Marconnet Thermal & Energy Conversion Lab Thermal Properties of Soft Nanomaterials: Materials Synthesis and Fabrication Marconnet Thermal & Energy Conversion Lab Meng Pan, Yu Han GM:Collier Miers, PI: Amy Marconnet Hello everyone, My name is Meng Pan and I am going to be a junior student in Mechanical Engineering. I am working as undergraduate research assistant in Marconnet Thermal and energy conversion lab. The topic I am talking about today is Thermal Properties of Soft Nanomaterials. We had a team of two and I am doing more about material fabrication while my partner yu was working on the measurement technique.

Introduction Soft nanomaterial: Hydrogel+Nanoparticle Objectives: A class of cross-linked polymers Nanoparticles allow tuning material properties Gelatinous state and mechanical instability Objectives: Engineer a soft nanomaterial Measure thermal properties First we need to know what soft nanomaterial we are making. In past papers, they show that the combination of soft materials (such as polymers and gels) with nanoparticles offers a unique platform for tuning multiple material properties. For this summer research, the soft nanomaterial we have been working on is hydrogel. Hydrogels are a class of cross-lined polymers that that can absorb large quantities of water changing their shape under the influence of various conditions such as humidity, temperature, and pH. On the bottom right hand corner is one of our hydrogels we fabricated. It feels just like jelly if you touch it. Currently, it is not possible to characterize the property and the property-structure relationships of soft nanomaterials accurately due to the Gelatinous state and mechanical instability. This research aims to develop a low cost, reliable, accurate method of measuring the thermal conductivity or thermal diffusivity of soft materials and seeks to engineer a material that has significant contrast in properties at different temperatures. See Yu Han’s poster for more details about measurement techniques

Challenge of Measuring Soft Materials How do you measure the temperature without thermocouples? Hydrogels swell, change shape, and need to be contained during synthesis and measurement IR Thermal Mapping Embedded Heater After Before Soft nanomaterial like hydrogel is water contained. If we insert thermal couple wire inside, it will short the circuit which can’t give us readings and might damage the equipment at the same time. Insulated thermocouple will impact temperature readings significantly. And we also considered that most non-electrical thermometers can’t give exact readings on temperatures. As I mentioned, the hydrogel preforms like jelly. However, they are hard to maintain steady shape for us to measure like what we did to measure hard solid objects. They can swell, change shape. Hydrogels we made are thin films and even more fragile to operate. 1 hour

Determining Temperature Infrared Microscopy: 2D Temperature mapping with high spatial resolution (1.7 to 12 µm/pixel depending on magnification) Movie mode allows 10 Hz frame rate time dependent temperatures to be captured The method we used to measure thermal properties is using Infrared Microscopy, commonly known as IR camera. This is a picture of our ir camera which we called ferrari. Set the sample on the stand, suspended by two wires and a heating line across the sample

Angstrom Method Thermal Diffusivity: x h, To Heat/Cool Modified Angstrom Method: (1) IR camera for 2D thermal movies  Higher accuracy and more data points The method we used to calculate thermal diffusivity is Angstrom Method. This method applies a sinusoidal heating to the sample at the edge. If you pay attention to the picture on the top right hand corner, the heating happens at the black dot which is the sine wave. The red and green dot are thermo couples that used to measure the temperature readings. In the diagram, you can see that the temperature become lower as heat transfer through the sample. And there is a heat delay in time between these sinusoidal waves. For soft nanomaterials, we are not using thermo couples. We use the IR camera for 2D movies which can give us higher accuracy and more data points to analyze compare to the old method. We also use square wave heating resource which gives us more power to heat the wire. Square waves are sinusoidal waves under fourier transform. The square wave heating can still give us a first harmonic wave for us to analyze. (2) Square-wave heating source + Fourier Transform to analyze data Ghasemi et al. “High Thermal Conductivity Ultra-High Molecular Weight Polyethylene (UHMWPE) Films”, ITHERM 2014, Orlando, FL. http://2.bp.blogspot.com/_okIcsBieX4U/TRKogEZh16I/AAAAAAAAAIY/spYsURzN3d0/s1600/squarewave32terms.png

Determining Temperature Using the thermal map software, we are able to convert the movie plot in to a huge matrix of temperature readings. And we can transfer these code into the information we need using matlab. 1 select time range for analysis 2 specify angle of heater 3 pic 1 point to specify heater edge which is defined as 0 line 4 specify angle of heater 5 pick the zero point on the bode plot ---- Magnitude of the temperature oscillations decrease as we move further from the heater line and the phase delay increases (because it takes time for the heat to diffuse from the heater along the sample).

Materials Synthesis Material Fabrication: Freeze-thaw method Materials fabricated Poly vinyl alcohol (PVA) PVA & Poly vinyl pyrrolidone (PVP) PVA & Dimethyl sulfoxide (DMSO) Above hydrogels with nanoparticles Freeze-thaw method Soluble materials Certain hours of freezing and certain hours of thawing as one cycle More cycles increase porosity Time cycle Temperature 25 ̊ C -20 ̊ C There are many types of hydrogels that we could synthesize and the each have unique material properites. I chose to fabricate PVA-based hydrogels because they swell based on temperature alone in the temp. range of interest. Specifically, they won’t have significant changes in volume based on pH level and water content and so we can focus on their temperature response. Based on my literature review, I found that the freeze-thaw method is the most used technique to fabricate PVA based hydrogels and there are many types of PVA-hydogels to choose from. The materials we fabricated are PVA plastic, PVA and PVP hydrogel, PVA and DMSO hydrogel, and the above hydrogels embedded with gold nanoparticles. These are the samples that are easier to fabricate and quicker to fabricate since we only have 11 weeks for research. Many hydrogels require freeze-thaw cycles of several hours repeated for several weeks, but the chosen materials can be fabricated in just a few days. Freeze-thaw method definitely requires soluble materials. The most important is certain hours of freezing and thawing. For example, the PVA and PVP hydrogel requires 21 hours of freezing while PVA-DMSO, which is easier to form gel shape, take only 8 hours. Cycles need to be repeated in order to acquire gel shape. More cycles will increase porosity which means that there are more spacings inside the hydrogel. Hydrogels with more cycles will swell more than hydrogels with fewer cycles. Separated Cross-linked http://kinki.chemistry.or.jp/pre/a-54.html

Materials Synthesis: Results PVA plastic PVA+PVP hydrogel PVA+DMSO hydrogel PVA+PVP Gold Nano hydrogel PVA plastic: Very thin film after fabrication Adhered to the glass surface easily PVA+PVP hydrogel: More cycles make the hydrogel stronger1 Hard to shrink and offers flat and uniform distribution on the surface PVA+DMSO hydrogel: Faster to fabricate but can’t stay as long as the other hydrogel PVA+PVP Gold Nano-hydrogel: Deep red: less than 30 nm of gold nanoparticle 1 Shrink faster than PVA+PVP hydrogel does Higher resistance than that of hydrogel without nanoparticle For small (~30nm) monodisperse gold nanoparticles the surface plasmon resonance phenomona causes an absorption of light in the blue-green portion of the spectrum (~450 nm) while red light (~700 nm) is reflected, yielding a rich red color. http://www.sigmaaldrich.com/materials-science/nanomaterials/gold-nanoparticles.html#sthash.T4wRh3dE.dpuf

IR Microscope: Results Thermal Diffusivity: Measurement in progress However, luckily, we have good results come out from IR scope. This is thermal diffusivity plot from our measurements. The hydrogel has lower diffusivity than the PVA plastic, but hydrogel with embedded gold nanoparticl has 15 times diffusivity than the hydrogel. DMSO hydrogel onlly give us one set of data and it is not funtioning anymore. So we might do more measurement on this material later. Finding the thermal diffusivity is not the end. Using the equation at the bottom, we are able to calculate more thermal properties. 𝛼= 𝐾 𝜌∗ 𝐶 𝑝 𝐾is thermal conductivity 𝜌 is density 𝐶 𝑝 Is heat capacity Other thermal properties

IR Microscope: Results There is a very interesting fact we observed when we are doing the measurement. The pvapvp gold nanoparticle hydrogel shrunk to that tiny size within 1 hour. The diffusivity goes up as the water evaporated from the sample. the fitting curve shows as polynomial function which is beyond my understanding. We definitely will go further on this part in our future work.

Conclusion Developed a method for fabricating hydrogels with and without gold nanoparticle for thermal characterization. IR results allow measurement of thermal diffusivity with low error (<10%). Need additional measurements of heat capacity and density to get thermal conductivity. Hydrogel with gold nanoparticles had >15x larger thermal diffusivity than pure hydrogel. As hydrogel with gold nanoparticles shrunk, the thermal diffusivity increased significantly Future Work: New Hydrogels: PVA based hydrogel with different combination Poly N-isopropylacrylamide (PNIPAM) based hydrogel Improved metrology taking into account hydrogel swelling and shrinking. The IR methods works well to find out the thermal diffusivity and provide data with less than10 percent error. However there are so much uncertain factor which affected our results, such as: The shape of hydrogel is not uniform; the current and voltage across the heating wire is different; the water evaporate as we measure the hydrogel and so on. We definitely will limit these factors and I am certain that we have more accurate results. Materials diffusivity is related with water content and other factors. For example, right now we are using 5 wt% of PVA in the solution. We may make 10 % 15 % 20% and see how it affects the material property. And the amount of nanoparticles is also considered. The most used hydrogel for testing is based on Poly N-isopropylacrylamide (PNIPAM). We will start the fabrication on such hydrogel and able to make comparisons with results from papers.

Acknowledgement Soham Ghosh and others from Prof. Bumsoo Han’s Biotransfer Phenomena Lab. Summer Undergraduate Research Fellowship NSF's Nanotechnology Undergraduate Education (NUE) program I would like to acknowledge all of the help from Soham and all the students in Professor Bumsoo Han’s Biotransport Phenomena Lab. This was the M-TEC lab’s first exploration of materials synthesis and we learned a lot from Prof Han’s students.

Thank you

Determining Temperature http://2.bp.blogspot.com/_okIcsBieX4U/TRKogEZh16I/AAAAAAAAAIY/spYsURzN3d0/s1600/squarewave32terms.png 𝑄 2𝜔 = (𝐼 1𝜔 ) 2 𝑅 𝑜 ↔ 𝑇 2𝜔 𝑉 3𝜔 = 𝐼 1𝜔 𝑅 2𝜔 3ω Method: 𝑅 𝑡𝑜𝑡𝑎𝑙 = 𝑅 𝑜 + 𝑅 𝑜 𝛼Δ 𝑇 2𝜔 𝑅 2𝜔 Subtract 1𝜔 Signal Voltage [V] For measurement, we picked the 3 omega measurement technique. This is a simple method and functional well. Basically, a metal line or wire inside the soft nanomaterial which is Rs acts as a heater and thermometer for the 3w method. We measure the temperature response to a sinusoidal heating input a frequency of 1w to the metal line. Current applied at 1ω causes heating (q~I2R) at 2ω leading to temperature oscillations at 2ω. Because the resistivity of the metal changes with temperature, the resistance of the metal wire oscillates at a frequency of 2ω We measure the voltage across the heater\thermometer, which is the current*the resistance (V=IR), and therefore the voltage changes at 3*w. 3ω which is related to the wire temperature which is what we need to acquire from 3 omega method. Unfortunately, our current lab equipment is not able to achieve high voltage or current using sinusoidal wave during the measurement. So we use the square waves. Square waves are functional because the square wave exported from function generator are several sinusoidal waves that pieced together. In math they are called fourier series. Here is a simple graph that shows how a square wave can be achieved through sinusoidal waves. We can that the light blue line is a sinusoidal function, when we add more functions in to fourier series, the red line came out. So more and more sinusoidal functions added in and built the square wave. Time [s] Use square waves to achieve the power we need. Miers, C. & Marconnet, A. D. (2014). “Simulation and Analysis of an Integrated Device to Simultaneously Characterize Thermal and Thermoelectric Properties”. International Thermal Conductivity Conference (ITCC) and the International Thermal Expansion Symposium (ITES), West Lafayette, IN, 2014.

Modified Angstrom Method Thermal Diffusivity: Phase Lag: Re-Arrange Equation for Fitting: Matlab Plot 1: Ghasemi et al. “High Thermal Conductivity Ultra-High Molecular Weight Polyethylene (UHMWPE) Films”, ITHERM 2014, Orlando, FL. Fit Slope We use matlab to help us decoding the matrix of temperature readings. We build a matlab script to help us calculate thermal diffusivity from those numbers. This also requires the movie we took from the IR camera. y = m x Matlab Plot 2: Fit Slope y = m x

3ω Method: Preliminary Results Stainless Wire SMS Heating Sample Unfortunately, even though we tried really hard on 3 omega method. The results came out doesn’t look nice at all. Here is preliminary results of our sample using 3 omega method. The blue line is the simulation results while the black dot are the samples we acquire. Preliminary results show significant electrical noise, which makes extraction of the thermal conductivity from the data challenging. Our next steps are to rebuild the circuit to remove the 1 component and amplify the 3 component for improved accuracy. For more information about the 3 omega method and its results, you may stop by my partner Yu Han’s poster.