Membranes made of stacked layers of graphene oxide (GO) hold the tantalizing promise of revolutionizing desalination and water filtration if selective transport of molecules can be controlled. We present the findings of an integrated study that combines experiment and molecular dynamics simulation of water intercalated between GO layers. We simulated a range of hydration levels from 1 wt.% to 23.3 wt.% water. The interlayer spacing increased upon hydration from 0.8 nm to 1.1 nm. We also synthesized GO membranes that showed an increase in layer spacing from about 0.7 nm to 0.8 nm and an increase in mass of about 15% on hydration. Water diffusion through GO layers is an order of magnitude slower than that in bulk water, because of strong hydrogen bonded interactions. Most of the water molecules are bound to OH groups even at the highest hydration level. We observed large water clusters that could span graphitic regions, oxidized regions and holes that have been experimentally observed in GO. Slow interlayer diffusion can be consistent with experimentally observed water transport in GO if holes lead to a shorter path length than previously assumed and sorption serves as a key rate-limiting step.

We demonstrated that Sc2O3 thin films deposited by plasma-assisted molecular-beam epitaxy can be used simultaneously as a gate oxide and as a surface passivation layer on AlGaN/GaN high electron mobility transistors (HEMTs). The maximum drain source current, IDS, reaches a value of over 0.8 A/mm and is ∼40% higher on Sc2O3/AlGaN/GaN transistors relative to conventional HEMTs fabricated on the same wafer. The metal–oxide–semiconductor HEMTs (MOS–HEMTs) threshold voltage is in good agreement with the theoretical value, indicating that Sc2O3 retains a low surface state density on the AlGaN/GaN structures and effectively eliminates the collapse in drain current seen in unpassivated devices. The MOS-HEMTs can be modulated to +6 V of gate voltage. In particular, Sc2O3 is a very promising candidate as a gate dielectric and surface passivant because it is more stable on GaN than is MgO.

A study of V/Al/Pt/Au and Ti/Al/Pt/Au contacts to n-Al0.3Ga0.7N/GaN heterostructures is presented. Vanadium was chosen as a potential replacement for Ti because V is expected to form a thermally stable nitride with a low work function. Low-resistance Ohmic contacts are achieved with V/Al/Pt/Au layers after annealing at 650 °C, which represents a decrease of 150 °C compared to the Ti/Al/Pt/Au counterpart. This contact exhibits two orders of magnitude lower specific contact resistance compared to the Ti/Al/Pt/Au contacts studied in this work when annealed at temperatures less than 800 °C, although the specific contact resistance of the Ti/Al/Pt/Au contacts is lower when annealed at higher temperatures. A contact resistance and specific contact resistance of 0.8±0.1 Ω mm and (1.4±0.3)×10−5 Ω cm2, respectively, are obtained after annealing at 650 °C for 45 s.

Executive SummarySolid oxide fuel cell (SOFC) power systems can be designed to produce electricity from fossil fuels at extremely high net efficiencies, approaching 70%. However, in order to penetrate commercial markets to an extent that significantly impacts world fuel consumption, their cost will need to be competitive with alternative generating systems, such as gas turbines.This report discusses a cost model developed at PNNL to estimate the manufacturing cost of SOFC power systems sized for ground-based distributed generation. The power system design was developed at PNNL in a study on the feasibility of using SOFC power systems on more electric aircraft to replace the main engine-mounted electrical generators [Whyatt and Chick, 2012]. We chose to study that design because the projected efficiency was high (70%) and the generating capacity was suitable for groundbased distributed generation (270 kW).The electricity costs for a mass manufactured solid oxide fuel cell could be competitive with centralized power production plants with costs estimated to be in the $0.07-0.08/kWh range based on a cost model using a standard approach to manufacturing solid oxide fuel cells. A process flow sheet was developed to understand the steps required to manufacture the units, as well as to estimate the materials, equipment, and labor required to make them. Equipment was sized to meet a production volume of 10,000 units per year. Appropriate material and equipment prices were collected.A sputtering approach was also examined using the model to project the decreases in costs associated with the process. The process not only reduces material costs but increases the power density of the fuel cell by 50%. The increased power density reduces the number of repeat units required to make up the 270 kW fuel cell stack. Stack costs decreased by 33%. However, due the BOP and the remainder of costs associated in power system manufacturing and installation, the cost of electricity was only reduced by $0.002/kWh.In addition, to the 10,000 units per year production scale model was adjusted to reflect the costs of production at 50, 250, 1000 and 4000 units of production per year. Material prices were adjusted to reflect purchase levels. Machinery and labor were adjusted to reflect the production scale.

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