comparison of SiOC and SiTiOC stimulating electrodes will be helpful to understand fundamental characteristics for the potential of this material as candidate for next-generation pacemaker electrodes. 相似文献
10.
Kh.V. Manukyan S.L. Kharatyan G. Blugan P. Kocher J. Kuebler 《Journal of the European Ceramic Society》2009,29(10):2053-2060
The fabrication method and the mechanical and electrical properties of different MoSi2–Si3N4 composite materials were investigated. Commercially available individual compounds, one-stage combustion synthesized MoSi2–Si3N4 and submicron MoSi2 powders were used as starting materials, followed by hot pressing. It was found that the sintering atmosphere used, nitrogen or argon, had a significant effect on the phase composition, mechanical and electrical properties of the final materials. It was shown that in some cases partial nitridation of MoSi2 occurred with the formation of MoSi2–Mo5Si3–Si3N4 ternary composites. The electrical conductivity of the composites depends also on the microstructure of materials. It was shown that the composites fabricated using combustion synthesized MoSi2 powders (500 nm) are characterized by higher flexural strength at room temperature compared to those from commercial powders. On the other hand, the composites fabricated from the commercial powders had higher strength and fracture toughness at elevated temperatures (up to 1200 °C). For all composites, the strength decreased significantly at temperatures over 1000 °C due to the brittle–ductile transition of the MoSi2 phase. 相似文献