Imide-aryl ether benzoxazole random copolymers

Novel imide-aryl ether benzoxazole copolymers were prepared and their morphology and mechanical properties investigated. A key feature of these copolymers is the incorporation of a benzoxazole moiety by the use of 2,2′-bis[4-(3-aminophenoxy)phenyl]-6,6′-bibenzoxazole or 2,2′-bis[4-(4-aminophenoxy)-phenyl]-6,6′-bibenzoxazole as co-diamines in polyimide syntheses. The preparation of these diamines involved the nucleophilic aromatic substitution of 2,2′-bis(4-fluorophenyl)-6,6′-bibenzoxazole with either 3- or 4-aminophenol in the presence of K₂CO₃. The diamines were co-reacted with various compositions of pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) to synthesize the desired poly(amic acids). Films were cast and cured (350°C) to effect the imide formation, affording films with elongations between 40 and 110% and moduli in the 2000–2750 MPa range. The copolymers exhibited good dimensional (T_g in excess of 300°C) and thermal stability. Wide-angle X-ray diffraction measurements on the copolymers showed that the ordered morphology characteristic of PMDA/ODA polyimide was retained. Improvements in the auto-adhesion were observed, particularly in those copolymers which displayed a T_g.     

HOMOGENEITY OF THE FRIABLE FLOUR OF MALTING BARLEY

A detailed comparison was made of the properties of the friable flours and non-friable residues of two samples of malted barley of different nitrogen contents. The friable flours were sieved and fractionated to give a range of particle sizes, and the intact malt, whole friable flour, non-friable residues and fractionated friable flours were subjected to a range of analyses. Endosperm fractionation studies showed that the pattern of enzymic degradation of proteins in the modified friable flour of low nitrogen malt was more uniform than the corresponding pattern of protein breakdown in the friable flour of high nitrogen malt. Examination of the non friable residues showed that cell wall breakdown in the high nitrogen malt was less extensive than the low nitrogen malt. It is proposed that the high protein levels in the endosperm caused starch/protein compacting which limited endosperm hydration and enzymic modification during malting. The friability scores of high nitrogen malts may given an over estimate of endosperm modification.     

Numerical Optimization of an Active Voltage Controller for High-Power IGBT Converters

Feedback control of insulated gate bipolar transistors (IGBTs) in the active region can be used to regulate the device switching trajectory. This facilitates series connection of devices without the use of external snubber networks. Control must be achieved across the full active region of the IGBT and must balance a number of conflicting system goals including diode recovery. To date, the choice of control parameters has been a largely empirical process. This paper uses accurate device models and formalized optimization procedures to evaluate IGBT active voltage controllers. A detailed optimization for the control of IGBT turn-on is presented in this paper     

Chemical Amplification of a Triphenylene Molecular Electron Beam Resist

Molecular resists, such as triphenylene derivatives, are small carbon rich molecules, and thus give the potential for higher lithographic resolution and etch durability, and lower line width roughness than traditional polymeric compounds. Their main limitation to date has been poor sensitivity. A new triphenylene derivative molecular resist, with pendant epoxy groups to aid chemically amplified crosslinking, was synthesized and characterized. The sensitivity of the negative tone, pure triphenylene derivative when exposed to an electron beam with energy 20 keV was ～ 6 × 10^–4 C cm^–2, which increased substantially to ～ 1.5 × 10^–5 C cm^–2 after chemical amplification (CA) using a cationic photoinitiator. This was further improved, by the addition of a second triphenylene derivative, to ～ 7 × 10^–6 C cm^–2. The chemically amplified resist demonstrated a high etch durability comparable with the novolac resist SAL 601. Patterns with a minimum feature size of ～ 40 nm were realized in the resist with a 30 keV electron beam.     

Annihilation density distribution calculations for medically important positron emitters

The effect of positron range on the image-plane resolution of tomographic images is evaluated through calculations based on a model which employs beta-decay energy spectra and an empirical range formula. Predicted range distribution functions are compared with published measurements for three medically important positron emitters: (11 )C, (68)Ga, and (82)Rb. The effect of tomographic slice thickness on point-source annihilation distribution functions is also demonstrated. Line-spread functions are calculated using the model, for the above isotopes as well as for (18)F, (15)O, and (13)N. Image-plane resolution predictions are made for high-resolution positron cameras for various positron emitting isotopes with end-point energies up to 4 MeV.     

Traveling-wave amplifiers with prescribed frequency response

Microwave transistor amplifier combinations that have controlled frequency response over a specified bandwidth were designed. Theoretical analysis of such an amplifier with an arbitrary number of sections is presented. The response of the amplifier is controlled by tapering the frequency selectivity or Q of each section of the amplifier. To verify the theory, a three-section amplifier with maximally flat time delay response was designed, constructed, and evaluated. Existing traveling-wave amplifiers were modeled as lossy transmission lines. Although the amplifiers were relatively broadband, a prescribed frequency response was not achieved, and each transistor did not receive an equal portion of the signal power. Resistive elements were required for impedance matching at the input and output. The design seeks to improve on previous techniques by trading bandwidth for controlled gain. By making the transmission line that connects the amplifier sections nonuniform, the frequency response was controlled over the design bandwidth. The designs can easily be implemented using familiar components     

Self‐Assembled Room Temperature Multiferroic BiFeO3‐LiFe5O8 Nanocomposites

Multiferroic materials have driven significant research interest due to their promising technological potential. Developing new room‐temperature multiferroics and understanding their fundamental properties are important to reveal unanticipated physical phenomena and potential applications. Here, a new room temperature multiferroic nanocomposite comprised of an ordered ferrimagnetic spinel α‐LiFe₅O₈ (LFO) and a ferroelectric perovskite BiFeO₃ (BFO) is presented. It is observed that lithium (Li)‐doping in BFO favors the formation of LFO spinel as a secondary phase during the synthesis of Li_xBi_1?xFeO₃ ceramics. Multimodal functional and chemical imaging methods are used to map the relationship between doping‐induced phase separation and local ferroic properties in both the BFO‐LFO composite ceramics and self‐assembled nanocomposite thin films. The energetics of phase separation in Li doped BFO and the formation of BFO‐LFO composites are supported by first principles calculations. These findings shed light on Li's role in the formation of a functionally important room temperature multiferroic and open a new approach in the synthesis of light element doped nanocomposites for future energy, sensing, and memory applications.     

Surface current mapping performance of bistatic and monostaticΔk-radars

A theoretical analysis of the surface ocean current mapping performance of land-based bistatic and monostatic Δk-radars is presented. Contour maps of the calculated surface current resolution achievable with both configurations are presented and compared. For the assumed surface conditions, both configurations are seen to be capable of resolving 5 cm/s currents with 100 m spatial resolution out to ranges of several kilometers using reasonable dwell times     

Two-step parameter extraction procedure with formal optimization for physics-based circuit simulator IGBT and p-i-n diode models

A practical and accurate parameter extraction method is presented for the Fourier-based-solution physics-based insulated gate bipolar transistor (IGBT) and power diode models. The goal is to obtain a model accurate enough to allow switching loss prediction under a variety of operating conditions. In the first step of the extraction procedure, only one simple clamped inductive load test is needed for the extraction of the six parameters required for the diode model and of the 12 and 15 parameters required for the nonpunch-through (NPT) and punch-through (PT) IGBT models, respectively. The second part of the extraction procedure is an automated formal optimization step that refines the parameter estimation. Validation with experimental results from various structures of IGBT demonstrates the accuracy of the proposed IGBT and diode models and the robustness of the parameter extraction method.     

The Extractable and Bound Condensed Tannin Content of Leaves from Tropical Tree,Shrub and Forage Legumes

The extractable, protein-bound and fibre-bound condensed tannin (CT) concentrations in the leaves of tropical legumes grown in both Colombia and Northern Australia were determined by the butanol–HCl method, whilst extractable CT was also determined by the vanillin–HCl method. With the exception of Senna siamea all species contained CT. The very high CT concentration found in many plants growing in Colombia may have been partly due to soil fertility being much lower at the Colombian than the Northern Australian site. Acacia boliviana , Arachis pintoi , Centrosema latidens , Senna velutina and Gliricidia sepium contained <55 g total CT kg⁻¹ DM, which suggests that they could comprise a reasonable proportion of ruminant diets. All other species grown in South America contained 100–240 g CT kg⁻¹ DM, which suggests that they should only be fed in small amounts as supplements to dilute the CT concentration. Leucaena species and Calliandra calothyrsus grown in Northern Australia contained intermediate concentrations of total CT (60–90 g kg⁻¹ DM). Most species contained 70–95% of total CT as extractable CT, with the exception of Flemingia macrophylla , where 60% was extractable and 40% bound, and Gliricidia sepium , where almost all the CT was bound to protein. Values for Flemingia macrophylla differed between accessions. Extractable CT determined with vanillin–HCl was generally higher than extractable CT determined with butanol–HCl. Three accessions showed negligible (<1 g kg⁻¹ DM) extractable CT with butanol–HCl but 10–12 g extractable CT kg⁻¹ DM with vanillin–HCl. Two accessions showed undetectable levels of extractable CT but substantial levels of protein-bound CT, illustrating the importance of using a bound CT method for identifying forages containing CT. Relative to freeze drying, oven drying of Leucaena species reduced the concentration of extractable CT and increased concentrations of bound CT. The significance of the results for the nutrition of ruminant livestock are discussed, including the possible roles of protein-bound and fibre-bound CT.