Model for Calculating J(H) Curves of Ni Coated Nd-Fe-B Magnets

An analytical model to describe the influence of surface degradation and the Ni layer itself on the magnetic properties of Ni coated Nd-Fe-B magnets is presented. Starting from the bulk magnetic properties, the dimensions, the thickness of Ni coating and the affected surface layer, J(H) demagnetization curve is calculated. Subsequently the expected values of (BH)_max, and the reversible permeability are deduced from the calculated J(H) curves. For flat magnets the surface effects lead to a decrease of B_r and an increase of the permeability which lowers (BH)_max. For strait magnets a step in the J(H) curve appears at H = 0. The deteriorating effect of Ni coating and the surface layer scale with the dimensions of magnet and the thickness of these layers, which depend on the processing and the grain size of magnet. These effects can not be neglected if one or more dimensions of a Ni coated magnet are less than about 5 mm. SmCo₅ magnets show similar effects but the coercivity of the damaged surface layer is higher. Pinning type Sm₂Co₁₇ magnets show almost no deterioration on surface due to machining. As a result, Sm-Co magnets are better suited for applications with dimensions smaller than about 2 mm.     

Overview of Recent Progress in Sm-Co Based Magnets

Superior magnetic properties of Sm-Co based magnets, especially their excellent high temperature stability and low temperature coefficient, have led to an exciting variety of applications. Despite the commercial success of sintered Nd-Fe-B magnets, Sm-Co magnets continue to play a dominant role in some critical applications, such as traveling wave tubes (TWT) for space exploration and satellite communication; inertial devices for accelerometers and gyroscopes; power tools for medical applications; and permanent magnet motors and generators for aircraft engines. We will review the recent development in ultra-high temperature magnets and zero RTC (reversible temperature coefficient) magnets.     

Domain Structure and Magnetic Properties of Epitaxial Rare Earth-Transition Metal Thin Films

Rare Earth-Transition Metal permanent magnet films were epitaxially grown by pulsed laser deposition on heated MgO single crystal substrates of different orientations. The epitaxial growth relation of film, buffer and substrate is studied by pole figure measurements and its consequences on the anisotropic magnetic behavior are discussed. In the investigated material systems, RCo₅ (R = Sm, Pr) and Nd-Fe-B, high anisotropies and coercivities are achieved, the film morphology, domain structure and the coercivity mechanism are, however, distinctly different. The small scaled domain structure found for SmCo₅ and PrCo₅ films is a consequence of the small grain sizes and the magnetization process is dominated by strong pinning. Nd₂Fe₁₄B based films, on the other hand, are nucleation type magnets and coercivity is influenced by film morphology and roughness.     

烧结钕铁硼镀镍防腐性能比较

采用不同浇铸工艺和破碎工艺制备三种烧结钕铁硼磁体,对三种磁体及其电镀镍后的防腐性能进行了比较.研究发现,三种磁体电镀镍后的防腐性能不仅与镍镀层厚度有关,同时也与基体本身耐蚀性能有关.经对比分析,采用速凝薄带配合氢气破碎工艺制备的镀镍磁体防腐性能最佳.     

晶界扩散Tb对烧结Nd-Fe-B组织与磁性能的影响

用磁控溅射法在烧结Nd-Fe-B磁体表面沉积Tb金属薄膜并进行晶界扩散处理,对比经不同热扩散温度及时间处理后的磁体组织和磁性能变化。结果表明,925℃×10 h+500℃×2 h为最佳晶界扩散工艺,可将磁体矫顽力提高到1630.9 kA·m^-1,较原始磁体提升50%,同时剩磁和磁能积无明显下降,磁体仍具有较高的退磁曲线方形度。晶界扩散处理后磁体取向度有所提高,主相晶粒表面形成了明显的富Tb壳层结构,其厚度随离开磁体表面距离的增加逐渐变薄,随热扩散温度升高和时间延长逐渐增厚。长时间热扩散处理使磁体内形成沿晶界分布的连续薄层富Nd相,将主相晶粒彼此分隔,有效降低磁性相颗粒间交换耦合作用。能谱(EDS)分析表明,适当的热扩散工艺可使Tb元素扩散至磁体芯部,渗透厚度4 mm的磁体。     

Improved Mechanical Properties and Thermal Stability of Sm-Co High Temperature Magnets Resulting From Surface Modifications

Improved mechanical properties and thermal stability of Sm-Co high temperature magnets resulting from surface modifications are reported. Compared to un-modified magnets, the modification with 15 μm sulfamate-Ni plating improved the fracture toughness by 76% and the thermal stability by 143% in vacuum and 861% in air for aging at 500 °C for up to 3000 hours. Microstructures of some specimens with and without modification were studied using SEM with EDS, and showed different fracture patterns. The improvements in mechanical properties were resulted by closing the infinite crack-origin sites on the surface of the brittle magnets with ductile metal. The striking improvement in thermal stability was achieved by sealing the Sm-Co magnets with corrosion resistive metal to limit oxidation. The adhesive bond strength between the magnet and the modified surface plays an important role in these improvements.     

Improvement of coercivity and corrosion resistance of Nd-Fe-B sintered magnets by doping aluminium nano-particles

Nd-Fe-B permanent magnets with a small amount of Al nano-particles doping were prepared by conventional sintered method. Effect of Al content on magnetic property, corrosion resistance and oxidation properties of the magnets were studied. Investigation showed that the coercivity rose gradually, while the remanence decreased simultaneously with increase of Al doping amount. Further investigation revealed that most Al element diffused into the main phase and some Al element diffused into the Nd-rich phase. The autoclave test results showed that the corrosion rate of the magnets decreased with Al content increasing. After oxidation, the maximum energy product losses of the magnets with 0.0 wt.% and 0.2 wt.% Al nano-particles doping were 6.13% and 3.99%, respectively. Therefore, Al nano-particles doping was a promising way to enhance the coercivity and corrosion resistance of sintered Nd-Fe-B magnet.     

Progress of Sm-Fe-N Anisotropic Magnets

A reduction and diffusion method (R/D) is used to make a mother alloy of Sm-Fe-N anisotropic magnets. Reduction of 0.5wt% of samarium content compared to the conventional powder increases magnetization. Milling condition and surface treatment improve the squareness of demagnetization curve, the aging property and the heat resistance. The maximum energy product of 292 kJ/m³ is obtained with the powder. High coercive force is maintainable even if the powder is exposed for 300h in 80 °C 90%RH. The maximum energy product of 141 kJ/m³ is obtained with an injection molded anisotropic magnet. The aging property estimated by irreversible flux loss is comparable to the conventional MQP-B magnets. The heat resistance temperature (T_−5%) at which die initial irreversible flux loss becomes −5% is 125-more than 150 °C for Sm-Fe-N magnets and 150–170 °C for hybrid magnets. The magnetic properties of bonded HDDR Nd-Fe-B magnet were improved by substituting for Nd-Fe-B powder with Sm-Fe-N powder. A new technology to make anisotropic bonded Sm-Fe-N thin cylinder magnets by an injection molding using unsaturated polyester (UP) resin was developed.     

Application of Magnetic Field in Fabrication Process of Anisotropic Bonded Nd-Fe-B Magnet

This work studied the application of the different magnetic field used in the compaction process for die fabrication of anisotropic Nd-Fe-B bonded magnet. The static field made from Nd-Fe-B permanent magnets was used in the blending process to separate the particles each other. The SEM observation gave intuitionistic results about it. The anisotropic Nd-Fe-B bonded magnets were fabricated with warm-compaction under the electromagnetic field about 2.5 T. It is known that magnetic field is necessary for anisotropic materials fabrication for alignment. And warm compaction was used to decrease the viscousness of binder, to enhance alignment magnetic particle while press, and to get high density materials. For coercivity of Nd-Fe-B magnets decrease largely with the temperature increasing, press in proper temperature and oriented field is benefit to the magnetic characteristics and the mechanical properties of the anisotropic bonded Nd-Fe-B magnets. Finally solidifying process was performed under the pulse field of 4 T. The increment for solidifying in the field was about 15% for maximum energy product of the bonded magnet. The magnetic properties of anisotropic bonded Nd-Fe-B magnets from d-HDDR powders compact at 90 °C in alignment field of 2.5 T were: B_r=8.55 kGs, _iH_c=12.0 kOe, (BH)_max=14.57 MGOe.     

Preparation and characterization of sodium silicate/epoxy resin composite bonded Nd-Fe-B magnets with high performance

As an organic binder for bonded Nd-Fe-B magnets, epoxy resin(EP) has poor heat resistance but good moisture resistance, while sodium silicate(SS) has poor moisture absorption but better heat resistance and corrosion resistance. In order to improve high temperature stability and decrease moisture absorption of bonded Nd-Fe-B magnets, EP/SS composites were applied as the binder to prepare bonded Nd-Fe-B magnets. The magnetic properties, moisture absorption, corrosion resistance, compressive strength and microstructure of composite bonded magnets were investigated. The results show that EP/SS bonded magnets can obtain excellent magnetic properties at room temperature, and even useable magnetic properties a thigh temperature environments at 200°C. EP/SS composite binder effectively improves heat resistance and corrosion resistance of bonded Nd-Fe-B magnets, and reduces the hygroscopic properties. The molecule of sodium silicateis rigid and keeps it original shape at high temperature environments. In addition, SS in composite binder improves the mobility of the magnetic powders during the pre-pressing process, which makes the magnetic powders attain a more regular structure. These two factors will increase the mechanical properties. Moreover, sodium silicate in the composite binder can also cover the surfaces protecting the magnetic powders from oxidation and corrosion. EP in composite binder can cover SS surface to reduce the water absorption of SS as epoxy is a hydrophobic material. The EDX analysis shows that the composite binder has accumulated in the gaps of the magnet powders, which not only improves heat resistance and corrosion resistance, but also increases the mechanical properties. Therefore, EP/SS composite binder endows bonded Nd-Fe-B magnets excellent comprehensive properties.     

Improvement of coercivity and thermal stability of Nd-Fe-B sintered magnets by intergranular addition of Tb80Fe20 alloy

To improve the coercivity and temperature stability of Nd-Fe-B sintered magnets for high-temperature applications, the eutectic Tb₈₀Fe₂₀ (wt%) alloy powders were added into the Nd-Fe-B sintered magnets by intergranular method to enhance the coercivity (H_cj) and thermal stability. The microstructure, magnetic properties and thermal stability of the Nd-Fe-B magnets with different Tb₈₀Fe₂₀ contents were studied. The experimental results demonstrate that the coercivity (H_cj) of the sintered Nd-Fe-B magnet is significantly enhanced from 14.12 to 27.78 kOe, and the remanence (B_r) decreases not obviously by introducing 4 wt% Tb₈₀Fe₂₀ alloy. Meanwhile, the reversible temperature coefficients of coercivity (β) and remanence (α) of the Nd-Fe-B magnets are increased from ?0.5634%/℃ to ?0.4506%/℃ and ?0.1276%/℃ to ?0.1199%/℃ at 20–170 ℃, respectively. The Curie temperature (T_C) of the Nd-Fe-B magnet is slightly enhanced with the increase of Tb₈₀Fe₂₀ content. Moreover, the irreversible flux magnetic loss (h_irr) is obviously reduced as Tb₈₀Fe₂₀ addition increases. Further analysis of the microstructure reveals that a modified microstructure, i.e. clear and continuous RE-rich grain boundary layer, is acquired in the sintered magnets by introducing Tb₈₀Fe₂₀ alloy. The associated mechanisms on improved coercivity and thermal stability were comprehensively researched.     

Progress of Research on Bonded Nd-Fe-B Magnets

Some progress of research on bonded Nd-Fe-B magnets in National University of Defense Technology(NUDT) is presented in this paper. The contents include B-rich R₂Fe₁₄B-based nano composite with good performance; a model to determine of the least amount of binder; resin for high temperature application; resin encapsulating magnetic powders for long-term storage; thermoplastic polymer used for mold-pressing magnets; hybrid bonded Nd-Fe-B/Sm₂Co₁₇ magnet with a potentially useful improvements in remanence and magnetic energy product.     

Properties of nanolayer erosion-resistant coatings based on metal carbides and nitrides

Vacuum-arc ion-assisted deposition is used to form nanolayer protective 2D coatings based on the nitrides or carbides of titanium and chromium, vanadium carbide, and aluminum nitride with a layer thickness of 5–80 nm and a total thickness up to 25 μm. The phase composition of the coatings is studied after deposition and tests. Titanium alloy VT1-0 (EP866 steel)-nanolayer coating compositions are subjected to hot-strength and rapid cyclic corrosion tests, and the erosion resistance of the 2D nanolayer coatings in a dust-air flux (the average fraction of quartz sand is 300–350 μm) is studied. Among the 2D nanolayer coatings on titanium and steel substrates, a composition of VT1-0 alloy with a TiN/CrN coating at a nanolayer thickness of 60–70 nm and a total thickness of 19 μm has the maximum erosion resistance. The erosion resistance of the TiN/CrN coating is shown to decrease with decreasing nanolayer thickness, and it has a high thermal stability after holding at 700°C for 100 h.     

Magnetic Microstructures of 2：17 Type Sm（Co,Fe,Cu,Zr）z Magnets Detected by Magnetic Force Microscopy

The magnetic microstructures of 2：17 type Sm （Co, Fe, Cu, Zr）z magnets were detected by magnetic force microscopy. Comparing the microstructures of the specimens eoated with and without Ta thin film before and after heat-treatment, it is found that： （a） as a protection layer, Ta coating layer about 20 nm thick can effectively restrain Sm volatilization under high temperature; （b） the stress built in the 2.17 type Sm-Co magnets during specimen preparation only affects some local parts of the domain structures; （c） the magnetic microstructures vary largely for specimens heat-treated at high temperature without Ta film coating due to Sm volatilization. In addition, by comparing with high coercivity Fe-Pt point tips, it is found that the Co-Cr thin-film tips are not suitable for detecting the magnetic microstructures of strong permanent magnets.     

Ni对热变形Nd-Fe-B磁体磁性能及力学性能的影响

为提高热压结合热变形工艺制备的各向异性Nd-Fe-B磁体的力学性能,向磁体中掺杂高熔点、弱磁性金属元素Ni,研究Ni含量对磁体的磁性能、力学性能以及微观结构的影响。掺杂Ni的质量分数在0~5%之间时,Nd-Fe-B复合磁体的抗弯强度先增大后减小,2%Ni含量的Nd-Fe-B复合磁体具有最高平均抗弯强度212 MPa,其最大磁能积保持在40 MGOe以上。从磁体断口形貌上看,Ni会在孔洞附近富集,强化增韧晶界相,从而提高晶界相的裂纹扩展阻力,使抗弯强度提高。     

Spark Plasma Sintering Nd-Fe-B Permanent Magnets With Good All-Around Property

In present study, sintered Nd-Fe-B permanent magnets with different compositions were fabricated by using both Spark Plasma Sintering (SPS) technique and conventional sintering technique. Microstructure and compositions of both magnets are observed by scanning electron microscope with energy dispersive X-ray detector. Magnetic properties, mechanical properties, and chemical stabilities of both Nd-Fe-B magnets are investigated. Compared with the conventional sintered magnets, SPS Nd-Fe-B magnets possess comparable magnetic properties, better corrosion resistance and mechanical properties. Further investigation shows that the good all-around properties of the SPS magnets result from their unique microstructure. In detail, the grain size of Nd₂Fe₁₄B main phase is fine and uniform, only a few Nd-rich phase forms along the grain boundaries of Nd₂Fe₁₄B, while most of them agglomerates into the triple junctions. As a result, SPS process is expected to be a promising method for the production of new Nd-Fe-B magnets with good all-around properties.     

Structure Analysis of Sm2(Fe,Mn)17NX Magnet

In order to understand the coercivity mechanism of Sm₂(Fe, Mn)₁₇N_x magnet, the structure of amorphous phase found in the crystal phase was clarified using a TEM-tomography. It was found that amorphous phases precipitated as a fine wire in the crystal phase that differs with the phase separation in Sm-Co magnets. It may say that different coercivity mechanism should be proposed in each Sm₂(Fe, Mn)₁₇N_x and Sm-Co magnets.     

The performance of bonded magnets used in the treatment of anterior open bite

This investigation examined the magnetic flux and repulsive force of neodymium-iron-boron (Nd-Fe-B) magnets based on Nd2Fe14B with acrylic coatings in different orientations. The flux was measured with a Hall probe and the force measured by electronic scales with the magnets mounted on a laboratory jack. Results show that there were no magnetic losses after embedding the magnets in acrylic bite-blocks, although there were significant flux losses when the magnet blocks were directly heated. The alignment of the magnets over each other was of the utmost importance and significantly affected the repulsive force between the magnets. When the magnets were mounted in an articulator to simulate jaw movement, the force levels between the magnets was further reduced as a result of the effects of the rotation of the articulator. It is concluded that Nd-Fe-B type magnets embedded in acrylic can be used to give predictable repulsive forces in the mouth. The dramatic reduction in force levels when the magnets are not in optimal alignment, however, makes their effectiveness in aiding intrusion of teeth doubtful.     

New types of coating systems for steel sheets by high‐rate evaporation in combination with plasma processes

High‐rate evaporation in combination with plasma processes is a promising approach to obtain new types of steel sheet coating with improved corrosion resistance and application properties. To estimate the potential for the application of PVD‐coatings (physical vapour deposition) different coating systems for steel sheet as well as for hot‐dip or electro‐galvanized steel sheet were designed. The samples were produced on a laboratory scale using PVD processes with very high deposition rates (in the order of 1 μm s^‐1) as well as high‐power plasma processes for the pre‐treatment. The relationship between the composition, microstructure and properties of the coating systems, in particular concerning corrosion protection, abrasion during forming, phosphating and paint adhesion, were studied. It was found that the corrosion resistance of galvanized steel sheets can be considerably improved by vapour deposition of metal or inorganic films with a thickness of several hundred nanometers. Investigations on vapour deposition of titanium and stainless steel coatings on steel sheets, for applications in a severely corrosive environment, showed that the corrosion resistance in relation to the coating thickness can be significantly enhanced by means of plasma activation during the vapour deposition process. Finally, an outlook on possible industrial applications including an estimation of the process costs will be presented. For certain coating systems the results look promising. Consequently, these particular coating systems will be investigated in more detail by means of using a large‐scale in‐line deposition plant for metallic strips and sheets.     

Effects of grain boundary diffusion process on magnetic properties enhancement and microstructure evolution of hot-deformed Nd-Fe-B magnets

Grain boundary diffusion(GBD) process is an important approach for producing Nd-Fe-B magnets with high coercivity and high thermal stability.The GBD for hot-deformed Nd-Fe-B magnets with nanocrystalline micro structure is more complicated compared to sintered magnets.Here,we investigated the effects of different GBD methods,i.e.,intergranular addition(in-situ GBD 1#),in-situ GBD from magnet surface during hot pressing and hot deformation(in-situ GBD 2#),and conventional GBD,on the magnetic prope...