High-temperature Annealing Step Research Articles

Solution process of graphene-induced ohmic contact between the metal and AlGaN/GaN for hemts application

This work demonstrates an AlGaN/GaN high electron mobility transistor (HEMT) with Cr/Graphene ohmic contacts constructed without heat treatment. The Cr/Graphene ohmic contact was fabricated using a spray-coated graphene nanoflakes solution and electron-beam-evaporated Cr. This method does not require a high-temperature annealing step in conventional Ti/Al/Ni/Au ohmic contact. It is suggested that the Cr/graphene combination acts similarly to a doped n-type semiconductor in contact with AlGaN/GaN heterostructures, enabling carrier transport to the AlGaN layer. The investigated Au/Cr/ Graphene/AlGaN/GaN HEMT device exhibits ohmic drain characteristics in the range of -4 V to 4 V of drain-source voltage with a calculated contact resistance density of 2.5 mΩcm2. Our results have important implications for the fabrication and manufacturing of AlGaN/GaN-based microelectronic and optoelectronic devices/sensors of the future.

Three-Step Process for Efficient Solar Cells with Boron-Doped Passivated Contacts

Crystalline silicon (c-Si) solar cells with passivation stacks consisting of a polycrystalline silicon (poly-Si) layer and a thin interfacial silicon dioxide (SiO2) layer show high conversion efficiencies. Since the poly-Si layer in this structure acts as a carrier transport layer, high doping of the poly-Si layer is crucial for high conductivity and the efficient transport of charge carriers from the bulk to a metal contact. In this respect, conventional furnace-based high-temperature doping methods are limited by the solid solubility of the dopants in silicon. This limitation particularly affects p-type doping using boron. Previously, we showed that laser activation overcomes this limitation by melting the poly-Si layer, resulting in an active concentration beyond the solubility limit after crystallization. High electrically active boron concentrations ensure low contact resistivity at the (contact) metal/semiconductor interface and allow for the maskless patterning of the poly-Si layer by providing an etch-stop layer in an alkaline solution. However, the high doping concentration degrades during long high-temperature annealing steps. Here, we performed a test of the stability of such a high doping concentration under thermal stress. The active boron concentration shows only a minor reduction during SiNx:H deposition at a moderate temperature and a fast-firing step at a high temperature and with a short exposure time. However, for an annealing time tanneal = 30 min and an annealing temperature 600 °C ≤ Tanneal≤ 1000 °C, the high conductivity is significantly reduced, whereas a high passivation quality requires annealing in this range. We resolve this dilemma by introducing a second, healing laser reactivation step, which re-establishes the original high conductivity of the boron-doped poly-Si and does not degrade the passivation. After a thermal annealing temperature Tanneal = 985 °C, the reactivated layers show high sheet conductance (Gsh) with Gsh = 24 mS sq and high passivation quality, with the implied open-circuit voltage (iVOC) reaching iVOC = 715 mV. Therefore, our novel three-step process consisting of laser activation, thermal annealing, and laser reactivation/healing is suitable for fabricating highly efficient solar cells with p++-poly-Si/SiO2 contact passivation layers.

Fabrication of Glass‐Ceramic 3D Micro‐Optics by Combining Laser Lithography and Calcination

AbstractThis perspective is an overview of a recent direction in optical 3D printing, where polymerization of crosslinkable materials and nanomaterial fillers can be guided to the final structures and new composites via high temperature annealing (HTA). Defining 3D nano/micro‐structures by ultrafast laser direct writing and tailoring their precursor composition with subsequent tunability of the final properties during 750–1500 °C HTA step takes place at the large surface‐to‐volume ratio conditions favoring efficient pyrolysis and calcination, which are required for exchange of chemical materials/gases between glass/ceramic phase and surrounding. Previously, unexplored inorganic material formation conditions in terms of fast thermal quenching, composition mixing and surface tension guided formation can be harnessed by glass making for creation of new materials endowed with preferable technical properties. An immediate application perspective for a high durability, integrated, and active 3D micro‐optics is foreseen.

Ni-Pd-Incorporated Fe3O4 Yolk-Shelled Nanospheres as Efficient Magnetically Recyclable Catalysts for Reduction of N-Containing Unsaturated Compounds

The use of metal-based heterogeneous catalysts for the degradation of N-containing organic dyes has attracted much attention due to their excellent treatment efficiency and capability. Here, we report the synthesis of heterometals (Ni and Pd)-incorporated Fe3O4 (Ni-Pd/Fe3O4) yolk-shelled nanospheres for the catalytic reduction of N-containing organic dyes using a facile combination of solvothermal treatment and high-temperature annealing steps. Benefiting from the magnetic properties and the yolk-shelled structure of the Fe3O4 support, as well as the uniformly dispersed active heterometals incorporated in the shell and yolk of spherical Fe3O4 nanoparticles, the as-prepared Ni-Pd/Fe3O4 composite shows excellent recyclability and enhanced catalytic activity for three N-containing organic dyes (e.g., 4-nitrophenol, Congo red, and methyl orange) compared with its mono metal counterparts (e.g., Ni/Fe3O4 and Pd/Fe3O4). In the 4-nitrophenol reduction reaction, the catalytic activity of Ni-Pd/Fe3O4 was superior to many Fe3O4-supported nanocatalysts reported within the last five years. This work provides an effective strategy to boost the activity of iron oxide-based catalytic materials via dual or even multiple heterometallic incorporation strategy and sheds new light on environmental catalysis.

SnO2-Based Ultra-Flexible Humidity/Respiratory Sensor for Analysis of Human Breath.

Developing ultraflexible sensors using metal oxides is challenging due to the high-temperature annealing step in the fabrication process. Here, we demonstrate the ultraflexible relative humidity (RH) sensor on food plastic wrap by using 808 nm near-infrared (NIR) laser annealing for 1 min at a low temperature (26.2-40.8 °C). The wettability of plastic wraps coated with sol-gel solution is modulated to obtain uniform films. The surface morphology, local temperature, and electrical properties of the SnO2 resistor under NIR laser irradiation with a power of 16, 33, and 84 W/cm2 are investigated. The optimal device can detect wide-range RH from 15% to 70% with small incremental changes (0.1-2.2%). X-ray photoelectron spectroscopy reveals the relation between the surface binding condition and sensing response. Finally, the proposed sensor is attached onto the face mask to analyze the real-time human breath pattern in slow, normal, and fast modes, showing potential in wearable electronics or respiration monitoring.

(Digital Presentation) Substrate Effects in GaN-on-Si Hemt Technology for RF FEM Applications

: GaN-on-Si HEMTs are emerging as a viable candidate for front-end-of-module (FEM) implementation in 5G and beyond user equipment and small-cell applications [1][2]. This is because GaN HEMTs based power amplifiers and switches have high power handling capability as well as excellent switch figure-of-merit (Ron × Coff). The cost-effective integration of GaN HEMTs on silicon substrates not only benefit from standard CMOS back-end-of-the-line processing but also wafer-level integration with Si-CMOS [1][3], enabling complex functionality and better performance than the standalone counterparts. An example can be a hybrid beamformer where GaN HEMTs can enable much smaller antenna array and therefore a smaller system form factor. For 5G wireless applications, standalone or co-integrated GaN HEMT based FEMs can lead to a more energy efficient and compact system as compared to standalone Si-CMOS technologies. However, for both amplifiers and switches, GaN-on-Si HEMTs present thermal management and substrate loss related issues. In this work, we study and model the impact of GaN HEMT integration on Si substrate on RF substrate losses and non-linearities. The growth of III-N buffer is the most significant factor in determining RF losses and harmonic distortion contribution from the substrate. High temperature annealing and ion implantation steps encountered during HEMT processing can also degrade the substrate performance. In addition, we demonstrate a direct co-relation between substrate losses and harmonic distortion analogous to silicon-on-insulator technologies (Figure 1). However, the bias dependence of RF losses and harmonics show a strong time dependence (memory effects) which is more complex to model [11]. We discuss the approaches to understand and model these effects.

A silicon carbide-based highly transparent passivating contact for crystalline silicon solar cells approaching efficiencies of 24%

A highly transparent passivating contact (TPC) as front contact for crystalline silicon (c-Si) solar cells could in principle combine high conductivity, excellent surface passivation and high optical transparency. However, the simultaneous optimization of these features remains challenging. Here, we present a TPC consisting of a silicon-oxide tunnel layer followed by two layers of hydrogenated nanocrystalline silicon carbide (nc-SiC:H(n)) deposited at different temperatures and a sputtered indium tin oxide (ITO) layer (c-Si(n)/SiO2/nc-SiC:H(n)/ITO). While the wide band gap of nc-SiC:H(n) ensures high optical transparency, the double layer design enables good passivation and high conductivity translating into an improved short-circuit current density (40.87 mA cm−2), fill factor (80.9%) and efficiency of 23.99 ± 0.29% (certified). Additionally, this contact avoids the need for additional hydrogenation or high-temperature postdeposition annealing steps. We investigate the passivation mechanism and working principle of the TPC and provide a loss analysis based on numerical simulations outlining pathways towards conversion efficiencies of 26%.

Recycling diesel soot nanoparticles for use as activated carbon in Li ion batteries

We report the successful capture and reuse of diesel exhaust soot particles as a conductive additive in lithium manganese oxide (LMO) and lithium iron phosphate (LFP) cathodes in Li-ion batteries. This approach enables an abundant toxic pollutant to be converted into a valuable material for energy storage devices. This study consists of an initial characterization of the diesel soot particles, a high-temperature annealing step to remove residual organics and unburned hydrocarbons, and characterization of the electrical performance in a Li-ion battery configuration. Here, composite electrodes are fabricated by mixing active materials (LFP or LMO) with conductive carbon and binders. The performance of the diesel soot particles as conductive additives is compared with that of commercially available activated carbon (i.e., Super P®). The current evolution of the composite electrode made with diesel soot particles demonstrates comparable performance to the electrodes containing the Super P® carbon. Based on high-resolution transmission electron microscope (HRTEM) images and scanning mobility particle sizer (SMPS) spectra, we find that these diesel soot nanoparticles follow a narrow log-normal distribution centered around 100 nm in diameter and consist of highly porous amorphous carbon, which provide a large surface-to-volume ratio, making them ideal candidates for electrode materials in Li ion batteries.

Gas Absorption in Package Using Au/Pt/Ti Bonding Layer

Vacuum packaging is required for the successful long-term operation of various kinds of micro-electromechanical system (MEMS) devices. The following points are important to complete and maintain encapsulated internal vacuum conditions: Degassing MEMS assemblies before packagingPackaging MEMS devices without leakageAbsorbing residual gases after packaging Our research group has developed a simplified MEMS packaging process using a Au/Ti layer (from top to bottom) as a bonding and gettering layer[1]. The assemblies were firstly metalized with thin Au/Ti films. The Au surfaces formed direct bonding without leakage even at room temperature. Moreover, when the packaged structure was annealed at 200 °C, the Ti atoms diffused to the inner surface of the package and absorbed residual gas molecules. This approach allows the simplification of the MEMS packaging process as the bonding and gettering layers can be simultaneously fabricated.However, we found that the Au/Ti film cannot form bonding after degas annealing at 200 °C because Ti oxides were formed on the surface. The annealing at 200 °C effectively removes the water molecules physi and chemi-sorbed on the MEMS assemblies, which deteriorate the packaged environment. The purpose of this study is to develop a metal multilayer that enables both the packaging after degassing at 200 °C and the absorption of residual gas at higher temperatures. The diffusion of Ti atoms can be controlled by fabricating a Pt barrier layer between the Au and Ti layers. A thick Pt layer enables bonding after degassing by preventing the diffusion of Ti during degassing. However, the gas absorption process requires a moderate thickness of the Pt layer because the Ti atoms should reach the surface at high temperatures. In this study, the packaging using thin Au/Pt/Ti multilayers were performed for the advanced MEMS packaging process.For the fabrication of packaged environments, a Si substrate with cavities was bonded with a flat Si substrate using the Au/Pt/Ti layers, as illustrated in Fig. 1. They were thermally oxidized before the bonding procedures. Firstly, the substrates were metalized with Au/Pt/Ti layers of 3/5/40 nm in thickness. The substrates were annealed at 200 °C under vacuum conditions for degassing. Immediately after degassing, the Au surfaces were brought in contact and pressed for the bond formation. Figure 2 shows the mapping of the interfacial gaps observed using a scanning acoustic microscope. Ultrasonic waves reflected in the bright areas, indicating the presence of unbonded areas at the interface. The bright square-shaped regions were due to the packaged areas. While particles on the surface were observed as tiny round voids, there was no leak path for the packages.For the absorption of residual gas molecules, the bonded specimens were annealed at 450 °C for 180 min. Figure 3 depicts the X-ray photoelectron spectra obtained from the inner surface of the annealed package. While the Ti atoms were absent on the surface before the high-temperature annealing step, Ti compounds were present after the high-temperature annealing. This is indicated that the Ti diffused through the Au and Pt layers and reacted with the residual gas molecules on the surface.These results suggested that the 3/5/40-nm-thick Au/Pt/Ti layers can form direct bonding after degassing and also absorb residual gas molecules in the package. We expect that this packaging technique would contribute to future MEMS manufacturing because high-vacuum sealing can be obtained by the simple metal multilayer structure.[1]. Y. Kurashima et al., 45th International Conference on Micro and Nano Engineering, 2019. Figure 1

Temperature-induced stoichiometric changes in thermally grown interfacial oxide in tunnel-oxide passivating contacts

A vital part of poly-Si passivating contacts is the ultrathin interfacial oxide layer sandwiched between heavily-doped poly-Si and c-Si absorber that will herein be investigated in detail. The stoichiometry of differently prepared interfacial oxides in the as-grown state was investigated by XPS. The findings are correlated with the thermal stability of the oxide layers integrated in tunnel-oxide passivating contacts (TOPCon) which experience further structural modification during the high-temperature annealing step. More specifically, it was observed that a more stoichiometric interfacial oxide is a more effective diffusion barrier for dopants and, thereby enhanced the thermal stability of TOPCon structures with respect to the passivation quality. Furthermore, stoichiometric changes in the oxide layer upon subsequent contact formation (thermal annealing) were analyzed by XPS after selective etch back of the doped poly-Si layer. It is found that the interfacial oxide becomes more stoichiometric during annealing. In addition, a slight increase in oxide thickness was observed which tentatively could be explained by a large amount of unbound interstitial oxygen in the interface region.

Multifunctional Self-Combustion Additives Strategy to Fabricate Highly Responsive Hybrid Perovskite Photodetectors.

To resolve the inherent trade-off issue between responsivity and detectivity in FA0.9Cs0.1PbI3 perovskite photodetectors, this paper proposes a novel strategy using multifunctional self-combustion additives (urea and ammonium nitrate). During the early stages of crystallization, urea allows for the formation of a strong Lewis complex-derived low-dimensional intermediate phase; this suppresses the formation of perovskite nuclei, while ammonium ions assist the preferred grain growth along the [110] direction. During the high-temperature annealing steps, a self-combusting exothermic reaction occurs between urea as a fuel and NH4NO3 as an oxidizer, through which a locally supplied heat facilitates the removal of residual urea and byproducts. These multifunctional roles of self-combustible additives facilitate the production of high-quality, enlarged grain-structured perovskite films with improved optoelectronic properties, as confirmed by various analyses, including impedance spectroscopy and intensity-modulated photocurrent spectroscopy. The resulting FA0.9Cs0.1PbI3-based photodiode-type photodetectors exhibit outstanding performance, such as a high responsivity (0.762 A W-1) and specific detectivity (over 5.08 × 1013 Jones) at a very low external reverse bias (-0.5 V). Our findings clearly suggest that the multifunctional self-combustion additives strategy could help realize the full potential of FA1-xCsxPbI3 as a photodiode-type photodetector.

Site specific nitrogen incorporation in reduced graphene oxide using imidazole as a novel reducing agent for efficient oxygen reduction reaction and improved supercapacitive performance

Nitrogen-functionalized reduced graphene oxide (NrGO) has emerged as an inexpensive and metal-free electrocatalyst for the oxygen reduction reaction (ORR) due to the presence of pyridinic, amine, and/or graphitic nitrogen on the reduced graphene oxide sheet. Herein we report a facile synthetic route to prepare NrGO at 90 °C (NrGO-90) using imidazole as a novel reducing agent as well as a targeted nitrogen precursor while circumventing the high-temperature annealing step. The substantial atomic percentage of pyridinic and amine nitrogen has been incorporated in NrGO-90, which facilitates the ORR via the four-electron pathway. NrGO-90 exhibits higher limiting-diffusion current density (−3.7 mA/cm2), onset potential (0.82 vs. RHE), and lower Tafel slope (67 mV/Decade), as compared to hydrazine hydrate reduced graphene oxide (rGO). In addition, our samples show higher performance in tolerance of methanol corrosion and longer cycling stability (90%) after 3.5 h compared to standard Pt/C. Furthermore, the NrGO-90 exhibits a specific capacitance of 287.5 F g-1 at 0.5 A g−1 with 90% retention after 10000 cycles suggesting facile interaction between electrolyte ions and active sites of rGO sheets. Such dual proficiency in a single rGO material has rarely been achieved so far employing simple solution chemistry.

Synthesis of diameter controlled multiwall carbon nanotubes by microwave plasma-CVD on low-temperature and chemically processed Fe nanoparticle catalysts

Properties of carbon nanotubes sensitively depend on the structural configuration, e.g., chirality, diameter, layer number and the compositional characteristics. The size of catalyst nanoparticles used to grow the CNTs significantly controls its diameter. Generally, catalyst nanoparticles are size-optimized via high-temperature annealing of thin metal films. Presently, formation of size-controlled Fe nano-particles as the efficient catalyst has been pursued via a low-temperature (TCG) wet chemical process that bypasses the high-temperature annealing step and leads to a low average size of catalyst nanoparticles. Suitable progress of the oxidation–hydrolysis reactions of metal-bearing ferrite entities controls the crystallinity via dehydration of the intermediate constituent and diminution of the oxide component. Optimally low diameter Fe nanoparticles have been obtained at temperature around TCG ~ +10 °C via simultaneous prominent aerial oxidation and neutralization in a single step. In this present work multiwall carbon nanotubes of controlled diameter (~15–25 nm) have been produced by relatively low-temperature (~300 °C) plasma processing, using pre-designed shadow-mask assembly to create diffused and remote-plasma of (CH4 + H2) and CO2 as a weak oxidizing gas to selectively remove the surplus amorphous components, and also by controlling the size of catalyst Fe-nanoparticles on the substrate via low-temperature non-plasma synthesis process.

Separating the two polarities of the POLO contacts of an 26.1%-efficient IBC solar cell

By applying an interdigitated back contacted solar cell concept with poly-Si on oxide passivating contacts an efficiency of 26.1% was achieved recently. In this paper the impact of the implemented initially intrinsic poly-Si region between p-type poly-Si and n-type poly-Si regions is investigated. Two recombination paths are identified: The recombination at the interface between the initially intrinsic poly-Si and the wafer as well as the recombination across the resulting p(i)n diode on the rear side which is aimed to be reduced by introducing an initially intrinsic region. By using test structures, it is demonstrated that the width of the initially intrinsic region ((i) poly-Si region) has a strong influence on the recombination current through the p(i)n diode and that this initially intrinsic region needs to be about 30 μm wide to sufficiently reduce the recombination across the p(i)n diode. Lateral and depth-resolved time of flight secondary ion mass spectrometry analysis shows that the high-temperature annealing step causes a strong lateral inter-diffusion of donor and acceptor atoms into the initially intrinsic region. This diffusion has a positive impact on the passivation quality at the c-Si/SiOx/i poly-Si interface and is thus essential for achieving an independently confirmed efficiency of 26.1% with 30 μm-wide initially intrinsic poly-Si regions.

CoS2 nanodots anchored into heteroatom-doped carbon layer via a biomimetic strategy: Boosting the oxygen evolution and supercapacitor performance

Development of efficient and durable electrocatalysts and electrode materials are significantly important to enhance the electrochemical performance for oxygen evolution reaction and supercapacitor applications. Herein, we design a hydrothermal-assisted biomimetic strategy to anchor CoS2 nanodots on heteroatom-doped carbon layer (CoS2@HADC). Numerous CoS2 nanodots and heteroatoms rich in N and S are in situ hybridized into the carbon layer in a homogeneously dispersed manner. The preparation procedure is quite simple and moderate, avoiding the high-temperature annealing step. The optimized hybrid not only act as effective electrocatalyst, in which overpotential of 226 mv is enough to drive a current density of 10 mA cm−2 with a Tafel slope of 56 mV Dec−1 during the OER process, but also serve as electrode material with specific capacitance of 565 F g−1 at a current density of 1 A g−1 for high-performance supercapacitor, compared with CoS2 (176 F g−1). Also, it exhibits superior cycling activity (~100% capability retention after 10000 cycles at 2 A g−1 in 6.0 M KOH) and remains a 57% high rate capability (320 F g−1 at 5 A g−1). The rational design concept for cobalt sulfides/heteroatom-doped carbon hybrid provide a simple strategy towards superior energy conversion and storage devices.

Impacts of calcination on surface-clean supported nanoparticle catalysts

Traditional methods of preparing size-controlled supported nanoparticle catalysts typically require the use of stabilizing ligands that passivate the nanoparticle surface to prevent overgrowth and aggregation. The presence of these ligands on catalytic surfaces can be detrimental to activity and are typically removed via a high-temperature annealing step, which ultimately results in changes to nanoparticle morphology and thus, reduced activity. We previously demonstrated that silylamine switchable surfactants can be used to synthesize and deposit highly active nanoparticles on to a support surface while preserving monodispersity. Here, we demonstrate using XPS that supported nanoparticles prepared in this manner are surface-clean after deposition, eliminating the need for traditional activation steps. Further, it is shown that even low-temperature calcinations have detrimental effects on the catalyst properties including changes in nanoparticle morphology that result in a significant decrease in activity, a change in surface hydrophilicity, a change in activation energy, and results in the formation of an induction time when utilized in the hydrogenation of 4-nitrophenol. While calcination remains a widely used method of catalyst activation, the detrimental effects of high temperatures on catalyst properties are often overlooked.

Impact of intermediate high temperature annealing on the properties of AlN/sapphire templates grown by metalorganic vapor phase epitaxy

In this work, we investigate AlN/sapphire templates grown by metalorganic vapor phase epitaxy (MOVPE) subjected to an intermediate high temperature annealing (HTA) step at different AlN film thicknesses. To avoid cracking of the MOVPE layers during HTA at 1700 °C these layers have to be grown under low incorporation of tensile strain. For each sample the growth is stopped at a certain thickness between 230 nm and 1.2 μm followed by HTA. X-ray rocking curve FWHM of 0002 and 10–12 reflections lead to an estimation of threading dislocation densities (TDD) as low as cm−2 for the thickest AlN layers of 880 nm and 1.2 μm after annealing. For all layer thicknesses MOVPE growth is subsequently continued to a total AlN layer thickness of 1.5 μm to reach for a smooth surface. The change of the AlN strain state after HTA leads to an increased wafer curvature at room temperature and compressive strain in the subsequently grown AlN layers increasing the wafer bow at room temperature. Hence, to serve as a base layer, e.g. for ultraviolet light emitting diodes, a trade-off between low TDD, surface smoothness and wafer bow has to be found.

Rapid photonic curing of solution-processed In2O3 layers on flexible substrates

In2O3 is one of the most important semiconducting metal oxides primarily because of its wide band gap, high electron mobility and processing versatility. To this end, high-quality thin films of In2O3 can be prepared using scalable and inexpensive solution-based deposition methods, hence making it attractive for application in a number of emerging electronic applications. However, traditional solution processing often requires high temperature and lengthy annealing steps, making it impossible to use in combination with temperature-sensitive plastic substrates, which would be desired for numerous emerging flexible device applications. Here, rapid photonic curing of In2O3 layers is explored as an alternative to thermal annealing. Oxide thin films are successfully prepared on a range of substrates, including glass, polyimide, and polyethylene naphthalate. The effect of substrate and post-processing treatment on the morphology, surface chemistry, and electronic properties is investigated by atomic force microscopy and X-ray photoelectron spectroscopy. Systematic trends are identified, particularly in the degree of conversion of the precursor and its influence on the electronic structure.

Preparation and Preliminary Nonlinear Optical Properties of BiFeO3 Nanocrystal Suspensions from a Simple, Chelating Agent-Free Precipitation Route

Preparation of stable BiFeO3 nanocrystal suspensions through a simple, low-cost precipitation technique is described. Amorphous precursors are first precipitated from metal nitrate salts in highly basic KOH solutions, and a short high-temperature annealing step is then performed to induce crystallization. Nanoparticles are characterized by X-ray diffraction (XRD), TEM, DLS and ζ-potential measurements, and the synthesis conditions optimized after a systematic variation of the KOH concentration within the range of 1–12 M. The presence of residual impurities (mainly Bi25FeO39 and Bi2Fe4O9) quantified from XRD and mean nanocrystal size is found to be strongly influenced by the initial KOH solution content. A concentration at about 3–4 M is optimal in terms of BiFeO3 phase-purity and nanocrystal size. Stability of aqueous dispersions of the amorphous precursors and of the purest crystallized nanoparticles is also characterized between pH = 2 and pH = 13. After preparation of stable, almost phase-pure BiFeO3 nanocrystal suspensions, second and third harmonic scattering (SHS and THS) at excitation wavelengths of 1064 nm and 1250 nm are reported from nonlinear optical scattering measurements and compared with other recently published literature values.

Durability of Pt-Co Alloy Cathode Catalysts for PEM Fuel Cells Under Cyclic Potentials

Alloying Pt with transition metals, such as Co and Ni, enhances the catalytic activity for the oxygen reduction reaction (ORR) at cathodes of polymer electrolyte fuel cells (PEFCs). The alloying process necessarily involves a high-temperature thermal annealing step that grows the size of catalyst nanoparticles above 4-5 nm. Particles of this size are known to be more stable under cyclic potentials as they are less prone to Pt dissolution and coalescence than finer scale 2-3 nm Pt particles. However, transition metals are unstable at operating fuel cell potentials and can leach out of the catalyst in the cathode and into the ionomer, resulting in loss of ORR activity. Three formulations of Pt-Co alloy catalysts supported on high surface area carbon were subjected to accelerated stress tests (ASTs) in H2-N2 environment. The AST protocol consisted of repeated square waves with 0.6 V lower potential limit (LPL), 0.95 V upper potential limit (UPL), and 3-s holds at LPL and UPL. Changes in cell performance were monitored by measuring polarization in H2-air, high frequency resistance (HFR), electrochemical surface area (ECSA), impedance in air and helox (79% He, 21% O2), and mass activity. These measurements were made at beginning of life (BOL) and after 1000 (1k), 20,000 (20k) and 30,000 (30k) cycles. The three catalysts had a different initial Co content (34%, 20%, and 15%) and electrode microstructure. At BOL, transmission electron microscopy (TEM) analysis of the high-Co content (H) catalyst electrode, 0.2 mg/cm2 Pt loading, showed a “spongy” porous (hollow) catalyst nanoparticle morphology with ~6.7 nm median Pt-Co particle size after conditioning. The BOL medium-Co content (M) catalyst electrode, 0.1 mg/cm2 Pt loading, showed a crystalline fully alloyed catalyst nanparticle morphology with ~4.4 nm particle size after conditioning. The BOL low-Co content (L) catalyst electrode, 0.1 mg/cm2 Pt loading, also showed catalysts with a fully alloyed crystalline morphology with ~4.5 nm median particle size.Post-mortem TEM analysis of aged electrode specimens showed different amounts of Co dissolution from the nanoparticles. There is an apparent direct relationship between the initial alloy Co content the amount of Co dissolved after potential cycles. WAXS analysis confirms significant Co loss from the nanoparticles and also indicates that the Pt-Pt d-spacing in Pt-Co alloy approaches the values typical of pure Pt particles. The effect of Co dissolution on the loss in specific activity for ORR is summarized in Fig. 1a. The data suggest that the specific activity depends not only on the initial Co content but also on the catalyst nanoparticle morphology and size. Even with 27-50% Co loss, the specific activity of Pt-Co alloy remains higher than 1000 mA/cm2-Pt, and exceeds the activity (650 mA/cm2-Pt) of pure Pt of similar particle size. The measured specific activities are consistent with a kinetic model and catalyst-specific choice of kinetic parameters.The modeled ORR kinetic losses depend on specific activity, ECSA, catalyst loading, and kinetic parameters (electrode morphology). As shown in Fig. 1b, the H-catalyst electrode shows the smallest kinetic overpotentials (ηc s) due to high Pt loading, but has the fastest increase in ηc s with ageing because of low Co retention in the electrode. The L-catalyst electrode shows higher ηc s due to lower Pt loading but has a smaller increase in ηc s with ageing because of high Co retention. For this catalyst, increase in is mostly due to loss of ECSA with ageing. The M-catalyst electrode shows similar increase in ηc s with ageing as the L-catalyst electrode, but this increase is due to the combined effects of Co dissolution and ECSA loss. The modeled increase in mass transfer overpotentials (ηm) with ageing correlates with Pt loading and ECSA loss and depends on the initial electrode morphology. There is no clear evidence of a relationship between Co dissolution (i.e., effect of Co2+ on O2 permeability in ionomer film) and the resulting increase in ηm. AcknowledgmentsThe authors wish to acknowledge the financial support of the Fuel Cell Technologies Office and Fuel Cell Component R&D Team Lead, Dr. Dimitrios Papageorgopoulus, and Technology Development Manager, Dr. Nancy Garland. The authors also wish to acknowledge General Motors, IRD and Umicore for supplying materials used in this study. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. Figure 1