138 research outputs found
Nonlinear Output Feedback Control: An Analysis of Performance and Robustness
No full textBy considering a non-singular performance cost functional, observer backstepping designs and adaptive observer backstepping designs are compared to high-gain observer designs for an output feedback system and a parametric output feedback system. For the output feedback system, if the initial error between the initial condition of the state and the initial condition of the observer is large, the high-gain observer design has better performance than the observer backstepping design. Whilst, for the parametric output feedback system, if the a-priori estimate for the bound of the uncertain parameter is conservative, the adaptive observer backstepping design has better performance than the high-gain observer design. In the sense of gap metric robustness, by a backstepping procedure, a robust state feedback controller is developed for the nominal plant in strick-feedback form. For the closed-loop, the controller achieves gain-function stability, and stability if the initial condition is zero. By the gap metric robustness theory, the controller achieves robustness to plant perturbations which are small in gap sense. In this way, it is shown that for any perturbed plant the controller stabilizes the closed-loop in the presence of input and measurement disturbances if the gap metric distance between the nominal and perturbed plant is less than a computable constant. For output feedback control, a nominal plant in output-feedback form is considered, and the observer backstepping procedure is amended to design a robust controller and an observer in the presence of input and measurement disturbances. The closed-loop is shown to be gain-function stable, and stable if the initial condition is zero. If the nonlinearities are only locally Lipschitz continuous, the results are only local to input and measurement disturbances; if the nonlinearities are globally Lipschitz continuous, then results are global to input and measurement disturbances. By gap metric robustness theory, if the initial condition is zero the controller is shown to be robust to plant perturbations in a gap metric sense. As an application, the theory is applied to a system with time delay, and it is shown that if the time delay is suitably small, the controller is able to achieve stability of the closed-loop. To investigate the robustness of high-gain designs to loop disturbances and plant perturbations, a restricted class of nonlinear nominal plant in normal form are considered. An amended high-gain observer control design is shown to be robust to loop disturbances and has a non-zero plant perturbation margin, which is independent of the high-gain factor
LiNi0.8Co0.15Al0.05O2: Enhanced Electrochemical Performance From Reduced Cationic Disordering in Li Slab
No full textAbstractSub-micron sized LiNi0.8Co0.15Al0.05O2 cathode materials with improved electrochemical performance caused by the reduced cationic disordering in Li slab were synthesized through a solid state reaction routine. In a typical process, spherical precursor powder was prepared by spray drying of a uniform suspension obtained from the ball-milling of the mixture of the starting raw materials. Then the precursor powders were pressed into tablets under different pressures and crushed into powder. The pressing treated powders were finally calcinated under oxygen atmosphere to obtain the target cathode materials. XRD investigation revealed a hexagonal layered structure without impurity phase for all samples and significant increase in the diffraction intensity ratio of I(003)/I(104) was observed. Rietveld refinement further confirmed the reduced cationic disordering in Li slab by such pressing treatment, and the smallest disordering was observed for sample S4 with only 1.3% Ni ions on Li lattice position. The electrochemical testing showed an improvement in electrochemical behavior for those pressing treated samples. The calculation of diffusion coefficients using EIS data showed improved Li diffusion coefficient after pressing treatment. The sample S4 presented a diffusion coefficient of 4.36 × 10−11 cm2·s−1, which is almost 3.5 times the value of untreated sample.</jats:p
Scaling relationships between the total number of leaves and the total leaf area per culm of two dwarf bamboo species
No full textAbstract Total leaf area per plant is an important measure of the photosynthetic capacity of an individual plant that together with plant density drives the canopy leaf area index, that is, the total leaf area per unit ground area. Because the total number of leaves per plant (or per shoot) varies among conspecifics and among mixed species communities, this variation can affect the total leaf area per plant and per canopy but has been little studied. Previous studies have shown a strong linear relationship between the total leaf area per plant (or per shoot) ( A T ) and the total number of leaves per plant (or per shoot) ( N T ) on a log–log scale for several growth forms. However, little is known whether such a scaling relationship also holds true for bamboos, which are a group of Poaceae plants with great ecological and economic importance in tropical, subtropical, and warm temperate regions. To test whether the scaling relationship holds true in bamboos, two dwarf bamboo species ( Shibataea chinensis Nakai and Sasaella kongosanensis ‘Aureostriatus’) with a limited but large number of leaves per culm were examined. For the two species, the leaves from 480 and 500 culms, respectively, were sampled and A T was calculated by summing the areas of individual leaves per culm. Linear regression and correlation analyses reconfirmed that there was a significant log–log linear relationship between A T and N T for each species. For S. chinensis , the exponent of the A T versus N T scaling relationship was greater than unity, whereas that of S. kongosanensis ‘Aureostriatus’ was smaller than unity. The coefficient of variation in individual leaf area increased with increasing N T for each species. The data reconfirm that there is a strong positive power‐law relationship between A T and N T for each of the two species, which may reflect adaptations of plants in response to intra‐ and inter‐specific competition for light
Protein Modifications and Structural Changesin Whey Protein upon Heat Treatments:Protein Oxidation, Desulfurization and Aggregation
No full textWhey protein, as a major by-product from cheese manufacture, is a versatile and nutritive food ingredient. In food and ingredient processing, thermal treatment is often applied to achieve specific product property, e.g. to increase product stability. However, undesirable modifications may also be promoted by heating such as protein oxidation and generation of off-flavor, leading to the decrease of nutritional value and/or consumer acceptance. To optimize the conditions of thermal treatment for specific product functionalities customization with minimized side effects on the final food applications, it is important and necessary to understand the mechanism of potential heat-induced protein modifications. In the present PhD study, whey protein modifications (e.g. disulfide rearrangement, protein oxidation and desulfurization) and structural changes were investigated under different heating conditions (i.e. 60-90 °C for 10 min, 90 °C for 120 min and 160 °C for 160 s (as a UHT-like condition)) using multiple analytical tools (e.g. liquid chromatography-mass spectrometry (LC-MS), size-exclusion chromatography (SEC) and gas chromatography-flame photometric detector (GC-FPD)). These modifications and changes were compared in different whey protein systems, including the two major whey proteins, i.e. beta- lactoglobulin (β-LG) and alpha-lactalbumin (α-LA), a whey-model system (α-LA + β-LG) and whey protein isolate (WPI). Heating of β-LG at 70 °C and above was found to induce disulfide rearrangement, which might lead to irreversible protein unfolding and protein aggregation. Although disulfide rearrangement was observed to start at both native disulfide bonds in β-LG, the surface-located Cys66-Cy160 was suggested to be more reactive towards rearrangement compared to Cys106-Cys119. At ≥80 °C, the disulfide-linked protein oligomers were increasing in size with increasing heat load, while Cys66 was proposed to be a key Cys residue participating in the rearranged inter-molecular disulfide bonds. Meanwhile, hydrogen sulfide (H2S) was also released (presumably via Cys β- elimination), which contributed to the sulfurous odor. Only minor level of heat-induced protein side-chain oxidation products was detected in β-LG after all employed heat treatments. Nevertheless, higher levels of heat-induced oxidation products, especially at Trp residues, were detected in α-LA as compared to β-LG after the severe heat treatment (i.e. 90 °C for 120 min). The presence of free Cys residue in β-LG was proposed to scavenge oxidants during heating, making it less susceptible towards oxidation than α-LA. Formation of non-native disulfide bond was only observed in α-LA once the temperature reached 90 °C, at which free Cys residue started to release from the native disulfide bonds. However, H2S was not detected in heated α-LA, which emphasized the major role of β-LG in H2S formation from heated whey. In mixed protein systems (i.e. whey-model and WPI), the higher level of total Cys and cystine residues facilitated both heat- induced disulfide rearrangement and H2S formation as compared to the single protein systems (i.e. pure α-LA and β-LG). The resulting larger disulfide-linked protein aggregates was suggested to provide further physical protection against protein oxidation, which is supported by the significantly lower oxidation occupancies found in the heated mixed protein systems compared to the single protein systems, especially in α-LA. Collectively, the formation of disulfide-linked protein aggregates, via disulfide rearrangement, was suggested to be a key outcome in WPI upon thermal treatment (60-90 °C), and the resulting compact protein structure was proposed to reduce the susceptibility of amino acid residues towards heat-induced changes (e.g. oxidation and formation of sulfur volatiles) by lowering their solvent accessibility. The wide range of interconvertible oxoforms of Cys derivatives (e.g. disulfide) was speculated to provide an in vitro redox-regulation mechanism in the free Cys-containing systems (i.e. β-LG, whey-model and WPI)
Protein Modifications and Structural Changes in Whey Protein upon Heat Treatments:Protein Oxidation, Desulfurization and Aggregation
No full textWhey protein, as a major by-product from cheese manufacture, is a versatile and nutritive food ingredient. In food and ingredient processing, thermal treatment is often applied to achieve specific product property, e.g. to increase product stability. However, undesirable modifications may also be promoted by heating such as protein oxidation and generation of off-flavor, leading to the decrease of nutritional value and/or consumer acceptance. To optimize the conditions of thermal treatment for specific product functionalities customization with minimized side effects on the final food applications, it is important and necessary to understand the mechanism of potential heat-induced protein modifications. In the present PhD study, whey protein modifications (e.g. disulfide rearrangement, protein oxidation and desulfurization) and structural changes were investigated under different heating conditions (i.e. 60-90 °C for 10 min, 90 °C for 120 min and 160 °C for 160 s (as a UHT-like condition)) using multiple analytical tools (e.g. liquid chromatography-mass spectrometry (LC-MS), size-exclusion chromatography (SEC) and gas chromatography-flame photometric detector (GC-FPD)). These modifications and changes were compared in different whey protein systems, including the two major whey proteins, i.e. beta-lactoglobulin (β-LG) and alpha-lactalbumin (α-LA), a whey-model system (α-LA + β-LG) and whey protein isolate (WPI).Heating of β-LG at 70 °C and above was found to induce disulfide rearrangement, which might lead to irreversible protein unfolding and protein aggregation. Although disulfide rearrangement was observed to start at both native disulfide bonds in β-LG, the surface-located Cys66-Cy160 was suggested to be more reactive towards rearrangement compared to Cys106-Cys119. At ≥80 °C, the disulfide-linked protein oligomers were increasing in size with increasing heat load, while Cys66 was proposed to be a key Cys residue participating in the rearranged inter-molecular disulfide bonds. Meanwhile, hydrogen sulfide (H2S) was also released (presumably via Cys β-elimination), which contributed to the sulfurous odor. Only minor level of heat-induced protein side-chain oxidation products was detected in β-LG after all employed heat treatments. Nevertheless, higher levels of heat-induced oxidation products, especially at Trp residues, were detected in α-LA as compared to β-LG after the severe heat treatment (i.e. 90 °C for 120 min). The presence of free Cys residue in β-LG was proposed to scavenge oxidants during heating, making it less susceptible towards oxidation than α-LA. Formation of non-native disulfide bond was only observed in α-LA once the temperature reached 90 °C, at which free Cys residue started to release from the native disulfide bonds. However, H2S was not detected in heated α-LA, whichVIemphasized the major role of β-LG in H2S formation from heated whey. In mixed protein systems (i.e. whey-model and WPI), the higher level of total Cys and cystine residues facilitated both heat-induced disulfide rearrangement and H2S formation as compared to the single protein systems (i.e. pure α-LA and β-LG). The resulting larger disulfide-linked protein aggregates was suggested to provide further physical protection against protein oxidation, which is supported by the significantly lower oxidation occupancies found in the heated mixed protein systems compared to the single protein systems, especially in α-LA.Collectively, the formation of disulfide-linked protein aggregates, via disulfide rearrangement, was suggested to be a key outcome in WPI upon thermal treatment (60-90 °C), and the resulting compact protein structure was proposed to reduce the susceptibility of amino acid residues towards heat-induced changes (e.g. oxidation and formation of sulfur volatiles) by lowering their solvent accessibility. The wide range of interconvertible oxoforms of Cys derivatives (e.g. disulfide) was speculated to provide an in vitro redox-regulation mechanism in the free Cys-containing systems (i.e. β-LG, whey-model and WPI)
Improved Electrochemical Performance of Rb<sub>x</sub>Li<sub>1.27-x</sub>Cr<sub>0.2</sub>Mn<sub>0.53</sub>O<sub>2</sub>Cathode Materials via Incorporation of Rubidium Cations into the Original Li Sites
No full textEnhanced Electrochemical Performance of Li1.27Cr0.2Mn0.53O2 Layered Cathode Materials via a Nanomilling-Assisted Solid-state Process
No full textLi1.27Cr0.2Mn0.53O2 layered cathodic materials were prepared by a nanomilling-assisted solid-state process. Whole-pattern refinement of X-ray diffraction (XRD) data revealed that the samples are solid solutions with layered α-NaFeO2 structure. SEM observation of the prepared powder displayed a mesoporous nature composed of tiny primary particles in nanoscale. X-ray photoelectron spectroscopy (XPS) studies on the cycled electrodes confirmed that triple-electron-process of the Cr3+/Cr6+ redox pair, not the two-electron-process of Mn redox pair, dominants the electrochemical process within the cathode material. Capacity test for the sample revealed an initial discharge capacity of 195.2 mAh·g−1 at 0.1 C, with capacity retention of 95.1% after 100 cycles. EIS investigation suggested that the high Li ion diffusion coefficient (3.89 × 10−10·cm2·s−1), caused by the mesoporous nature of the cathode powder, could be regarded as the important factor for the excellent performance of the Li1.27Cr0.2Mn0.53O2 layered material. The results demonstrated that the cathode material prepared by our approach is a good candidate for lithium-ion batteries
Effect of sol aging time on the anti-reflective properties of silica coatings templated with phosphoric acid
No full textAbstractSilica anti-reflective coatings have been prepared by a sol–gel dip-coating process using the sol containing phosphoric acid as a pore-forming template. The effect of the aging time of the sol on the anti-reflective properties has been investigated. The surface topography of the silica AR coatings has been characterized. With increasing sol aging time, more over-sized pores larger than 100nm are formed in the silica coatings. These could act as scattering centers, scattering visible light and thereby lowering transmittance. The optimal aging time was identified as 1day, and the corresponding silica coatings showed a maximum transmittance of 99.2%, representing an 8% increase compared to the bare glass substrate
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