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

Ultra-Fast High Temperature Sintering (UHS) of Strontium Titanate

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Highly conductive grain boundaries in cold-sintered barium zirconate-based proton conductors

Proton-conducting barium zirconate ceramics have shown large potential for efficient electrochemical conversion and separation processes at intermediate operation temperatures. The high energy efficiency, robustness, and intermediate-temperature operation (500-650 °C) make proton-conducting cells promising candidates for future energy conversion systems. However, the major disadvantages of these materials are the inevitable high-sintering temperatures (>1500 °C), leading to Ba-evaporation and formation of high-resistance grain boundaries, which dominate the electrochemical performance. Here, we introduce a novel processing route for proton-conducting barium zirconates, which, on the one hand, significantly lowers the maximum processing temperature and, on the other hand, overcomes the dominating influence of grain boundaries on total conductivity. The key step of this novel processing route is the cold sintering of the powder using pure water as a sintering aid to consolidate BaZrCeYO (BZCY) at 350 °C. We show that clean grain boundaries with a high acceptor-dopant concentration are preserved thanks to the recovery of the perovskite phase during thermal treatment at 1300 °C. This compensates the interfacial core charge, resulting in highly conductive grain boundaries, which do not limit the total conductivity. Consequently, dense BZCY electrolytes produced by our novel approach outperform the conductivity of conventionally sintered BZCY irrespective of the significantly lower maximum processing temperature and its nanocrystalline microstructure. Our presented approach opens up new possibilities for grain boundary engineering and might facilitate novel co-sintering pathways for barium zirconate-based components.The authors acknowledge Dr Doris Sebold for help with SEM investigations and Dr Yoo Jung Sohn for assistance with HT-XRD measurements. M. K. acknowledges financial support from the DFG under project number MA 1280/69-1. Additionally, D. J. and W. R. thank the DFG for funding within the Emmy Noether program (RH 146/1-1). A. V. expresses gratitude to Dr Ivan Povstugar for his insightful discussions on the quality of APT data and its reconstruction. The authors thank Hitachi High-Technologies for providing access to the HF5000 STEM located at ER-C

Field assisted sintering of yttria ceramics for plasma etching applications

Advanced ceramics materials like yttrium oxide (Y$_{2}$O$_{3}$) are of high interest for critical manufacturing processes in the semiconductor industry due to their high chemical stability in contact with fluorine based etching plasmas. However, until now Y$_{2}$O$_{3}$ gets primarily applied as a functional coating deposited by thermal spray or aerosol deposition technique due to complications associated with the manufacturing of bulk ceramic components. Yttria exhibits a low sinterability when conventionally processed and fabricating large scale samples with high performance concerning chemical purity and relative density is a challenging task. Complex processing routes using conventional or vacuum sintering and an additional post-compaction step by hot isostatic pressing are necessary to achieve relative densities which are matching the requirements of the semiconductor industry. These major drawbacks have prevented the application of bulk Y$_{2}$O$_{3}$ components in state of the art semiconductor manufacturing devices. Therefore, in this work, field assisted sintering technique /spark plasma sintering (FAST/SPS) is investigated as a straight forward processing technique which enables to consolidate high performance, dense ceramic components in a single processing step. In two separate parts, applied and fundamental research questions are going to be addressed. The first part of this work focuses on evaluating the direct processability of commercial powders, solving challenges during the upscaling of sample sizes, processing of complex shaped components as well as characterizing the impact of rare earth doping on sintering and grain growth. Upscaling of ceramic samples is generally hindered by the formation of thermal inhomogeneities in the tool setup. Therefore the application of carbon fibre reinforced carbon spacers and their optimal position in the FAST/SPS tool was investigated by coupling experiments and simulations. Furthermore, graphite powder bed assisted FAST/SPS was applied to study the possibilities of sintering multiple complex shaped Y$_{2}$O$_{3}$ samples in one step. Lastly, the influence of rare earth doping with La$^{3+}$ and Gd$^{3+}$ on sintering and grain growth of Y$_{2}$O$_{3}$ during FAST/SPS processing was studied thoroughly. The segregation of La$^{3+}$ decelerated both sintering and grain boundary kinetics through a solute drage ffect, effectively preventing pore detachment at high sintering temperatures and leading to enhanced densification in the final stage of sintering. [...

High-velocity water vapor corrosion of Yb-silicate: Sprayed vs. sintered body

The water vapor corrosion of Yb-silicates is of interest to their application as environmental barrier coatings in gas turbine technology. In this study, densified samples from the Yb-silicate powder, as well as plasma-sprayed free-standing Yb-silicate coating were tested at a high-velocity steam rig (T = 1400 °C, v = 90 m/s, PH2O = 0.19 atm) for microstructural comparison. After the test, the measured weight losses of the coatings were larger than that of the densified sample. At the same time, the thicknesses of the corroded scales at the coating surfaces were found to be thicker than that of the sintered sample by a factor of two

Processing map to control the erosion of Y 2 O 3 in fluorine based etching plasmas

Due to the increasing number of applications for ceramic components in reactive etching processes, the interest in the specific erosion behavior of highly etch-resistant materials like yttrium oxide (Y2O3) has increased in the past years. Despite the large number of investigations already existing in this field, a more general understanding of the erosion mechanisms still lacks due to the limited comparability of these investigations. The huge difference in the kind of etching setups, processing parameters (bias voltage and plasma gas composition), and sample microstructures prevented consistent conclusions so far. To achieve a more general understanding, this study investigates the erosion behavior Y2O3 under a broad spectrum of plasma etching parameters. Therefore, the bias voltage is increased from 50 to 300 V and the plasma gas composition is gradually changed from Ar-rich to CF4-rich compositions. This systematic approach allows to directly correlate the morphology changes caused by plasma erosion with the related plasma etching parameters and enables to better understand their influence on the depth of physical and chemical interactions, surface damage, and etching rate. We discovered three distinct erosion regimes, which exhibit specific erosion characteristics. Using these observations, a schematic processing map for Y2O3 was developed, which could help to estimate the severity of the erosion attack dependent on the processing parameters

Reactive FAST/SPS sintering of strontium titanate as a tool for grain boundary engineering

A high-pressure FAST/Spark Plasma Sintering method was used to produce dense SrTiO3 ceramics at temperaturesof 1050 ◦C, more than 250 ◦C below typical sintering temperatures. Combining SPS with solid-state reactivesintering further improves densification. The process resulted in fine-grained microstructures with grain sizes of~300 nm. STEM-EDS was utilized for analyzing cationic segregation at grain boundaries, revealing no cationicsegregation at the GBs after SPS. Electrochemical impedance spectroscopy indicates the presence of a spacecharge layer. Space charge thicknesses were calculated according to the plate capacitor equation and the Mott-Schottky model. They fit the expected size range, yet the corresponding space charge potentials are lower thantypical values of conventionally processed SrTiO3. The low space charge potential was associated to low cationicGB segregation after SPS and likely results in better grain boundary conductivity. The findings offer a path totailor grain boundary segregation and conductivity in perovskite ceramics

The role of fluorination during the physicochemical erosion of yttria in fluorine-based etching plasmas

A physicochemical mechanism acting between the reactive plasma and the material surface controls the erosion of polycrystalline ceramics in fluorine containing etching plasmas. In this study, a Y2O3/YOF composite was exposed to a fluorine etching plasma. Relocalization enables the direct correlation of crystalline orientation with material response. Our study reveals an orientation dependent surface fluorination of Y2O3, which controls the etching resistance and morphology formation. Orientations near the low index planes (001), (010) and (100) exhibit the lowest stability due to a homogeneous surface reaction. The presented results help to extend the mechanistic understanding of the plasma-material interaction of Y2O3