Field Assisted Sintering Technology Research Articles

Thermal reliability of copper alloy–diamond composites produced by field-assisted sintering technology

This paper highlights the thermal properties of copper–diamond composites fabricated by field assisted sintering technology. The samples were fabricated using a matrix of copper alloyed with silver and zirconium. Previous reports have shown that these samples possess high thermal conductivity. This work examines the thermal conductivity of the samples after being subjected to thermal cycling. Coefficient of thermal expansion is also reported in this work.

Evaluation of Incoherent Interface Strength of Solid-State-Bonded Ti64/Stainless Steel Under Dynamic Impact Loading

Ti/steel interfaces are produced using field-assisted sintering technology, a technique known to bring about full consolidation of materials using much lower sintering temperatures and durations. The interface thickness is verified using the energy-dispersive x-ray analysis exhibiting the extent of diffusion in interface regions. The interface mechanical strength is characterized using dynamic indentation experiments at strain rates approaching 400 s−1. The experiments were conducted on the interfaces within the spatial error tolerance of less than 3 µm. The measurements of dynamic hardness values, strain rates, and plastic-residual depths were correlated to show the relation of interface mechanical strength with the bulk-phase mechanical strength properties of Ti and steel. The Johnson–Cook model is fitted to the obtained interface normal stress–normal strain data based on the nanoimpact experiments. The coefficient of restitution in the mechanical loading and its dependence on the interface dynamic hardness and interface impact velocity validate the experimental results. The results show that interfacial properties are affected by the rate of loading and are largely dependent upon the interface structural inhomogeneity.

Preparation of cubic Li7La3Zr2O12 solid electrolyte using a nano-sized core–shell structured precursor

Nano-sized Al-doped Li7La3Zr2O12 (LLZO) precursor is successfully synthesized by a novel two-step precipitation method. Microstructure and thermal behavior of the prepared LLZO precursor powders and properties of the sintered LLZO pellets are comprehensively investigated. Results show that the obtained precursor powders have a special core–shell structure that a coating of Li2C2O4 covers on the surface of the lanthanum and zirconium co-precipitate products. Pure cubic LLZO powders can be obtained at a low calcination temperature of 900°C. Sample sintered by field assisted sintering technology at 1000°C has a high relative density of 96.5% with a total ionic conductivity of as high as 3.32×10−4Scm−1 (corresponding to the activation energy of 0.32eV) at room temperature. The reported method provides a simple and low-temperature synthesis of high quality LLZO solid electrolytes.

Field assisted sintering of graphene reinforced zirconia ceramics

Threemol. % yttria stabilized zirconia (3Y-TZP) with Graphene platelets (GNPs) powder mixture was produced by ball milling using dimethyl formamide (DMF) as dispersion media. The GNPs-reinforced 3Y-TZP composites were densified (~99%) by field assisted sintering technology (FAST). A homogeneous dispersion of GNPs in 3Y-TZP is observed from the sintered samples, and the GNPs are bent and embedded between the grains, which results in increasing the contact area. The trans-granular mechanism of crack propagation becomes increasingly dominant by adding GNPs. The fracture toughness reaches 15.3MPa·m1/2, as tested by single-edge notched beam method, which reflects an increase of 61% compared to GNPs-free 3Y-TZP. On the other hand, with higher GNPs content, the improvement in fracture toughness is limited. Toughening mechanisms, as determined through micro-hardness testing, including pull out, crack deflection, crack bridging, and micro-cracks, are observed and discussed.

Field assisted sintering of dense Al-substituted cubic phase Li7La3Zr2O12 solid electrolytes

High performance lithium ion conducting Li7La3Zr2O12 solid electrolytes are prepared for the first time by field assisted sintering technology (FAST). The effect of sintering temperature on the phase compositions, microstructure and Li ionic conductivity is systematically investigated. The results show that pure cubic phase LLZO can be obtained at a range of temperatures from 1100 to 1180 °C for no more than 10 min. For the sample sintered at 1150 °C, a maximum relative density of 99.8% with a total ionic conductivity as high as 5.7 × 10−4 S cm−1 are obtained at room temperature. This value is the highest among the present research. Compared with the traditional preparation methods, the current FAST is very promising to obtain high performance LLZO for its advantages of very short sintering time, a single preparation step of reaction-densification processing, and relatively lower sintering temperature.

Study on parameter optimisation for field-assisted sintering of fully-dense, near-nano WC–12Co

A sintering route to fully-dense near-nanocrystalline WC–12Co without grain growth inhibitors was developed. The influence of sintering parameters on the densification, microstructure, and mechanical properties of the material compacted by Field Assisted Sintering Technology (FAST) was investigated. High and low-effect sintering parameters were identified, and a process to consolidate nearly fully-dense cemented carbide samples with mean grain sizes of about 200nm was elaborated. The parameters of the sintering process were adapted in order to maximise densification on the one hand and, on the other hand, to avoid growth of the WC grains during the sintering process. Furthermore, the reproducibility of the sintering process with respect to densification was tested. Microscopic examination of the sintered samples showed a minimisation of inhomogeneity combined with maximisation of the relative density, while the impact of the sintering parameters on the microstructural parameters was found to be insignificant. X-ray diffraction (XRD) was applied to determine the phase composition and crystallite size of the sintered WC.

Effect of Liquid Phase Additions on Microstructure and Thermal Properties in Copper and Copper-Diamond Composites

This study details a new approach to creating copper-diamond composite materials for thermal management applications by using a two-phase (solid-liquid) approach in powder metallurgy using Field Assisted Sintering Technology (FAST). Silver-copper alloyed powder at eutectic compositions was used as a nonreactive liquid phase while Cu5Si was used as a reactive liquid phase. Microstructure results are reported favorably comparing the additions of a small amount of liquid phase to pure solid state sintering. Additionally, EDX results indicate that the liquid phase material fills gaps at the interface of the matrix and diamond particle resulting in improved microstructure and density. Thermal conductivity results show that liquid phase additions improve the thermal conductivity of composites compared to composites without any liquid phase, but Si additions cause a severe drop in baseline conductivity.

Ceramic windows and gain media for high-energy lasers

Abstract. Recent progress in high-quality transparent ceramic window materials ( MgAl 2 O 4 spinel and β -SiC) and high-power solid-state laser materials ( Yb 3 + : Y 2 O 3 , Yb 3 + : Lu 2 O 3 , and Ho 3+ : Lu 2 O 3 is reported. Spinel ceramic demonstrates a record low-absorption loss of 6 ppm/cm at 1.06 μ m. The capability of fabricating various shapes and sizes of spinel ceramics is also demonstrated. We also report optical transparency from a β -SiC ceramic fabricated by field assisted sintering technology (FAST). We report lasing in hot-pressed Yb 3+ : Y 2 O 3 and Yb 3+ : Lu 2 O 3 ceramic made from coprecipitated powder. The highest ever reported output power and efficiency from 10% doped Yb 3+ : Lu 2 O 3 ceramic is also presented. Lasing oscillation from hot pressed composite of five-layered Yb 3+ : Y 2 O 3 ceramic is also demonstrated for the first time.

Alloy development for highly conductive thermal management materials using copper-diamond composites fabricated by field assisted sintering technology

Improving thermal management materials is critical to many industries including the microelectronics where smaller components lead to larger heat densities. Performance of thermal management materials are largely governed by the thermal conductivity of the material. Commercial diamond particles have a very high thermal conductivity, but their inclusion into conductive metals such as copper is challenging due to a lack of compatibility which creates a poor interface between materials. This study uses a powder metallurgy approach employing an alloyed powder blend to act as an agent to improve the interface leading to improved thermal properties. Results show that minor additions of zirconium to the copper matrix is successful in improving the interface and allows for improvement of thermal conductivity up to a maximum of 533 W/m K with 40 vol.% diamond. Energy-dispersive X-ray spectroscopy mapping analysis shows that additions of zirconium are vastly better than chromium for improvement of interface. It is shown that caution must be taken when adding alloying elements because small additions result in a rapid decrease in conductivity of the alloy before diamonds are added.

Sintering of tungsten powder with and without tungsten carbide additive by field assisted sintering technology

Tungsten powder (0.6–0.9μm) was sintered by field assisted sintering technology (FAST) at various processing conditions. The sample sintered with in-situ hydrogen reduction pretreatment and pulsed electric current during heating showed the lowest amount of oxygen. The maximum relative density achieved was 98.5%, which is from the sample sintered at 2000°C, 85MPa for 30min. However, the corresponding sintered grain size was 22.2μm. To minimize grain growth, nano tungsten carbide powder (0.1–0.2μm) was used as sintering additive. By mixing 5 and 10vol.% WC with W powder, densification was enhanced and finer grain size was obtained. Relative density above 99% with grain size around 3μm was achieved in W–10vol.% WC sintered at 1700°C, 85MPa, for 5min.

Field assisted sintering of SiC using extreme heating rates

Several challenges still exist in the fabrication of silicon carbide (SiC) ceramic components associated with using multiple step processes and sintering at high temperature which result in long lead times. This challenge is being addressed by exploring an emerging sintering technology called field assisted sintering technology (FAST), also known as spark plasma sintering. The objective of this study is to sinter SiC to near theoretical density using the FAST technique and study the effect of using extreme heating rates during sintering. All samples were sintered under identical temperature and pressure of 2000 °C and 45 MPa. Resulting samples were characterized for density, microhardness, grain size, and microstructure evolution. The results showed that density decreased slightly and grain size increased as heating rate was varied from 50 to 400 °C/min.

Engineered chemistry of Cu–W composites sintered by field-assisted sintering technology for heat sink applications

High performance microelectronics require superior thermal management systems. Heat sink plates that dissipate the heat generated by the microelectronic components are of critical importance for this purpose. The two primary concerns with heat sink materials are the coefficient of thermal expansion (CTE), which should be low to match that of silicon (4 × 10−6/°C), and the thermal conductivity, which should be as high as possible to move the heat efficiently. This study created copper–tungsten composite materials using the field assisted sintering technology (FAST) process. The amount of tungsten used in the composites was varied from 0 to 70 wt% to study the effects of tungsten as well as to show the ability of the composite’s properties to be tailored based on the percentage of tungsten in the composite. It is shown that increasing pressure has a positive effect on final density while heating rate has no effect. High densities of >97.5% were achieved in all of the composites. As predicted, the CTE decreases as the percentage of tungsten increases. Thermal conductivities are also reported for each sample.

Temperature distribution for electrically conductive and non-conductive materials during Field Assisted Sintering (FAST)

During Field Assisted Sintering Technology (FAST) the temperature differences at two different positions were investigated using two pyrometers, an internal and an external one. Two substances, an electrically conductive (tungsten carbide) and a non-conductive material (96 wt.% silicon nitride with 2 wt.% alumina and yttria) were used to monitor the temperature differences between both pyrometers during heating, sintering shrinkage and dwell time by varying die geometry and heating rate. It was shown that the temperature distribution is strongly influenced by the electrical conductivity of the material as well as by tool design and setup. The alpha–beta transformation of silicon nitride was analyzed to predict the radial temperature distribution within the sample. For comparison and for visualization a dynamical FE model including piston movement for simulating sintering shrinkage was introduced. With this, a complete time dependent FAST run could be simulated. The modeled differences in temperature distribution are in good agreement with real temperature measurements as well as phase analyses.

Binder-Free Tungsten Carbide Fabricated by Pulsed Current Sintering

In this presentation, we show some experimental results of binder-free WC with Nano WC sintered by Pulsed Electric Current Sintering (PECS) process also known as Field Assisted Sintering Technology (FAST). The particle size of WC powder is almost 80 nm. These binder-free WC have extremely hardness and stiffness. However, these mechanical properties are dependent on the sintering condition, e.g., maximum temperature, applied presssure, etc. We show some relationship between mechanical properties and sintering condition to improve to sinter the binder-free WC

Binder-Free Tungsten Carbide Fabricated by Pulsed Electric Current Sintering

In this paper, we show some experimental results of binder-free WC sintered by Pulsed Electric Current Sintering (PECS) also known as Field Assisted Sintering Technology (FAST). These binder-free WC have extremely high hardness and stiffness. However, these mechanical properties are dependent on the sintering condition, e.g., maximum temperature, applied pressure, etc. We show some relationship between mechanical properties and sintering condition to improve to sinter the binder-free WC.