Using Fusion for Simultaneous Electrical and Heating: Electric-Fiel Assisted Sintering of ZrO2 Nanoparticles

Sintering is the process of densification and consolidation of powders into desired shapes and geometries. Densification of ceramics or powder metals and firing of pottery into preferred shapes to increase material strength are examples of the sintering process.

The sintering process occurs at temperatures below the melting point of a material, saving material and energy. The process also has other benefits such as high precision fabrication of complicated geometries and reduction of post-processing requirements, so parts are ready to use.

However, pores and voids often remain in the material from the original powder, leading to undesired behavior and poor performance.

Using heat alone, sintering occurs at high temperatures (~80% of melting point) and requires several hours to complete. Ceramic or metal powders undergo a three-stage evolution during the sintering process: the particles make contact, then neck, and finally the porosity decreases considerably.

Diffusion is the dominant driving mechanism for the sintering process. In addition to temperature, electrical current is also used to lower the sintering temperature, the time taken, and improve the properties of the final product. However, the contribution of electrical current – whether sintering enhancement is due to the applied electric field surrounding the particles or the current flowing through the particles – is not clearly known.

With the transmission electron microscope (TEM), researchers can image and examine materials at the atomic scale. The advent of new, sophisticated tools to apply stimuli in situ, such as heat and electrical bias has expanded TEM capabilities, enabling researchers to gain a better understanding of dynamic mechanisms in their samples.

The Protochips Fusion system can heat from room temperature to 1200 °C and take electrical measurements down to the picoAmp level. The system is controlled with user-friendly, advanced workflow-based software known as Clarity.

Semiconductor MEMS devices, known as E-chips, enable ultra-stable heating experiments to be carried out with best in class temperature uniformity and accuracy, and low noise electrical measurements. An advanced electrothermal E-chip now integrates heating and electrical biasing to enable simultaneous electrical and heating stimuli.

Figure 1. (a) Side-view and (b) top-view schematics of the Protochips electrothermal E-chips. Sample is heated by SiC heating membrane. Electric field is applied via two parallel W electrodes to generate a homogenous noncontacting electric field.

Fusion is the only in situ instrument available on the market today with such a unique and useful feature. The electrical contacts on the new electrothermal E-chips are patterned over a superior quality, insulating silicon nitride membrane, directly on a silicon carbide membrane, which acts as the sample support and heater.

Fusion E-chips are available in different geometries to suit a variety of experiments and assist scientists to conduct studies that were not previously possible.

Experiment

Scientists in the van Benthem group at University of California, Davis, used an aberration-corrected JEOL JEM 2100F/Cs in STEM mode to investigate the sintering mechanisms in 3 mol% yttria-stabilized ZrO2 (3YSZ) nanoparticles.

The experiments were conducted with thermal E-chips at temperatures up to 1200 °C, and with electrothermal E-chips to 900 °C, and under an electric field of 500 V/cm.

In both studies, the microstructural evolution of the agglomerates during densification was monitored using the in situ TEM and STEM images. Densification curves were generated using MATLAB to measure pore shrinkage as a function of time and applied electric field strength.

Discussion

The consolidation process of 3YSZ particles upon heating, with and without the application of an electric field is shown in Figure 2. Using heating chips, there is no change in the particle structure when heated at 900 °C for 106 minutes (Figure 2(a-c)) and the pore size reduces after raising the temperature to 1200 °C, as indicated by arrows in Figure 2d and 2e.

The sintering effects were compared under an electric field by heating a fresh 3YSZ sample with electrothermal chips to 900 °C and applying a homogeneous 500 V/cm noncontacting electric field.

Figure 2. Under no external electrical field: (a-c) Heating to 900 °C does not show significant structural changes after 106 min. (d) and (e) represent pores annihilation upon heating to 1200 °C. Upon application of electrical field: (f-h) show significant morphological changes at 900 °C after only 4 minutes.

Pore shrinkage and particle coalescence took place after just four minutes. The insets in Figure 2f-h display a close up view of the interparticle neck growth and particle coalescence seen in the presence of the electric field.

Image analysis by MATLAB revealed that the projected area of the agglomerate was decreased by 3% after 106 minutes when heating to 900 °C without the application of the electric field. However, when the electric field was applied, the projected area was decreased by 7% after four minutes.

The authors suggest that enhanced mass transport is attributed to defect formation between two adjacent powder particles, facilitating diffusion. As a result, electrical conductivity is increased, and neck formation and consolidation is promoted at lower temperatures.

However, the densification rate of current-assisted processes is faster than non-contacting electrical field assisted processes. The reason could be the additional Joule heating due to the electrical current flowing through the sample.

Applications

Simultaneous temperature and electrical current stimuli are essential in a variety of fields, including the study of domain switching at high diffusion rates, temperatures, and directions dependence upon aging, heating, and degradation of electronics, etc. For example, studies of lithium-ion batteries and fuel cells at high temperature can explain the role of diffusion paths that are not activated at room temperature.

The Fusion system provides a unique and innovative approach via E-chip devices that are ideally suited for a range of experiments and studies with almost all microscopes from Hitachi, JEOL, and FEI.

This information has been sourced, reviewed and adapted from materials provided by Protochips.

For more information on this source, please visit Protochips.

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