| 1. |
- Ghai, Viney, 1989, et al.
(författare)
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Orientation of graphene nano sheets in magnetic fields
- 2024
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Ingår i: Progress in Materials Science. - 0079-6425. ; 143
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Forskningsöversikt (refereegranskat)abstract
- Aligning anisotropic nanoparticles using external fields is one of the major obstacles to unlocking their enormous potential for novel applications. The most famous such example is graphene, a 2D family of nanomaterials that has received enormous attention since its discovery. Using graphene to enhance mechanical, thermal, electric or gas barrier properties, imparting antibacterial properties etc., relies to a great extent on the ability to control their orientation inside a matrix material, i.e., polymers. Here we summarize the latest advances on graphene orientation using magnetic fields. The review covers the underlying physics for graphene interaction with magnetic fields, theoretical continuum mechanics framework for inducing orientation, typical magnetic field orientation setups, and a summary of latest advances in their use to enhance the performance of materials. Current trends, limitations of current alignment techniques are highlighted and major challenges in the field are identified.
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| 2. |
- Ghanbari, Reza, 1984, et al.
(författare)
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Painting Taylor vortices with cellulose nanocrystals: supercritical spectral dynamics
- 2023
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We study the flow stability and spatio-temporal spectral dynamics of cellulose nanocrystal (CNC) suspensions in a custom Taylor-Couette flow cell using the intrinsic shear induced birefringence and liquid crystalline properties of CNC suspensions for flow visualizations for the first time. The analysis is performed at constant ramped speed inputs of the independently rotating cylinders for several cases ranging from only inner or outer rotating cylinders to three counter-rotation cases. All CNC suspensions have measurable elastic and shear thinning, both increasing with CNC concentration. We show that the flow patterns recorded are essentially Newtonian-like, with non-Newtonian effects ranging from a decrease in wavenumbers to altering the critical parameters for the onset of instability modes. Outer cylinder rotation flow cases are stable for all concentrations whereas inner cylinder rotation flow cases transition to axisymmetric and azimuthally periodic secondary flows. However, unstable counter-rotation cases become unstable to asymmetric spiral modes. With increasing CNC concentration a counter-rotation case was found where azimuthally periodic wavy patterns transition to asymmetric spiral modes. In contrast to polymeric solutions of similar low to moderate elasticity and shear thinning, the shear-thinning region of CNC suspensions is expected to lead to the breakdown of the chiral nematic phase, whose elastic constants constitute the dominant structural elasticity mechanism. Thus, we interpret the Taylor-Couette stability of the CNC suspensions as dominated by their shear-thinning character due to the expected loss of elasticity in nonlinear flow conditions.
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| 3. |
- Ghanbari, Reza, 1984, et al.
(författare)
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Painting Taylor vortices with cellulose nanocrystals: Suspension flow supercritical spectral dynamics
- 2024
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Ingår i: Physics of Fluids. - 1089-7666 .- 1070-6631. ; 36:4
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Tidskriftsartikel (refereegranskat)abstract
- We study the flow stability and spatiotemporal spectral dynamics of cellulose nanocrystal (CNC) suspensions in a custom Taylor-Couette flow cell using the intrinsic shear induced birefringence and liquid crystalline properties of CNC suspensions for flow visualizations, for the first time. The analysis is performed at constant ramped speed inputs of the independently rotating cylinders for several cases ranging from only inner or outer rotating cylinders to three counter-rotation cases. All CNC suspensions have measurable elasticity and shear thinning, both increasing with CNC concentration. We show that the flow patterns recorded are essentially Newtonian-like, with non-Newtonian effects ranging from a decrease in wavenumbers to altering the critical parameters for the onset of instability modes. Outer cylinder rotation flow cases are stable for all concentrations whereas inner cylinder rotation flow cases transition to axisymmetric and azimuthally periodic secondary flows. However, counter-rotation cases become unstable to asymmetric spiral modes. With increasing CNC concentration, a counter-rotation case was found where azimuthally periodic wavy patterns transition to asymmetric spiral modes. Based on rheo-SAXS measurements, the shear-thinning region of CNC suspensions is expected to lead to the breakdown of the chiral nematic phase, whose elastic constants constitute the dominant structural elasticity mechanism. Thus, we interpret the Taylor-Couette stability of the CNC suspensions as dominated by their shear-thinning character due to the expected loss of elasticity in nonlinear flow conditions.
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| 4. |
- Pashazadehgaznagh, Sajjad, 1994, et al.
(författare)
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An Intriguing Array of Extrudate Patterns in Long-Chain Branched Polymers During Extrusion
- 2023
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Ingår i: Macromolecular Materials and Engineering. - 1439-2054 .- 1438-7492. ; 308:8
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Tidskriftsartikel (refereegranskat)abstract
- The present study highlights a range of surface and volume extrudate patterns that can be detected during the extrusion flow of long-chain branched polymers. Thus, four linear low-density polyethylenes (LDPEs) have been extruded using a single-screw extruder coupled to an inline optical imaging system. The selected LDPEs are selected to outline the influence of molecular weight and long-chain branching on the types of melt flow extrusion instabilities (MFEI). Through the inline imaging system, space–time diagrams are constructed and analyzed via Fourier-transformation using a custom moving window procedure. Based on the number of characteristic frequencies, peak broadness, and whether they are surface or volume distortions, three main MFEI types, distinct from those typically observed in linear and short-chain branched polymers, are identified. The higher molecular weight, low long-chain branching LDPEs exhibited all three instability types, including a special type volume instability. Independently of the molecular weight, higher long-chain branching appeared to have a stabilizing effect on the transition sequences by suppressing volume extrudate distortions or limiting surface patters to a form of weak intensity type.
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| 5. |
- Pashazadehgaznagh, Sajjad, 1994
(författare)
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Highly-filled biocomposites: modelling, processing, and characterization
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The use of polymer with natural fiber reinforcements is expected to increase significantly. This can be noted in several industrial sectors of strategic importance for Sweden, such as automotive, packaging, construction, and furniture industries. In addition to the reinforcement effect, with improved stiffness and strength at a low price, part of the plastics is replaced by a renewable material. A current drive for, e.g., wood plastic composites (WPCs) is to maximize the use of renewable content in mass production systems. However, while varying processing conditions using existing processing elements, e.g., dies, screws, moulds, designed for conventional plastics may allow processability of WPCs to a certain extent, their full potential as reinforcement cannot be reached without the development of dedicated processing components. This task required a fundamental understanding of WPC flow dynamics during processing that can only be achieved through combined experimental-numerical work. In this framework, this PhD project focuses on the processing, optimizing, and modeling of WPCs in single screw extrusion. For processing optimization, a study of inline extrudate analysis was carried out by analyzing temporal and spatial characteristic periodicities via a 2D Fourier transform. Spectral dynamics of space-time diagrams were determined as a function of the die shear rate and moisture content of the WPCs. Based on the spatio-temporal spectrograms, we assessed for the first time with unprecedented detail the onset and decay of surface instabilities in WPC. A combined numerical-experimental methodology for predicting orientation in WPCs flowing through a film die at three screw speeds of 21, 66, and 91 rpm is presented to take the initial steps toward extrusion modeling. In addition, material functions for validating the in-situ melt pressure and fiber orientation from SEM micrograph analysis were studied. The results show a promising agreement between the experimental and numerical data, but also challenges that need further exploration in terms of boundary conditions and constitutive models.
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| 6. |
- Pashazadehgaznagh, Sajjad, 1994, et al.
(författare)
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Mapping surface defects in highly-filled wood fiber polymer composite extrusion from inline spectral analysis
- 2023
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Ingår i: Composites Science and Technology. - 0266-3538. ; 242
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Tidskriftsartikel (refereegranskat)abstract
- Surface defects in highly-filled wood polymer composites (WPCs) are mapped in single-screw extrusion via inline optical spectral analysis for the first time. The effects of wood fiber content and drying on the dynamics of surface defects are spatio-temporally resolved via space–time inline optical imaging. Surface tearing appeared from the lowest shear rates investigated followed by a gradual decay in spectral intensity with increasing shear rates/slip velocities. This is accompanied by broadening of the surface tearing characteristic frequency while the average wavelength is estimated to remain constant within the experimental conditions. Increasing shear rates, drying and increasing wood fiber content showed mitigating effects on surface tearing. However, surface tearing in undried samples was still present even at the highest shear rates and high wall slip velocities. A regime where the extrudate surface is dominated by bubbles at high shear rates and low wood fiber contents in undried WPCs was identified.
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| 7. |
- Pashazadehgaznagh, Sajjad, 1994, et al.
(författare)
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Surface instability detection in highly-filled biocomposites from inline imaging during extrusion
- 2023
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Ingår i: AIP Conference Proceedings. - 1551-7616 .- 0094-243X. ; 2997
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Konferensbidrag (refereegranskat)abstract
- The processing of wood fiber biocomposites, and in particular, the extrusion, is accompanied by multiple challenges, among which agglomeration, entanglement, slip, or surface instabilities being the most common ones. In the current work, we focus on the dynamics of surface instabilities during the single screw extrusion of highly filled wood fiber biocomposites. The biocomposites are polypropylene based with up to 40 wt% wood fiber content of custom compositions based on the commercial-grade by Stora Enso. To detect and quantify the dynamics of surface instabilities, inline image analysis was applied using an optical visualization system positioned at the die exit. Therefrom, space-time diagrams were constructed, and after that, via 2D-Fourier transform analysis, the spatio-tempoal spectral dynamics of the surface instabilities were determined as a function of the die (apparent) shear rate. The spectral dynamics show that melt instabilities of 40 wt% for example, detected via their characteristic (temporal) frequency and (spatial) wavenumber, dissipate with increasing the shear rate, such that at shear rates above ca. 90 1/s, no characteristic frequency and wavenumber can be distinguished i.e. instabilities can no longer be observed on the surface of the extrudates.
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