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Then the next question is, now that you got it, can you actually deal with it and capture that energy and not have the entire apparatus melt down or disintegrate on the spot. Likely you'll need daily cartridges that are destroyed in the capture process, and \"breakeven\" needs to account for that, too.
Tokamaks are torus-shaped devices for nuclear fusion research and are a leading candidate for the generation of sustainable electric power. A main direction of research is to study the effects of shaping the distribution of the plasma into different configurations3,4,5 to optimize the stability, confinement and energy exhaust, and, in particular, to inform the first burning-plasma experiment, ITER. Confining each configuration within the tokamak requires designing a feedback controller that can manipulate the magnetic field6 through precise control of several coils that are magnetically coupled to the plasma to achieve the desired plasma current, position and shape, a problem known as the tokamak magnetic control problem.
Permanent magnets (PMs) produce magnetic fields and maintain the field even in the presence of an opposing magnetic field. Electrical machines using permanent magnets are more efficient than those without. Currently, all known strong magnets contain rare earth (RE) elements, and they are core components of a wide range of applications including electric vehicles and wind turbines. RE elements such as Nd and Dy have become critical materials due to the growing demand and constrained supply. Improving the manufacturing process is effective in mitigating the RE criticality issue by reducing waste and improving parts consistency. In this article, the state of the industry for PM is reviewed in detail considering both the technical and economic drivers. The importance of RE elements is discussed along with their economic importance to green energy. The conventional sintering and casting manufacturing processes for commercial magnets, including Nd-Fe-B, Sm-Co, Alnico, and ferrite, are described in detail.
Several comprehensive reviews exist which extensively cover the development of rare earth (RE) magnets and the factors determining their coercivity.3,4,5,6,7,8,9 Figure 3 shows the historical development and commercialization of permanent magnets based on their (BH)max.10 It is noteworthy that the main discoveries of new commercial hard magnetic materials and the advancement in (BH)max have occurred exclusively during the twentieth century. No major new magnetic material has been introduced since Nd-Fe-B in the early 1980s; in fact, it is now more than 38 years since the announcement of Nd-Fe-B magnets at the 29th MMM Conference held in Pittsburgh, PA, in November 1983.
The search for a RE-iron based permanent magnet began in the late 1970s following the cobalt raw material supply crisis. This eventually led to the simultaneous development of magnets based on the Nd2Fe14B tetragonal compound by both Sumitomo Special Metals (SSM) in Japan and General Motors (GM) in the US in the early 1980s.12 SSM was later to form a joint venture with Hitachi and eventually merged as Hitachi Metals Ltd. in 2007. GM spun off the Nd-Fe-B magnet business as Magnequench, today part of Neo Performance Materials. The Hitachi production method is based on powder metallurgical processing, and the Magnequench process uses melt spinning. However, both types of magnets are based on the same Nd2Fe14B tetragonal compound, but have vastly different microstructures and use different processing routes.12
The permanent magnet market is relatively small at $21 billion (2020) compared to other industrial markets. However, permanent magnets are critical and enabling for many high-value downstream products that represent many 100s of billion dollars of market value.
Other future major market growth drivers include electric bicycles, drones, wind/tidal energy generators, and robotics. Tables I and II shows the growth projections of the major magnet material in 2030 and 2040.
Almost all fully dense REPMs are produced using the same basic powder metallurgical processes. However, there are some detailed differences between SmCo5, Sm2Co17, and Nd2Fe14B magnets, and each manufacturer has its own customized variant. The high reactivity of the RE elements and their alloys and the critical dependence of the magnetic properties on the chemical composition require effective suppressions of contamination during the alloy preparation and subsequent powder metallurgical processing. In particular, oxidation of the RE components by air and moisture must be kept to a minimum through all fine-powder handling, sintering, and heat treatment stages.
The basic process steps for the Sm-Co-based magnets are shown in Fig. 6.23 The general process consists of alloy preparation, powder production, particle alignment and pressing, sintering and heat treatment, machining, and finally magnetizing.
Depending on the method used to prepare the alloy, the material may require a size reduction stage prior to final milling. For example, after vacuum melting and casting, the Sm-Co alloy is in the form of chill cast lumps. They are typically crushed, under a nitrogen atmosphere in a high energy hammer mill, to a particle size range of
During the sintering operation, the pressed product volume reduces to the final magnet body. This shrinkage depends upon production factors and the final magnet shape and size. This results in some variation in magnet size, and therefore a machining operation is necessary. REPMs are in general hard and brittle, although Nd-Fe-B magnets are tougher and less susceptible to breakage and chipping than Sm-Co magnets. Magnetic chucks are therefore not used to hold pieces down directly. Small series items are fastened by special adhesives to steel backing plates and then ground on conventional grinding machines fitted with either silicon carbide or diamond grinding wheels. Large series production is ground on double disc machines where the pieces are moved between two grinding wheels set the required distance apart. Small blocks can be slit using diamond impregnated wheels. Machined surfaces are required to give the necessary magnetic contact with the associated components in the final assembly.
Full second quadrant demagnetization curves are required from a representative sample from each batch of magnets. A batch is typically defined as the load from the final heat treatment furnace. A larger sample should be magnetized and then measured using a Helmholtz coil and flux meter against an agreed standard magnet. It may also be necessary to measure the side-to-side flux using a Hall probe and Gauss meter. Depending on the variation of magnetic properties (both batch to batch and within a batch), it may be necessary to classify the magnets within specific flux bands.
The microstructure of the starting alloy used for producing sintered magnets is critically important.12 The major challenge is that the Nd2Fe14B intermetallic compound forms by a peritectic reaction, where a liquid and solid phase reacts to form a second phase. In this instance, Nd2Fe14B forms by peritectic reaction from a liquid plus γ-Fe. At normal cooling rates, this reaction does not go to completion, and the cast alloy is found to contain a mixture of the Nd2Fe14B, NdFe4B4, and α-Fe [the (FCC) γ-Fe converts to (BCC) α-Fe at 910C]. The presence of the α-Fe is a problem for sintered magnet producers because it is a comparatively ductile phase compared to the brittle Nd2Fe14B alloy and makes crushing and grinding of the ingot into a powder much more difficult. Another serious problem for sintered Nd magnets is that the α-Fe is magnetically soft and results in a reduction in magnetic performance (kinked B-H loop) in the finished magnets. Elimination of secondary phases can be accomplished by a lengthy high-temperature annealing during which the secondary phases react together to form the desired Nd2Fe14B intermetallic phase. However, this annealing or homogenization process is slow and costly.
The best way to circumvent the formation of the α-Fe phase is to rapidly cool the alloy through the peritectic temperature, so that formation of the α-Fe is suppressed. In addition, the rapid cooling allows alloy to be produced with lower Nd content, resulting in higher remanence in the finished magnets. In the early stages of the development of sintered Nd magnets, the ingot was cast into book molds, whose casting cavities are slots with a cross section on the order of 1 cm or less in width, resulting in more rapid cooling of the ingot. This helped but did not solve the problem of α-Fe precipitates. The problem was finally solved by the development of strip casting, a process where the molten alloy is first produced in a standard melt furnace and then poured into a trough-shaped tundish, which contains a long narrow slot-shaped nozzle. A sheet of molten alloy pours from the nozzle and is quenched on a rotating water-cooled drum to form a continuous sheet of cast alloy. This alloy sheet is typically
The strip cast flakes are then broken down using hydrogen decrepitation process.25 Hydrogen embrittles metals by entering the grain boundaries and creating uneven stress to grains. This causes micro-cracks that begin to propagate through the grain structure and makes the flakes friable. This is followed by jet milling to