Guide to Magnetic Materials

Transcription

1 Guide to Magnetic s Abstract The purpose of this guide is to present the various materials and their performance, and also to give you the basic knowledge in choosing the right magnet material for your application. for Permanent Magnets Most sintered magnets can be produced by cutting from large blocks to the suitable shape, which implies no cost for special tools. What should I choose? Today, permanent magnet materials for almost any application exists. Thanks to the many grades of materials, rigorous demands on stability, performance, and flux can be complied. When choosing a permanent magnet material, several of the following parameters have to be considered. Flux Magnetic properties Maximum working temperature Method of assembly Plastic bonded or sintered There are several different materials for producing permanent magnets, Alnico, Ferrite, Samarium, and are the most common for commercial manufacuring. SmNFE is commercially available only as bonded magnet. Each main group contains several grades. The different grades and their respective properties can be found at our website: Most materials are anisotropic. Anisotropic magnets have higher magnetic properties than isotropic magnets. In the production, the anisotropic magnets gets a predetermined direction of magnetization. The advantage with isotropic materials is the possibility to magnetize in any direction, which makes complex magnet patterns possible. Cost Corrosion resistance Magnetic stability Mechanical strength Limitations in size and weight 1

2 Block diagram You can compare the properties of different materials in the block diagram below. The material properties are explained in detail in later sections. NEODYMIUM SAMARIUM ALNICO FERRITE FLEXIBLE Relative cost Coercivity Maximal energy product Maximum work temperature 2

3 Demagnization Curves Below is a graph showing the magnetic properties for different magnetic materials. Sintered NdFeB Sintered SmCo 2:17 Sintered SmCo 1:5 Polymer bonded NdFeB anisotropic Polymer bonded NdFeB isotropic Polymer bonded SmCo anisotropic Sintered ferrite anisotropic Polymer bonded ferrite anisotropic Sintered ferrite isotropic Polymer bonded ferrite isotropic Alnico 3

4 Magnetic Properties Comparison Below is a comparison of the most important properties for a permanent magnet.study also the block diagram in part 1 where some of the tables is shown graphically. Comparing Magnetic Performance The most simple method to compare different typs of material and classes is to compare their maximum energy product (BHmax). MGO kj/m3 Ferrite (anisotropiq) Alnico (anisotropic) Samarium (1:5) Samarium(2:17) This is the point on the demagnetization curve where B*X has its maximum and thus also the point where the magnet has its highest magnetic energy for the smallest volume. Effect of Temperature Temperature effects are separated in two major groups: irreversible losses and reversible losses. A reversible loss depends on the material properties of the used material. The shape of the magnet, size, working point on demagnization curve does not matter on this respect. The original properties returns at room temperature. Irreversible losses depends on material, size, shape, magnetic circuit etc. The losses are not regained at room temperature Ferrite (anisotropiq) Alnico (anisotropic) Samarium (1:5) Samarium(2:17) Temperature %/ C %/ C -0.2 %/ C %/ C %/ C Losses due to Temperature Ferrite (anisotropiq) Alnico (anisotropic) Samarium (1:5) Samarium(2:17) Flux density 100 mt 130 mt 270 mt 320 mt 410 mt Temperature Ferrite (anisotropiq) 250 C Alnico (anisotropic) 540 C Samarium (1:5) 250 C Samarium(2:17) 300 C 100 C Typical values for the flux density of polytan when working around BHmax. (100mT = 1000 Gauss) from all directions. These magnets gets the highest possible magnetic values. Maximum Working Temperature Magnet Stability Temperature affects the magnetic stability, but strong external magnetic fields may also change the magnetic stability. The following table describes how much an external magnetic field affects the magnetic stability. 4

5 Ferrite (anisotropiq) Alnico (anisotropic) Samarium (1:5) Samarium(2:17) Affect little much very little very little very little Corrosion Resistance for non Coated Magnets The following table describes corrosion resistance for non coated magnets compared to each other. Alnico Magnets Alnico magnets are manufactured by casting or sintering. For many years, cast Alnico magnets were the most commonly used material, but as new materials got introduced the use of Alnico magnets have decreased significantly. The Alnico magnets are very stable for temperature changes and can be used in temperatures up tp 500 c. The remanence for some qualities are comparable to Samarium cobalt and. However, the coercive force are very low. Ferrite Alnico Samarium Resistance excellent fairly good excellent bad Manufacturing Cost The cost for permanent magnets is certainly influenced by the shape and its tolerance, but the dominant cost is the material cost. The following table describes approximate costs for the materials compared to each other. Cost Muliple factor Ferrite (anisotropic) low 1 Alnico (anisotropic) medium 5 Samarium (1:5) high 15 Samarium(2:17) very high 20 medium high 10 Chemical Composition Alnico is an alloy that contains aluminium, nickel, cobalt, and iron. Typical Applications Typical applications are measuring instruments, holding magnets for high temperature use, and some types of loudspeakers. The classic horse shoe magnets are made of Alnico. Ferrite Magnets The most commonly used materials for permanent magnets are Sintered Barium and Strontium Ferrite. These materials offers a cost effective solution for most applications, thanks to cheap raw material. Ferrite magnets are able to stand large variations in working temperature and are suitable for large scale manufacturing. The ferrite magnets are ceramic in its nature and have a very high resistance to corrosion. They have high resistivity and are suitable for high frequency applications. 5

6 Chemical Composition The composition of ferrite magnet are Ba- Fe12O19 or SrFe12O19 and are therefore ceramics. They consists of about 80% ferric oxide (Fe2O3) and of about 20% Barium oxide (BaO) or Strontium oxide (SrO). The raw materials are easy to get and are cheap to prepare. The magnets are resistant to many chemicals such as: solvents, alkaline solutions, and diluted acids. The ferrite magnets are fragile and brittle for shocks and bends because of its chemical nature. Since they are hard (6-7 Mohs scale), they must be machined with diamond tools. Typical Applications Loudspeakers, toys, magnetic filtering, magnetic separators, DC motors, and holding magnets. Sura Magnets has the knowledge an equipment to construct and manufacture complex assemblies. All you need to know is the function of the assembly. When constructing an assembly, the following issues has the be considered: Samarium Magnets The sintered Samarium magnets are produced by a powder metallurgic process and most magnets are cut out of an isostatic compressed block. Compression moulded Samarium magnets are produced by compression in a magnetic field. Both Samarium 1:5 and Samarium 2:17 offers a considerable higher flux density than Ferrite and Alnico magnets. This gives the possibility to make the magnets physically smaller, hence the construction size decreases. However, Samarium magnets are extremely brittle, which limits the range of applications. Samarium magnets needs to be handled with considerable care, especially after magnetization. Bonded Samarium Cobalt can be produced with with a variety of magnetic properties and polymer base. For high temperature use PPS is a suitable polymer. Sura Magnets is one of the largest producers of Bonded samarium cobalt Chemical Composition Samarium magnets exists in two compositions, SmC05 (Samarium 1:5) and Sm2Co17 (Samarium 2:17). Typical Applications DC motors, generators, tachometers, headphones, loudspeakers, magnetic couplings, measuring instruments, high temperature applications.does not affect the magnets. Magnets Sintered magnets are produced by a powder metallurgic process, and is the most powerful magnet available. However, magnets are sensitive to high temperatures and corrosion. Their highest working temperature is about 180 C. This is the reason for the many different magnet qualities. To raise the maximum working temperature, mostly cobalt is added. The remanence is often decreased by adding other materials but the coercive force can be increased, which makes 6

7 this a good choice for use in DC motors. These magnets are not as fragile as the Samarium magnets. Although, because the attractive forces are very strong, handling the magnets should be done with caution. Plastic bonded NdFeB can be compression molded or injection-moulded. Bonded magnets have lower magnetic properties, but they are easy to produce in desired shape. They have high resistivity, which counteracts eddy currents. This is especially important in high frequency applications. Chemical Composition The chemical composition of magnets is Nd2Fe14B. Mechanical Properties magnets can be machined by EDM or by grinding with standard tools. The magnet density is approximately the same as for steel, i.e. 7.5 g/cm3. The mechanical properties for plastic bonded magnets are mainly depending of the polymer base. Plastic bonded magnets are easy to machine with standard cutting tools. 7