Zhaoxi Chen

Kinetics of evaporation and gel formation in thin films of ceramic precursors.

Precursors derived from the hydrolysis of organic or inorganic salts have been widely used to produce ceramic coatings for a broad variety of applications. When applying the liquid precursors to the substrates, it is extremely challenging to control the film uniformity and homogeneity. The rate of solvent evaporation at different locations is different, causing the viscosity variation and flows in the film. There is very limited knowledge about the viscosity change in evaporating ceramic precursors. Therefore, it is crucial to understand the effect of evaporation on viscosity variation in thin films and droplets. We use magnetic rotational spectroscopy to study the time dependence of viscosity in mullite precursors. A correlation between the viscosity change and evaporation kinetics is revealed. This correlation was used to relate the change of viscosity to the concentration of mullite. A master curve relating viscosity to the mullite concentration was constructed and used to propose a possible scenario of the viscosity increase during solvent evaporation.

Measuring flexural rigidity of mullite microfibers using magnetic droplets

Flexural rigidity of many microfibers is known to deviate from the Bernoulli-Euler predictions that neglect shear deformations. We examine mullite microfibers formed by electrospinning of sol-gel precursors. The formed fibers have diameters smaller than 10 μm. A magnetic drop was placed on the free end of a dangling fiber, and the fiber was flexed by applying a non-uniform magnetic field. By applying different magnetic fields, we generated a series of different fiber profiles and filmed the process of fiber bending. Mullite microfibers were found to follow the Bernoulli-Euler predictions, and the shear deformations in the material were insignificant. This was confirmed by employing the Euler elastica model to describe the fiber profiles. The bending test provided a Young modulus of E = 100 GPa, which appeared to be very close to that found from the tensile test.

Permeation and optical properties of YAG:Er3+ fiber membrane scintillators prepared by novel sol–gel/electrospinning method

An electrospinning method for fabrication of the YAG:Er3+ fibrous membrane is developed and the scintillation properties of the obtained membranes were examined. A homogeneous precursor YAG sol was synthesized allowing to control the sol–gel transition. The synthesized precursor allows one to achieve the 5 wt.% level of fiber doping with Er without formation of any undesired crystalline phases. It was found that the relative humidity had a strong impact on the fiber microstructure. The fibers obtained at the low relative humidity level (~30%) had almost straight cylindrical shape with an average diameter of ~590 nm, their surface was smooth. The shape of fibers obtained at the high relative humidity level (~50%) deviated from the straight cylindrical shape and the average diameter was larger, ~1.12 µm. The fluid permeability of membranes, K, obtained at the low relative humidity level was measured using an upward wicking experiment to give K~10−13 m2. The YAG:Er membrane presented a strong green photoluminescence under ultraviolet excitation and intense radioluminescence dominated by emission lines at 398 and 467 nm under the X-ray excitation. The properties of these materials make them promising candidates as porous scintillators for the detection of ionizing radiation of flowing fluids.Graphical Abstract

Magnetic submicron mullite coatings with oriented SiC whiskers.

Addressing the challenge of making ceramic thin films with the in-plane-oriented nanorods, we propose to decorate the nanorods with magnetic nanoparticles and orient them using the external magnetic field. As an illustration, the mullite thin films with embedded and oriented SiC nanorods were synthesized. The SiC nanorods were decorated with the Fe3O4 nanoparticles. A two-step processing route was developed when the nanorods are first oriented in a sacrificial polymer layer. Then, the polymer film with the aligned nanorods was removed by heat-treatment. In the second step, a sol-gel/dip-coating method was applied to produce the mullite composite film. The main challenge was to guarantee that all of the nanorods that were initially randomly distributed in the polymer would have time to rotate toward the field direction before complete solidification of the sacrificial layer. Theoretical and experimental analyses of the orientational distribution of the nanorod axes were conducted to identify a relationship between the polymer viscosity and processing parameters of the system. In contrast to the ferromagnetic nanorods, the rate of rotation of paramagnetic nanorods and their time of alignment are more sensitive to the magnetic field. This methodology allows manufacturing of different ceramic films with aligned nanorods and making nonmagnetic ceramic coating magnetic.