Clouds influence Earth’s climate by reflecting incoming solar radiation and reducing outgoing thermal radiation. As Earth’s surface warms, the net radiative effect of clouds also changes, contributing either a dampening (negative, cooling) or amplifying (positive, heat-trapping) feedback to the climate system. The amount of global warming from increased carbon dioxide (CO2) is critically dependent on the cloud feedback.
In research appearing in the October 31, 2016, edition of Nature Geoscience, Lawrence Livermore researchers identified a mechanism that causes low clouds—and their influence on Earth’s energy balance—to respond differently to global warming depending on the spatial pattern of the warming. The researchers showed that the strength of the cloud feedback as predicted by a climate model fluctuates depending on the observed time period. Despite having a positive cloud feedback in response to long-term projected global warming, the model exhibits a strong negative cloud feedback over the last 30 years. At the heart of this difference are low-level clouds in the tropics, which strongly cool the planet by reflecting solar radiation to space.
The feedback from low-level clouds in tropical regions between the 1980s and 2000s was found to be substantially more dampening compared to long-term cloud feedback. “With a combination of climate model simulations and satellite observations, we found that the trend of low-level cloud cover over the last three decades differs substantially from that under long-term global warming,” says Chen Zhou, lead author of the paper. The results imply that studies relying solely on recent observed trends may underestimate how much Earth will warm from increased CO2.
Contact: Mark Zelinka (925) 423-5146 (zelinka1@llnl.gov).
Organic–inorganic halide perovskite materials have emerged as attractive alternatives to conventional silicon solar cell technology. Their high light absorption and long diffusion lengths result in high power-conversion efficiencies. New research led by Professor Alex Zetti from the University of California at Berkeley, in collaboration with Lawrence Livermore research scientist Marcus Worsley, shows that using the mineral perovskite in graded bandgap solar cells achieves record-setting parameters, including an average steady-state efficiency rate of 21.7 percent. The study appears in the November 7, 2016, online issue of Nature Materials.
The new solar cell configuration contains two perovskite layers (incorporating gallium nitride), a monolayer hexagonal boron nitride, and Livermore-developed graphene aerogel. “The graphene aerogel is incorporated in the hole transport layer under the perovskite absorber layer and serves several critical roles in enhancing the performance of the solar cell,” says Worsley. The study demonstrated that the graded bandgap perovskite device achieved a 18.4 percent steady-state efficiency rate and a peak rate of 26 percent. Typical silicon solar panels have a maximum efficiency rate of about 20 percent.
In addition, unlike silicon cells, which require expensive, multistep processes involving a clean room and high vacuum, perovskite cells can be made in traditional wet chemistry laboratories. “These advantages have made perovskite solar cells commercially attractive alternatives to silicon-based solar cells,” says Worsley. “If perovskite cells reach their potential in terms of efficiency and can be reliable and produced at large-scale, they could ultimately displace silicon.”
Contact: Marcus Worsley (925) 424-4831 (worsley1@llnl.gov).
Lawrence Livermore scientists, in collaboration with theorists at the University of South Florida (USF), recently reported the synthesis and equation of state of a long sought-after five-membered ring nitrogen (N5) compound. The ring-structure compounds, known as pentazolates, are the last all-nitrogen members of the azole series. The research appears in the December 6, 2016, online edition of Chemistry of Materials.
Starting from a mixture of cesium azide (CsN3) and molecular nitrogen (N2), the Livermore scientists synthesized a stable cesium pentazolate salt compound at pressures just above 40 gigapascals (400,000 times atmospheric pressure) and temperatures near 2,000 kelvins. Surprisingly, the experiments revealed that CsN5 is stable at room temperature down to much lower pressures. USF theorists used evolutionary structural search algorithms to generate the roadmap required for the Livermore experimentalists to synthesize and verify the ring-shaped molecular structure. “This work provides critical insight into the role of extreme conditions in exploring unusual bonding routes that ultimately lead to the formation of novel high-nitrogen-content compounds,” says Elissaios (Elis) Stavrou, the lead Livermore physicist for this study.
Understanding the chemical processes governing the synthesis of single-bonded, nitrogen-rich compounds is one key required to unlock viable production strategies of high-energy-density chemical propellant and explosive formulations. Stavrou says, “The knowledge gained through this study brings our community closer to understanding how to make stable nitrogen-rich energetic materials. We aim to pursue alternative synthesis routes derived from our recent results.”
Contact: Elissaios Stavrou (925) 423-7474 (stavrou1@llnl.gov).