Heat and Mass Transfer in Wood Composite Panels During Hot-Pressing: Part I. A Physical-Mathematical Model

A theoretical model for heat and moisture transfer in wood composite mats during hot-pressing is developed based on the basic principles of mass conservation, momentum of gas flow, energy conservation, and resin curing kinetics. The model provides a mathematical description of the coupled physical phenomenon in hot-pressing process as a closure problem in which fifteen governing equations are rigorously derived to solve fifteen unknown variables of mat environmental conditions. These variables, including mat temperature, gas pressure, and moisture content are linked to basic material properties such as mat density, thermal conductivity, and permeability. Initial and boundary conditions for solving the governing equations are also discussed

Theoretical Modeling of Bonding Characteristics and Performance of Wood Composites. Part IV. Internal Bond Strength

A mechanistic model was developed to predict the internal bond (IB) strength of wood composites. Based on the earlier models reported in this series, the IB model integrates the mechanisms of inter-element contact, resin distribution, and localized bond development and debonding failure. Experimental tests were also conducted, and the results compare favorably with model predictions. It was discovered that a composite product having large horizontal density variations typically realizes less than 50% of the bond strength attainable between its constituent elements. The loss of bonding strength is attributed to the premature debonding of low-density regions, and the subsequent load concentration and failure-acceleration of the higher-density regions. The model predicts IB as a function of product density, wood density, resin content, and element dimensions. The IB improves with an increase of product density, resin content, and element thickness. The relationships are monotonic and nonlinear, resulting from their interactions on the contact development or/and the resin coverage. The relationship between IB and wood density is also nonlinear and dependent upon the product density. Implications of the predictive results on fundamental understanding and optimization of wood composite bonding are discussed

Theoretical Modeling of Bonding Characteristics and Performance of Wood Composites. Part I. Inter-Element Contact

This is the first of a four-paper series aiming to model fundamental bonding characteristics and performance of wood composites. In this paper, a mathematical model and a computer simulation model are developed to predict the variation of inter-element (strand) contact during mat consolidation. The mathematical predictions and the computer simulations agree well with each other. The results show that the relationship between the inter-element contact and the mat density is highly nonlinear and is significantly affected by the wood density and the element thickness. The effects of the element length and width are less significant. The models imply that use of less dense wood species and thinner strands are beneficial to lower product density while achieving adequate inter-element contact for bonding

Heat and Mass Transfer in Wood Composite Panels During Hot Pressing. Part II. Modeling Void Formation and Mat Permeability

Theoretical models have been developed to predict the porosity and permeability of wood strand mats during consolidation. Based on the Poisson distribution of mat formation, the porosity model predicts the formation of both inside- and between-strand void volumes. It is proposed and predicted that the between-strand voids consist of voids between non-contact strand faces and voids around strand edges, with the former dominating in the early stage of consolidation and the latter dominating in the latter stage of consolidation. The permeability model is developed based on the Carman-Kozeny theory for porous materials. The model is compared and agrees with experimental results obtained from this study and from the literature. The results show mat permeability is mainly controlled by voids between strands instead of those inside strands. Mat density has a primary effect and strand size has a secondary but very important effect on mat porosity and permeability especially in the later stage of consolidation. Strand thickness has a stronger impact than strand width and length. Strand dimensions and mat permeability are shown to have significant effects on internal environmental conditions in wood composites during hot-pressing

l-2,3-Diaminopropionate: One of the building blocks for the biosynthesis of Zwittermicin A in Bacillus thuringiensis subsp. kurstaki strain YBT-1520

AbstractZwittermicin A (ZwA) is a hybrid polyketide–non-ribosomal peptide that is thought to be biosynthesized from five proposed building blocks, including the 2,3-diaminopropionate. Candidate genes for de novo biosynthesis of 2,3-diaminopropionate, zwa5A and zwa5B, have been identified in a previous study. In this research, zwa5A was interrupted and chemically synthesized 2,3-diaminopropionate was used to feed the zwa5A− mutant. Results showed that feeding with 2,3-diaminopropionate restored the ability of the zwa5A− mutant to produce ZwA. Another non-ribosomal peptide synthase gene, designated orf3, was identified. Amino acid dependent PPi release assay showed that the adenylation domain ZWAA2 of ORF3 acyl-adenylated l-2,3-diaminopropionate effectively. Taken together, it can be concluded that l-2,3-diaminopropionate is indeed one of the building blocks for the biosynthesis of Zwittermicin A

Semiconducting nonperovskite ferroelectric oxynitride designed ab initio

Recent discovery of HfO2-based and nitride-based ferroelectrics that are compatible to the semiconductor manufacturing process have revitalized the field of ferroelectric-based nanoelectronics. Guided by a simple design principle of charge compensation and density functional theory calculations, we discover HfO2-like mixed-anion materials, TaON and NbON, can crystallize in the polar Pca21 phase with a strong thermodynamic driving force to adopt anion ordering spontaneously. Both oxynitrides possess large remnant polarization, low switching barriers, and unconventional negative piezoelectric effect, making them promising piezoelectrics and ferroelectrics. Distinct from HfO2 that has a wide band gap, both TaON and NbON can absorb visible light and have high charge carrier mobilities, suitable for ferroelectric photovoltaic and photocatalytic applications. This new class of multifunctional nonperovskite oxynitride containing economical and environmentally benign elements offer a platform to design and optimize high-performing ferroelectric semiconductors for integrated systems

The spatial variability of streambed vertical hydraulic conductivity in an intermittent river, northwestern China

Abstract Streambed vertical hydraulic conductivity (K) plays an important role in river water and groundwater interaction. The K at the ten transects (Ts1-Ts10) at the Donghe River (an intermittent river) in the Ejina Basin, northwestern China, was measured to investigate its spatial variation. Based on the sediment characteristics and vertical hydraulic conductivity of the riverbed, the entire riverbed of the Donghe River could be divided arbitrarily into two parts: an upper part (starting at Ts1 and ending at Ts9, without an obvious and continuous clogging layer) and a lower part (the remaining riverbed, with an obvious and continuous clogging layer). In the upper part, although the K varied with depth within the 0-30 cm layer, the variability with depth could be ignored in practice. The arithmetic mean K of the upper part ranged from 12 to 27.6 m/ day, three orders of magnitude larger than that of the lower part (0.06 m/day). The change of K along the river cross section was significant, and larger values of K often occurred in the parts of the channels with greater water depth. However, there were no consistent patterns of the variability of K at transects across the river, which was influenced by the variation in streambed characteristics. The results could be useful for the estimation of groundwater recharge from river and groundwater resources evaluation in the Ejina Basin