Kaplesh Kumar

RETM5 and RE2TM17 permanent magnets development

RETM5 and RE2TM17 (RE=rare earth, TM=transition metal) permanent magnets are discussed in this review. The scientific and technological considerations which have guided their development are described. Along with a discussion of the various research efforts reported in the literature, they help establish the framework within which most of the work can be integrated and understood. In regard to the RETM5 alloys, the review limits itself to the technologically significant SmCo5 materials; others are discussed minimally, and only to the extent that they contributed to overall magnet development efforts. Likewise, from among the fabrication technologies, the review is limited to those processes which have been demonstrated as capable of producing magnets in bulk useful form. Areas covered in this review include phase diagrams, crystal structures, magnetocrystalline anisotropy, coercivity mechanisms, fabrication technologies, processing‐microstructure‐property interrelationships, and thermal effects related to...

High coercivity, isotropic plasma sprayed samarium‐cobalt magnets

A value of 67.5 kOe has been measured for the room temperature coercivity of an isotropic, heat treated, 1‐5 samarium‐cobalt plasma sprayed deposit. This value is believed to be the largest published, so far, for a permanent magnet. Sprayed magnets are generally found to be better than 93 percent dense with coercivities of 35 to 60 kOe. Deposits comprised of fine microstructures yield broad, unresolved x‐ray diffraction peaks. Low temperature crystallization of these deposits yields diffraction patterns representative of the high temperature equilibrium structure. The emergence of peaks, ascribed to SmCo5, from a low temperature 700 °C heat treatment implies both formation and stability of the SmCo5 phase at this temperature. These results dispute the existence of the generally accepted eutectoid decomposition reaction for SmCo5 at 750–800 °C. If such a reaction were present the products of crystallization should have included Sm2Co7 and not SmCo5. High temperature investigations on sprayed magnets indica...

Magnetic properties and microstructures of sprayed SmCo5 magnets exposed to intermediate temperatures

The hysteresis observed in Hci from cycling SmCo5 magnets at temperatures of 650–800 °C has been shown to result from an inability to completely redissolve the precipitated phase (Sm2Co17) at the higher temperature in the short time that has been allowed. It has, therefore, been concluded that the rate of Sm2Co17 precipitation is higher than that of dissolution at these temperatures. Metallographic observations made on plasma‐sprayed SmCo5 magnets (after exposing them for time periods of a few hours to several weeks at these temperatures) show that the precipitated phase exists as discrete second‐phase particles in the material unlike in sintered magnets where extensive Sm2Co17 precipitation is observed at grain boundaries. It is suggested that the striations observed in micrographs of SmCo5 magnets (after exposure to these temperatures) are the result of strain processes related to thermal‐expansion mismatch of neighboring grains.

Chemically homogeneous fine‐grained Mn‐Zn ferrites by spray drying

Chemically and structurally uniform Mn‐Zn ferrites have been produced using powders synthesized by the spray decomposition of mixed, aqueous Mn, Fe, and Zn nitrate solutions. The particle size of the as‐formed powder depended strongly on the metal ion concentration in solution; higher concentrations produced larger sizes. The as‐formed particles are spherical, internally hollow, and consisted mostly of Fe2O3. A 900 °C, 6‐h argon treatment removed the internal void space and converted the particles mostly to the ferrite spinel phase. Sintering of compacts from heat‐treated powders produced microstructures superior to those from as‐formed powders. Uniform, fine‐grained materials, with a densification level comparable to that of commercial sintered ferrite, have been produced at the very low sintering temperature of 1100 °C. Preliminary work indicates that a higher initial permeability is obtained when a higher sintering temperature was used and the level of its disaccommodation depended on the oxygen partia...

Aligned, plasma sprayed SmCo5 deposits

Highly aligned SmCo5 deposits were produced using plasma spraying. c‐axis alignment, normal to the plane of the deposit, was achieved by depositing the Sm‐Co alloys on steel substrates maintained at high temperatures. The substrates were heated by the plasma flame to obtain the high temperatures. The attainment of a range of substrate temperatures was made possible through control over the geometry of the substrate.

Reply to ’’Comment on ’High coercivity, isotropic plasma sprayed samarium‐cobalt magnets ’ ’’

The alternative interpretation of x‐ray results of Kumar et al. by Buschow and den Broeder, suggesting possible support for eutectoid decomposition of SmCo5, is questioned in view of the large body of published experimental evidence that clearly demonstrates that the SmCo5 phase must be stable at the low temperatures.

Chemically homogeneous fine‐grained Mn‐Zn ferrites by spray drying (abstract)

Chemically and structurally uniform Mn‐Zn ferrites have been produced using powders synthesized by the spray decomposition of mixed, aqueous Mn, Fe, and Zn nitrate solutions. The particle size of the as‐formed powder depended strongly on the metal ion concentration in solution; higher concentrations produced larger sizes. The as‐formed particles are spherical, internally hollow, and consisted mostly of Fe2O3. A 900 °C, 6‐h argon treatment removed the internal void space and converted the particles mostly to the ferrite spinel phase. Sintering of compacts from heat‐treated powders produced microstructures superior to those from as‐formed powders. Uniform, fine‐grained materials, with a densification level comparable to that of commercial sintered ferrite, have been produced at the very low sintering temperature of 1100 °C. Preliminary work indicates that a higher initial permeability is obtained when a higher sintering temperature was used and the level of its disaccommodation depended on the oxygen partial pressure present during sintering. Both ‘‘accommodation’’ and disaccommodation were observed in the permeability; their magnitudes depended on the peak value of the applied excitation field.Chemically and structurally uniform Mn‐Zn ferrites have been produced using powders synthesized by the spray decomposition of mixed, aqueous Mn, Fe, and Zn nitrate solutions. The particle size of the as‐formed powder depended strongly on the metal ion concentration in solution; higher concentrations produced larger sizes. The as‐formed particles are spherical, internally hollow, and consisted mostly of Fe2O3. A 900 °C, 6‐h argon treatment removed the internal void space and converted the particles mostly to the ferrite spinel phase. Sintering of compacts from heat‐treated powders produced microstructures superior to those from as‐formed powders. Uniform, fine‐grained materials, with a densification level comparable to that of commercial sintered ferrite, have been produced at the very low sintering temperature of 1100 °C. Preliminary work indicates that a higher initial permeability is obtained when a higher sintering temperature was used and the level of its disaccommodation depended on the oxygen partia...

Role of atmosphere in the crystallization of amorphous plasma‐sprayed Sm‐Co deposits

Thick, machinable, and amorphous 1Sm‐5Co plasma‐sprayed deposits show a high resistance to magnetization and demagnetization at room temperature. This behavior is believed to be related to large internal stresses and variations in chemical composition present in the as‐sprayed material. Thermal treatments of these deposits, in argon, at temperatures substantially lower than those required for crystallization, produce little change in their magnetic behavior. Similar treatments in hydrogen, however, result in an easy approach to saturation and very low coercivity values. This different response of the material to hydrogen has been attributed to an increased mobility of the atoms in hydrogen, which permits sufficient rearrangement of the atoms so that high magnetization and low coercivity are achieved in the amorphous state. Use of a hydrogen atmosphere also results in a considerable increase in the temperature required for crystallization. Whereas crystallization occurs at about 500 °C in argon, temperatur...

Electromagnetic Containerless Reaction of Samarium with Cobalt for the Formation of Samarium-Cobalt Alloys

The electromagnetic levitation technique has been used to obtain nearly stoichiometric SmCo5, with the reaction temperature controlled by a gas jet. The results of several experiments carried out at a 450 kHz, 25 kw RF power levitation facility using different reaction times and cooling rates are presented. It is shown that reaction rates achieved with the levitation technique are larger than the expected diffusion rate in the system liquid samarium-solid cobalt. It is also shown that substantial mixing occurs in the RF-levitated melt.

Homogeneous fine‐grained Mn‐Zn ferrites by sintering of colloidally precipitated powders

Colloidally precipitated powders were investigated for the synthesis of chemically homogeneous, uniformly fine‐grained Mn‐Zn ferrites. The process consists of the atomized dispersion of an aqueous solution of Mn, Zn, and Fe nitrates in an agitated solution of 1M ammonium hydroxide. This results in the coprecipitation of a fine colloidal mixture of Mn, Zn, and Fe hydroxides. The mixture is filtered, washed, dried, and heat treated to form agglomerated powders. The agglomerated powders are reduced in size and sintered after cold compaction. Sintering was performed at temperatures of 1100–1250 °C with oxygen partial pressures from 0 to 500 ppm in argon. Magnetic permeability and disaccommodation were measured on selected samples after an initial 0.5–5.0 Oe decaying sinusoidal field was applied to the specimens to excite time varying permeability. The permeability variation depended strongly on the magnitude of this excitation; both disaccommodation and accommodation were observed. The duplex structure typica...