塑性加工法制备各向异性稀土永磁材料的研究进展

塑性加工是一种可制备各向异性材料并可实现近终形成形的方法,已有一系列尝试将其应用于制备高性能各向异性稀土永磁材料的研究报导.介绍了用于制备各向异性磁体或磁粉的塑性加工方法,包括热压、模压、轧制、正挤压、反挤压、等径角挤压等,和用各种塑性加工方法提高永磁材料性能的基本原理、效果,以及工业化应用进展.     

Design and study of an enhanced filament ion source

Filament ion sources (FISs) have been used in the production of high current of H⁻ ions for a wide variety of research fields and applications. The major deficiency in FIS performance is the escape of energetic electrons produced by filament striking with interior metal surfaces of the FIS. To avoid this problem, magneto-static fields produced by permanent magnets are used. Some kinds of same polarity FIS with superior performance than traditional alternating polarity FIS have been reported. Here, we propose a novel FIS and a method to compare different FISs. The purpose of this design is to recapture more energetic electrons with lower number of permanent magnets. Performance simulations of the proposed FIS show considerable improvement over other reported FISs.     

Design and study of an enhanced filament ion source

Filament ion sources (FISs) have been used in the production of high current of H⁻ ions for a wide variety of research fields and applications. The major deficiency in FIS performance is the escape of energetic electrons produced by filament striking with interior metal surfaces of the FIS. To avoid this problem, magneto-static fields produced by permanent magnets are used. Some kinds of same polarity FIS with superior performance than traditional alternating polarity FIS have been reported. Here, we propose a novel FIS and a method to compare different FISs. The purpose of this design is to recapture more energetic electrons with lower number of permanent magnets. Performance simulations of the proposed FIS show considerable improvement over other reported FISs.     

Perspective and Prospects for Rare Earth Permanent Magnets

Rare earth permanent magnets constitute a mature technology, but the shock of the 2011 rare earth crisis led to the re-evaluation of many ideas from the 1980s and 1990s about possible new hard magnets containing little or no rare earth (or heavy rare earth). Nd–Fe–B magnets have been painstakingly and skillfully optimized for a wide range of applications in which high performance is required at reasonable cost. Sm–Co is the material of choice when high-temperature stability is required, and Sm–Fe–N magnets are making their way into some niche applications. The scope for improvement in these basic materials by substitution has been rather thoroughly explored, and the effects of processing techniques on the microstructure and hysteresis are largely understood. A big idea from a generation ago—which held real potential to raise the record energy product significantly—was the oriented exchange-spring hard/soft nanocomposite magnet; however, it has proved very difficult to realize. Nevertheless, the field has evolved, and innovation has flourished in other areas. For example, electrical personal transport has progressed from millions of electric bicycles to the point where cars and trucks with electrical drives are becoming mainstream, and looks ready to bring the dominance of the internal combustion engine to an end. As the limitations of particular permanent magnets become clearer, ingenuity and imagination are being used to design around them, and to exploit the available mix of rare earth resources most efficiently. Huge new markets in robotics beckon, and the opportunities offered by additive manufacturing are just beginning to be explored. New methods of increasing magnet stability at elevated temperature are being developed, and integrated multifunctionality of hard magnets with other useful properties is now envisaged. These themes are elaborated here, with various examples.     

Additive Manufacturing of Hard Magnetic Passive Shims to Increase Field Homogeneity of a Halbach Magnet

Obtaining a highly homogeneous magnetic field is desired for field-controlled applications. For example, the resolution of magnetic analysis methods can be improved by generating a stronger and more homogeneous field over the region of interest (ROI). A set of 3D-printed passive shims is fabricated using additive manufacturing to improve the magnetic field homogeneity of a Halbach magnet assembly. The feedstock is a custom acrylonitrile butadiene styrene (ABS)-hard magnet composite filament filled with 60% wt. isotropic NdFeB. Additionally, a method for investigating the remanence is developed and validated. The result reveals a good agreement between the new method and existing measurement techniques for the remanence of permanent magnets. It is also shown that the additive manufacturing procedure has negligible effects on the magnetic properties. Performing a parametric study over a rectangular ROI, an optimized shim configuration is achieved. In the optimized and 3D-printed configuration, the average norm of the magnetic flux density, B_norm, is increased by 13% and, more importantly, a 43% increase in the magnetic uniformity is obtained. These results highlight the great potential of freeform manufacturing, namely, additive manufacturing, to tailor the properties of magnet structures.     

Structure and mechanical properties of polyethylene-fullerene composites

The microhardness of films of fullerene-polyethylene composites prepared by gelation from semidilute solution, using ultrahigh molecular weight polyethylene (PE) (6×10⁶), has been determined. The composite materials were characterized by optical microscopy and X-ray diffraction techniques. The microhardness of the films is shown to increase notably with the concentration of fullerene particles within the films. In addition, a substantial hardening of the composites is obtained after annealing the materials at high temperatures (T _a=130 °C) and long annealing times (t _a=10⁵s). The hardening of the composites with annealing temperature has been identified with the thickening of the PE crystalline lamellae. Comparison of X-ray scattering data and the microhardness values upon annealing leads to the conclusion of phase separation of C₆₀ molecules from the polyethylene crystals within the material. The temperature dependence is discussed in terms of the independent contribution of the PE matrix of the C₆₀ aggregates to the hardness value.     

Fibrillar polymer–polymer composites: morphology, properties and applications

Micro- or nano-fibrillar composites (MFCs or NFCs) are created by blending two homopolymers (virgin or recycled) with different melting temperatures such as polyethylene (PE) and poly(ethylene terephthalate) (PET), and processing the blend under certain thermo-mechanical conditions to create in situ fibrils of the polymer that has the higher-melting temperature. These resulting fibrillar composites have been reported to possess excellent mechanical properties and can have wide ranging applications with suitable processing under controlled conditions. However, the properties and applications very much depend on the morphology of created polymer fibrils and their thermal stability. The present paper develops an understanding of the mechanism of micro-/nano-fibril formation in PE/PET and polypropylene (PP)/PET blends by studying their morphology at various stages of extrusion and drawing. It is revealed that this subsequent mechanical processing stretches the polymer chains and creates fibrils of very high aspect ratios, thus resulting in superior mechanical performance of the composites compared to the raw blends. The study also identifies the primary mechanical properties of the main types of MFCs, as well as quantifying their enhanced resistance to oxygen permeability. Furthermore, the failure phenomena of these composites are studied via application of the modified Tsai–Hill criterion. In addition to their usage as input materials in different manufacturing processes, possible applications of these fibrillar composites in two different areas are also discussed, namely food packaging with controlled oxygen barrier properties and biomedical tissue scaffolding. Results indicate a significant scope for using these materials in both areas.     

Thermomechanical processing of aluminium-based particulate composites

Thermomechanical processing of aluminium-based particulate composites containing dispersions of various soft and hard particles such as graphite, zircon, glass, alumina and mica in aluminium alloy matrices, with a view to improving strength and ductility for structural applications, has been discussed. The existing literature on the subject has been critically reviewed and analysed, and broad guidelines for optimum thermomechanical processing have been presented. Considerable improvements in strength and ductility of these composite materials have been reported after rolling, forging and extrusion due to fragmentation/fibrization of particles together with the refinement of matrix microstructure, annihilation of defects such as porosity, and texture hardening, etc. The influence of process variables, and of volume fraction, size and morphology of the particles on the strengthening mechanisms, fracture toughness and work-hardening behaviour of worked composites has been discussed.     

Chemical,morphological, and mechanical analysis of rice husk/post-consumer polyethylene composites

Composites were obtained from post-consumer high-density polyethylene (PE) reinforced with different concentrations of rice husk. PE and rice husk were chemically modified to improve their compatibility in composite preparation. Rice husk was mercerized with a NaOH solution and acetylated. The chemically modified fibers were characterized by FTIR and ¹³C NMR spectroscopy. The composites were prepared by extrusion of modified and unmodified materials containing either 5 or 10 wt.% fibers. The morphology of the obtained materials was analyzed by SEM. The chemical modification of the fiber surface was found to improve its adhesion with matrix. Flexural and impact tests demonstrated that PE/rice husk composites present improved mechanical performance comparatively to the pure polymer matrix, on the contrary no benefit is observed in the tensile strength over the pure PE.     

Biaxial testing of high strength carbon fiber composite cylinders for pulsed magnet reinforcement

A methodology is introduced to test carbon-fiber-reinforced, hoop-wound composite cylinders for their biaxial mechanical properties under axial compression and hoop tension. The understanding of the behavior of these composites under biaxial loads is extremely important in the design of pulsed magnets. These composites are used as reinforcements for both the inner conducting layers and as an overall exterior reinforcement. Testing of actual pulsed magnets to ascertain design change effects of composite reinforcement schemes on the maximum attainable field can be expensive; hence, a standard biaxial testing method is desirable which is relevant to the design of pulsed magnets. In this investigation, an attempt was made to produce a standard testing procedure aimed at measuring the biaxial mechanical properties (elastic, plastic, and failure envelope) of composite materials. This methodology was applied to two different carbon/epoxy based composites. The results of these tests (elastic properties and failure points) are compared with theoretical predictions, specifically those due to Tsai-Wu.     

Characterisation of the effects of a titanium micro particle filler on a polyether-<Emphasis Type="Italic">block</Emphasis>-amide host matrix

The body of work described in this research article outlines the use of titanium microparticles as fillers in the production of a polyether-block-amide (Pebax 5533) based composite for medical applications. Virgin polyether-block-amide was compared with titanium filled composites with loadings of 40 and 60% by weight prepared using twin screw extrusion and compression moulding. The materials were characterised using a range of mechanical, thermal, toxicological and surface analysis techniques. Fourier transform infrared spectroscopy indicated that no chemical interaction occurred between the filler particles and the host polymer matrix. Thermal analysis of the composites indicated that as the blend composition varied, so too did the melting behaviour. The inclusion of the titanium microparticles was observed to increase the flow viscosity, tensile strength, hardness and Young’s modulus of the composites whilst also resulting in a rougher surface with lower surface energy.     

Life cycle energy analysis of fiber-reinforced composites

Life cycle assessment is a technique to assess environmental aspects associated with a product or process by identifying energy, materials, and emissions over its life cycle. The energy analysis includes four stages of a life cycle: material production phase, manufacturing phase, use phase, and end-of-life phase. In this study, the life cycle energy of fiber-reinforced composites manufactured by using the pultrusion process was analyzed. For more widespread use of composites, it is critical to estimate how much energy is consumed during the lifetime of the composites compared to other materials. In particular, we evaluated a potential for composite materials to save energy in automotive applications. A hybrid model, which combines process analysis with economic input–output analysis, was used to capture both direct and indirect energy consumption of the pultrusion process in the material production and manufacturing stages.     

Direct-write 3D printing of NdFeB bonded magnets

We report a method to fabricate Nd–Fe–B (NdFeB) bonded magnets of complex shape via extrusion-based additive manufacturing (AM), also known as 3D-printing. We have successfully formulated a 3D-printable epoxy-based ink for direct-write AM with anisotropic MQA NdFeB magnet particles that can be deposited at room temperature. The new feedstocks contain up to 40 vol.% MQA anisotropic NdFeB magnet particles, and they are shown to remain uniformly dispersed in the thermoset matrix throughout the deposition process. Ring, bar, and horseshoe-type 3D magnet structures were printed and cured in air at 100°C without degrading the magnetic properties. This study provides a new pathway for fabricating NdFeB bonded magnets with complex geometry at low temperature, and presents new opportunities for fabricating multifunctional hybrid structures and devices.     

Manufacture and evaluation of bioactive and biodegradable materials and scaffolds for tissue engineering

For tissue regeneration and tissue engineering applications, a number of bioactive and biodegradable composites, either porous or non-porous, were fabricated. The newly developed materials included tricalcium phosphate reinforced polyhydroxybutyrate and its copolymer, poorly crystallized hydroxyapatite reinforced chitin, and plasma sprayed hydroxyapatite reinforced poly(L-lactic acid). It was shown that these new materials could be successfully produced using the manufacturing techniques adopted. In vitro experiments revealed that the incorporation of bioceramic particles in biodegradable polymers rendered the composites bioactive and significantly improved the ability of composites to induce the formation of bone-like apatite on their surfaces. Degradation of composite scaffolds in simulated body fluid was observed and could be due to the simultaneous degradation of polymer matrix and dissolution of bioceramic particles.     

Mechanical properties of novel aluminum metal matrix metallic composites: Application to overhead conductors

The mechanical behavior and microstructural evolution of aluminum metal matrix metallic composites fabricated under various process conditions were investigated to understand their process-structure–property relations. The novel techniques for arranging the matrix and reinforcement materials and controlling the processing atmosphere were applied to the extrusion process. The composites were comprised of matrix 1050 and reinforcement 6061 aluminum alloys with varying percent weight compositions and were arranged in a tailorable concentric annular pattern. The composites were shown to substantially increase compressive strength when the atmosphere of composite arrangement was evacuated prior to extrusion. Mechanical response of the composites were compared to the pre-extruded 1050 and 6061 aluminum alloys. The yield strengths of each composite, with varying percent weight compositions, were found to lie between those of matrix and reinforcement alloys, and abided by a simple rule-of-mixtures when considering weight composition. Highly elongated grains were oriented in the as-extruded composites along the extrusion direction and grains near the interface between two constituent alloys showed higher aspect ratio than in the interior region. The present study could lead to the optimum composite design for various industrial applications including all aluminum alloy overhead conductors with high strength and improved electric conductivity.     

Design and 3D Printing of Scaffolds and Tissues

A growing number of three-dimensional(3D)-print- ing processes have been applied to tissue engineering. This paper presents a state-of-the-art study of 3D-printing technologies for tissue-engineering applications, with particular focus on the development of a computer-aided scaffold design system; the direct 3D printing of functionally graded scaffolds; the modeling of selective laser sintering(SLS) and fused deposition modeling(FDM) processes; the indirect additive manufacturing of scaffolds, with both micro and macro features; the development of a bioreactor; and 3D/4D bioprinting. Technological limitations will be discussed so as to highlight the possibility of future improvements for new 3D-printing methodologies for tissue engineering.     

Tunable,Flexible Composite Magnets for Marine Monitoring Applications

This paper presents flexible NdFeB‐PDMS composite magnets with tunable magnetic and mechanical properties optimized for applications in corrosive marine environments. The magnetic and mechanical properties are studied for different NdFeB powder concentrations and the performance of the magnetic composites for different exposure times to sea water investigated systematically. The remanence and saturation magnetizations could be tailored by the powder concentration, whereby up to 70 wt% concentration could be employed without compromising the integrity of the magnets. The elastic modulus of the composite magnets is about 10⁵ times lower than the one of a bulk permanent magnet. This ensures a high bending flexibility, which allows the magnets to be attached to curved surfaces as illustrated for a giant clam, crab, and turtle. At the same time, the weight of the composite magnets is reduced by a factor of about 10, which poses less burden to animals’ natural free movement. Without a protective layer, the composite magnets lose more than 50% of their magnetization after 51 days in seawater. However, the durability of the composite magnets has been improved considerably by using polymer coatings. Parylene C is the most effective for this, providing corrosion resistance, flexibility, and enhanced biocompatibility. Parylene C films of 2 and 4 μm thicknesses provided excellent protection of the magnetic composite in corrosive aqueous environments for 65 and 82 days, respectively. By combining the composite magnets with tunnel magnetoresistance sensors, a magnetic animal monitoring system is established that is used to track the behavior of giant clam, crab, and turtle.

     

Green polypropylene/waste paper composites with superior modulus and crystallization behavior: Optimizing specific energy in solid-state shear pulverization for filler size reduction and dispersion

Solid-state shear pulverization (SSSP) is a continuous process that overcomes challenges in producing well-dispersed polymer composites that cannot be made by twin-screw melt extrusion. We use SSSP to produce 85/15 wt% polypropylene/waste paper biocomposites with polypropylene pellets and 2-cm-square waste paper pieces as starting material. Single-pass SSSP achieves effective filler size reduction and dispersion within the polypropylene matrix. We determine how waste paper size reduction and composite properties are functions of specific energy input and tune specific energy input by SSSP screw design and throughput. Composites made at moderate to high specific energy input (14–35 kJ/g) have 25 to nearly 50% of filler particles at sub-micron size; relative to neat polypropylene, composites exhibit a 70% increase in Young’s modulus, retention of neat polypropylene yield strength, and a ∼50% reduction in crystallization half-time. Estimates indicate that the cost of such biocomposite materials made by SSSP is less than that of virgin polypropylene.     

Cosmic radiation shielding tests for UHMWPE fiber/nano-epoxy composites

Cosmic radiation shielding properties are important for spacecraft, and hydrogenous materials such as polyethylene have been shown to be effective in shielding against galactic cosmic rays and solar energetic particles. Ultrahigh molecular weight polyethylene (UHMWPE) fibers, which are effective in such shielding, also have advanced mechanical and physical properties, which potentially are very valuable for NASA space missions both as a radiation shield and as vehicle structure. In our previous studies, we fabricated a nano-epoxy matrix with reactive graphitic nanofibers that showed enhanced mechanical (including strength, modulus and toughness) and thermal properties (higher T_g, stable CTE, and higher ageing resistance), as well as wetting and adhesion ability to UHMWPE fibers. In this work, the radiation shielding performance of the UHMWPE fiber reinforced nano-epoxy composite was characterized by radiation tests at the NASA Space Radiation Laboratory at Brookhaven National Laboratory. The results showed that the high radiation shielding performance associated with UHMWPE was not degraded by the addition of graphitic nanofibers in the matrix. Together with the previous studies showing higher mechanical properties, these new studies validate the importance of the UHMWPE fiber/nano-epoxy composite for potential applications in more durable space composites and structures, and offer reduced manufacturing costs and wider design applications through avoidance of specialized and in some cases ineffective UHMWPE fiber surface treatment processes.