Application of steel as an alternative tool material for field assisted sintering in SPA

The Field Assisted Sintering Technique/Spark Plasma Sintering (FAST/SPS) is a new method for fast processing of variety metallic and ceramic materials with unique microstructure frequently far from equilibrium. Despite an essential progress in fundamental understanding and in upscaling of this technique from laboratory to industrial scale, some challenges are still remaining. One of them is application of a tool made from material other then traditionally used graphite. The advantages of graphite are high electrical conductivity, low temperature dependence of strength and good machinability. At the same time, the strength of graphite is relatively low. This property restricts the pressure used during FAST/SPS to 50-100 MPa resulting in a reduced sintering rate, residual porosity, enhanced sintering temperature and time and, correspondingly, in increased grain growth. Besides, graphite trends to react with many materials at sintering temperatures building carbides, carbonates, reducing oxides and others. This can lead to an essential change in phase composition particularly in chemically instable materials, loss in functional properties, embrittlement, chemical expansion and disintegration of the material during sintering or subsequent cooling. Therefore, investigation of new tool materials replacing the graphite is an actual working task in FAST/SPS. Graphite tool can be potentially replaced with a tool manufactured from hot working steel, nickel or molybdenum based alloy, hard metal, conductive ceramics or composites. The inserts manufactured from these materials can be used in the tool design as well. Additionally, coating of contacting surfaces can be applied to eliminate or to reduce chemical interaction between tool elements (die and punch) and between tool and sintered preform. Obviously, suitable choice of an alternative tool material depends on sintering temperature, used pressure, chemical affinity to the sintered material and economical consideration. The experience in the application of alternative tool materials for FAST/SPS is nowadays limited to only several case studies. Therefore, the aim of the present work was to extend the knowledge by applying a hot working steel Böhler W360-Isobloc as a tool material at low-temperature field assisted sintering. Two types of experiments were carried out. Firstly, the risk of diffusion bonding or even partial melting between two steel discs during current assisted sintering at various temperatures was studied. The coating of discs with TiN by PVD was found to be a very promising measure to reduce this risk. Another effective approach is the insertion of a graphite foil between the contacting surfaces. This option can be used if there is no chemical interaction between graphite foil and sintered material. Finally, the coating by BN-spray was investigated. This coating reduces the interaction between steel discs to some extent, but was found to be quite inhomogeneous and frequently instable. Moreover, BN-spray coating can be easily removed from contacting surfaces during densification when the punch moves into the die. Second type of experiments was related to the FAST/SPS of ZnO in an alternative steel tool. In this case, a special adjustment of the PID controller was strictly required to follow the prescribed temperature profile. The influence of sintering temperature, pressure and holding time on sintering kinetics, density and microstructure of samples was investigated. Our experience in application of a steel tool for FAST/SPS is summarized in form of concluding remarks

Field assisted sintering of larger scaled ceramic parts using adapted tool design and hybrid heating

Field Assisted Sintering/Spark Plasma Sintering (FAST/SPS) is a promising technology for the energy efficient sintering of ceramic, composite and metal powders. The combination of direct current heating and applied pressure enables high heating rates, rapid densification and offers the potential to decrease the sintering temperature significantly. FAST/SPS is of special interest for materials, which are difficult to densify by conventional methods like pressure less sintering. To establish this processing technology on industrial scale, fundamental studies are required to better understand the relationship between processing parameters, specific FAST/SPS boundary conditions and resulting material properties. A challenging task – especially for non-conductive oxide ceramics – is the decrease of thermal gradients during FAST/SPS cycles to a minimum and to suppress interface reactions with the tool material. In the present work, a systematic study was conducted in our FAST/SPS device aiming on to homogeneously densifying commercial yttria (Y2O3) powder to discs with diameter up to 100 mm. Specific attention was laid on the formation of thermal gradients during the cycle and to investigate their influence on the resulting microstructure. Therefore, different tool set ups were used. Amongst others, carbon fiber reinforced carbon (CFC) inlays were implemented to adjust thermal conductivity of the tool. Furthermore, the effect of hybrid heating was evaluated. For this experimental series, an additional induction coil was mounted in the FAST/SPS device. For evaluating the efficiency of hybrid heating, total energy consumption of the FAST/SPS device – operated with and without induction coil – was measured. The experimental studies were accompanied by finite element modelling to estimate the temperature distribution of non-conductive yttria sample during FAST/SPS processing. The modelling results will be correlated with the grain size distribution along the cross section of the 100 mm disc. Additionally, Vickers hardness measurements were done to investigate how thermal gradients tend to influence mechanical properties

Quantum dynamics of wave packets in a non-stationary parabolic potential and the Kramers escape rate theory

At sufficiently low temperatures, the reaction rates in solids are controlled by quantum rather than by thermal fluctuations. We solve the Schrödinger equation for a Gaussian wave packet in a non-stationary harmonic oscillator and derive simple analytical expressions for the increase of its mean energy with time induced by the time-periodic modulation. Applying these expressions to the modified Kramers theory, we demonstrate a strong increase of the rate of escape out of a potential well under the time-periodic driving, when the driving frequency of the well position equals its eigenfrequency, or when the driving frequency of the well width exceeds its eigenfrequency by a factor of ~2~2. Such regimes can be realized near localized anharmonic vibrations (LAVs), in which the amplitude of atomic oscillations greatly exceeds that of harmonic oscillations (phonons) that determine the system temperature. LAVs can be excited either thermally or by external triggering, which can result in strong catalytic effects due to amplification of the Kramers rate

Will person detection help bag-of-features action recognition?

Bag-of-feature (BoF) models currently achieve state-of-the-art performance for action recognition. While such models do not explicitly account for people in video, person localization combined with BoF is expected to give further improvement for action recognition. The purpose of this paper is to validate this assumption and to quantify the improvements in action recognition expected from current and future person detectors. Given locations of people in video, we find that---somewhat surprisingly---background suppression leads only to a limited gain in performance. This holds for actions in both simple and complex scenes. On the other hand, we show how spatial locations of people enable to incorporate strong geometrical constraints in BoF models and in this way to improve the accuracy of action recognition in some cases. Our conclusions are validated with extensive experiments on three datasets with varying complexity, basic KTH, realistic UCF Sports and challenging Hollywood

Conceptual Design of Beryllium Target for the KLF Project

The Kaon Production Target (KPT) is an important component of the proposed K-Long facility which will be operated in JLab Hall~D, targeting strange baryon and meson spectroscopy. In this note we present a conceptual design for the Be-target assembly for the planned K-Long beam line, which will be used along with the GlueX spectrometer in its standard configuration for the proposed experiments. The high quality 12-GeV CEBAF electron beam enables production of a K$_L$ flux at the GlueX target on the order of $1\times 10^4 K_L/sec$, which exceeds the K$_L$ flux previously attained at SLAC by three orders of magnitude. An intense K$_L$ beam would open a new window of opportunity not only to locate "missing resonances" in the strange hadron spectrum, but also to establish their properties by studying different decay channels systematically. The most important and radiation damaging background in K$_L$ production is due to neutrons. The Monte Carlo simulations for the proposed conceptual design of KPT show that the resulting neutron and gamma flux lead to a prompt radiation dose rate for the KLF experiment that is below the JLab Radiation Control Department radiation dose rate limits in the experimental hall and at the site boundary, and will not substantially affect the performance of the spectrometer.Comment: 9 pages, 9 figure