Although passive building envelope systems dominate contemporary building design, active building envelope (ABE) research, development, and deployment are rapidly growing and present many new ways to address building energy efficiency at the façade level. This paper presents a comprehensive review on the state-of-art-research on ABEs for improving building energy efficiency. First, a clarified concept of ABE is put forward based on two conditions: The ability of lowering cooling/heating loads in buildings, and performing energy transformation as the key judging factors. Second, four major categories of ABEs, namely air-based, water-based, solid-based and kinetic façades, are discussed in terms of their system structural and functional features as well as system performance. In addition, a statistical analysis is performed for a better understanding of current research focus, general trends, as well as research methods. It is found that technical research on ABEs has dramatically increased but the ratio of ABE to general façade studies remains stable. In terms of research methods and approaches, numerical simulations are dominant. Some specific comments on limitations of current ABE studies and general suggestions for the future studies are discussed. Future work suggests the need for contributions from a wide range of scientists, engineers, and architects in the building industry and beyond to push forward building energy efficiency.
The globe thermometer has been considered a reliable instrument to quantify mean radiant temperature (MRT) since Bedford & Warner isolated its readings from air movement in their 1934 paper so that radiation could be quantified. Recent expanded use of radiant heating and cooling systems has presented new challenges for the usage of globe thermometers in the built environment by causing additional radiant asymmetries and performance expectations. Therefore, we replicate the original Bedford & Warner work to reconsider and develop a more holistic understanding of black globe performance and the determination of MRT in buildings. We recreate the MRT and air temperature separation to investigate the accuracy of globe thermometers on measuring MRTs. A radiantly heated open-plan laboratory and a radiantly cooled conference room were selected and measured with multiple globe thermometers and non-contacting infrared sensors. The globe temperature results were then corrected with air movement to produce MRTs and compared against MRTs simulated from measured surface temperatures. We demonstrate a significant impact of air speed on the MRTs obtained from globe thermometers. We also illustrate a less-investigated non-graybody emissivity variation and spatial variation of MRTs of up to 5 °C at the same height. We believe the increasing temporal and spatial resolution of digital sensors may create new challenges for using globe thermometers to measure MRTs, since fluctuating readings may camouflage potential MRT changes. Through a validation of our spatial MRT distribution with experimental results, we believe there is a need for better sensors that could spatially resolve MRTs, and recognize issues with both air speed and emissivity.
Graphene is a one-carbon atom thick sheet with a hexagonal lattice. The chemical structure of graphene is dependent on its processing and synthesis methods. Indeed, graphene obtained via mechanical exfoliation of graphite may yield defect-free single layer sheets, whereas chemically produced graphene results in defective graphene that is highly functionalized with different chemical groups. In the first part of my talk, I will review the major techniques employed for the production of graphene sheets in laboratory and industrial scales. Then, I will discuss their resulting physical and chemical characteristics. In the second part of my talk, I will review and discuss the broad applications of graphene and the most recent achievements.
The built environment is positioned uniquely at the intersection of all three pillars of sustainability: society, economy and the environment. Systems of materials and physical interactions persist through all these domains that can only be leveraged through interdisciplinary investigations. We present a survey of projects and concepts that have been developed and studied at the CHAOS (Cooling and Heating for Architecturally Optimized Systems) Laboratory at Princeton University.
Elastomer applications for graphene, a versatile carbon-based nanomaterial, have been explored in several past studies. Use of Vor-x graphene strongly enhances compound stiffness and decreases hysteresis while maintaining or improving properties such as strength, abrasion, and fatigue life. In this study of elastomer reinforcement using functionalized graphene sheet nanomaterial (Vor-x), a control natural rubber + polybutadiene rubber blend tread compound was compared to a similar compound with 2 phr Vor-x replacing 16 phr carbon black. The results show good potential for improvement of performance properties. Comparable levels of stiffness, hardness, and Mooney viscosity were achieved. Tensile strength and elongation at break were maintained. Hysteresis was reduced by 20-30%, indicating reduced rolling resistance. Fatigue-to-failure also improved by 20-30%, and DeMattia crack growth resistance improved by 250%. Transmission electron microscopy studies showed excellent dispersion of Vor-x as well as carbon black.
We have covalently grafted tetrazine derivatives to graphene oxide through nucleophilic substitution. Since the tetrazine unit is electroactive and nitrogen-rich, with a reduction potential sensitive to the type of substituent and degree of substitution, we used electrochemistry and X-ray photoelectron spectroscopy to demonstrate clear evidence for grafting through covalent bonding. Chemical modification was supported by Fourier transform infrared spectroscopy and thermal analysis. Tetrazines grafted onto graphene oxide displayed different mass losses compared to unmodified graphene and were more stable than the molecular precursors. Finally, a bridging tetrazine derivative was grafted between sheets of graphene oxide to demonstrate that the separation distance between sheets can be maintained while designing new graphene-based materials, including chemically bound, redox structures.
Elastic instabilities, when properly implemented within soft, mechanical structures, can generate advanced functionality. In this work, we use the voltage-induced buckling of thin, flexible plates to pump fluids within a microfluidic channel. The soft electrodes that enable electrical actuation are compatible with fluids, and undergo large, reversible deformations. We quantified the onset of voltage-induced buckling, and measured the flow rate within the microchannel. This embeddable, flexible microfluidic pump will aid in the generation of new stand-alone microfluidic devices that require a tunable flow rate.
Composites of carbon-based fillers and elastomeric matrices are at the heart of developing technologies such as high-strain sensors/actuators and stretchable electronics due to their unique combination of electrical and mechanical properties. Production of these composites typically includes dispersion of filler particles into an uncross-linked polymer matrix such as liquid polydimethylsiloxane (PDMS) and a subsequent cross-linking of that matrix. We show here that, in the cross-linking of PDMS elastomer, carbon-based fillers such as carbon blacks and functionalized graphene can diminish the extent of cross-linking via a deactivation of small molecule catalysts and cross-linking agents. This deactivation is evidenced by the relationship between the filler loading, the composition at which gelation is observed, and the elastomer cure time. We have studied composite mechanical properties over a broad range of cure mixture compositions, and we demonstrate that materials with a high degree of cross-linking can be obtained when corrections are applied for this deactivation effect. Mechanical and electrical properties of these composites are explored with stretchable conductor applications in mind.
Dielectric elastomers are well-known for their superior stretchability and permittivity. A fully-clamped thin elastomer will buckle when it is compressed by applying sufficient electric potentials to its sides. When embedded within soft, silicone rubbers, these advanced materials can provide a means for a biocompatible pumping mechanism that can be used to inject bio-fluids with desired flow rates into microfluidic devices, tissues, and organs of interest. We have incorporated a dielectric film that is sandwiched between two thin, flexible, solid electrodes into a microfluidic device and utilized a voltage-induced out-of-plane buckling instability for pumping of fluids. We experimentally quantify the voltage-induced plate buckling and measure the fluid flow rate when the structure is embedded in a microchannel. Additionally, we offer an analytical prediction that uses plate buckling theory to estimate the flow rate as a function of applied voltage.
Fluid flow can be directed and controlled by a variety of mechanisms within industrial and biological environments. Advances in microfluidic technology have required innovative ways to control fluid flow on a small scale, and the ability to actively control fluid flow within microfluidic devices is crucial for advancements in nanofluidics, biomedical fluidic devices, and digital microfluidics. In this work, we present a means for microfluidic control via the electrical actuation of thin, flexible valves within microfluidic channels. These structures consist of a dielectric elastomer confined between two compliant electrodes that can be actively and reversibly buckle out of plane to pump fluids from an applied voltage. The out-of-plane deformation can be quantified using two parameters: net change in surface area and the shape of deformation. Change in surface area depends on the voltage, while the deformation shape, which significantly affects the flow rate, is a function of voltage, and the pressure and volume of the chambers on each side of the thin plate. The use of solid electrodes enables a robust and reversible pumping mechanism that will have will enable advancements in rapid microfluidic diagnostics, adaptive materials, and artificial muscles.
Buckling of plates and rods are often avoided in design of structures; however, proper implementation of these instabilities lead to advanced material functionality. In this work, we use the buckling of circular plates for pumping fluids with a specific flow rate. Dielectric elastomers have been widely used in many applications such as soft robotics, opto-electro-mechanical systems, and energy harvesting devices due to their unique mechanical and electrical properties as well as their ability to convert electrical signals to mechanical actuations and vice versa. If we confine a thin dielectric elastomeric plate at its edge and expose it to an electric field, it will undergo buckling. When embedded within a microfluidic device, the out-of-plane deformation can be used as a pumping mechanism to inject the fluids above or below the plate into channels. In order for the device to be compatible with fluids, the dielectric film was sandwiched between two thin, flexible, solid electrodes that can not only be in direct contact with fluids, but also undergo significant deformation without losing their functionality. We conducted experiments to quantify the voltage-induced buckling instability and measured the flow rate as a function of voltage. In addition, the effect of other boundary conditions such as volume and pressure difference between two sides of the thin film was investigated. We also show that these pumps can be used in series and/or parallel to enhance the flow rate besides the pump efficiency. Finally, we offer an analytical prediction that uses plate buckling theory to estimate the critical voltage for buckling of confined dielectric plates.
A carbon-nanotube architecture based on ceramic microparticles allows for strikingly reducing the number of thermal contact resistances between carbon nanotubes (CNT). The result is a 130% enhancement of the thermal conductivity of the nanocomposites at a remarkably low CNT mass fraction of 0.15 wt%.
Broad-frequency dielectric behaviors of multiwalled carbon nanotubes MWCNTs embedded in room temperature vulcanization silicone rubber RT-SR matrix were studied by analyzing alternating current ac impedance spectra, which would make a remarkable contribution for understanding some fundamental electrical properties in the MWCNT/RT-SR nanocomposites. Equivalent circuits of the MWCNT/RT-SR nanocomposites were built, and the law of polarization and mechanism of electric conductance under the ac field were acquired. Two parallel RC circuits in series are the equivalent circuits of the MWCNT/RT-SR composites. At different frequency ranges, dielectric parameters including conductivity, dielectric permittivity, dielectric loss, impedance phase, and magnitude present different behaviors.
A cavity microelectrode (CME) was used to perform an electrochemical synthesis of hybrid materials made of carbon nanotubes (CNTs) and conducting polymers. The confinement of the CME is used to produce a uniform nanometric coating of an electronically conducting polymer such as poly(N-methylpyrrole) (Pmpy) on multiwalled carbon nanotubes. The CME also allows easy characterization of the presence of the polymer layer on the surface of the CNTs by cyclic voltammetry. Transmission electron microscopy allowed us to measure the thickness and confirm the homogeneity of the Pmpy coating around the CNTs. Finally Raman spectroscopy brings additional information on the electrogenerated hybrid materials.
Carbon nanotubes (CNTs) are ideal candidates to reinforce thermoset polymers due to their exceptional intrinsic properties. The resulting multifunctional nanocomposite has electrical, thermal and mechanical properties sensitively higher than pristine polymer. Therefore, this new material possesses various potential applications, and particularly in the domain of electronics and aerospace. The aim of this PhD thesis is oriented towards two directions. In the first one, we establish efficient techniques to produce composite materials with multifunctional properties. Then, the objective consists in the enhancement of these properties by proposing valuable alternatives to previous results cited in the litterature. In the first chapter, we present the state of the art research concerning the materials studied during this work. Among these, there are in particular: CNTs, hybrids constituted of CNTs and alumina microparticles, electronically conducting and thermoset polymers. Moreover, this chapter deals with the characteristics of each material, i.e. elaboration techniques, structures and properties. The second chapter of the manuscript contains first, the elaboration techniques allowing the synthesis of high quality nanocomposites according to international standards. Then, we analyze the properties of these nanomaterials, and particularly in terms of electrical and thermal transports. Further characterization procedures allow better understanding of the obtained structures in a domain ranging from macroscopic to atomic scales. This is realized using scanning/transmission electron microscopy, Raman spectroscopy, EELS, XPS, and AFM. Electrical and thermal conductivity measurements obtained on these new materials give prominence to the necessity of some improvements. Thereby, we have focused our research on the physico-chemical phenomena at the matrix/filler interface. We have proposed to modify the surface of CNTs, in order to favour the matrix/filler cohesion, but also and mainly to decrease contact resistances between the randomly distributed CNTs within the polymer matrix. Finally, the last chapter deals with the surface functionalization of CNTs using electrochemistry. First, we have implemented an accurate technique to deposit a nanometric layer of electronically conducting polymer on the surface of CNTs. This conducting polymer, namely polypyrrole (Ppy) is in the meantime biocompatible. The accuracy and efficiency of our approach are demonstrated through various characterization techniques, and particularly using transmission electron microscopy. Further studies using AFM coupled with a resiscope indicate the electrical resistance distribution performed on CNT-Ppy hybrids. In the second part of this chapter, we present our method to control precisely the thickness of the Ppy layer around the CNTs.