Dielectric properties of polymer nanocomposites
In modern electronics and electric power systems for applications such as electrical energy storage or filtering, polymer-based capacitors are becoming very attractive for their intrinsic characteristics as low manufacturing cost, low dielectric loss, and high reliability under high voltage. Coupled to this, the rapid miniaturization and damanding functionality of such devices require capacitors depicting high energy densities and capable to operate in a wide range of temperatures within certain range of fluctuations. It is known that for polymer-based capacitors the energy density is linearly proportional to the dielectric constant, thus, enhancing the dielectric constant while maintaining the dielectric loss low could potentially increase the energy density making this type of capacitor more appealing. To obtain the increase in the dielectric properties of the polymers, composites (inorganic nanofillers) with large dielectric constants can be added to the polymer matrix. Nonetheless, the inclusion of a high volume of these high dielectric constant nanofillers results in local electric fields in the polymer interface near the fillers which may lead to a large reduction of the electric breakdown strength, being this the major drawback in the practical application of dielectric organic/inorganic composites. This drawback is majorly related with high volume concentration of the nanofillers within the polymer matrix, but this problem may be overcome after recent experimental results presented large increments in the dielectric constant of polymers with significantly low volume concentration of inorganic nanofillers. At the IPHEL lab, we are focused in the development of theories and algorithms, to help in the explanation of the observed behavior in dielectric properties, and in the numerical exploration at the atomistic scale of the dielectric response and its relationship with the interfacial phenomena.
This research has been generously supported by the Materials for Enhancing Energy and Environmental Stewardship Seed Grant Program and the ICS Seed Grant Program.
Thermal management of high-power electronic packages
The rapid miniaturization and large energy requirements of high-power density electronic devices have posed a colossal challenge for the adequate thermal management. High operating temperatures are associated to decreased performance and decrements in the durability of electronic devices; moreover, malfunction or the complete loss of the devices can be occasioned by irregular temperature distributions or hotspots due to the non-uniform power distribution in chip packages. Thus, demonstrating that both the maximum surface temperature and the uniformity of the temperature distribution are important factors to control for the proper thermal management of high-performance chip packages. While many investigations are conducted at the chip level to eliminate this problem, active cooling techniques are currently being developed at the device and system level in order to speed up the rate of thermal energy removal from the chips. Several air-based forced convection approaches have been proposed but conventional air-cooling techniques are found to be insufficient for high performance electronics. In recent years, liquid-cooling has been receiving attention for its notorious advantages over conventional air-cooling approaches, as it offers superior cooling performance, decrement of performing noise, and enhanced packability.
At the IPHEL lab, we focus our research on active liquid cooling solutions for high power electronics and the design novel manifolds based on the principles of entropy generation minimization and from nature inspire systems for uniform flow distribution. Our goal is to use computational fluid dynamics models to optimize the flow conduits embedded in liquid-cooled heat sinks and to characterize the uniformity of flow distribution generated from each manifold as well as the pumping power requirements for the operation of complete systems in order to allow for higher flow rates (higher cooling rates) at the lowest power consumption. Furthermore, using low force stereolithography (resin based 3-D printing) techniques, we are able to manufacture the intricated 3-dimensional distributors product of the computational optimization to put to the test its cooling capabilities in comparison with commercially available products. Our cooling devices are completely functional and depict similar performance in comparison with widely used cooling kits proving its impressive cooling capabilities with lower production cost and faster production rates.
- C. Ulises Gonzalez-Valle, Saurabh Samir, and Bladimir Ramos-Alvarado. Experimental investigation of the cooling performance of 3-D printed hybrid water-cooled heat sinks, Applied Thermal Engineering 138 (2020) 114823.
- Luis E. Paniagua-Guerra, Shitiz Sehgal, C. Ulises Gonzalez-Valle, and Bladimir Ramos-Alvarado.Fractal manifolds for microjet liquid-cooled heat sinks, International Journal of Heat and Mass Transfer 138 (2019) 257-266.
- Bladimir Ramos-Alvarado, Bo Feng, and G. P. Peterson.Comparison and optimization of single-phase liquid cooling devices for the heat dissipation of high-power LED arrays, Applied Thermal Engineering 59, pp. 648-659, 2013.
- Bladimir Ramos-Alvarado, Peiwen Li, Hong Liu, and Abel Hernandez-Guerrero.CFD study of liquid-cooled heat sinks with microchannel flow field configurations for electronics, fuel cells, and concentrated solar cells, Applied Thermal Engineering 31(14-15), pp. 2494-2507, 2011.
Thermal transport across solid-liquid interfaces
The main limitations in the development of micro/nano scale electronic chips come from the difficulties to avoid overheating during the operation of these devices. Overheating occurs due to the low thermal conductivity of size-affected materials and the creation of many interfaces (thermal resistances) during the fabrication of the chips; therefore, the thermal transport across interfaces has been widely investigated over several decades. The rapid development of nanofluidics devices (electronics and fluids systems at nanoscale) and the lack of understanding of the thermal transport across hard-soft interfaces place the study of solid-liquid interfaces as a rich area of investigation for the thermal/fluid community. Unlike solid-solid interfaces, solid-liquid interfaces present an extra challenge: the formation of interfacial solid-like structures in the liquid phase depending on the affinity between the solid and the liquid atoms and the atomic arrangement within the solid phase. At the IPHEL lab, it is our goal to unravel the underlying mechanisms of thermal transport at solid-liquid interfaces by looking at the affinity between phases, interfacial liquid structuring and confinement, and the effect these factors and their interplay have on the phonon modes contributing to thermal energy transport. The main applications of our research are biomedical in nature such as thermal treatment of cancer using microparticles, nanofludic devices where chips interact with aqueous environments, among others.
- C. Ulises Gonzalez-Valle, Luis E. Paniagua-Guerra, and Bladimir Ramos-Alvarado.Implications of the interface modelling approach on the heat transfer across graphite-water interfaces, The Journal of Physical Chemistry C 123, pp 22311-22323, 2019
- C. Ulises Gonzalez-Valle, Satish Kumar and Bladimir Ramos-Alvarado.Heat transfer across SiC-water interfaces, ACS Applied Materials & Interfaces 10, pp 29179-29186, 2018.
- Bladimir Ramos-Alvarado, Satish Kumar, and G. P. Peterson.Solid-liquid thermal transport and its relationship with wettability and the interfacial liquid structure, The Journal of Physical Chemistry Letters 7, pp 3497-3501, 2016.
Characterization of solid-liquid interactions via wettability simulations
Overlapping surface chemistry and surface physics is how we explain the behavior of different phases interacting at interfaces. One of the objectives for scientists and engineers is to develop theories and techniques to quantify the affinity between a given set of materials forming an interface. Particularly, solid-liquid interfaces are of significant importance to many engineering applications in the fields of nanofluidics and heat transfer as the solid-liquid interaction strength plays a significant role in the energy, mass, and momentum transport processes. Furthermore, for this kind of interfaces, the larger mobility of the liquid phase and its ability to morph allow the formation of solid-like structures influenced by the solid phase. These structures are characterized for displaying larger molecular concentration and depicting drastic modifications on its vibrational properties severely affecting the transport phenomena. Our current efforts at the IPHEL lab are focused in the development of theory and evidences for supporting it with atomistic simulations to further understand the fundamental physics of solid-liquid interactions and how these affect the structure of interfacial liquid and hence, the transport phenomena. Additionally, we investigate techniques to generate appropriate solid-liquid interaction potentials for classical atomistic simulations based on experimental wettability information and multibody electronic structure methods (e.g., DFT, RPA), and at the same time we determine whether the obtained models are reliable for its application.
- Bladimir Ramos-Alvarado. Water wettability of graphene and graphite, calibration of solid-liquid interaction force fields, and insights from mean-field modelling, The Journal Chemical Physics 151, 114701, 2019.
- C. Ulises Gonzalez-Valle, Satish Kumar and Bladimir Ramos-Alvarado.Investigation on the Wetting Behavior of 3C-SiC Surfaces: Theory and Modelling, The Journal of Physical Chemistry C 122, pp 7179-7186, 2018.
- Bladimir Ramos-Alvarado, Satish Kumar, and G. P. Peterson.On the wettability transparency of graphene-coated silicon surfaces, The Journal of Chemical Physics 144(1), 014701, 2016.
- Bladimir Ramos-Alvarado, Satish Kumar, and G. P. Peterson.Wettability of graphitic-carbon and silicon surfaces: MD modeling and theoretical analysis, The Journal of Chemical Physics 143(4), 044703, 2015.
Slip in nanoconfined liquids (nanochannels)
The Navier-Stokes equations in combination with the no-slip boundary condition represent a cornerstone of classical fluid dynamics. While the Navier-Stokes equations are derived from a physically sound model, the no-slip boundary condition is an empirical assumption only verified at the macroscale. For years now, experiments and simulations have predicted the existence of a slip boundary condition in nanoconfined flows e.g., a finite velocity, different from the wall velocity, at the solid-liquid interface. An interface slip velocity indicates that a lower than a classical friction flow can be developed. For nanofluidic devices this represents a major area of opportunity for further technological development, as friction is the main limitation in these devices. We use atomistic simulations to predict the flow conditions in nanoconfined liquids by looking at the influence of the solid-liquid affinity, the interfacial liquid layering, and the absorption of liquid particles into the solid materials. This multifactor investigation allows us to develop a new approach to control the flow conditions in nanoconfined liquids for future applications in general nanofluidics devices, drug delivery systems, cell sorting devices, micro/nano gap lubrication, among others.
- Bladimir Ramos-Alvarado, Satish Kumar, and G. P. Peterson. Hydrodynamic slip length as a surface property,, Physical Review E 93, 023101, 2016.
- Bladimir Ramos-Alvarado, Satish Kumar, and G. P. Peterson. Wettability transparency and the quasiuniversal relationship between hydrodynamic slip and contact angle, Applied Physics Letters,108(7),074105, 2016.