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Open Access Publications from the University of California

Scholarly Works (29 results)

  • Thesis
  • Peer Reviewed
UC Irvine Electronic Theses and Dissertations (2015)

Wireless sensor networks (WSNs) have been widely studied for detection and estimation problems. When a coherent multiple access channel is employed between the sensor nodes and fusion center (FC), each sensor takes a noisy measurement of the signal of interest, amplifies and forwards the measurement to a FC through a wireless fading channel, and the FC makes a decision about the presence of the signal and estimates its parameters based on the coherent sum of the signals from all the sensor nodes. To minimize estimation error or maximize probability of detection, the transmit power at the sensors is optimized under either sum or individual power constraints.

Most of the existing works assume that the FC is configured with a single antenna. It is well-known that multiple antennas can effectively increase the throughput of a wireless link, and in this thesis, we investigate how to exploit the benefit of the multiple antennas in WSN, and we study the detection and estimation performance of a coherent amplify-and-forward WSN, in which the sensor node has single antenna and the FC is configured with a massive number of antennas. When the perfect channel state information (CSI) is available at FC, we derive optimal closed-form sensor transmission gains to optimize the performance of Neyman-Pearson (NP) detector and the linear minimum mean-squared error estimator (LMMSE), and if CSI is unknown at FC, we find the optimal sensor transmission gains to maximize the deflection coefficient of the energy detector (ED). Regarding the energy efficiency, our analysis show that the performance of NP detector and LMMSE estimator remain asymptotically constant if the sensor transmit power decreases proportionally with the increase in the number of antennas, and for the ED which does not require CSI, we show that a constant deflection can be asymptotically achieved if the sensor transmit power scales as the inverse square root of the number of FC antennas.

Additionally, we consider the problem of optimize the sensor phase to minimize the estimation error at FC, when the FC has a limited number of antennas. Two phase optimization algorithms are proposed and the sensor selection problem is formulated and solved. In addition to the case with multi-antenna FC, we also investigate the optimal power allocation for the WSN with single-antenna FC, when the FC use sensor measurements as input for a Kalman filter to track a dynamic parameter of interest.

When a fixed network infrastructure is not available (e.g. in military or disaster response scenarios), we investigate how to use the multi-antenna unmanned aerial vehicles (UAVs) as a relay to improve the connectivity between the mobile sensor nodes and the FC, which may be separated by a distances greater than their communication range. Several algorithms are proposed to optimize the trajectory of UAV.

    Cover page: Physical Layer Optimization for Wireless Sensing and Network Connectivity
    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2014)
    Silver nanoprisms (AgNPs) were robustly synthesized using TEMPO-oxidized cellulose nanofibrils (CNFs) as a dual capping and shape-regulating agent for the first time. Reducing AgNO3 with NaBH4 in CNF suspensions produced smaller but more uniform Ag nanospheres (AgNSs) with increasing Ag(+)/CNF ratios. CNF bound AgNSs were facilely transformed to AgNPs by etching with H2O2, supporting the capping and shape-regulating capability of CNFs. AgNPs could also be synthesized directly in a one-shot reduction reaction with NaBH4 in the presence of both CNFs and H2O2. The AgNPs transformed from CNF bound AgNSs are similar to those synthesized directly, but more stable against H2O2. Successful synthesis of AgNPs with 80-320 nm truncated edges was confirmed by light blue solution color, sharp out-of-plane quadruple resonance peak at 334 nm and prominent in-plane dipole resonance peaks at 762-900 nm. The [111] lattice plane of AgNP was clearly evident by its predominant XRD peak at 38°, confirming the unique shape-regulating ability of the nearly fully surface carboxylated CNFs. The CNF surface bound AgNPs were easily fabricated into freestanding CNF/AgNPs films that showed excellent surface enhanced Raman scattering of Rhodamine 6G with analytical enhancement factor of 5 × 10(3) in contrast to none from the CNF/AgNSs film.
      Cover page: Synthesis of Cellulose Nanofibril Bound Silver Nanoprism for Surface Enhanced Raman Scattering
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2018)
      Cellulose fibrous aerogels have been fabricated by a facile and aqueous process that disintegrated electrospun cellulose fibers (ECFs) and reassembled via freezing/freeze-drying with significantly improved dry resiliency and spontaneous 89% shape recovery from ca. 70% compressive strain. Owing to the resilient and 200-300 nm wide ECFs, the cellulose fibrous aerogels exhibited excellent dual dry and wet resiliency as well as improved pore accessibility. The fibrous cellular walls interconnect the aerogel pore structure to allow extraordinary liquid absorption capacity up to 373 g/g, accounting for 95% of the theoretical absorption capacity. Both highly dry resilient and absorbent properties of the ECF aerogel are highly advantageous for hydrocarbon/oil contamination removal and for hydrocarbon/water separation applications. In addition, the ECF aerogel could be carbonized into carbon aerogel in supercapacitors for energy storage.
        Cover page: Dual Wet and Dry Resilient Cellulose II Fibrous Aerogel for Hydrocarbon–Water Separation and Energy Storage Applications
        • Article
        • Peer Reviewed
        UC Davis Previously Published Works (2014)
        Mechanically robust cellulose nanofibril (CNF) aerogels with ultralow density (8 mg cm-3), superior porosity (99.5%), super water absorbency (104 g g-1 water/dried mass), high crystallinity (68.5%) as well as exceptional wet resilience and water activated shape recovery were facilely fabricated for the first time by ice-crystal templated self-assembly of TEMPO oxidized CNFs via cyclic freezing-thawing. With ultrathin widths (1-2 nm), high aspect ratios (several hundreds) and numerous surface polar hydroxyls and carboxyls, TEMPO oxidized CNFs behaved similar to aqueous soluble polymers to form strong freestanding hydrogels by repetitive freezing (-20°C, 15 h) and thawing (room temperature, 9 h). The spaces occupied by the several hundred microns wide ice crystals were well preserved upon freeze-drying, deriving macroporous CNF aerogels with over 99% porosity of interconnected pores. The freezing induced self-assembling of CNFs was observed at a low concentration of 0.05%, whilst more ordered macroporous honeycomb structures were observed at and above 0.2%. Exchanging water in the CNF hydrogel with tert-butanol generated hierarchical CNF aerogels containing several hundred microns sized macroscopic as well as mesoscopic pores ranging from 2 to 90 nm with further improved specific surface area (117.8 m2 g-1), pore volume (1.19 cm3 g-1) and water absorption (116 g g-1). All CNF aerogels demonstrated super water absorbency, fast water-activated shape recovery in 4 s and reusability for at least 20 times. © 2013 The Royal Society of Chemistry.
          Cover page: Super water absorbing and shape memory nanocellulose aerogels from TEMPO-oxidized cellulose nanofibrils via cyclic freezing–thawing
          • Article
          • Peer Reviewed
          UC Davis Previously Published Works (2014)
          Ultra-light (1.7 to 8.1 mg cm-3) and ultra-porous (99.5 to 99.9%) aerogels have been assembled from cellulose nanofibrils (CNFs) that were defibrillated from rice straw cellulose at 96.8% yield. The as-prepared aerogels, amphiphilic super-absorbents, absorbing 210 and 375 times water and chloroform, respectively, are far superior to any previously reported cellulose aerogel. Vapor deposition with triethoxyl(octyl) silane turned the amphiphilic aerogel more hydrophobic and oleophilic, capable of absorbing 139-356 times non-polar hydrocarbons, polar aprotic solvents and oils, surpassing all previously reported polymeric, cellulosic and carbonaceous aerogels by 2 to nearly 20 times. These aerogels are excellent amphiphilic super-absorbents for selective oil removal and recovery. This journal is © the Partner Organisations 2014.
            Cover page: Amphiphilic superabsorbent cellulose nanofibril aerogels
            • Article
            • Peer Reviewed
            UC Davis Previously Published Works (2015)
            Amphiphilic holoCNCs were derived from rice straw holocellulose by sulfuric acid hydrolysis (64%, 45 °C, 45 min) at 11.6% yield (8.8% of rice straw). HoloCNCs are similar in lateral dimensions (4.1 ± 1.6 nm thick, 6.4 ± 1.8 nm wide), but shorter and more heterogeneous in lengths (113 ± 70 nm long), less negatively charged (0.128 mmol/g) and less crystalline (CrI 84.4%) than CNCs. HoloCNCs were also more thermally stable (Tmax =284 °C), attributed to the presence of residual lignosulfonate, hemicellulose, and silica. Most remarkable, the amphiphilic holoCNCs were more hydrophobic than CNCs, exhibiting distinct surface active behaviors and lowering equilibrium surface tension to 49.2 mN/m at above 0.57% critical aggregation concentration. HoloCNCs not only stabilize 30% more oil-in-water (O/W) emulsion and formed droplets (1.2-1.6 μm) doubled the sizes of those with CNCs, but also self-assembled into highly mesoporous structures with up to 3× higher specific surface (111 m(2)/g) and total pore volume (0.40 cm(3)/g) than that from CNCs upon freeze-drying. The unique surface active, amphiphilic, and less self-assembling properties of holoCNCs offer new desirable characteristics but without additional isolation process nor surface modification of CNCs.
              Cover page: Holocellulose Nanocrystals: Amphiphilicity, Oil/Water Emulsion, and Self-Assembly
              • Article
              • Peer Reviewed
              UC Davis Previously Published Works (2017)
              A facile gelation cross-linking approach was devised to fabricate meso- and macroporous cellulose nanofibril (CNF) aerogels with multiple improved properties. CNF hydrogels made using a freezing-thawing method with a 94 kPa modulus were solvent exchanged with acetone and then cross-linked with methylene diphenyl diisocyanate (MDI) to produce aerogels with significantly improved compressive properties that follow a power law increment against aerogel density with impressive 1.69, 2.49, and 1.43 scaling factors for Young's modulus, yield stress, and ultimate stress, respectively. The optimally cross-linked aerogels had nearly tripled specific surface area (228 m2/g) and doubled pore volume (1 m3/g) from numerous new 9-12 nm wide mesopores as well as significantly improved thermal stability (43% char residue at 500 C vs 9.1% for un-cross-linked aerogel). Cross-linking also made the amphiphilic CNF aerogel highly hydrophobic and capable of completely separating chloroform from water via simple filtration. These nanocellulose aerogels show great promise for efficient and continuous separation of oils and hydrophobic liquids from water.
                Cover page: Cellulose Nanofibril Aerogels: Synergistic Improvement of Hydrophobicity, Strength, and Thermal Stability via Cross-Linking with Diisocyanate
                • Article
                • Peer Reviewed
                UC Davis Previously Published Works (2016)
                Self-assembling of cellulose nanofibrils (CNFs) as affected by varying extent of protonation on C6 surface carboxyls was investigated under freeze-drying and air-drying processes. Surface carboxyls were protonated from 10.3 to 100%, all on the same TEMPO oxidized and mechanically blended CNFs with identical geometries and level of oxidation. Upon freeze-drying, all CNFs assembled into amphiphilic mass. The mostly charged CNFs assembled into the finest and most uniform fibers (φ = 137 nm) that absorbed significantly more nonpolar toluene than water; whereas the fully protonated CNFs assembled extensively into porous and more thermally stable ultrathin filmlike structures that absorbed water and toluene similarly. Ultrafiltration and air-drying induced cyrstallization led to more thermally stable, semitransparent, and hydrophilic films that showed no affinity toward nonpolar toluene. In essence, CNFs could be tuned by varying the degree of surface carboxyl protonation, along with drying processes, to create fibrous to film morphologies, amphiphic to hydrophilic properties, and higher thermal stability.
                  Cover page: Self-assembling of TEMPO Oxidized Cellulose Nanofibrils As Affected by Protonation of Surface Carboxyls and Drying Methods
                  • Article
                  • Peer Reviewed
                  UC Davis Previously Published Works (2015)
                  Pure cellulose has been successfully isolated from tomato peels by either acidified sodium chlorite or chlorine-free alkaline peroxide routes, at 10.2-13.1% yields. Negatively charged (ζ = -52.4 mV, 0.48 at% S content) and flat spindle shaped (41:2:1 length:width:thickness) cellulose nanocrystals (CNCs) were isolated at a 15.7% yield via sulfuric acid hydrolysis (64% H2SO4, 8.75 mL/g, 45 °C, 30 min). CNCs could be facilely assembled from dilute aqueous suspensions into highly crystalline (80.8%) cellulose Iβ fibrous mass containing mostly sub-micron fibers (ϕ = 260 nm) and few interconnected nanofibers (ϕ = 38 nm), with 21.7 m(2)/g specific surface and 0.049 m(3)/g pore volume. More uniformly nanofibers with average 42 nm width and significantly improved specific surface area (101.8m(2)/g), mesoporosity and pore volume (0.4m(3)/g) could be assembled from CNCs in 1:1 v/v tert-butanol/water mixture.
                    Cover page: Cellulose nanocrystal isolation from tomato peels and assembled nanofibers
                    • Article
                    • Peer Reviewed
                    UC Davis Previously Published Works (2014)
                    Self-assembling of sulfuric-acid-hydrolyzed cellulose nanocrystals (CNCs, 6.4 nm wide) and TEMPO oxidized cellulose nanofibrils (CNFs, 2.1 nm wide) from aqueous suspensions was induced by rapid freezing (-196 °C, 10 min) and slow lyophilization (-50 °C, 0.05 mbar, 2 days). The assembled structures contain submicron (200-700 nm) wide and tens of micrometer long fibers at up to 0.1-0.5% and 0.01-0.05%, the critical fiber-to-film transformation concentrations for CNCs and CNFs, respectively. The assembled fiber widths were significantly reduced to ∼40 nm, that is, by 1 order of magnitude, when 10% of the aqueous media was replaced with tert-butanol. Further increasing tert-butanol contents in the media to 93/7 (CNCs) and 50/50 (CNFs) tert-butanol/water, both at 0.1% nanocellulose concentration, reduced longitudinal assembling for CNCs and lateral assembling for CNFs as well as increased critical fiber-to-film transformation concentration for CNFs. While all assembled structure could be redispersed in water, those from tert-butanol/water could also be easily redispersed in DMF aided with brief 2 min ultrasonication. None of the assembled structures could be redispersed in the lower dielectric constant ethanol, acetone or chloroform.
                      Cover page: Assembling and Redispersibility of Rice Straw Nanocellulose: Effect of tert-Butanol
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