Nano-enabled Energy Conversion, Storage and Thermal Management Systems (NEXT) Group

University of Colorado at Boulder

Director: Dr. Ronggui Yang, Associate Professor of Mechanical Engineering

Office Location: ECME 136, Engineering Center

Post-Doctor Offices: ECME 251A, ECME 251B

Student Offices and Labs: ECME 165, ECME 219

Tel: 303-735-1003 (O), 303-735-1763 (Lab); Fax: 303-492-3498

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Professor Ronggui Yang’s group controls ~2000 square feet lab space for the Nano-enabled Energy Conversion, Storage and Thermal Management Systems (NEXT) Group. Yang’s group is specialized in the following areas:

A). Low-Cost Large-Scale Nanofabrication using Templates

B). Thermal Conductivity Measurement of Nanostructured Materials

C). Phase-Change Heat Transfer on Micro/Nano-Structured Surfaces

D). Modeling and Simulation of Nanoscale Thermal and Thermoelectric Transport

A). Low-Cost Large-Scale Nanofabrication using Templates

A1. High Power DC supply for Large Scale Fabrication of Porous Anodic Alumina Template

This system consists with an Altech DC power supply and a Sorenson DLM 300-2 DC power supply. The Altech DC power provides up to 20 A with a constant 24V voltage output, which is used to polish Al foils with area larger than 10*10 cm2. The Sorenson DLM 300-2 DC power supply gives a stable and tunable DC output with voltage up to 300V and current up to 2A, which can be used to fabricate the porous anodic alumina template with tunable size of 20-300nm.

A2. Langmuir Blodgett Through

Langmuir troughs are used to create, modify and study monolayer at either the gas-liquid or liquid-liquid interface. This system is computerized to monitor surface pressure and precisely control the moving speed for both barriers and dipper in real time. Using this equipment, we are able to create ordered monolayer or multilayer polystyrene arrays with controllable density.

A3. Multifunctional Electrochemical Workstation

The CHI 760C electrochemical workstation is computerized potentiostat/ galvanostat with precise control. The potential control range is ±10V and the current range is ±250mA. This instrument is capable of measuring current down to picoamoperes. Moreover, this instrument is equipped with more than 30 electrochemical techniques, including: cyclic voltammetry, multi-potential steps, multi-current step, etc.. Thus, it is well suitable for the electrochemical properties study and complex electrodeposition process. Cu nanowire arrays, Bi2Te3 nanowire arrays, Bi2Te3/Sb heterostructure nanowire arrays have been fabricated based on this equipment.

A4. High Frequency Ultrasonicator

The misonix ultrasonic liquid processor (s-4000) is the world's most advanced ultrasonic processor. It comes with digital signal processing enables precise control of amplitude and output power and the output amplitude is controlled from 1-100% giving a greater degree of resolution, which lead to the consistent and reproducible sample processing. This system can be used to uniformly disperse nanostructures in liquid, such as: polystyrene particles, Cu or Ni nanowires.

B). Thermal Conductivity Measurement of Nanostructured Materials

A range of electrical and optical measurement systems have been developed in the NEXT lab for the measurement of thermal, thermoelectric and electrical properties of both bulk and nanostructured materials from low temperature to high temperature including the conventional four-probe measurement systems, 3-omega system for thermal measurement of nanostructures, the transient hot wire measurement system, and the very complicated two color and EUV femtosecond optical pump-and-probe systems, which are described in detail below.

B1. Two-Color Pump-and-Probe system for both bulk and nanostructure thermal property characterization

Figure 1 (a) and (b) show the schematics and a snapshot of the ultrafast thermoreflectance setup in our lab [[i]]. The setup uses a femtosecond laser (oscillator) and a 0.6m mechanical delay stage (8ns optical delay with two round trips). The Spectra-Physics Tsunami femtosecond Ti-sapphire laser, pumped by a 10W diode laser, emits a train of 150 fs pulses at a repetition rate of 80 MHz. The central wavelength is 800 nm and the power per pulse is roughly 19 nJ. The laser pulse is split into pump and probe beams. The pump beam passes through an electro-optic modulator (EOM) that modulates the beam at a frequency between 0.1 and 20 MHz. The modulation frequency serves as the reference for a lock-in amplifier which extracts the thermoreflectance signal from the background. The second-harmonic generator (SHG) is used to double the frequency of the probe pulses, which produces a light train with a central wavelength of 400 nm and are time-delayed relative to the pump pulses with the mechanical stage.

(a)

(b)

Figure 1 (a) and (b) the schematics and a snapshot of the ultrafast thermoreflectance setup in our lab.

B2. A femtosecond EUV Pump-and-Probe System using high resolution X-ray CCD camera configured for two-dimensional nanoscale thermal imaging.

EUV light or soft X-ray is the electromagnetic radiation at wavelengths 10-to-100 times shorter than visible light, typically 1-50nm wavelength. Such short-wavelength radiation makes it possible to "see" smaller features and to "write" smaller patterns for such applications as lithography or microscopy. Table-top ultrafast short wavelength light sources using high harmonic generation (HHG) can be used in a new generation of applications in nanotechnology, material sciences, biology, chemistry, plasma physics and so on. Indeed High harmonic generation (HHG) is an ideal coherent light source for studying nanoscale thermal transport effects for many reasons. First, the spatial coherence of HHG in a waveguide enables highly sensitive interferometric measurements of the temperature-dependent surface profile. Moreover, the short wavelength of HHG (tunable from 100-1 nm) allows for tighter focusing and better phase sensitivity in interferometric experiments. Finally, the femtosecond and sub-femtosecond duration of HHG pulses holds promise for achieving unprecedented temporal resolution in dynamics experiments. Figure 2 shows the schematics of the experiment setup for probing nanoscale thermal transport using EUV light [[ii],[iii]]

Figure 2: Experimental geometry: an infrared laser beam at 800 nm light heats the nanostructure, and a soft x-ray beam at 29 nm monitors the heat flow from the nanostructure into the substrate.

B3. Laser Flash System and DSC System.

We have standard measurement facilities for specific heat and thermal diffusivity for bulk materials. Figure 3(a) shows the Differential Scanning Calorimetry (DSC), the facility using a thermo-analytical technique which measures energy directly and allows precise measurements of heat capacity. Figure 3(b) shows the Laser System for measuring thermal diffusivity.

(a)

(b)

Figure 3 (a) Netzsch DSC 204 F1 Phoenix in our lab (b) Netzsch LFA 457 Microflash in our lab

C). Phase-Change Heat Transfer on Micro/Nano-Structured Surfaces

A Pool Boiling Test System has been set up to test the pool boiling performance of a variety of structured and functionalized surfaces. Both traditional heating bar method and the novel integrated heating/sensing method can switch easily on this system. A high speed camera Photran APX with a speed up to 250000 fps is used to visualize the nucleate boiling process. A Flow Boiling Testing Loop has also been set up to investigate the flow boiling heat transfer characteristics in single/multi- microchannels. This setup shares the high speed image system with the pool boiling system. A Condensation Test Loop is currently being constructed.

D). Computational and Simulation Tools:

Our research group has a record of successful development of algorithms and tools for studying microscopic multi-physics problems.

We have access to several commercial software packages and open source codes for constructing, displaying, animating and analyzing large molecular systems using three-dimensional graphics and built-in scripting. These tools are very helpful for the investigation into all kinds of material systems, such as soft materials (biomolecules, polymers) and inorganic materials (metal, semiconductors).

For ab-initio quantum mechanical molecular dynamics simulation (QD-MD), we have the licenses of the VASP package and have some experiences using free softwares such as ABINIT, WIEN2K, and Gaussian. For the classical molecular dynamics simulation, we use both open source code LAMMPS developed by Sandia National Laboratories and our in-house code.

We have developed our in-house equilibrium molecular dynamics code for the calculation of thermal conductivity of bulk and nanostructures based on the Green-Kubo formula. We also have developed code for lattice dynamics simulation and nonequilibrium Green’s function (NEGF) calculation. The lattice dynamics code is used to calculate the phonon dispersion and density of state of bulk and nanostructures. Nonequilibrium Green's function is has been developed for studying phonon transmission of material interfaces.

We have also developed phonon transport simulation code by solving the Boltzmann transport equation (BTE) for two- and three-dimensional problems using both discrete ordinate method and Monte Carlo simulations. This code has proved to be an effective tool to study thermal problems in nanoscale transistors and nanocomposites.

Computer Systems:

Our research group has access to a variety of high-performance computing platforms which will allow the development, verification and application of the proposed multiscale simulation and design tools. We have access to two clusters for the large-scale parallel computation.

One is the Prospero Cluster in Department of Mechanical Engineering, which is jointly owned by the PI Yang and two other faculty colleagues in the department, has 32 computing nodes and 384 cores. The cluster operates with ROCKS+ 5.3 OS, including 20 TB RAID 6 storage, ellanox MIS5030Q 36Port, 40Gb/s per port Infiniband Switch, and Supermicro 48 Port Layer 3 1/10 Gigabit Ethernet Switch. The processors used here are dual Xeon X5650 CPUs 6 core Westmere with 24GB ECCR memory and 250GB hard drives.

The other one is the supercomputer system funded by the National Science Foundation. The instrument combines Sun’s highest density Constellation assemblies, Intel’s next generation Nehalem-EP microprocessors, a custom InfiniBand QDR-based mesh interconnect, NVIDIA accelerators, and Sun Thumper storage modules to produce a highly balanced I/O rich, and cost-effective computing system.

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[i] J. Zhu, D. Tang, W. Wang, J. Liu, K. W. Holub, and R. Yang, “Ultrafast Thermoreflectance Techniques for Measuring Thermal Conductivity and Interface Thermal Conductance of Thin Films”, Journal of Applied Physics, 108, 094315 (2010).

[ii] M. E. Siemens, Q. Li, M. M. Murnane, H. C. Kapteyn, R. G. Yang, E. H. Anderson, and K. A. Nelson, “High-frequency surface acoustic wave propagation in nanostructures characterized by coherent extreme ultraviolet beams”, Applied Physics Letters, 94, 093103 (2009).

[iii] M. E. Siemens, Q. Li, R. Yang, K. A. Nelson, E. H. Anderson, M. M. Murnane, and H. C. Kapteyn, “Quasi-ballistic thermal transport from nanoscale interfaces observed using ultrafast coherent soft X-ray beams”, Nature Materials, 9, 26-30(2010).