Mapping variation of handsheet properties within loblolly pine trees

Within-tree variation of four handsheet properties (burst index, sheet density, STFI short-span compression strength (STFI) and tensile index) was mapped for loblolly pine trees aged 13 and 22 years using NIR predicted handsheet property data (representing an average of 18 trees for each age). All within-tree maps were similar demonstrating a radial decrease in handsheet properties at all heights, with sheet density and tensile index having the greatest within-tree variation. The corewood zone had the highest values for all properties, while the lowest values were observed in a region consistent with juvenile and transitional outerwood as defined by Burdon et al. (2004). The maps are also similar to, but the inverse of, maps reported in prior studies for density and tracheid coarseness and wall thickness. Relationships amongst the examined handsheet properties and wood and tracheid properties explains the overall similarities of the different maps. The maps provide a representation of within-tree variation of important paper properties at two different ages. An understanding of how these properties vary within loblolly pine trees can aid in better utilization of forest resources.

An analytical model for the current voltage characteristics of GaN-capped AlGaN/GaN and AlInN/GaN HEMTs including thermal and self-heating effects

We present an analytical model for the I-V characteristics of AlGaN/GaN and AlInN/GaN high electron mobility transistors (HEMT). Our study focuses on the influence of a GaN capping layer, and of thermal and self-heating effects. Spontaneous and piezoelectric polarizations at Al (Ga,In)N/GaN and GaN/Al(Ga,In)N interfaces have been incorporated in the analysis. Our model permits to fit several published data. Our results indicate that the GaN cap layer reduces the sheet density of the two-dimensional electron gas (2DEG), leading to a decrease of the drain current, and that n+-doped GaN cap layer provides a higher sheet density than undoped one. In n+GaN/AlInN/GaN HEMTs, the sheet carrier concentration is higher than in n+GaN/AlGaN/GaN HEMTs, due to the higher spontaneous polarization charge and conduction band discontinuity at the substrate/barrier layer interface.

Density development in foam forming: wet pressing dynamics

The compression behaviour of both foam and water formed wet sheets was studied in the laboratory. The development of sheet thickness was followed for different pressing dynamics including both short and long pulses. The immediate recovery of sheet thickness after the first short pressing pulse was clearly better for foam than for water. The bulk advantage of foam forming gradually reduced as the number of pressing pulses increased. The solids content after wet pressing became higher for foam than water forming. The differences in sheet density and dewatering for the two forming methods should be taken into account when developing industrial processes for lightweight fibre-based products.

Effect of sheet temperature on web densification during calendering

We used two different experimental methods to study the effects of sheet temperature at a given moisture content on web densification and sheet smoothness. Commercially prepared paperboard obtained from the WestRock Mahrt mill in Cottonton, AL, USA, was used in this study. Laboratory made handsheets prepared from Mahrt high yield kraft pine pulp were also used for portions of this work. Sheets were sealed in moisture-proof bags, heated to various temperatures, and calendered at multiple nip loads. The resulting changes in sheet density were determined. Additionally, a pilot-scale calender was modified to preheat the non-print side of the sheet before entering the nip. The modified sheet run allowed for an increase in the bulk temperature of the board with minimal changes in the moisture content of the print surface. Increasing the bulk temperature increased board density at a given print surface smoothness. We determined that increasing the board temperature by 14°C resulted in about a 1% increase in board density. The results of this study help explain the commercial results obtained with thermal gradient calendering.