Patent Publication Number: US-2022223346-A1

Title: Dielectric material, ceramic electronic device, manufacturing method of dielectric material, and manufacturing method of ceramic electronic device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2021-003742, filed on Jan. 13, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     A certain aspect of the present disclosure relates to a dielectric material, a ceramic electronic device, a manufacturing method of a dielectric material, and a manufacturing method of a ceramic electronic device. 
     BACKGROUND 
     Ceramic electronic devices such as multilayer ceramic capacitors are being used in devices for high frequency communication such as mobile phones, in order to remove noise. The devices need ceramic electronic devices having a small size (a thin size) and a large capacity. Ceramic electronic devices having a high rated voltage and high reliability are necessary in devices mounted on a vehicle. In order to meet the needs, a highly reliable dielectric material securing reliability even if electric field load per a single dielectric layer increases is necessary. And so, there is disclosed a technology in which a rare earth element is added to a perovskite (for example, see International Publication 2008/105240). An effect of Eu acting as a rare earth element is disclosed (for example, see Japanese Patent Application Publication No. 2016-169130). 
     SUMMARY OF THE INVENTION 
     However, since the load on the dielectric materials increases, the reliability is not sufficient. Means for further improving the reliability are requested. 
     According to a first aspect of the embodiments, there is provided a dielectric material including: a perovskite as a main phase, an A site of the perovskite including at least Ba, a B site of the perovskite including at least Ti; and Eu having +2 valence and +3 valence, wherein a ratio of +2 valence of Eu is 21% or more. 
     According to a second aspect of the embodiments, there is provided a ceramic electronic device including: a multilayer structure in which a plurality of dielectric layers and a plurality of internal electrode layers are alternately stacked, wherein a material of the plurality of dielectric layers is the dielectric material. 
     According to a third aspect of the embodiments, there is provided a manufacturing method of a dielectric material including: firing a dielectric material in which Eu is added to powder of a perovskite of which an A site includes at least Ba and of which a B site includes at least Ti; and annealing a fired dielectric material at a temperature lower than a temperature of the firing process and at an oxygen partial pressure lower than an oxygen partial pressure of the firing process, wherein a condition of the annealing process is adjusted so that a ratio of +2 valence of Eu is 21% or more. 
     According to a fourth aspect of the embodiments, there is provided a manufacturing method of a ceramic electronic device including: firing a multilayer structure in which a sheet of a dielectric material and a pattern of metal conductive paste are stacked, Eu being added to powder of a perovskite of the dielectric material, an A site of the perovskite including at least Ba, a B site of the perovskite including at least Ti; and annealing a fired multilayer structure at a temperature lower than a temperature of the firing and at an oxygen partial pressure lower than an oxygen partial pressure of the firing, wherein a condition of the annealing process is adjusted so that a ratio of +2 valence of Eu is 21% or more. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional perspective view of a multilayer ceramic capacitor; 
         FIG. 2  is a cross-sectional view taken along line A-A in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along line B-B in  FIG. 1 ; 
         FIG. 4  is a flowchart of a method of manufacturing a multilayer ceramic capacitor; 
         FIG. 5  illustrates XANES spectrum near L 3  absorption edge of Eu with respect to an example 2; 
         FIG. 6  illustrates XANES spectrum near L 3  absorption edge of Eu with respect to an example 5; and 
         FIG. 7  illustrates a relationship between a ratio of +2 valence of Eu and a lifetime. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an exemplary embodiment will be described with reference to the accompanying drawings. 
     Exemplary Embodiment 
       FIG. 1  illustrates a perspective view of a multilayer ceramic capacitor  100  in accordance with an embodiment, in which a cross section of a part of the multilayer ceramic capacitor  100  is illustrated.  FIG. 2  is a cross-sectional view taken along line A-A in  FIG. 1 .  FIG. 3  is a cross-sectional view taken along line B-B in  FIG. 1 . As illustrated in  FIG. 1  to  FIG. 3 , the multilayer ceramic capacitor  100  includes a multilayer chip  10  having a rectangular parallelepiped shape, and external electrodes  20   a  and  20   b  that are respectively provided on two edge faces of the multilayer chip  10  facing each other. Among four faces other than the two edge faces of the multilayer chip  10 , two faces other than the top face and the bottom face in the stack direction are referred to as side faces. Each of the external electrodes  20   a  and  20   b  extends to the top face and the bottom face in the stack direction and the two side faces of the multilayer chip  10 . However, the external electrodes  20   a  and  20   b  are spaced from each other. 
     The multilayer chip  10  has a structure designed to have dielectric layers  11  and internal electrode layers  12  alternately stacked. The dielectric layer  11  contains a ceramic material acting as a dielectric material. End edges of the internal electrode layers  12  are alternately exposed to a first edge face of the multilayer chip  10  and a second edge face of the multilayer chip  10  that is different from the first edge face. The external electrode  20   a  is provided on the first edge face. The external electrode  20   b  is provided on the second edge face. Thus, the internal electrode layers  12  are alternately electrically connected to the external electrode  20   a  and the external electrode  20   b . Accordingly, the multilayer ceramic capacitor  100  has a structure in which a plurality of the dielectric layers  11  is stacked with the internal electrode layers  12  interposed therebetween. In the multilayer structure of the dielectric layers  11  and the internal electrode layers  12 , the outermost layers in the stack direction are the internal electrode layers  12 , and cover layers  13  cover the top face and the bottom face of the multilayer structure. The cover layer  13  is mainly composed of a ceramic material. For example, the main component of the cover layer  13  is the same as the main component of the dielectric layer  11 . 
     For example, the multilayer ceramic capacitor  100  may have a length of 0.25 mm, a width of 0.125 mm, and a height of 0.125 mm. The multilayer ceramic capacitor  100  may have a length of 0.4 mm, a width of 0.2 mm, and a height of 0.2 mm. The multilayer ceramic capacitor  100  may have a length of 0.6 mm, a width of 0.3 mm, and a height of 0.3 mm. The multilayer ceramic capacitor  100  may have a length of 1.0 mm, a width of 0.5 mm, and a height of 0.5 mm. The multilayer ceramic capacitor  100  may have a length of 3.2 mm, a width of 1.6 mm, and a height of 1.6 mm. The multilayer ceramic capacitor  100  may have a length of 4.5 mm, a width of 3.2 mm, and a height of 2.5 mm. However, the size of the multilayer ceramic capacitor  100  is not limited to the above sizes. 
     The internal electrode layer  12  is mainly composed of a base metal such as nickel (Ni), copper (Cu), or tin (Sn). The internal electrode layer  12  may be composed of a noble metal such as platinum (Pt), palladium (Pd), silver (Ag), or gold (Au) or alloy including one or more of them. 
     The dielectric layer  11  is mainly composed of a ceramic material having a perovskite structure expressed by a general formula ABO 3 . The perovskite structure includes ABO 3 -a having an off-stoichiometric composition. In the embodiment, an A site of the ceramic material includes at least Ba (barium), and a B site of the ceramic material includes at least Ti (titanium). The B site may include Zr (zirconium). 
     As illustrated in  FIG. 2 , the section where the internal electrode layer  12  connected to the external electrode  20   a  faces the internal electrode layer  12  connected to the external electrode  20   b  is a section where capacity is generated in the multilayer ceramic capacitor  100 . Thus, this section is referred to as a capacity section  14 . That is, the capacity section  14  is a section where two adjacent internal electrode layers  12  connected to different external electrodes face each other. 
     The section where the internal electrode layers  12  connected to the external electrode  20   a  face each other with no internal electrode layer  12  connected to the external electrode  20   b  interposed therebetween is referred to as an end margin  15 . The section where the internal electrode layers  12  connected to the external electrode  20   b  face each other with no internal electrode layer  12  connected to the external electrode  20   a  interposed therebetween is also the end margin  15 . That is, the end margin  15  is a section where the internal electrode layers  12  connected to one of the external electrodes face each other with no internal electrode layer  12  connected to the other of the external electrodes interposed therebetween. The end margin  15  is a section where no capacity is generated. 
     As illustrated in  FIG. 3 , in the multilayer chip  10 , the section from each of the two side faces of the multilayer chip  10  to the internal electrode layers  12  is referred to as a side margin  16 . That is, the side margin  16  is a section that covers each of the edges, extending toward the respective side faces of the multilayer structure, of the stacked internal electrode layers  12 . The side margin  16  is a section where no capacity is generated. 
     It is thought that a rare earth element is added to the dielectric layers  11  in order to improve the reliability of the dielectric layers  11 . However, the rare earth element such as Ho (holmium), Dy (dysprosium), Y (yttrium), or the like tends to be solid-solved into the A site more than into the B site. When a large amount of the rare earth element is solid-solved into the B site, the amount of acceptors is excessive. This causes oxygen vacancy causing degradation of lifetime. Therefore, the improvement of lifetime is limited. 
     Therefore, the present inventors have studied a rare earth element having a large ionic radius which is easily solid-solved into the A site of BaTiO 3 . The present inventors have found that Eu (europium) improves the high temperature load lifetime of the dielectric layers  11  more than other rare earth element such as Ho, Dy, Y or the like, by one digit or so. 
     Normally, the rare earth elements have a valence of +3. As a rare earth element of +3 valence, Eu is also used for modifying the perovskite. However, Eu has a special characteristic in a point that Eu has another stable valence of +2, in addition to +3. The present inventors have found that when the amount of Eu having the +2 valence, the high temperature load lifetime is remarkably improved. The factor of the effect of improving the lifetime is not solved yet. However, the Eu ion of +2 valence has the largest ionic radius in the rare earth elements which can be solid-solved into the A site. It is therefore thought that, with high possibility, the Eu ions largely distort the perovskite lattice and reduces the energy (activation energy) necessary for movement of the oxygen vacancy which may be a main factor of lifetime degradation. Actually, when the rare earth elements other Eu which are not capable of having the stable+2 valence are applied to the embodiment, the lifetime is not improved. 
     Table 1 shows ionic radiuses of rare earth elements of which a coordination number is 6. Exhibition of Table 1 is “R. D. Shannon, Acta Crystallogr., A32, 751(1976)”. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 IONIC RADIUS(Å) 
                   
               
            
           
           
               
               
               
               
            
               
                   
                   
                 COORDINATION 
                 COORDINATION 
               
               
                   
                 VALENCE 
                 NUMBER IS 6 
                 NUMBER IS 12 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Ba 
                 +2 
                   
                 1.610 
               
               
                 Ti 
                 +4 
                 0.605 
               
               
                 Eu 
                 +2 
                 1.170 
               
               
                 La 
                 +3 
                 1.032 
               
               
                 Tm 
                 +2 
                 1.030 
               
               
                 Yb 
                 +2 
                 1.020 
               
               
                 Ce 
                 +3 
                 1.010 
               
               
                 Pr 
                 +3 
                 0.990 
               
               
                 Nd 
                 +3 
                 0.983 
               
               
                 Pm 
                 +3 
                 0.970 
               
               
                 Sm 
                 +3 
                 0.958 
               
               
                 Eu 
                 +3 
                 0.947 
               
               
                 Gd 
                 +3 
                 0.938 
               
               
                 Tb 
                 +3 
                 0.923 
               
               
                 Dy 
                 +3 
                 0.912 
               
               
                 Ho 
                 +3 
                 0.901 
               
               
                 Y 
                 +3 
                 0.900 
               
               
                 Er 
                 +3 
                 0.890 
               
               
                 Tm 
                 +3 
                 0.880 
               
               
                 Yb 
                 +3 
                 0.868 
               
               
                 Lu 
                 +3 
                 0.861 
               
               
                 Sc 
                 +3 
                 0.745 
               
               
                   
               
            
           
         
       
     
     The present inventors have found that when an amount of Eu having +2 valence in the all of Eu added to the dielectric layers  11  is 21% or more, the lifetime of the dielectric layers  11  is improved and the reliability of the dielectric layers  11  is improved. It is thought that a large amount of Eu can be solid-solved into the A site. 
     From a viewpoint of improving the reliability of the dielectric layers  11 , it is preferable that the ratio of Eu having +2 valence in all of Eu added to the dielectric layers  11  (the ratio of +2 valence) is 21% or more. It is more preferable that the ratio is 26% or more. 
     It is favorable to reduce a large amount of Eu having +3 valence into Eu having +2 valence, in order to increase the ratio of +2 valence. However, grain growth may occur in the dielectric layers  11  during reducing Eu having +3 valence into Eu having +2 valence in an annealing process for reducing Eu. When the grain growth occurs in the dielectric layers  11 , the lifetime of the dielectric layers  11  may be degraded. Therefore, when the grain growth occurs in the dielectric layers  11 , the effect of shortening the lifetime caused by the grain growth may cancel the effect of improving the lifetime caused by the +2 valence of Eu. The grain growth may crush the structure of the internal electrode layers  12 . This may cause short. It is therefore preferable that the ratio of +2 valence of Eu has an upper limit. For example, it is preferable that, in the dielectric layers  11 , the ratio of +2 valence of Eu is 80% or less. It is more preferable that the ratio is 70% or less. It is still more preferable that the ratio is 59% or less. 
     When the A site of the perovskite forming the main phase of the dielectric layers  11  has vacancies, a sufficient amount of vacancy is formed in the A site. And, Eu having a large ionic radius and +2 valence is easily solid-solved into the A site. It is therefore preferable that the A site has vacancies. For example, it is preferable that an atomic concentration ratio of the A site/the B site in the perovskite forming the main phase of the dielectric layers  11  is 0.980 or less. It is more preferable that the atomic concentration ratio is 0.970 or less. It is still more preferable that the atomic concentration ratio is 0.960 or less. When the A site element is only Ba and the B site element is only Ti, the atomic concentration ratio of the A site/the B site is the atomic concentration ratio of Ba/Ti. When the A site element is only Ba and the B site element is only Ti and Zr, the atomic concentration ratio of the A site element/the B site element is the atomic concentration ratio of Ba/(Ti+Zr). 
     When the atomic concentration ratio of the A site/the B site is excessively small, the dielectric constant of the dielectric layers  11  may be reduced because the tetragonal characteristic is degraded. It is preferable that the atomic concentration ratio of the A site/the B site has a lower limit. It is preferable that the atomic concentration ratio of the A site/the B site in the perovskite forming the main phase of the dielectric layers  11  is 0.920 or more. It is more preferable that the atomic concentration ratio is 0.940 or more. It is still more preferable that the atomic concentration ratio is 0.950 or more. 
     The perovskite forming the main phase of the dielectric layers  11  can maintain insulation characteristic when the B site includes Zr. From a viewpoint of achieving sufficient effect of maintaining insulation characteristic, it is preferable that the Zr/Ti ratio (atomic concentration ratio) has a lower limit. It is preferable that the Zr/Ti ratio is 0.01 or more. It is more preferable that the Zr/Ti ratio is 0.02 or more. It is still more preferable that the Zr/Ti ratio is 0.04 or more. 
     When the amount of Zr is excessively large in the case where the B site of the perovskite forming the main phase of the dielectric layers  11  includes Zr, grain growth may occur during the firing of the dielectric layers  11 . And long lifetime may not be necessarily achieved. It is preferable that the Zr/Ti ratio has an upper limit. For example, it is preferable that the Zr/Ti ratio is 0.14 or less. It is more preferable that the Zr/Ti ratio is 0.10 or less. It is still more preferable that the Zr/Ti ratio is 0.08 or less. 
     When the amount of Eu added to the dielectric layers  11  is excessively small, sufficient lifetime may not be necessarily achieved. And so, it is preferable that the amount of Eu added to the dielectric layers  11  has a lower limit. For example, it is preferable that an atomic concentration ratio of Eu with respect to the amount of the B site element (Eu/B site) in the dielectric layers  11  is 0.001 or more. It is more preferable that the atomic concentration ratio is 0.005 or more. It is still more preferable that the atomic concentration ratio is 0.010 or more. When the B site element is only Ti, the atomic concentration ratio (Eu/B site) means Eu/Ti. When the B site element is Ti and Zr, the atomic concentration ratio (Eu/B site) means the atomic concentration ratio Eu/(Ti+Zr). 
     When the amount of Eu is excessively large in the dielectric layers  11 , the insulation characteristic may be degraded and sufficient lifetime may not be necessarily achieved. It is therefore preferable that the amount of Eu added to the dielectric layers  11  has an upper limit. For example, it is preferable that the atomic concentration ratio of Eu/B site is 0.100 or less. It is more preferable that the atomic concentration ratio is 0.060 or less. It is still more preferable that the atomic concentration ratio is 0.040 or less. 
     When the amount of rare earth elements other than Eu is excessively large in the dielectric layers  11 , the effect of improving the lifetime gets smaller. And, sufficient lifetime may not be necessarily achieved. It is therefore preferable that the amount of the rare earth elements other than Eu has an upper limit. In concrete, it is preferable that the amount of the rare earth element other than Eu is smaller than the amount of Eu. When the number of the types of the rare earth element other than Eu is two or more, it is preferable that the total amount of the rare earth elements is smaller than the amount of Eu. 
     Next, the manufacturing method of the multilayer ceramic capacitor  100  will be described.  FIG. 4  is a flowchart of the manufacturing method of the multilayer ceramic capacitor  100 . 
     [Making of Raw Material Powder] 
     A dielectric material for forming the dielectric layer  11  is prepared. The A site element and the B site element contained in the dielectric layer  11  are contained in the dielectric layer  11  typically in the form of a sintered compact of ABO 3  particles. For example, BaTiO 3  is a tetragonal compound having a perovskite structure, and exhibits high dielectric constant. This BaTiO 3  can be obtained typically by reacting a titanium raw material such as titanium dioxide with a barium raw material such as barium carbonate to synthesize barium titanate. Various methods have been known as a synthesizing method of ceramic constituting the dielectric layer  11 . For example, the solid phase method, the sol-gel method, the hydrothermal method, and the like are known. Any one of the above methods can be employed in the present embodiment. 
     Additive compound is added to the resulting ceramic powder in accordance with purposes. The additive compound may be an oxide of Zr (zirconium), Mg (magnesium), Mn (manganese), V (vanadium), Cr (chromium) or Eu (europium), or an oxide of Co (cobalt), Ni (nickel), Li (lithium), B (boron), Na (sodium), K (potassium) or Si (silicon), or glass. If necessary, an oxide of a rare earth element other than Eu. The rare earth element is such as Sc (scandium), Y (yttrium), La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium), Sm (samarium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), or Lu (lutetium). 
     For example, a compound including an additive is wet-blended with the resulting ceramic material powder. The resulting ceramic material powder is dried and crushed. For example, the resulting ceramic material is crushed if necessary. Thus, a particle diameter is adjusted. Alternatively, the particle diameter may be further adjusted by a classification process. Thus, a dielectric material is obtained. 
     [Stacking Process] 
     Next, a binder such as polyvinyl butyral (PVB) resin, an organic solvent such as ethanol or toluene, and a plasticizer are added to the resulting dielectric material and wet-blended. With use of the resulting slurry, a strip-shaped dielectric green sheet with a thickness of, for example, 0.5 μm or more is painted on a base material by, for example, a die coater method or a doctor blade method, and then dried. 
     Next, an internal electrode layer pattern is formed on the surface of the dielectric green sheet by printing a metal conductive paste for forming the internal electrode with use of screen printing or gravure printing. The metal conductive paste for forming the internal electrode contains an organic binder. A plurality of internal electrode layer patterns are alternately exposed to a pair of external electrodes. Ceramic particles are added as a co-material to the metal conductive paste. The main component of the ceramic particles is not particularly limited, but is preferably the same as the main component ceramic of the dielectric layer  11 . For example, BaTiO 3  of which an average grain diameter is 50 nm or less may be evenly dispersed. 
     After that, the dielectric green sheet on which the internal electrode layer pattern is printed is stamped into a predetermined size, and a predetermined number (for example, 100 to 1000) of stamped dielectric green sheets are stacked while the base material is peeled so that the internal electrode layers  12  and the dielectric layers  11  are alternated with each other and the end edges of the internal electrode layers  12  are alternately exposed to both edge faces in the length direction of the dielectric layer so as to be alternately led out to a pair of external electrodes of different polarizations. Cover sheets to be the cover layers  13  are clamped on the upper face and the lower face of the stacked dielectric green sheets in the stacking direction. And, the resulting multilayer structure is cut into a predetermined chip size (for example, 1.0 mm×0.5 mm). 
     [Firing Process] 
     The binder is removed from the resulting ceramic multilayer structure in N 2  atmosphere. After that, Ni paste to be the base layer of the external electrodes  20   a  and  20   b  is painted by a dipping method. The resulting ceramic multilayer structure is fired in a reducing atmosphere with an oxygen partial pressure of 10 −12  to 10 −9  atm in a temperature range of 1160° C. to 1280° C. for 5 minutes to 10 minutes. 
     [Annealing Process] 
     After that, an annealing process is performed at a temperature which is lower than that of the firing process and in an atmosphere which is more reductive than that of the firing process. For example, the annealing process is performed at an oxygen pressure of 10 −14  MPa to 10 −13  MPa, in a reductive atmosphere of 1100 degrees C. to 1150 degrees C., for 1 hour to 2 hours. It is preferable that the temperature of the annealing process is lower than the maximum temperature of the firing process by approximately 50 degrees C. 
     When the annealing process of which the atmosphere is more reductive than that of the firing process is performed, it is possible to reduce Eu of +2 valence into +3 valence. The condition of the annealing process may be adjusted so that the ratio of +2 valence becomes 21% or more. 
     During the firing process, it is possible to reduce Eu in the strong reductive atmosphere like the annealing process. However, in this case, the barium titanate acting as the main phase of the dielectric layer  11  may be reduced. And, a large amount of oxygen vacancy and a large amount of electron may be generated. The oxygen vacancy is a main factor of shortening the lifetime of the dielectric layer  11 . Therefore, the reliability may be further degraded. Actually, when the atmosphere which sufficiently reduces Eu into +2 valence is used, the barium titanate itself is strongly reduced. In this case, the barium titanate may become an n-type semiconductor. When the barium titanate is subjected to a high temperature in the strong reductive atmosphere for a long time, the grains of the barium titanate grow. In this case, the internal electrode layers  12  may be broken. And, the capacity may be degraded. It may be possible to achieve the resistance to reduction of the barium titanate and reduction of Eu, by strictly designing the condition of the high temperature firing and controlling the furnace atmosphere with high accuracy. However, the process is not a general process for mass-producing multilayer ceramic capacitors. The process for reducing Eu is not limited. However, a process including the above-mentioned strong reductive annealing in the temperature falling process is proposed in order to securely achieving the effect of improving the lifetime. The method for increasing the ratio of +2 valence of Eu is not limited to the above-mentioned method. 
     [Re-Oxidation Process] 
     In order to return the oxygen into the barium titanate acting as the main phase of the dielectric layers  11  which are fired in the reductive atmosphere and is partially reduced, a thermal process may be performed in a mixed gas of N 2  and moisture at approximately 1000 degrees C. or in a normal atmosphere at 500 degrees C. to 700 degrees C. so that the internal electrode layers  12  are not oxidized. The process is called a re-oxidation process. However, the general re-oxidation process is performed, Eu having +2 valence may be oxidized into +3 valence. 
     Accordingly, in the embodiment, the re-oxidation process is performed in N 2  gas atmosphere at approximately 900 degrees C. When the re-oxidation process having a weak oxidizing condition, it is possible to return oxygen into barium titanate while the high ratio of +2 valence of Eu is kept. 
     [Plating Process] 
     After that, metal layers such as Cu, Ni, Sn or the like may be formed on the base layers of the external electrodes  20   a  and  20   b  by plating. With the processes, the multilayer ceramic capacitor  100  is fabricated. 
     It is possible to evaluate the ratio of +2 valence of Eu of the dielectric layers  11  by using XANES (X-ray Absorption Near Edge Structure) or the like. In concrete, spectrum near L 3  absorption edge of Eu (6977 eV) is obtained by a fluorescence method using a synchrotron light source. A background is removed. The ratio of +2 valence (%) is calculated from a ratio of a peak area of +2 valence (hatch is illustrated) and a peak area of +3 valence (hatch is illustrated). The ratio of the peak areas is expressed as follows. 
       (peak area of+2valence)/{(peak area of+2valence)+(peak area of+3valence)}×100(%)   (1)
 
     The ratio of +2 valence of Eu may be measured from a multilayer ceramic capacitor. Alternatively, powder is obtained by crushing the multilayer ceramic capacitor, and the powder may be deformed into a shape which is appropriate for a sample for the evaluation. The shape may be a bulk body which is a compact of a disc or a rectangular board of the dielectric material and is fired together with the multilayer ceramic capacitor. 
     EXAMPLES 
     The multilayer ceramic capacitors in accordance with the embodiment were made. And, property of the multilayer ceramic capacitors was measured. 
     (Example 1) Barium titanate powder was prepared as a dielectric material. Additives such as ZrO 2 , Eu oxide, MnCO 3 , SiO 2  were added to the barium titanate powder. The dielectric material and the additives were mixed and crushed with use of zirconia bead of φ of 0.5 mm. Thus, the dielectric material was obtained. 1.0 mol of Eu was added with respect to 100 mol of a main phase (BaTiZrO 3 ). 0.5 mol of MnCO 3  was added with respect to 100 mol of the main phase (BaTiZrO 3 ). 1.0 mol of SiO 2  was added with respect to 100 mol of the main phase (BaTiZrO 3 ). Ba/(Ti+Zr) was 0.960. 
     An organic binder and a solvent were added to the resulting dielectric material. With use of the resulting slurry, dielectric green sheets were made by a doctor blade method. Polyvinyl butyral (PVB) or the like was used as the organic binder. Ethanol, toluene or the like was used as the solvent. And a plasticizer was added to the resulting dielectric material. 
     Next, metal conductive paste for forming internal electrode layers was made with use of a planetary ball mill. The metal conductive paste included Ni acting as the main component metal of the internal electrode layers  12 , the co-material, a binder (ethyl cellulose), a solvent, and an additive when necessary. 
     The metal conductive paste for forming internal electrode layers was printed on each of the dielectric green sheets by a screen printing. Sheet members in which the metal conductive paste was printed on the dielectric green sheet were stacked. Cover sheets were stacked on the stacked green sheets and under the stacked green sheets. After that, a multilayer structure was obtained by a thermal clamping. The multilayer structure was cut into a predetermined shape. 
     The binder was removed from the resulting multilayer structure in N 2  atmosphere. After that, the metal conductive paste including a metal filler of which a main component is Ni, the co-material, the binder, the solvent and so on was painted from both edge faces of the multilayer structure to each side faces and was dried. The metal conductive paste for base layers and the multilayer structure were fired in a reducing atmosphere at a temperature of 1200° C. for 10 minutes and at an oxygen partial pressure of 10 10  MPa. The oxygen partial pressure was adjusted by a mixture ratio of N 2 —H 2 -H 2 O. After that, the temperature was changed to 1140 degrees C. The oxygen partial pressure was changed to 10 −13  MPa. The atmosphere was kept for 2 hours. Thus, the annealing process was finished. After that, the re-oxidation process was performed in N 2  gas atmosphere at approximately 900 degrees C. 
     The size of the multilayer ceramic was 1005 size (The length, the width and the height of the sintered structure were 1.0 mm, 0.5 mm and 0.5 mm). The average thickness of the dielectric layers was 2.0 μm. The stacked number of the internal electrode layers was 100. 
     (Example 2) The temperature of the annealing process was 1150 degrees C. in an example 2. Other conditions were the same as those of the example 1. 
     (Example 3) The temperature of the annealing process was 1160 degrees C. in an example 3. Other conditions were the same as those of the example 1. 
     (Example 4) The oxygen partial pressure of the annealing process was 10 −14  MPa. Other conditions were the same as those of the example 1. 
     (Example 5) The temperature of the annealing process was 1150 degrees C., and the oxygen partial pressure of the annealing process was 10 −14  MPa in an example 5. Other conditions were the same as those of the example 1. 
     (Example 6) The temperature of the annealing process was 1160 degrees C., and the oxygen partial pressure of the annealing process was 10 −14  MPa, in an example 6. Other conditions were the same as those of the example 1. 
     (Example 7) The temperature of the annealing process was 1150 degrees C., and Ba/(Ti+Zr) was 0.940, in an example 7. Other conditions were the same as those of the example 1. 
     (Example 8) The temperature of the annealing process was 1150 degrees C., and Ba/(Ti+Zr) was 0.920, in an example 8. Other conditions were the same as those of the example 1. 
     (Example 9) The temperature of the annealing process was 1150 degrees C., and Ba/(Ti+Zr) was 0.980, in an example 9. Other conditions were the same as those of the example 1. 
     (Comparative example 1) The annealing process was not performed in a comparative example 1. Other conditions were the same as those of the example 1. 
     (Comparative example 2) The time period of the firing process was 20 minutes in a comparative example 2. Other conditions were the same as those of the example 9. 
     (Comparative example 3) La was used instead of Eu, in a comparative example 3. Other conditions were the same as those of the example 2. 
     (Comparative example 4) Gd was used instead of Eu, in a comparative example 4. Other conditions were the same as those of the example 2. 
     (Comparative example 5) Dy was used instead of Eu, in a comparative example 5. Other conditions were the same as those of the example 2. 
     (Comparative example 6) Ho was used instead of Eu, in a comparative example 6. Other conditions were the same as those of the example 2. 
     (Comparative example 7) Er was used instead of Eu, in a comparative example 7. Other conditions were the same as those of the example 2. 
     (Evaluation of the valence) The ratio of +2 valence of Eu in the dielectric layers was evaluated with respect to each of the examples 1 to 9 and the comparative examples 1 and 2. The ratio of +2 valence of Eu was evaluated by using XANES.  FIG. 5  illustrates XANES spectrum near the L 3  absorption edge of Eu (6977 eV) with respect to the example 2. In addition to +3 valence, a peak of +2 valence clearly appears. From the result of  FIG. 5 , in the example 2, the ratio of +2 valence was 26%.  FIG. 6  illustrates XANES spectrum near the L 3  absorption edge of Eu (6977 eV) with respect to the example 5. From the result of  FIG. 6 , in the example 5, the ratio of +2 valence was 43%. 
     The ratio of +2 valence of the example 1 was 21%. The ratio of +2 valence of the example 3 was 29%. The ratio of +2 valence of the example 4 was 32%. The ratio of +2 valence of the example 6 was 59%. The ratio of +2 valence of the example 7 was 28%. The ratio of +2 valence of the example 8 was 27%. The ratio of +2 valence of the example 9 was 24%. Table 2 shows the results. From the results, the ratio of +2 valence was high when the temperature of the atmosphere during the annealing process was high or the oxygen partial pressure of the atmosphere during the annealing process was low. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
               
               
                   
                 RARE 
                   
                   
                 DIELECTRIC 
                   
                 RATIO  
                   
               
               
                   
                 EARTH 
                 CONDITION OF 
                 Ba 
                 CONSTANT 
                 tan δ  
                 OF + 2 
                 RELATIVE 
               
               
                 SAMPLE 
                 ELEMENT 
                 ANNEALING 
                 /(Ti + Zr) 
                 (%) 
                 (%) 
                 VALENCE 
                 LIFETIME 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 COMPARATIVE 
                 Eu 
                 — 
                 0.960 
                 3500 
                 3.8 
                 4 
                 1.0 
               
               
                 EXAMPLE 1 
                   
                   
                   
                   
                   
                   
                   
               
               
                 EXAMPLE 1  
                 Eu 
                 1140° C., 10 −13  MPa 
                 0.960 
                 3500 
                 3.8 
                 21 
                 6.6 
               
               
                 EXAMPLE 2 
                 Eu 
                 1150° C., 10 −13  MPa 
                 0.960 
                 3400 
                 4.0 
                 26 
                 10.8 
               
               
                 EXAMPLE 3  
                 Eu 
                 1160° C., 10 −13  MPa 
                 0.960 
                 3300 
                 4.2 
                 29 
                 21.1 
               
               
                 EXAMPLE 4 
                 Eu 
                 1140° C., 10 −14  MPa 
                 0.960 
                 3400 
                 4.0 
                 32 
                 37.6 
               
               
                 EXAMPLE 5 
                 Eu 
                 1150° C., 10 −14  MPa 
                 0.960 
                   
                 4.1 
                 43 
                 68.2 
               
               
                 EXAMPLE 6 
                 Eu 
                 1160° C., 10 −14  MPa 
                 0.960 
                 3800 
                 4.6 
                 59 
                 73.3 
               
               
                 EXAMPLE 7 
                 Eu 
                 1150° C., 10 −13  MPa 
                 0.940 
                 3300 
                 3.8 
                 28 
                 12.0 
               
               
                 EXAMPLE 8 
                 Eu 
                 1150° C., 10 −13  MPa 
                 0.920 
                 2900 
                 3.1 
                 27 
                 1 5.7 
               
               
                 EXAMPLE 9 
                 Eu 
                 1150° C., 10 −13  MPa 
                 0.980 
                 3900 
                 4,9 
                 24 
                 5.0 
               
               
                 COMPARATIVE 
                 Eu 
                 1150° C., 10 −13  MPa 
                 0.980 
                 4100 
                 5.4 
                 18 
                 4.1 
               
               
                 EXAMPLE 2 
                   
                   
                   
                   
                   
                   
                   
               
               
                 COMPARATIVE 
                 La 
                 1150° C., 10 −13  MPa 
                 0.960 
                 2900 
                 3.2 
                 — 
                 0.3 
               
               
                 EXAMPLE 3 
                   
                   
                   
                   
                   
                   
                   
               
               
                 COMPARATIVE 
                 Gd 
                 1150° C., 10 −13  MPa 
                 0.960 
                 3300 
                 3.6 
                 — 
                 0.1 
               
               
                 EXAMPLE 4 
                   
                   
                   
                   
                   
                   
                   
               
               
                 COMPARATIVE 
                 Dy 
                 1150° C., 10 −13  MPa 
                 0.960 
                 3400 
                 3.8 
                 — 
                 0.2 
               
               
                 EXAMPLE 5 
                   
                   
                   
                   
                   
                   
                   
               
               
                 COMPARATIVE 
                 Ho 
                 1150° C., 10 −13  MPa 
                 0.960 
                 3400 
                 3.9 
                 — 
                 0.1 
               
               
                 EXAMPLE 6 
                   
                   
                   
                   
                   
                   
                   
               
               
                 COMPARATIVE 
                 Er 
                 1150° C., 10 −13  MPa 
                 0.960 
                 3600 
                 4.4 
                 — 
                 0.1 
               
               
                 EXAMPLE 7 
               
               
                   
               
            
           
         
       
     
     (Method for evaluating lifetime) With respect to each of the examples 1 to 9 and the comparative examples 1 to 7, a mean time to failure of highly-accelerated lifetime test (125 degrees C. and 50 V/μm) was measured as a lifetime. The lifetime of the sample (the comparative example 1) made by adding Eu through the normal firing condition was considered as 1.0. Ratios of lifetimes with respect to the lifetime of the comparative example 1 were measured with respect the examples 1 to 9 and the comparative examples 2 to 7. Table 1 shows the results. When the measured lifetime of a sample of the ratio is five times or more, the sample was determined as good because the lifetime was sufficiently long. When the measured lifetime of a sample of the ratio is less than five times, the sample was determined as bad because the lifetime was not sufficiently long. 
       FIG. 7  illustrates a relationship between the ratio of +2 valence of Eu and the lifetime. As illustrated in  FIG. 7 , when the ratio of +2 valence is 21% or more, the lifetime becomes 5 times as that of the comparative example 1 or more. When the ratio of +2 valence is 26% or more, the lifetime becomes 10 times as that of the comparative example 1 or more. 
     In comparison, the lifetimes of the comparative examples 3 to 7 were not long. From the results, rare earth elements other than Eu do not withstand the high load such as 125 degrees C. and 50 V/m. It is thought that this is because the stable valence of the rare earth elements other than Eu is +3 and the valence of the rare earth elements is not changed into +2. 
     When the examples 7 and 8 are compared with the example 2, the effect of improving the lifetime of the examples 7 and 8 is superior to that of the example 2. It is thought that this is because Ba/(Ti+Zr) of the examples 7 and 8 was smaller than that of the example 2, the amount of vacancy of the A site increased, and the amount of the solid-solved Eu increased. 
     (Evaluation of dielectric constant) With respect to each of the examples 1 to 9 and the comparative examples 1 to 7, the dielectric constant and tans were measured. Table 2 shows the results. When the comparative example 1 is compared with the example 1, a large difference does not occur in the dielectric constant and the tans. Therefore, even if the annealing process is performed, remarkable structure change such as grain growth does not occur. When the comparative example 1 is compared with the example 2, a large difference does not occur in the dielectric constant and the tans. Therefore, even if the annealing process of the example 2 is performed, remarkable structure change such as grain growth does not occur. 
     In the example 6, the dielectric constant and tan δ increased. An indication of the grain growth appeared. When the reduction process is progressed more than the example 6, the structure may be changed. It is therefore preferable that the ratio of +2 valence of Eu is 59% or less. 
     When the example 7 was compared with the example 8, lowering of the dielectric constant of the example 8 started. This means that the capacity may be reduced even if long lifetime is secured. It is therefore preferable that Ba/(Ti+Zr) is 0.940 or more. 
     When the example 1 is compared with the comparative example 2, the difference of the ratios of +2 valence was small. However, the relative lifetime of the comparative example 2 was not 5.0 times or more. The dielectric constant and tans of the comparative example 2 were larger than those of the example 1. It is thought that this means grains of the barium titanate grew and the effect of shortening lifetime caused by the grain growth cancels the effect of the ratio of +2 valence. It is thought that when Ba/(Ti+Zr) gets close to 1.000 of stoichiometry, the reactivity of barium titanate rapidly increases. It is therefore preferable that Ba/(Ti+Zr) is 0.960 or less. 
     Although the embodiments of the present invention have been described in detail, it is to be understood that the various change, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.