Cure kinetics of wood phenol-formaldehyde systems

This project aims to develop kinetic models for chemical and mechanical cure development and correlate chemical and mechanical degrees of cure in order to create a comprehensive cure model that encompasses both of these tasks. With these objectives, the cure processes of two commercial phenol-formaldehyde (PF) resol resins with differing molecular weights were evaluated using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) under isothermal and linear heating regimes. For both resins, cure was characterized in the neat state with DSC, in mixtures of PF/wood flour with DSC, and as a bondline between two wood substrates with DMA. The synergy of DSC and DMA techniques picked up the phase transitions of PF curing processes and characterization were comparable between the two techniques. During a DSC temperature scan, PF resols typically exhibited two exotherms, while in wood/PF mixtures another small exotherm appeared in a lower temperature range, indicating the impact of wood/PF interactions. In contrast, DMA offered a quantitative view of the adhesion mechanics from which the glass transition of uncured resin, gelation, and vitrification points were inferred. An analytical solution was developed to estimate the in situ shear modulus of the adhesive layer during the curing process, a change estimated from 0.01 to 16MPa. The maximum storage modulus and the ratio of maximum to minimum storage modulus were recommended for direct evaluation of wood-PF system.; Model-fitting kinetics of nth order and autocatalytic models can be reasonably applied to the DSC data, while autocatalytic, Prout-Tompkins, and Avrami-Erofeev models have been successfully applied to describe cure development in the DMA. The activation energy of PF curing processes in the neat state was around 85-100 kJ/mol and decreased to 50-70 kJ/mol in the presence of wood. However, it was the model-free kinetics of the Kissinger-Akhira-Sunnose, Friedman and Vyazovkin methods that offered insight into the cure mechanisms of commercial PF resoles and predicted the cure development under isothermal and linear heating regimes for both mechanical and chemical degrees of cure. Mechanical cure development has been correlated with chemical advancement in an empirical equation analog to the Weibull cumulative function. After either mechanical or chemical cure development is characterized, the other can be estimated through connection of the correlation equation between the mechanical and chemical degrees of cure

Abelian flux induced magnetic frustrations of spinor boson superfluids on a square lattice

Inspired by recent experimental advances to generate Abelian flux for neutral cold atoms and photons moving in a lattice, we investigate the possible effects of the $\pi$ flux through a unit cell in the pseudo-spin 1/2 spinor boson Hubbard model in a square lattice. We find that the $\pi$ flux induces a dramatic interplay between the charge and the spin which leads to a frustrated superfluid. We develop a new and systematic "order from quantum disorder" analysis to determine not only the true quantum ground state, but also the excitation spectrum. The superfluid ground state has a 4 sublattice $90^{\circ}$ coplanar spin structure which supports 4 linear gapless modes with 3 different velocities. We speculate the transition from the weak coupling frustrated SF to the strong coupling Ferromagnetic Mott state to be in a new universality class of non-Ginsburg Landau type. These novel phenomena may be observed in these recent cold atom and photonic experiments.Comment: 5 pages, REVTEX-4, 3 figure

Control System Design of Threshing Separator Based on ARM

In order to realize the safe operation of the amphibious multifunctional threshing separator, designing the control system based on the ARM embedded processor, the system through the CAN industrial field bus, connected with the temperature sensor, the infrared sensor, the stroke sensor, the revolution speed sensor, control the automatic clutch and the brake system through the electric relay. When the equipment overheating, personnel illegal operation, speed anomalies, equipment location errors, etc., the system can be timely control of the device brake and shut-down, to avoid serious safety accidents

Microtension Test Method for Measuring Tensile Properties of Individual Cellulosic Fibers

A microtension testing system was devised to measure mechanical properties of individual cellulosic fibers. To avoid specimen gripping and to enhance fiber alignment during testing, a self-aligning ball and socket gripping assembly was used in the microtensile tester design. A resolution of 0.098 mN was obtained for the tensile load measurement with this microtensile tester. Fiber strain was determined from high-precision stepper motor movement with 0.078-μm resolution or by in situ video photography. Cross-sectional areas of a single fiber cell wall were measured with a confocal laser scanning microscope. Results obtained from this system indicated a linear stress-strain curve until fatal failure for mature latewood fibers, whereas juvenile latewood fibers displayed curvilinear stress-strain relationships. Average values of tensile strength, tensile modulus, and elongation at break were 1258 MPa, 19.9 GPa, and 6.6% for mature latewood fiber and 558 MPa, 8.5 GPa, and 9.9% for juvenile latewood fiber, respectively. These values agreed with published data. The preliminary test indicated the usefulness of the integrated environmental chamber for investigating moisture effect on fiber engineering properties, but further investigation is needed to obtain statistically significant data

In-plane mechanical behavior of novel auxetic hybrid metamaterials

We present in this paper two novel concepts of hybrid metamaterials that combine a core unit cell of re-entrant or cross-chiral shape and lateral missing ribs. The first topology is a hybrid between an anti-tetrachiral and a missing rib (cross-chiral) configuration; the second one has a variable cross-chiral layout compared to the classical missing rib square structure. Their in-plane mechanical properties have been investigated from a parametric point of view using finite element (FE) simulations. The two classes of metamaterials have been benchmarked to obtain optimized designs and specific effective properties. Nonlinear simulations and experimental tests of the new re-entrant missing rib metamaterials featuring optimized geometry parameters have been performed to understand the behavior of these architectures under large deformations

In-Plane Mechanical Behavior of a New Star-Re-Entrant Hierarchical Metamaterial

A novel hierarchical metamaterial with tunable negative Poisson’s ratio is designed by a re-entrant representative unit cell (RUC), which consists of star-shaped subordinate cells. The in-plane mechanical behaviors of star-re-entrant hierarchical metamaterial are studied thoroughly by finite element method, non-dimensional effective moduli and effective Poisson’s ratios (PR) are obtained, then parameters of cell length, inclined angle, thickness for star subordinate cell as well as the amount of subordinate cell along x, y directions for re-entrant RUC are applied as adjustable design variables to explore structure-property relations. Finally, the effects of the design parameters on mechanical behavior and relative density are systematically investigated, which indicate that high specific stiffness and large auxetic deformation can be remarkably enhanced and manipulated through combining parameters of both subordinate cell and parent RUC. It is believed that the new hierarchical metamaterial reported here will provide more opportunities to design multifunctional lightweight materials that are promising for various engineering applications

3D phase field modeling of multi-dendrites evolution in solidification and validation by synchrotron x-ray tomography

© 2021 by the authors. Licensee MDPI, Basel, Switzerland. In this paper, the dynamics of multi-dendrite concurrent growth and coarsening of an Al-15 wt.% Cu alloy was studied using a highly computationally efficient 3D phase field model and real-time synchrotron X-ray micro-tomography. High fidelity multi-dendrite simulations were achieved and the results were compared directly with the time-evolved tomography datasets to quantify the relative importance of multi-dendritic growth and coarsening. Coarsening mechanisms under different solidification conditions were further elucidated. The dominant coarsening mechanisms change from small arm melting and interdendritic groove advancement to coalescence when the solid volume fraction approaches ~0.70. Both tomography experiments and phase field simulations indicated that multi-dendrite coarsening obeys the classical Lifshitz–Slyozov–Wagner theory Rn − Rn0=kc(t − t0), but with a higher constant of n = 4.3

Quantum generated vortices, dual singular gauge transformation and zero temperature transition from d-wave superconductor to underdoped regime

By extending the original Anderson singular gauge transformation for static vortices to two mutual flux-attaching singular gauge transformations for moving vortices, we derive an effective action describing the zero temperature quantum phase transition from d-wave superconductor to underdoped regime. Neglecting the charge fluctuation first, we find that the mutual statistical interaction is exactly marginal. In the underdoped regime, the quasi-particles are described by 2+1 dimensional QED; in the superconducting regime, they are essentially free. However, putting back the charge fluctuation changes the physical picture dramatically: both the dynamic Doppler shift term and the mutual statistical interaction become {\em irrelevant} short-ranged interactions on both sides of the quantum critical point. There are no spin-charge separation and {\em no} dynamic gapless gauge field in the Cooper-pair picture. The formalism developed at $T=0$ is applied to study thermally generated vortices in the vortex plasma regime near the finite temperature KT transition.Comment: 17 pages, 7 figure