Centrifuge-aided Micromolding and Sintering of Micron- and Submicron-sized Ceramic Features
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Microfabrication of ceramic features has become a critical issue in realizing the miniaturization of devices. Micromolding and sintering play critical roles in fabricating micron- and submicron-sized ceramic features using nanoparticles. Developed from soft lithography, replica molding has been proven a good method to prepare micron- and submicron-sized features. However, the fidelity of the features can be compromised by incomplete feature cavity filling and feature shrinkage during the forming process. In this study, centrifuge-aided micromolding is developed to prepare micron- and submicron-sized ZnO features. By introducing a centrifugal force, the shear-thinning behavior of the suspensions is utilized, and the cavity filling process and the diffusion of trapped air out of the features are accelerated. The drying shrinkage is decreased by increasing the density of the wet nanoparticle packing from the centrifugal process. The centrifugal force improves the fidelity of all the designed features. ZnO ridges from 0.4 μm to 2 μm size and rods of 1.6 μm size are prepared successfully. The wide applicability of this strategy has been demonstrated by preparing ZrO2 features via the same method. Sintering process has a significant influence on the morphology and microstructural evolution of micron-sized ceramic features. When ceramic features decrease to much smaller sizes, such as in the micron range, the dominating sintering mechanism(s) can be different from those of the bulk at large scales. However, limited effort has been devoted to understanding the sintering behaviors. In this study, the as-prepared micron-sized ZnO ridges and rods were sintered at 950oC for different time in air atmosphere. The sintering process destructs the ZnO features via abnormal grain growth and surface roughening. Destruction prediction of features using sintering time is established based on grain growth. Feature surface roughening is further analyzed with respect to thermodynamic fundamentals. Because of the evaporation tendency during zinc oxide sintering, sintering atmosphere has a significant influence on the sintering behavior and feature fidelity. In this study, micron-sized ZnO ridge features were sintered under air and argon atmospheres. Ridge size, line edge roughness, and grain size were characterized. Quantitative calculation of sintering behaviors was performed in order to obtain fundamental understating of the micron-sized ZnO feature sintering. It is found that oxygen partial pressure is the deciding factor for the ridge feature evolution. ZnO evaporation and defects diffusion are responsible for the ZnO bulk and ridge sintering behavior differences.
General Audience Abstract
In order to produce portable devices with small sizes, novel techniques are required to make small components, which is called microfabrication. Since ceramic materials are widely used in various electronic devices, microfabrication of small ceramic features has become an important issue. When ceramic nanoparticles are used as the raw material, the fabrication of ceramic features mainly consists of two processes: micromolding and sintering, which are the problems that this thesis focuses on. In the micromolding process, the loose nanoparticles are packed to form features with specific shapes. In the sintering process, the nanoparticles in as-prepared features are bonded into a coherent and dense feature. For the micromolding process, a suspension made from the nanoparticles is poured into a mold with as-designed feature shape, and the dry feature is obtained after a drying process. In this study, the factors that will affect the shape of the features are studied. It is found that the major factors include completeness of the filling process and shrinkage during the drying process. By completing the micromolding process in a centrifugal machine, the micromolding process is accelerated, and the shrinkage during the drying process is decreased. Both the two aspects will benefit the feature quality. By using this technique, zinc oxide ridges from 0.4 μm to 2 μm size and rods of 1.6 μm size are fabricated successfully. It is also demonstrated that this technique can be applied to other ceramic materials. Sintering process can convert packed nanoparticles into a coherent object, which can help us to obtain dense ceramic features. However, the sintering process will cause the change in feature shape. For large size ceramic bulks, the sintering theory has been well established to explain these changes. When the size of ceramic materials decreases to very small scale, such as micron size, new sintering theory is needed to explain the change of ceramic features in the sintering process. In this study, micron-sized zinc oxide ridges and rods were sintered at 950oC for different time. It was found that the sintering process will distort the shape of the zinc oxide features. Based on thermodynamic views, the corresponding new theory was established. Because zinc oxide is relatively easy to evaporate during sintering, sintering atmosphere will also affect the shape of the features. In this study, micron-sized zinc oxide ridge features were sintered under air and argon atmospheres. It was found that oxygen content was the major factor that will affect the shape change. The corresponding theory was established to explain the effect of the sintering atmosphere based on thermodynamic views.
- Masters Theses