Low dimensional materials such as thin films and nanowires exhibit unique energy transport behaviors compared to bulk counterpart.
For instance, classical as well as quantum size effect may occur at low dimensions that lead to reduced thermal conductivity. Moreover, artificial nanostructuring may offer further modification of transport properties that is far from bulk. We explore fundamental aspects of heat transport in various nanostructured materials to seek for possible utilizations in diverse technological applications. Using in-house built 3-ω method, we are capable of measuring and analyzing the thermal conductivity of thin films.
Ordered mesoporous silica and nitrogen-doped GeSbTe alloy are few good examples for modifying the physical properties including thermal conductivity via nanostructuring. Few nanometers or ordered mesopores in mesoporous silica can significantly change its thermal transport behavior while providing structural integrity and maintaining dielectric constant as low as possible. With its excellent thermal conductivity as well as mechanical and dielectric properties, ordered mesoporous silica thin films can be a perfect candidate for low-k dielectric applications.
Also, nitrogen doping in Ge1Sb4Te7 can effectively suppress the crystalline growth behavior during its phase change.
This leads to reduced thermal as well as electrical conductivity which are favorable for reducing energy consumption and reset current in the phase-change memory device.
In the last decade, nanowires have attracted a considerable amount of attention, especially in the fields of electronics and energy applications because of their unique transport properties. Among many fabrication methods, template-assisted electrodeposition is regarded as an attractive route for the synthesis of nanowire arrays because of its simplicity, cost-effectiveness, room temperature fabrication, etc.
Moreover, in the feasibility aspect, template-assisted nanowires have an extraordinary advantage over other nanowires that are grown by different methods. In detail, the template can be readily used as a supporting matrix without any further fabrication process.
Supporting matrix, which is essential in fabricating nanowire-based electronic devices, provides mechanical robustness, electrical insulation among adjacent nanowires, and fabrication feasibility when contacting the electrodes at the ends of the nanowires. Ideally, template-assisted electrodeposited nanowires should all be connected to the conducting electrodes at the both ends.
Unfortunately, nonuniform ion transport near and inside the template pores results in nonuniform growth of the nanowires that creates only a few numbers of nanowires that are fully grown to be exposed out of the template and completely block the other remaining pores having shorter nanowires.
This is extremely critical in fabricating the nanowire-based devices since only a limited numbers of the nanowires are actually in contact at the both ends. We overcome such problem by lowering the deposition temperature down to subzero centigrade.
Even with highly disordered commercial porous anodic aluminum oxide template and conventional potentiostatic electrodeposition, length uniformity over 96.5% can be achieved when the deposition temperature is lowered down to -2.4°C.
Decreased diffusion coefficient and ion concentration gradient due to the lowered deposition temperature effectively reduces ion diffusion rate, thereby favors uniform nanowire growth. Moreover, we can artificially tune the morphology of electrodeposited nanowires by controlling the growth processes during electrodeposition. By changing the temperature and controlling the rates of two different growth mechanisms, i.e. kinetics and thermodynamics, nanowires having various classes of morphologies can be obtained. For instance, a rough nanowire with an irregular wire diameter along the wire axis is obtained at a high deposition temperature, whereas a smooth, compact nanowire is obtained as the temperature is lowered.
However, as the temperature is dropped further down to subzero degrees, the nanowires exhibit rough, dendritic morphologies with relatively uniform wire diameters. These artificial modifications of morphologies in electrodeposited nanowires can be applied to various state-of-the-art applications such as photovoltaics, thermoelectrics, phase-change memory devices, two-phase heat transfer applications, etc.
Surface roughness is promotive of increasing their hydrophilicity or hydrophobicity to the extreme according to the intrinsic wettability determined by the surface free energy characteristics of a base substrate.
Top-down etched silicon nanowires are used to create superhydrophilic surfaces based on the hemi-wicking phenomenon. Using fluorine carbon coatings, surfaces are converted from superhydrophilic to superhydrophobic to maintain the Cassie-Baxter state stability by reducing the surface free energy to a quarter compared with intrinsic silicon.
Considering the schemes for hemi-wicking and Cassie-Baxter state, we present the criteria for superhydrophilic and superhydrophobic conditions and design guidelines as a function of geometric variables of the silicon nanowires. The morphology of the silicon nanowires is used to demonstrate their critical height exceeds several hundred nanometers for superhydrophilicity, and surpasses a micrometer for superhydrophobicity.
Boiling heat transfer based on phase-change phenomena is essential as a cooling technology on lots of energy consumption and conversion devices.
Improvement of boiling heat transfer performance is promising via nanoscale surface manipulation technology. We propose design technique on nanoscale structures in the light of surface geometry and wettability characteristics. By controlling artificial surface geometries and its local surface free energy, it is also possible to manipulate static contact angle on the surface from superhydrophilic to superhydrophobic regime.
Considering the nanoscale surface geometries and the wettability characteristics, we demonstrate the improvement of boiling heat transfer based on qualitative local heat transfer measurements including critical heat flux (W/cm2) and heat transfer coefficient (W/m2K).
Using the nanostructure-decorated surface, it is possible to decrease surface wall superheat significantly. In addition, we conduct visualization for dynamics of vapour bubble generated by phase-change in order to demonstrate the improvement of boiling performance by comparing the vapour behaviours on nanostructure-applied surfaces with those on the plain surface.