Materials Challenges for High Performance Magnetocaloric Refrigeration Devices

Magnetocaloric materials with a Curie temperature near room temperature have attracted significant interest for some time due to their possible application for high‐efficiency refrigeration devices. This review focuses on a number of key issues of relevance for the characterization, performance and implementation of such materials in actual devices. The phenomenology and fundamental thermodynamics of magnetocaloric materials is discussed, as well as the hysteresis behavior often found in first‐order materials. A number of theoretical and experimental approaches and their implications are reviewed. The question of how to evaluate the suitability of a given material for use in a magnetocaloric device is covered in some detail, including a critical assessment of a number of common performance metrics. Of particular interest is which non‐magnetocaloric properties need to be considered in this connection. An overview of several important materials classes is given before considering the performance of materials in actual devices. Finally, an outlook on further developments is presented.     

The Resource Basis of Magnetic Refrigeration

Emerging economies such as China and India are currently experiencing a “refrigeration revolution.” Energy spent for domestic cooling is expected to outreach that for heating worldwide over the course of the twenty‐first century. Magnetic refrigeration is an alternative cooling technology that works without gas‐based refrigerants and has the potential to be significantly more energy efficient. We evaluate to what extent the raw materials needed to produce this kind of technology on a mass‐market scale are critical in terms of demand and supply, thus identifying potential supply bottlenecks that might hinder the breakthrough of this promising technology. We assess the criticality of three promising magnetocaloric materials, that is, Gd₅(SiGe)₄, La(FeSi)₁₃, and (MnFe)₂P), as well as of Nd₂Fe₁₄B, as the candidate permanent magnet material to drive the cooling cycle. The Gd‐based alloys are disqualified as a mass‐market refrigerant in terms of resource criticality, whereas La‐ and Mn‐based alloys are much less problematic. Given the current state of technology and projected resource supply, Nd in Nd₂Fe₁₄B magnets would experience a significant bottleneck only at a later innovation stage, that is, when magnetic cooling technology would largely dominate the domestic refrigerator and air‐conditioning market.     

Reduced Graphene Oxide Thin Films as Ultrabarriers for Organic Electronics

Encapsulation of electronic devices based on organic materials that are prone to degradation even under normal atmospheric conditions with hermetic barriers is crucial for increasing their lifetime. A challenge is to develop ultrabarriers that are impermeable, flexible, and preferably transparent. Another important requirement is that they must be compatible with organic electronics fabrication schemes (i.e., must be solution processable, deposited at room temperature and be chemically inert). Here, a lifetime increase of 1300 h for poly(3‐hexylthiophene) (P3HT) films encapsulated by uniform and continuous thin (≈10 nm) films of reduced graphene oxide (rGO) is reported. This level of protection against oxygen/water vapor diffusion is substantially better than conventional polymeric barriers such as Cytop, which degrades after only 350 h despite being 400 nm thick. Analysis using atomic force microscopy, X‐ray photoelectron spectroscopy, and high‐resolution transmission electron microscopy suggest that the superior oxygen gas/moisture barrier property of rGO is due to the close interlayer distance packing and absence of pinholes within the impermeable sheets. These material properties can be correlated to the enhanced lag time of 500 h. The results provide new insight for the design of high‐performance and solution‐processable transparent ultrabarriers for a wide range of encapsulation applications.     

Rational Design of Metal Oxide–Based Cathodes for Efficient Dye‐Sensitized Solar Cells

Recently, there is an urgent need for alternative energy resources due to the nonrenewable nature of fossil fuels and increasing CO₂ greenhouse gas emissions. The photovoltaic technologies which directly utilize the abundant and sustainable solar energy are critical. Among various photovoltaic devices (solar cells), dye‐sensitized solar cells (DSSCs) have gained increasing attention due to their high efficiency and easy fabrication process in the past decade. The cathode is a critical part in DSSCs while the benchmark Pt cathode suffers from high cost and scarcity. Thus, the development of alternative Pt‐free cathodes has attracted significant attention with the aim to heighten the cost competitiveness of DSSCs. Among various cathodes, metal oxides are of growing interest due to their superior activity, robust stability, and low cost. Simple oxides such as WO₃ and SnO₂ are used as cathodes for DSSCs. Considering the fixed atomic environment in simple oxides, complex oxides are more attractive as cathodes because of their more flexible physical and chemical properties. This review attempts to present the rational design of simple/complex metal oxide–based cathodes in DSSCs and then to provide useful guidance for the future design of Pt‐free cathodes. The demonstrated design strategies can be extended to other electrocatalysis‐based applications.     

Dynamics of the First‐Order Metamagnetic Transition in Magnetocaloric La(Fe,Si)13: Reducing Hysteresis

The influence of dynamics and sample shape on the magnetic hysteresis in first‐order magnetocaloric metamagnetic LaFe_13–xSi_x with x = 1.4 is studied. In solid‐state magnetic cooling, reducing magnetic and thermal hysteresis is critical for refrigeration cycle efficiency. From magnetization measurements, it is found that the fast field‐rate dependence of the hysteresis can be attributed to extrinsic heating directly related to the thickness of the sample and the thermal contact with the bath. If the field is paused partway through the transition, the subsequent relaxation is strongly dependent on shape due to both demagnetizing fields and thermal equilibration; magnetic coupling between adjacent sample fragments can also be significant. Judicious shaping of the sample can both increase the onset field of the ferromagnetic–paramagnetic (FM–PM) transition but have little effect on the PM–FM onset, suggesting a route to engineer the hysteresis width by appropriate design. In the field‐paused state, the relaxation from one phase to the other slows with increasing temperature, implying that the process is neither thermally activated or athermal; comparison with the temperature dependence of the latent heat strongly suggests that the dynamics reflect the intrinsic free energy difference between the two phases.     

Si‐Doped Cu(In,Ga)Se2 Photovoltaic Devices with Energy Conversion Efficiencies Exceeding 16.5% without a Buffer Layer

In this communication, novel and simplified structure Cu(In,Ga)Se₂ (CIGS) solar cells, which nominally consist of only a CIGS photoabsorber layer sandwiched between back and front contact layers but yet demonstrate high photovoltaic efficiencies, are reported. To realize this accomplishment, Si‐doped CIGS films grown by the three‐stage coevaporation method, B‐doped ZnO transparent conductive oxide front contact layers deposited by chemical vapor deposition, and heat–light soaking treatments are used. Si‐doping of CIGS films is found to modify the film surfaces and grain boundary properties and also affect the alkali metal distribution profiles in CIGS films. These effects are expected to contribute to improvements in buffer‐free CIGS device performance. Heat–light soaking treatments, which are occasionally performed to improve conventional buffer‐based CIGS device performance, are found to be also effective in enhancing buffer‐free CIGS photovoltaic efficiencies. This result suggests that the mechanism behind the beneficial effects of heat–light soaking treatments originates from CIGS bulk issues and is independent of the buffer materials. Consequently, over 16.5% efficiencies, including an independently certified value, are demonstrated from completely buffer‐free CIGS photovoltaic devices.     

Integration in a depot‐based decentralized biorefinery system: Corn stover‐based cellulosic biofuel

The current or “conventional” paradigm for producing process energy in a biorefinery processing cellulosic biomass is on‐site energy recovery through combustion of residual solids and biogas generated by the process. Excess electricity is then exported, resulting in large greenhouse gas (GHG) credits. However, this approach will cause lifecycle GHG emissions of biofuels to increase as more renewable energy sources (wind, solar, etc.) participate in grid‐electricity generation, and the GHG credits from displacing fossil fuel decrease. To overcome this drawback, a decentralized (depot‐based) biorefinery can be integrated with a coal‐fired power plant near a large urban area. In an integrated, decentralized, depot‐based biorefinery (IDB), the residual solids are co‐fired with coal either in the adjacent power plant or in coal‐fired boilers elsewhere to displace coal. An IDB system does not rely on indirect GHG credits through grid‐electricity displacement. In an IDB system, biogas from the wastewater treatment facility is also upgraded to biomethane and used as a transportation biofuel. The GHG savings per unit of cropland in the IDB systems (2.7–2.9 MgCO₂/ha) are 1.5–1.6 fold greater than those in a conventional centralized system (1.7–1.8 MgCO₂/ha). Importantly, the biofuel selling price in the IDBs is lower by 28–30 cents per gasoline‐equivalent liter than in the conventional centralized system. Furthermore, the total capital investment per annual biofuel volume in the IDB is much lower (by ~80%) than that in the conventional centralized system. Therefore, utilization of biomethane and residual solids in the IDB systems leads to much lower biofuel selling prices and significantly greater GHG savings per unit of cropland participating in the biorefinery system compared to the conventional centralized biorefineries.     

Magnetic Entropy as a Proposed Gating Mechanism for Magnetogenetic Ion Channels

Magnetically sensitive ion channels would allow researchers to better study how specific brain cells affect behavior in freely moving animals; however, recent reports of “magnetogenetic” ion channels based on biogenic ferritin nanoparticles have been questioned because known biophysical mechanisms cannot explain experimental observations. Here, we reproduce a weak magnetically mediated calcium response in HEK cells expressing a previously published TRPV4-ferritin fusion protein. We find that this magnetic sensitivity is attenuated when we reduce the temperature sensitivity of the channel but not when we reduce the mechanical sensitivity of the channel, suggesting that the magnetic sensitivity of this channel is thermally mediated. As a potential mechanism for this thermally mediated magnetic response, we propose that changes in the magnetic entropy of the ferritin particle can generate heat via the magnetocaloric effect and consequently gate the associated temperature-sensitive ion channel. Unlike other forms of magnetic heating, the magnetocaloric mechanism can cool magnetic particles during demagnetization. To test this prediction, we constructed a magnetogenetic channel based on the cold-sensitive TRPM8 channel. Our observation of a magnetic response in cold-gated channels is consistent with the magnetocaloric hypothesis. Together, these new data and our proposed mechanism of action provide additional resources for understanding how ion channels could be activated by low-frequency magnetic fields.     

An egg‐laying device to estimate the induction of sterility in Ceratitis capitata (Diptera: Tephritidae) sterile insect technique programmes

Area‐wide environmentally friendly pest control methods such as the sterile insect technique (SIT) are being developed and improved to contribute in managing agricultural, environmental and public health problems. A key aspect to evaluate performance of sterile males is to directly measure sterility induction in the field. Sterility induction has been estimated for tephritid fruit flies by recovering egg from host fruit in the field, the method is, however, impractical, and past efforts to develop artificial egg‐laying devices have not prospered. Here, we evaluated response of wild gravid Ceratitis capitata (Medfly) females to long‐distance fruit‐based chemical attractants, visual and tactile stimuli to develop an artificial egg‐laying device. The device combining the most attractive features was further tested under two deployment schemes. Finally, devices and deployment tactics were used to compare fertility levels between feral Medfly females under conventional management and under SIT. Agar spheres wrapped in plastic film, baited with pressed peach juice and visually enhanced with yellow discs received more egg than other combinations of attractive features. Such devices also received more eggs when deployed on fruitless trees and when placed on the orchard perimeter. The egg hatch in an orchard under conventional management was estimated at 86%, whilst egg hatch in an area under SIT was reduced to 31%. The egg‐laying devices are therefore useful to measure sterility induction and can be further improved by refining long‐distance attraction and deployment schemes.     

Controllable ZnMgO Electron‐Transporting Layers for Long‐Term Stable Organic Solar Cells with 8.06% Efficiency after One‐Year Storage

Currently, one main challenge in organic solar cells (OSCs) is to achieve both good stability and high power conversion efficiencies (PCEs). Here, highly efficient and long‐term stable inverted OSCs are fabricated by combining controllable ZnMgO (ZMO) cathode interfacial materials with a polymer:fullerene bulk‐heterojunction. The resulting devices based on the nanocolloid/nanoridge ZMO electron‐transporting layers (ETLs) show greatly enhanced performance compared to that of the conventional devices or control devices without ZMO or with ZnO ETLs. The ZMO‐based OSCs maintain 84%–93% of their original PCEs over 1‐year storage under ambient conditions. An initial PCE of 9.39% is achieved for the best device, and it still retains a high PCE of 8.06% after 1‐year storage, which represents a record high value for long‐term stable OSCs. The excellent performance is attributed to the enhanced electron transportation/collection, reduced interfacial energy losses, and improved stability of the nanocolloid ZMO ETL. These findings provide a promising way to develop OSCs with high efficiencies and long device lifetime towards practical applications.     

Assessing the performance of the photo‐acoustic infrared gas monitor for measuring CO2, N2O,and CH4 fluxes in two major cereal rotations

Rapid, precise, and globally comparable methods for monitoring greenhouse gas (GHG) fluxes are required for accurate GHG inventories from different cropping systems and management practices. Manual gas sampling followed by gas chromatography (GC) is widely used for measuring GHG fluxes in agricultural fields, but is laborious and time‐consuming. The photo‐acoustic infrared gas monitoring system (PAS) with on‐line gas sampling is an attractive option, although it has not been evaluated for measuring GHG fluxes in cereals in general and rice in particular. We compared N₂O, CO_2, and CH₄ fluxes measured by GC and PAS from agricultural fields under the rice–wheat and maize–wheat systems during the wheat (winter), and maize/rice (monsoon) seasons in Haryana, India. All the PAS readings were corrected for baseline drifts over time and PAS‐CH₄ (PCH₄) readings in flooded rice were corrected for water vapor interferences. The PCH₄ readings in ambient air increased by 2.3 ppm for every 1000 mg cm^?3 increase in water vapor. The daily CO₂, N₂O, and CH₄ fluxes measured by GC and PAS from the same chamber were not different in 93–98% of all the measurements made but the PAS exhibited greater precision for estimates of CO₂ and N₂O fluxes in wheat and maize, and lower precision for CH₄ flux in rice, than GC. The seasonal GC‐ and PAS‐N₂O (PN₂O) fluxes in wheat and maize were not different but the PAS‐CO₂ (PCO₂) flux in wheat was 14–39% higher than that of GC. In flooded rice, the seasonal PCH₄ and PN₂O fluxes across N levels were higher than those of GC‐CH₄ and GC‐N₂O fluxes by about 2‐ and 4fold, respectively. The PAS (i) proved to be a suitable alternative to GC for N₂O and CO₂ flux measurements in wheat, and (ii) showed potential for obtaining accurate measurements of CH₄ fluxes in flooded rice after making correction for changes in humidity.     

Top‐illuminated Organic Photovoltaics on a Variety of Opaque Substrates with Vapor‐printed Poly(3,4‐ethylenedioxythiophene) Top Electrodes and MoO3 Buffer Layer

Organic photovoltaics devices typically utilize illumination through a transparent substrate, such as glass or an optically clear plastic. Utilization of opaque substrates, including low cost foils, papers, and textiles, requires architectures that instead allow illumination through the top of the device. Here, we demonstrate top‐illuminated organic photovoltaics, employing a dry vapor‐printed poly(3,4‐ethylenedioxythiophene) (PEDOT) polymer anode deposited by oxidative chemical vapor deposition (oCVD) on top of a small‐molecule organic heterojunction based on vacuum‐evaporated tetraphenyldibenzoperiflanthene (DBP) and C₆₀ heterojunctions. Application of a molybdenum trioxide (MoO₃) buffer layer prior to oCVD deposition increases the device photocurrent nearly 10 times by preventing oxidation of the underlying photoactive DBP electron donor layer during the oCVD PEDOT deposition, and resulting in power conversion efficiencies of up to 2.8% for the top‐illuminated, ITO‐free devices, approximately 75% that of the conventional cell architecture with indium‐tin oxide (ITO) transparent anode (3.7%). Finally, we demonstrate the broad applicability of this architecture by fabricating devices on a variety of opaque surfaces, including common paper products with over 2.0% power conversion efficiency, the highest to date on such fiber‐based substrates.     

High Performance Thermoelectric Materials: Progress and Their Applications

Thermoelectric (TE) materials have the capability of converting heat into electricity, which can improve fuel efficiency, as well as providing robust alternative energy supply in multiple applications by collecting wasted heat, and therefore, assisting in finding new energy solutions. In order to construct high performance TE devices, superior TE materials have to be targeted via various strategies. The development of high performance TE devices can broaden the market of TE application and eventually boost the enthusiasm of TE material research. This review focuses on major novel strategies to achieve high‐performance TE materials and their applications. Manipulating the carrier concentration and band structures of materials are effective in optimizing the electrical transport properties, while nanostructure engineering and defect engineering can greatly reduce the thermal conductivity approaching the amorphous limit. Currently, TE devices are utilized to generate power in remote missions, solar–thermal systems, implantable or/wearable devices, the automotive industry, and many other fields; they are also serving as temperature sensors and controllers or even gas sensors. The future tendency is to synergistically optimize and integrate all the effective factors to further improve the TE performance, so that highly efficient TE materials and devices can be more beneficial to daily lives.     

High stability of self‐assembled peptide nanowires against thermal,chemical, and proteolytic attacks

Understanding the self‐assembly of peptides into ordered nanostructures is recently getting much attention since it can provide an alternative route for fabricating novel bio‐inspired materials. In order to realize the potential of the peptide‐based nanofabrication technology, however, more information is needed regarding the integrity or stability of peptide nanostructures under the process conditions encountered in their applications. In this study, we investigated the stability of self‐assembled peptide nanowires (PNWs) and nanotubes (PNTs) against thermal, chemical, proteolytic attacks, and their conformational changes upon heat treatment. PNWs and PNTs were grown by the self‐assembly of diphenylalanine (Phe–Phe), a peptide building block, on solid substrates at different chemical atmospheres and temperatures. The incubation of diphenylalanine under aniline vapor at 150°C led to the formation of PNWs, while its incubation with water vapor at 25°C produced PNTs. We analyzed the stability of peptide nanostructures using multiple tools, such as electron microscopy, thermal analysis tools, circular dichroism, and Fourier‐transform infrared spectroscopy. Our results show that PNWs are highly stable up to 200°C and remain unchanged when incubated in aqueous solutions (from pH 1 to 14) or in various chemical solvents (from polar to non‐polar). In contrast, PNTs started to disintegrate even at 100°C and underwent a conformational change at an elevated temperature. When we further studied their resistance to a proteolytic environment, we discovered that PNWs kept their initial structure while PNTs fully disintegrated. We found that the high stability of PNWs originates from their predominant β‐sheet conformation and the conformational change of diphenylalanine nanostructures. Our study suggests that self‐assembled PNWs are suitable for future nano‐scale applications requiring harsh processing conditions. Biotechnol. Bioeng. 2010; 105: 221–230. © 2009 Wiley Periodicals, Inc.     

Energy Applications of Magnetocaloric Materials

The need for energy‐efficient and environmentally friendly refrigeration, heat pumping, air conditioning, and thermal energy harvesting systems is currently more urgent than ever. Magnetocaloric energy conversion is among the best available alternatives for achieving these technological goals and has been the subject of substantial basic and applied research over the last two decades. The subject is strongly interdisciplinary, requiring proper understanding and efficient integration of knowledge in different specialized fields. This review article presents a historical and up‐to‐date account of the energy‐related applications of magnetocaloric materials and information about their processing and magnetic fields, thermodynamics, heat transfer, and other relevant characteristics. The article also discusses the conceptual design of magnetocaloric refrigeration and power generation systems and some guidelines for future research in the field.     

Integrating ZnO Microwires with Nanoscale Electrodes Using a Suspended PMMA Ribbon for Studying Reliable Electrical and Electromechanical Properties

A simple method for making electrical connections to ZnO microwires is reported. By using a suspended poly(methyl methacrylate) (PMMA) ribbon, it is shown that it is possible to electrically contact 1–2 μm diameter ZnO microwires with metal electrodes that are only 90 nm thick. The contact resistances of ZnO microwire‐based electronic devices fabricated by this method are lower than those of devices fabricated by standard electron‐beam lithography and evaporation processes. As one of the possible device applications from this fabrication method, suspended ZnO microwire‐based electromechanical devices are produced and their piezoelectric properties are investigated. Piezoelectric‐induced current is detected when the suspended microwires are induced to vibrate at their resonant frequency. This fabrication method can be readily and generally applied to prepare nanoscale electrodes on micrometer‐sized materials and provides a convenient means for studying their electrical and electromechanical phenomena in a reliable manner.     

Cancer Liquid Biopsy Using Integrated Microfluidic Exosome Analysis Platforms

Liquid biopsies serve as both powerful noninvasive diagnostic tools for early cancer screening and prognostic tools for monitoring cancer progression and treatment efficacy. Exosomes are promising biomarkers for liquid biopsies, since these nano‐sized extracellular vesicles (EVs) enrich proteins, lipids, mRNAs, and miRNAs from cells of origin, including cancer cells. Although exosomes are abundantly present in various bodily fluids, conventional exosome isolation and detection methods that rely on benchtop equipment are time‐consuming, expensive, and involve complicated non‐portable procedures. As an alternative, recently developed microfluidic platforms can perform effective exosome separation and detection for liquid biopsies using a single device. Such methods offer advantages of integrity, speed, cost‐efficiency, and portability over conventional benchtop and early microfluidic‐based single‐functional methods which can only separate or detect exosomes separately. These advances have made exosome‐based point‐of‐care (POC) applications possible. This review outlines recent integrated microfluidic‐based exosomal detection strategies to guide future development of such devices for use in liquid biopsies for early cancer screening, prognostic monitoring, and other potential POC applications.     

Aptamer‐based downstream processing of his‐tagged proteins utilizing magnetic beads

Aptamers are synthetic nucleic acid‐based high affinity ligands that are able to capture their corresponding target via molecular recognition. Here, aptamer‐based affinity purification for His‐tagged proteins was developed. Two different aptamers directed against the His‐tag were immobilized on magnetic beads covalently. The resulting aptamer‐modified magnetic beads were characterized and successfully applied for purification of different His‐tagged proteins from complex E. coli cell lysates. Purification effects comparable to conventional immobilized metal affinity chromatography were achieved in one single purification step. Moreover, we have investigated the possibility to regenerate and reuse the aptamer‐modified magnetic beads and have shown their long‐term stability over a period of 6 months. Biotechnol. Bioeng. 2011;108: 2371–2379. © 2011 Wiley Periodicals, Inc.     

Enhanced Cyclability of Lithium–Oxygen Batteries with Electrodes Protected by Surface Films Induced via In Situ Electrochemical Process

Although the rechargeable lithium–oxygen (Li–O₂) batteries have extremely high theoretical specific energy, the practical application of these batteries is still limited by the instability of their carbon‐based air‐electrode, Li metal anode, and electrodes, toward reduced oxygen species. Here a simple one‐step in situ electrochemical precharging strategy is demonstrated to generate thin protective films on both carbon nanotubes (CNTs), air‐electrodes and Li metal anodes simultaneously under an inert atmosphere. Li–O₂ cells after such pretreatment demonstrate significantly extended cycle life of 110 and 180 cycles under the capacity‐limited protocol of 1000 mA h g^?1 and 500 mA h g^?1, respectively, which is far more than those without pretreatment. The thin‐films formed from decomposition of electrolyte during in situ electrochemical precharging processes in an inert environment, can protect both CNTs air‐electrode and Li metal anode prior to conventional Li–O₂ discharge/charge cycling, where reactive reduced oxygen species are formed. This work provides a new approach for protection of carbon‐based air‐electrodes and Li metal anodes in practical Li–O₂ batteries, and may also be applied to other battery systems.     

A comparative analysis of the equilibrium dynamics of a designed protein inferred from NMR,X‐ray,and computations

A detailed analysis of high‐resolution structural data and computationally predicted dynamics was carried out for a designed sugar‐binding protein. The mean‐square deviations in the positions of residues derived from nuclear magnetic resonance (NMR) models and those inferred from X‐ray crystallographic B‐factors for two different crystal forms were compared with the predictions based on the Gaussian Network Model (GNM) and the results from molecular dynamics (MD) simulations. GNM systematically yielded a higher correlation than MD, with experimental data, suggesting that the lack of atomistic details in the coarse‐grained GNM is more than compensated for by the mathematically exact evaluation of fluctuations using the native contacts topology. Evidence is provided that particular loop motions are curtailed by intermolecular contacts in the crystal environment causing a discrepancy between theory and experiments. Interestingly, the information conveyed by X‐ray crystallography becomes more consistent with NMR models and computational predictions when ensembles of X‐ray models are considered. Less precise (broadly distributed) ensembles indeed appear to describe the accessible conformational space under native state conditions better than B‐factors. Our results highlight the importance of using multiple conformations obtained by alternative experimental methods, and analyzing results from both coarse‐grained models and atomic simulations, for accurate assessment of motions accessible to proteins under native state conditions. Proteins 2009. © 2009 Wiley‐Liss, Inc.