60th Annual Report on Research 2015

The objectives of this research project are to obtain fundamental mechanistic understanding of interfacial thermal transport in metal organic frameworks (MOFs), and to convert this insight into strategies for minimizing boundary thermal resistance, or for making interfacial thermal resistance an externally controllable property. The project is subdivided into four tasks: (1) identify the low energy structure of interfaces; (2) determine the interfacial boundary resistance; (3) develop a method for fast comparison of boundary resistance; and (4) apply engineering design methods to engineer interfaces with tailored thermal properties. In the first year of the project we have made progress on the first three of these research tasks.

We are specifically interested in the interfaces between contacting grains of MOF-5 in a powder compact. MOF-5 forms cubic grains bounded by (001) surfaces, and we are thus focused on understanding the structure of these (001) faces. In this years research we have created the simplest structure of (001) surface — a surface terminated after the MOF’s nodal units (in the language of MOF the secondary building units or SBU) with the exposed oxygen atoms left un passivated. In parallel we are using this simple surface to develop methods to elucidate heat transport processes at interfaces, while also seeking the low energy structures of more realistic but more complicated surfaces which are terminated with dangling terephthalic acid linkers.

Calculations of boundary interfacial resistance: Using the Green-Kubo method we have computed the effective boundary thickness of our node-terminated test interface to be ∼ 200 nm. This yields a bound- ary conductance of be 1.8 MW.m−2.K−1. While the uncertainty in this prediction is large, the value of the thermal conductance is extremely low — on the order of the lowest known interfacial conductances. This is especially note worthy as the highest interfacial resistances arrise between acoustically mismatch materials which different heat carriers (phonons in one material electrons in the other). For this MOF interface the heat transport modes and the spectrum of heat carriers on both sides of the interface are the same. As the particle size of MOF-5 crystals in used in H2 sorption beds is on the order of 5–1 μm, this very low interfacial conductance can considerably degrade heat transport through the sorption bed.

To dissect the heat transport mechanism and elucidate the heat transport channels across the interface we are pursuing three sets of complementary analysis: We have computed the atom by atom velocity cross-correlation matrix for atom pairs across the interface. This reveals the atoms active in com- municating across the boundary, and also the distance over which atomic motion is correlated. The calculations reveal that it is the motion of the atoms in nodes (rather than linker atoms) that contribute most to the heat transport. We expect the lowest frequency vibrational modes to provide that most efficient channels for heat transport across the interface, and the, and these tend to involve vibrations of the nodes which are much heavier than the linkers. We are currently extending this analysis to examine the cross-correlation of the species by species heat flux. We are also working now to examine the character of the vibrational modes that span the interfaces, and are analyzing the heat transport predicted using Allen and Feldman theory.

The nature of the synthesis process of MOF-5 means that it is most likely that its free surfaces are terminated with a carpet of dangling linkers. As the carboxylate connection to the these SBU can act like a hinge it is probable that these dangling linker molecules undergo a surface reconstruction, with linkers folding down to interact with the linker diagonally opposite it. We are using MD simulations to explore the low energy structures that can be formed in this way.

The first portion of the year was spent developing a method using classical molecular dynamics (MD) simulations for fast comparison of boundary resistance. The approach is based on watching the transient dissipation of a locally heated region of a material. The simulation is performed of a material containing a periodically repeated grain boundary. A slice of the material roughly one third of the way through the slab is heated above the ambient temperature, and the system then simulated in NVE and the temperature profile in the slab computed as the hot spot dissipates. The asymmetry of the profile as it hits the interface is used to compare the heat flux across the boundary with the heat flux in the slab. We have also employed this method in MOFs without internal boundaries to examine if there as a two temperature transient phenomenon due to separate heat baths of fast and slow propagating vibrations.

The research has supported one masters student. The student is currently working on their masters thesis. The research has resulted in no publications thus far, but the results of the cross-correlation analysis of the heat transport channels that are being prepared for the masters thesis will be presented at the Spring MRS meeting and will also be written up for journal publication. The PI recently moved to the University of California, Riverside, and it is planned that the student working on this project will also move to UCR for his PhD as soon as he has completed his masters at OSU. This research project has enabled the PI to strengthen his experience in both MOFs and thermal transport, and he is now working to establish a collaboration with experimentalists in Southern California with the capability of measuring thermal conductivity of powders.

In this coming year will apply the analysis tools that we have developed to interfaces with various arrangements of dangling linker molecules, will examine the effect of pressure and misorientation, before moving on to task (4), designing interfaces with improved thermal transport properties.