Bio-based carbon fiber from biorefinery lignin and lignin-derived bio-oil

Due to its light weight and superior mechanical properties, carbon fiber has many attractive applications. However, PAN-based carbon fiber is too costly, which prevent its wide applications. On the other hand, lignin is considered as a low-cost alternative of PAN for carbon fiber production. As ever-increasing amount of lignin is produced from emerging biorefineries as a byproduct, lignin-based carbon fiber could contribute to economic sustainability of the lignin producing industries. In the first section of this work, the potential of producing carbon fiber from a high-ash containing corn stover lignin is investigated. The lignin was pretreated with methanol fractionation and partially acetylation and melt-spun at 165-170 ℃. After stabilization and carbonization, carbon fiber with tensile strength of 0.454 GPa was obtained. In following serious of study, a new concept called “depolymerization followed repolymerization” is proposed to prepare structurally modified lignin precursors in order to improve carbon fiber properties. Specifically, red oak lignin was thermally depolymerized into crude bio-oil via fast pyrolysis, and then repolymerized into carbon fiber precursors through different reaction routes. The aim of the work is to produce a lignin-based carbon fiber precursor with modified structure and molecular orientation. In first route, the bio-oil, so called pyrolytic lignin (PL) was repolymerized in the presence of acid catalyst to prepare a precursor. The precursor had a Tg of 101 ℃, could be easily melt-spun at 115-120 ℃, and stabilized at 0.3 ℃/min. The resulting carbon fiber had smooth surface and void-free cross section. The average tensile strength of the fiber was 0.855 GPa. In the second route, PL was thermally co-treated with 5 or 20% of polyethylene terephthalate (PET), prior to the PL-PET precursor was melt-spun. Results from FTIR, NMR, thermal, and rheological tests all indicated that PL and PET chemically bonded to improve molecular orientation of the resulting precursor. As a result, PL-PET precursor had decreased spinning temperatures and higher thermal stabilities. Also, PL-PET based carbon fiber had lower ID/IG from Raman spectra for improved crystallinity and higher mechanical properties compared to PL-based carbon fiber. In third route, PL was converted to lignin-based polymers through free radical polymerization. To convert PL into radically polymerizable material, PL was functionalized with different amounts of methacryloyls and acetyls and then polymerized by applying Reversible Addition-Fragmentation chain Transfer (RAFT) technique. Three polymers namely PLMAP1, PLMAP2, and PLMAP3 were synthesized. PLMAP1 (fully methacylated PL) was a cross-linked polymer. PLMAP2 (partially methacrylated PL) was a thermoplastic polymer with glass transition temperature (Tg) of 161 ℃ and thermal decomposition temperature (Td) of 241 ℃. PLMAP3 (partially methacrylated and the fully acetylated PL) had reduced Tg of 130 ℃, while its Td increased to 250 ℃. PLMAP3 is melt-spinnable and also demonstrated highly attractive properties as ideal carbon fiber precursor. It is carbonizable at 1000 ℃, and has much improved molecular orientation

Exploiting Data Skew for Improved Query Performance

Analytic queries enable sophisticated large-scale data analysis within many commercial, scientific and medical domains today. Data skew is a ubiquitous feature of these real-world domains. In a retail database, some products are typically much more popular than others. In a text database, word frequencies follow a Zipf distribution with a small number of very common words, and a long tail of infrequent words. In a geographic database, some regions have much higher populations (and data measurements) than others. Current systems do not make the most of caches for exploiting skew. In particular, a whole cache line may remain cache resident even though only a small part of the cache line corresponds to a popular data item. In this paper, we propose a novel index structure for repositioning data items to concentrate popular items into the same cache lines. The net result is better spatial locality, and better utilization of limited cache resources. We develop a theoretical model for analyzing the cache behavior, and implement database operators that are efficient in the presence of skew. Our experiments on real and synthetic data show that exploiting skew can significantly improve in-memory query performance. In some cases, our techniques can speed up queries by over an order of magnitude

Conditionally Risk-Averse Contextual Bandits

Contextual bandits with average-case statistical guarantees are inadequate in risk-averse situations because they might trade off degraded worst-case behaviour for better average performance. Designing a risk-averse contextual bandit is challenging because exploration is necessary but risk-aversion is sensitive to the entire distribution of rewards; nonetheless we exhibit the first risk-averse contextual bandit algorithm with an online regret guarantee. We conduct experiments from diverse scenarios where worst-case outcomes should be avoided, from dynamic pricing, inventory management, and self-tuning software; including a production exascale data processing system

Teachers’ Attitudes to The Implementation of Bhutan Professional Standards (BPST) For Teachers in Bhutan

Implementation of Bhutan Professional Standards for Teachers in Bhutan is a key impetus for teachers across the country to enhance impeccable teacher competency. Studies on teachers’ attitudes towards professional standards show a positive as there was a significant correlation between teachers’ competencies, learners’ academic achievement, and the quality of education. However, Bhutan Professional Standards for Teachers is relatively new and there is little study exists relative to Bhutan. The study on Teachers’ Attitudes to the Implementation of Bhutan Professional Standards for Teachers in Bhutan aimed to explore and get a snapshot view of teachers on Bhutan Professional Standards for Teachers. Through a cross-sectional survey design, data were collected from 155 participants using the Five-Pointed Likert Scale and two open-ended questionnaires. The descriptive analysis revealed that teachers have fairly positive attitudes toward the implementation of Bhutan Professional Standards for Teachers. Associated challenges, the implication of the findings, and recommendations are also discussed

Difference in Temperature between Shallow and Deep Valleys of the Bhutan Himalaya

The differences in temperature between Khasadrupchu and Lumitsawa and between Yusipang and Lamperi, located at a similar altitude along different valleys, are analyzed to clarify the difference in thermal condition between the shallow and deep valleys. The annual mean temperature is almost same between the shallow and deep valleys. However, the annual mean maximum and minimum temperature, the annual variation in monthly temperature and the diurnal variation in temperature are found to differ between the shallow and deep valleys. One of the main causes of differences in temperature between Khasadrupchu and Lumitsawa and between Yusipang and Lamperi is the relative height from the valley bottom, as the altitudinal difference between the shallow and deep valleys varies. The diurnal range of temperature is found to be larger along the shallow valley than along the deep valley. One of the causes of large diurnal range of temperature along the shallow valley is due to the valley wind of the deep valley that invades the shallow valley and cause föhn like phenomena

High-Performance Heterostructured Cathodes for Lithium-Ion Batteries with a Ni-Rich Layered Oxide Core and a Li-Rich Layered Oxide Shell

The Ni-rich layered oxides with a Ni content of >0.5 are drawing much attention recently to increase the energy density of lithium-ion batteries. However, the Ni-rich layered oxides suffer from aggressive reaction of the cathode surface with the organic electrolyte at the higher operating voltages, resulting in consequent impedance rise and capacity fade. To overcome this difficulty, we present here a heterostructure composed of a Ni-rich LiNi0.7Co0.15Mn0.15O2 core and a Li-rich Li1.2-xNi0.2Mn0.6O2 shell, incorporating the advantageous features of the structural stability of the core and chemical stability of the shell. With a unique chemical treatment for the activation of the Li2MnO3 phase of the shell, a high capacity is realized with the Li-rich shell material. Aberration-corrected scanning transmission electron microscopy (STEM) provides direct evidence for the formation of surface Li-rich shell layer. As a result, the heterostructure exhibits a high capacity retention of 98% and a discharge- voltage retention of 97% during 100 cycles with a discharge capacity of 190 mA h g(-1) (at 2.0-4.5 V under C/3 rate, 1C = 200 mA g(-1)).ope