Patent Publication Number: US-2005142022-A1

Title: Method for producing magnetostrictive element, sintering container and magnetostrictive element

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a method for producing a magnetostrictive element, sintering container used for the same method, and magnetostrictive element.  
      2. Description of the Related Art  
      Magnetostrictive elements have been used for various devices, e.g., linear actuators, vibrators, pressure torque sensors, vibration sensors and gyro sensors.  
      When a magnetostrictive element is used for a linear actuator, vibrator or the like, a driving force is generated by changing a magnetic field applied to change the dimensions of the magnetostrictive element. When a magnetostrictive element is used for a pressure torque sensor, vibration sensor, gyro sensor or the like, on the other hand, it detects permeability changing with dimensional changes of the magnetostrictive element caused by external pressure and thereby the pressure is detected.  
      These magnetostrictive elements are produced by compacting an alloy powder of given composition into a compact in a magnetic field and sintering it in an inert gas atmosphere (See, for example, Japanese Patent Laid-Open No. 2003-3203, Page  4 ).  
      During the sintering step, a compact to be sintered into a magnetostrictive element tends to be oxidized, and also discolored by radiation heat from the heater as the heat source for sintering. Therefore, a compact  1  is contained in a closed container  2 , as shown in  FIG. 7 , while it is sintered in a sintering furnace. The container  2  is provided with a setter  3  therein, on which the compact  1  of a stick shape is laid down while being sintered.  
      However, when a material containing a rare earth element (so-called super magnetostrictive material) is processed to produce a magnetostrictive element, the compact  1  is so active to cause problems of producing reaction products on the side surface  1   a  at which it comes into contact with the setter  3 .  
      The reaction products, when formed on the side surface  1   a , strain the compact  1  by a mechanical stress produced while it is sintered to contract, which causes uneven spacing of lattice planes in the body, thereby magnetic properties of the compact  1  and hence magnetostrictive element are deteriorated.  
      One of the attempts to solve these problems is to structure the setter  3  to support the compact  1  only by both ends, in order to decrease contact area between the compact  1  and setter  3 . This structure, however, causes thermal deformation of the compact  1  during the sintering step, and does not solve effectively the problems.  
     SUMMARY OF THE INVENTION  
      The present invention is developed to solve these technical problems. It is an object of the present invention to provide a method for producing a magnetostrictive element of higher performance. It is another object of the present invention to provide a container used for the same method.  
      The method of the present invention for producing a magnetostrictive element, developed to achieve the above objects, comprises the steps of compacting a starting material powder into a shape in a magnetic field to prepare a compact, and sintering the compact kept upright in a container.  
      During the sintering step, it is preferable to support the compact kept upright in the container by a supporting member that is not in contact with the compact when the compact contracts following sintering during the sintering step. This prevents the compact from reacting with the supporting member to produce a reaction product thereon.  
      The compact, which contains Tb, Dy and Fe, can be sintered into a magnetostrictive element.  
      Shape of the compact is not limited. It may be stick-shaped, for example.  
      For the method for producing a magnetostrictive element, the sintering container of the present invention, described below, is suitably used.  
      The sintering container of the present invention holds an object to be sintered into a magnetostrictive element. It comprises a container body having a basal plane and an opening, a freely detachable lid to cover the opening of the container, and a setter arranged in the container body. The setter is provided with a cavity. The object can be set in the cavity in such a way that the object extends along the direction in which a magnetostrictive element is driven after the object is sintered into the magnetostrictive element. The object, when longitudinally shaped, can be set in the cavity in such a way that its major axis extends almost vertically. Thickness of the setter can be designed to be almost same as the longitudinal length of the object to be sintered.  
      The setter cavity can be directed perpendicular to the basal plane of the container body.  
      It is also preferable that the setter is made of a material which reacts with the object to be sintered at temperature higher than temperature at which the object contracts as a result of sintering. This makes the object non-contacting with the cavity surface when it contracts as the sintering proceeds, allowing the object kept upright while coming into contact with the setter or container body only at the basal plane. Therefore, the object is not in contact with the cavity surface when temperature reaches a level at which the sintering reaction occurs, preventing formation of a reaction product by the sintering reaction between them.  
      The cavity may be a closed or through hole so long as it is concave upward to hold the object to be sintered. When it is a through-hole, the object to be sintered stands on the basal plane of the container body.  
      When the object to be sintered has a composition represented by Formula (2) Tb a Dy (1-a) T y  (wherein, 0.27&lt;a≦0.50, T is one or more transition metal elements, and 1&lt;y&lt;4), it is preferable that the setter is made of a material which reacts with the object to be sintered at temperature higher than temperature at which the object contracts as a result of sintering. Dy 2 O 3  is a suitable material for the setter, because it is inert to the object to be sintered. The setter may be totally made of Dy 2 O 3 , or partly only for the cavity portion which may be in contact with the object to be sintered.  
      In the magnetostrictive element produced by employing the above method and the above sintering container, the spacing of lattice planes is almost uniform, because formation of a product by its reaction with the setter is prevented.  
      The magnetostrictive element of the present invention can be regarded as the one comprising a sintered body having a composition represented by Formula (1) RT y  (wherein, R represents one or more rare earth elements (providing that the rare earth elements include Y), T represents one or more transition metal elements, and 1&lt;y&lt;4), spacing of lattice planes in the magnetostrictive element as-sintered, is almost uniform. T may be at least one of Fe, Co and Ni.  
      The magnetostrictive element may be also regarded as the one, wherein in the [222] orientation in the X-ray intensity distribution, a half width of the magnetostrictive element is between 0.05 and 0.70.  
      When the magnetostrictive element of the present invention is composed of a sintered body having a composition represented by Formula (2) Tb a Dy (1-a) T y  (wherein, 0.27&lt;a≦0.50, T is one or more transition metal elements and 1&lt;y&lt;4), it may be regarded as the one having a half width of 0.05 to 0.70, inclusive, in the [222] orientation in the X-ray intensity distribution. It can exhibit a magnetostrictive value of 1100 ppm or more in a magnetic field of 1 kOe.  
      Fe is a preferable choice as T. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an oblique view showing an exterior of a sintering container in one embodiment of the present invention;  
       FIG. 2A  is a cross-sectional view of a sintering container, holding compacts, and  FIG. 2B  is a top view of a setter arranged in the container body of a sintering container;  
       FIG. 3  is a cross-sectional view showing that compacts contract in a sintering container;  
       FIG. 4  shows the degree of orientation, determined by X-ray analysis, of the sintered body obtained;  
       FIG. 5  shows a relationship between half width in the X-ray intensity distribution and magnetostrictive value;  
       FIG. 6  shows a relationship between half width in the X-ray intensity distribution and magnetostrictive value; and  
       FIG. 7  is a cross-sectional view of a conventional sintering container. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention is described in detail, based on the embodiments illustrated in the attached drawings.  
      First, the method for producing a magnetostrictive element of one embodiment is described.  
      In this embodiment, an alloy powder having a composition represented by Formula (1) RT y  (wherein, R is one or more rare earth elements, T is one or more transition metal elements and 1&lt;y&lt;4) is sintered to prepare a magnetostrictive element.  
      R is at least one selected from rare earth elements of lanthanoids and actinoids, where the rare earth elements represents a concept including Y. Of these elements, Nd, Pr, Sm, Tb, Dy and Ho are particularly preferable as R, and Tb and Dy are more preferable. They may be used in combination. T is at least one selected from transition metal elements. Of these, transition metal elements of Fe, Co, Ni, Mn, Cr and Mo are particularly preferable as T, and Fe, Co and Ni are more preferable. Fe, Co and Ni may be used in combination.  
      In the alloy represented by the Formula (1) RT y , 1&lt;y&lt;4. RT 2  (RT y  with y=2) as a laves type intermetallic compound is suitable for a magnetostrictive element because of its high Curie temperature and high magnetostrictive value. When y is 1 or less, the R-rich phase deposits in the alloy during heat treatment which follows sintering and leads to decrease its magnetostrictive value. When y is 4 or more, on the other hand, the RT 3  or RT 5  phase increases also leads to decrease a magnetostrictive value of the alloy. It is therefore preferable to keep the relationship 1&lt;y&lt;4 to increase the RT 2  phase, more preferably 1.5≦y≦2.0, still more preferably 1.8≦y≦1.95.  
      R may be a mixture of rare earth elements, in particular, a mixture of Tb and Dy is preferable. In the alloy represented by Formula (3) Tb a Dy (1-a) , a further preferably satisfies the relationship 0.27&lt;a≦0.50. This ensures a high saturation magnetostrictive coefficient and hence a high magnetostrictive value of the alloy represented by Formula (2) Tb a Dy (1-a) T y . When a is 0.27 or less, the alloy may not have a sufficient magnetostrictive value at room temperature or lower. When a is above 0.50, on the other hand, the alloy may not have a sufficient magnetostrictive value at around room temperature. It is more preferably 0.27≦a≦0.40, still more preferably 0.28≦a≦0.34.  
      As T, particularly Fe is prefereable, which forms with Tb or Dy an intermetallic compound ((Tb, Dy) Fe 2 ) having a high magnetostrictive value and high other magnetostrictive properties. Fe may be partly substituted by Co or Ni. In this case, however, Fe preferably accounts for 70% by weight or more, more preferably 80% or more, because Co decreases permeability although increasing magnetic anisotropy, while Ni decreases Curie temperature to decrease magnetostrictive value at room temperature and in a high magnetic field.  
      The alloy powder as a starting material is preferably treated to absorb hydrogen partially or totally. The alloy powder, when absorbs hydrogen, produces a strain in the particles that constitute the powder, the resulting internal stress cracking the particles. The particles in the mixture are cracked into fractions, when exposed to pressure during the compacting step, and the resulting compact can be sintered more densely. Rare earth elements, e.g., Tb and Dy, are easily oxidated, and coated with an oxide film of high melting point in the presence of oxygen even in a trace quantity, to retard sintering. They are more resistance to oxidation, when absorb hydrogen. Therefore, the alloy powder can be sintered more densely, when treated to absorb hydrogen partially or totally.  
      The hydrogen-absorbing starting material preferably has a composition represented by Formula (4) Dy b T (1-b)  with b satisfying the relationship of 0.37≦b≦1.00. T may be Fe itself or Fe partly substituted by Co or Ni. This allows the starting alloy powder to be sintered more densely.  
      In this embodiment, the starting material powder is sintered in a mixed hydrogen/inert gas atmosphere with the relationship of hydrogen gas: argon (Ar) gas=X: 100−X (Formula 5) with X (vol %) satisfying  0 &lt;X&lt;50. The mixed hydrogen/inert gas atmosphere is provided in a heating-up period in a range of 650° C. or higher or/and in a stable temperature range of 1150 to 1230° C., inclusive.  
      For the alloy represented by Formula (1) RT y , the starting material powder is placed in the mixed hydrogen/inert gas atmosphere at least in a heating-up period at 650° C. or higher.  
      The compact of the starting material powder is heated at 3 to 20° C./minute in a furnace for sintering. At below 3° C./minute, productivity will go down. At above 20° C./minute, on the other hand, the compacted powder may not be heat-treated uniformly in the furnace to cause problems, e.g., segregation or production of a dissimilar phase. The reason to set the temperature range for the above mixed hydrogen/inert gas atmosphere at 650° C. or higher is to avoid oxidation of the rare earth element by oxygen remaining in a very small quantity.  
      It is preferable to sinter the compact for a period in which temperature is essentially kept at a constant level. The stable temperature is preferably in a range of 1150 to 1230° C. At below 1150° C., the sintering step needs a longer time to remove the internal strain and hence is not effective. At above 1230° C., on the other hand, which is near a melting point of the alloy represented by RT y , problems, e.g., melting of the sintered body or precipitation of another phase, e.g., RT 3  occur.  
      Moreover, it is preferable to carry out the sintering in a non-oxidative atmosphere, more specifically in a hydrogen gas atmosphere or mixed hydrogen/inert gas atmosphere with the relationship of hydrogen gas: argon (Ar) gas=X: 100−X (Formula 5) with X (vol %) satisfying  0 &lt;X&lt;50.  
      R readily reacts with oxygen to form a stable rare earth oxide, which little exhibits magnetic properties for a practical magnetic material. Oxygen, although present at a very low content in a sintering atmosphere, can greatly deteriorate magnetic properties of the sintered body prepared at high temperature. Therefore, heat treatment such as sintering is carried out preferably in a hydrogen-containing atmosphere. Oxidation is also controlled in an inert gas atmosphere, but an inert gas alone is difficult to completely remove oxygen, allowing it to react with a rare earth element, which is highly reactive with oxygen, to form the oxide. Therefore, the sintering atmosphere is preferably of a hydrogen/inert gas mixture to prevent the oxidation of rare earth element.  
      For a hydrogen gas-containing reducing atmosphere, X (vol %) preferably satisfies  0 &lt;X&lt;50 in Formula 5 of hydrogen gas: argon (Ar) gas=X: 100−X. An Ar gas is inert and does not oxidize R. Therefore, it can form a reducing atmosphere when mixed with hydrogen gas. In order to obtain a reducing atmosphere, X (vol %) preferably satisfies  0 &lt;X. At 50≦X, the reducing atmosphere is saturated. Therefore, X&lt;50 is preferable. It is preferable to keep a mixed hydrogen/Ar gas atmosphere during the heating-up step carried out at 650° C. or higher. It is more preferable to keep the mixed hydrogen/Ar gas atmosphere in the stable temperature range.  
      Flow of the magnetostrictive element production process will be described in detail below.  
      First, Tb, Dy and Fe are weighed and then melted in an inert gas atmosphere of Ar to prepare the alloy (hereinafter referred to as Starting Material A) as one of starting materials. Starting Material A has a composition of Tb 0.4 Dy 0.6 Fe 1.94 , for example. The Starting Materials A is annealed in order to make concentration distribution of the elements uniform and remove a dissimilar phase when it deposits, and is then milled by, e.g., an atomizer.  
      Then, Dy and Fe are weighed and then melted in an inert gas atmosphere of Ar to prepare the alloy (hereinafter referred to as Starting Material B) as one of starting materials. Starting Material B has a composition of Dy 2.0 Fe, for example. It is similarly milled by, e.g., an atomizer.  
      Fe, as one of starting materials, is reduced in a hydrogen gas atmosphere to remove oxygen, and is then milled by, e.g., an atomizer (hereinafter referred to as Starting Material C).  
      Starting Materials A, B and C are weighed, milled and mixed with each other to prepare the alloy powder (starting material powder) having a composition of Tb 0.3 Dy 0.7 Fe 1.88 , for example.  
      The alloy powder thus obtained is compacted in a mold in a magnetic field of a given intensity, e.g., 8 kOe, to produce the compact.  
      The compact is heated in a furnace to produce a sintered body, where temperature in the furnace is programmed to have a given profile. For example, it is sintered in a mixed hydrogen/argon gas atmosphere (35/65% by volume) in a stable temperature range of 1150 to 1230° C. to produce a sintered body.  
      The sintered body is aging-treated and then divided into pieces of given size to produce magnetostrictive elements.  
       FIGS. 1 and 2  structurally illustrate a sintering container  10  used in the method for producing a magnetostrictive element, where  FIG. 1  is an oblique view showing the container  10  exterior, and  FIG. 2A  is a cross-sectional view of the container  10  holding compacts  100  and  FIG. 2B  is a top view of a setter  20  arrnged in a container body  11  of the sintering container  10 .  
      In this embodiment, the compact  100  to be sintered into a magnetostrictive element is sintered while being held in the sintering container  10 .  
      As illustrated in  FIG. 1 , the container  10  comprises the container body  11  and a lid  12 .  
      As illustrated in  FIG. 2 , the setter (supporting member)  20  is arranged in the container body  11  to support the compact  100  during the sintering step.  
      The setter  20  is provided with a plurality of holes (cavities)  21  extending in the direction perpendicular to a basal plane of the container body  11 , that is, extending almost vertically. Each hole  21  has an inner diameter slightly larger than the outer diameter of the compact  100  before sintering. In this embodiment, thickness of the setter  20 , i.e., depth of the hole  21 , is designed to be almost same as the longitudinal length of the compact  100 .  
      It is preferable that the setter  20  is made of a material hardly reactive with the compact  100 . It is also preferable that the setter  20  is made of a material which reacts with the compact  100  at temperature (sintering reaction temperature) higher than temperature at which the compact  100  contracts as a result of sintering. These materials include CaO and Dy 2 O 3 , of which the latter is particularly preferable for the setter  20 .  
      A given number of the compacts  100  are set in the corresponding holes  21 , when they are sintered in the sintering container  10  provided with the setters  20 . This allows each of the compacts  100  to be supported by the corresponding setter  20  in such a way that its major axis extends almost vertically. In this position, the compact  100  comes into contact with the setter  20  only partly. Dy 2 O 3  constituting the setter  20  differs in linear expansion coefficient from the compact  100 . The hole diameter of the setter  20  is designed to be larger by around 1% than the outer diameter of the compact  100 .  
      The sintering container  10  is heated in a furnace, where temperature is programmed to have a given profile, to sinter each of the compacts  100  held in the sintering container  10  into a magnetostrictive element. The compact to be sintered into a magnetostrictive element can be designed to have dimensions of 2 to 20 mm in diameter and 20 to 40 mm in length.  
      The compact  100  contracts while being sintered in a furnace to form a gap between the contracted compact  100  and the holes  21 , as shown in  FIG. 3 , and leads to secure the compact  100  to be non-contacting with the setter  20 . Contraction of the compact  100  starts at, e.g., around 800 to 850° C. On the other hand, the sintering reaction of the compact  100  with the setter  20  starts at, e.g., around 1150° C. Therefore, they are kept away from each other at a temperature level at which the sintering reaction starts between them. Even when they are in contact with each other, extent is limited to a mere point contact.  
      As discussed above, each setter  20  in the sintering container  10  is provided with the holes  21  to keep each compact  100  kept upright. During sintering, this keeps the compact  100  and setter  20  away from each other at a temperature level at which the sintering reaction starts between them because of contraction of the compact  100 . This prevents deposition of the sintering reaction product on the compact  100  due to the sintering reaction between the compact  100  and setter  20 . There ensures that no mechanical stress (internal stress) is produced by the reaction product to act on the compact  100  and secures that the spacing of lattice planes in the sintered body obtained is almost uniform. Therefore, the resulting sintered body has better magnetic properties than the one sintered while being laid down. Moreover, the setter  20  can be simply structured only to have the holes  21 , producing a significant effect at a minimum cost.  
      In the above embodiment, depth of the hole  21  is set at almost the same as longitudinal length of the compact  100 . However, depth of the hole  21  may be shorter than length of the compact  100  to project the latter over the former. It is however preferable that they have dimensions in such a way that the compacts  100  are firmly held by the setters  20  not to fall down when the sintering container  10  is moved after the compacts  100  contract, e.g., after the sintering treatment is completed.  
      In this embodiment, each compact  100  is kept upright on the basal plane of the container body  11 , where each hole  21  in the setter  20  may not be a through-hole but a closed one.  
     EXAMPLE 1  
      In EXAMPLE 1, the compact was sintered in the sintering container  10  to confirm how nests were produced in the magnetostrictive element. The results are described below.  
      As for Starting Material A, Tb, Dy and Fe were weighed and then melted in an inert gas atmosphere of Ar to prepare the alloy having a composition Tb 0.4 Dy 0.6 Fe 1.94 . It was annealed to secure an even concentration distribution of each metal element of the prepared alloy and remove a dissimilar phase when it deposits, and was then milled by a Brown mill. Next, as for Starting Material B, Dy and Fe were weighed and then melted in an inert gas atmosphere of Ar to prepare the alloy having a composition Dy 2.0 Fe, which was treated in a hydrogen gas atmosphere at 150° C. for 1 hour to absorb hydrogen. Then, as for Starting Material C, Fe is reduction-treated in a hydrogen gas atmosphere to remove oxygen.  
      Starting Materials A, B and C were weighed, milled and mixed with each other by an atomizer to prepare the alloy powder having a composition of Tb 0.3 Dy 0.7 Fe 1.9 .  
      The alloy powder was compacted in a mold in a magnetic field of 8 kOe, to prepare the compact  100 . It had a stick shape, 7 mm in diameter and 100 mm in length.  
      The compacts  100  put in the sintering container  10  were heated in a mixed hydrogen/argon (35/65% by volume) atmosphere in a stable temperature range of 1150 to 1230° C. to prepare the sintered bodies.  
     EXAMPLE  
      Each of the compacts  100  was kept upright by the setter  20  in the sintering container  10  during the sintering process.  
     Comparative Example  
      Each of the compacts  100  was laid down (arranged horizontally) during the sintering process, following the conventional manner.  
      A total of 10 compacts  100  were sintered in each of EXAMPLE and COMPARATIVE EXAMPLE, to measure their magnetic properties (magnetostrictive value) for the sintered bodies obtained.  
      The results are given in FIGS.  4  to  6 .  
       FIG. 4  shows the measurement result of the X-ray-analyzed degree of orientation, and magnetostrictive value of each sintered body was evaluated in the [222] orientation shown in  FIG. 4 .  
       FIGS. 5 and 6  show the relationship between half width of the X-ray intensity distribution and magnetostrictive value, where the half width of the X-ray means a width of spread of the X-ray intensity peak in the [222] orientation at half of the peak height, and magnetostrictive value λ 1.0  means the value in a magnetic field of 1 kOe.  
      As shown in  FIG. 6 , the sintered bodies laid down in COMPARATIVE EXAMPLE had a half width of the X-ray intensity distribution in a range from 0.74 to 1.15, whereas those kept upright in EXAMPLE had a half width of the X-ray intensity distribution in a range from 0.35 to 0.70, which means that the sintered bodies prepared in EXAMPLE had a sharper X-ray intensity peak and hence the quantity of contaminants smaller than that of COMPARATIVE EXAMPLE.  
      It was also observed that the sintered bodies laid down in COMPARATIVE EXAMPLE had a magnetostrictive value of 1030 to 1122 ppm, whereas those kept upright in EXAMPLE had a value of 1132 to 1231 ppm, at least 10% higher.  
      As discussed above, it is apparent that keeping the compact  100  upright during the sintering process suppresses formation of a contaminant, i.e., product by the reaction between the compact  100  and setter  20 , and greatly improves magnetostrictive value of the sintered body.  
      The sintered bodies prepared in EXAMPLE and COMPARATIVE EXAMPLE were analyzed for element dispersion conditions by an electron probe X-ray micro analyzer (EPMA). The results indicate that the carbides and oxides are produced less and dispersed more uniformly in the sinter prepared in EXAMPLE than in the sintered body prepared in COMPARATIVE EXAMPLE, and that Dy and Fe are dispersed more uniformly in the sintered body prepared in EXAMPLE.  
      It is thus confirmed that keeping the compact  100  upright during the sintering process produces carbides and oxides less and gives a more uniform composition with Dy and Fe dispersed more uniformly.