Master sintering curve applied to the Field-Assisted Sintering Technique
- First Online:
- Cite this article as:
- Guillon, O. & Langer, J. J Mater Sci (2010) 45: 5191. doi:10.1007/s10853-010-4556-7
- 313 Views
The master sintering curve approach was extended theoretically to the Field-Assisted Sintering Technique (or SPS). Experimental data from constant heating rate testing confirm the general applicability of this framework. An apparent activation energy of ~290 kJ/mol was found for ultrafine alumina below 1200 °C. As alumina densifies by grain boundary diffusion in the present conditions, this low value of the apparent activation energy might be due to thermal gradients in the specimens induced by rapid heating.
The effect of an applied load during hot pressing (HP) has been further taken into account by An and Johnson, leading to 3D maps including pressure  and was successfully applied to alumina. Experiments were conducted at a given heating rate (10 °C/min) and apparent constant pressures of 6.9–34.5 MPa were used. Here, we propose to develop equations for the Field-Assisted Sintering Technique (FAST, also known as SPS, Spark Plasma Sintering ) as function of the heating rate. This technique, which presents large similarities with HP, enables the manufacturing of dense  and transparent  ceramics as well as nanocrystalline metals . It enjoys a rapid development and enables the use of a large range of heating rates (up to several hundreds of degrees per minute). A series of constant heating rate experiments complements this theoretical framework and validates the present approach.
All experiments were made with the same FAST equipment (HP D 25/1, FCT Systeme, Germany). The powder used is an ultrafine α-Al2O3 (purity of 99.99% and median particle size of 150 nm; TMDAR, Taimei Chem., Tokyo, Japan). Raw powder was sieved through a 100 μm mesh, allowing better packing and reproducibility of the green bodies. The interior of the graphite die and the surfaces of the punches were covered by a compressible graphite foil to maximize the contact area between the rough powder compact surface and the punches and reduce friction as well as temperature gradients. Moreover, the exterior of the die was covered by graphite felt with a thickness of ~10 mm to reduce temperature gradients. The initial sample height was ~11 mm with a relative green density of 55 ± 2% (measured geometrically). Various heating rates from 35 to 150 K min−1 were used up to a temperature of 1200 °C under vacuum. Isothermal step at the maximum temperature lasted 10 min. A pulse pattern of 25:5 was adopted for heating. A load of 15.7 kN, corresponding to 50 ± 1.6 MPa, was kept constant. Axial shrinkage was measured with a resolution of 10 μm and the thermal expansion of the machine was taken into account for every heating rate through reference measurements obtained by placing a dense alumina sample in the pressing tool and subtracting these measured values from the original sintering curves. Data points were recorded every 1 s. The temperature was measured with an axial pyrometer, and calibrated with melting copper . The densities of all sintered specimens were measured by the Archimedes method in water at room temperature.
Results and discussion
In the case of submicron alumina (and more generally electrically insulating powders), the densification mechanism was found to be the same during FAST as in HP, i.e., grain boundary diffusion . For that, a rigorous methodology based on the use of the same processing parameters (powder, heating schedule, pressure, and atmosphere) and thorough measurement calibration was developed. No effect of electric field could be highlighted, and activation energies were found to be equal. Differences in densification curves were attributed to differences in real temperatures seen by the specimens. The value of activation energy measured from isothermal measurements lies typically in the range of 420–480 kJ/mol [17, 18]. An and Johnson  used an activation energy for pure fine alumina powder of 477 kJ/mol for their study case of HP, which showed an excellent agreement. Therefore, the lower value found in the present study needs to be rationalized. By coincidence, it corresponds to literature values of the activation energy for surface diffusion (230–280 kJ/mol ), which is a nondensifying mechanism. However, contrasting values resulting from different constant heating rates experiments have been very recently obtained, all on TMDAR alumina [9, 20, 21]. Depending on the processing method for the sample shaping as well as the heating rate, different activation energies characterizing pressureless sintering were measured. On the one hand, specimens sintered by dry pressing and pressure filtration showed different apparent activation energies (700 and 605 kJ/mol, respectively), both higher than expected . The analysis included heating rates between 5 and 25 K/min. On the other hand, very low-heating rates below 2 K/min lead to an enhanced neck growth due to surface diffusion during heating, which in turn seems to give high values of ~1000 kJ/mol . In contrast, higher heating rates may lead to temperature gradients in the densifying specimen. Motivated by FEM simulations of the temperature distribution, Raether et al.  proposed that too high heating rates (for example 25 K/min in their case) may lead to temperature heterogeneity within the specimen and consequently to artificially lowered activation energy values. When using FAST, it is clear that temperature gradients are expected at the high heating rates available. Temperature measurement, control, overheating are critical issues with this technique and a specimen heated at 1000 °C/min will meet entirely the required isothermal temperature only after a few minutes, and larger specimens will experience larger gradients. It was already shown that with a heating rate of 10 K/min, the temperature within the sample becomes homogeneous only after 60 s . On the other hand, Olevsky et al.  have shown that high heating rates are expected to enhance the densification rates by leaving more quickly the temperature regime for which surface diffusion is dominant and avoiding too early pore spheroidization. These findings coincide with the lower value of the activation energy characterizing the whole sintering cycle measured here.
In this study, we have shown that the MSC approach can be applied to the Field-Assisted Sintering Technique (as well as HP, but with reduced range of heating rates). This complements other modeling methodologies like finite element simulations as proposed by Maizza et al. . Experimental data confirm the general applicability of this theoretical framework and gives an apparent activation energy of ~290 kJ/mol. As alumina densifies by grain boundary diffusion in the present conditions, this low value of the apparent activation energy might be due to thermal gradients in the specimens induced by rapid heating. However, even for the wide range of heating rates investigated, it seems that the temperature measurement and homogeneity within the specimen is still good enough to be described by the MSC framework. If relevant, other densification mechanisms could be implemented in the model in order to adequately describe the densification behavior of electrically conductive materials induced by the electric field or current.
This work was financially supported by the Deutsche Forschungsgemeinschaft (Emmy Noether Program GU993-1/1). Michael Hoffmann is acknowledged for providing access to his FAST setup.