JP5207675B2 - Dielectric porcelain and multilayer ceramic capacitor using the same - Google Patents

Description

  The present invention relates to a dielectric ceramic composed of crystal particles mainly composed of barium titanate and a multilayer ceramic capacitor using the dielectric ceramic as a dielectric layer.

  In recent years, with the widespread use of mobile devices such as mobile phones and the high speed and high frequency of semiconductor devices, which are the main components of personal computers and the like, multilayer ceramic capacitors mounted on such electronic devices are required to be smaller and have higher capacities. For this reason, the dielectric layer constituting the multilayer ceramic capacitor is required to be thin and highly laminated.

Therefore, a fine barium titanate powder that can be used for a dielectric layer to be thinned has been developed. For example, in Patent Document 1, a barium hydroxide powder and a titanium oxide powder having a large specific surface area are used. By heating these mixed powders under a pressure lower than atmospheric pressure, barium titanate powder having an average particle size of 0.1 μm (100 nm) or less is synthesized, and such fine barium titanate powder is laminated. Application to a dielectric layer of a ceramic capacitor is disclosed (for example, see Patent Document 1).
JP 2003-2738 A

  However, even if an attempt is made to obtain a dense dielectric ceramic using the fine barium titanate powder disclosed in Patent Document 1, since the barium titanate powder, which is a raw material powder, tends to grow during firing, the average particle size It is difficult to obtain a dielectric ceramic composed of crystal grains of 100 nm or less.

  Further, crystal grains mainly composed of barium titanate tend to lower the relative dielectric constant when the average particle diameter is 100 nm or less, and it is difficult to increase the dielectric constant.

  Accordingly, an object of the present invention is to provide a dielectric ceramic having a high dielectric constant even if the average crystal grain size is 100 nm or less, and a high-capacity multilayer ceramic capacitor having such a dielectric ceramic as a dielectric layer. And

The dielectric porcelain of the present invention fills a partially opened ceramic container with a mixed powder containing barium carbonate powder and titanium oxide powder, and at a temperature range of 500 ° C. or lower under a pressure lower than atmospheric pressure. Of crystal particles mainly composed of barium titanate obtained by calcining a calcined powder of barium titanate obtained by heating so as to be accompanied by a change in weight of −5 mass% / ° C. or more. In the X-ray diffraction chart having an average crystal grain size of 30 to 100 nm and using Cukα rays of dielectric ceramic, each of the diffraction peaks of the (200) plane and (002) plane of the barium titanate at room temperature The half width is 0.2 to 0.26 deg. It is characterized by being.

  In the dielectric ceramic, the coefficient of variation of the grain size of the crystal particles is preferably 45% or less.

  In addition, the multilayer ceramic capacitor of the present invention is characterized in that it is composed of a laminate of a dielectric layer made of the dielectric ceramic and an internal electrode layer.

According to the dielectric ceramic of the present invention, the mixed powder containing the barium carbonate powder and the titanium oxide powder is filled in a partially opened ceramic container, and at 500 ° C. or less under a pressure lower than atmospheric pressure . It is mainly composed of barium titanate obtained by calcining a calcined powder of barium titanate obtained by heating in a temperature range so as to have a weight change of −5% by mass / ° C. or more. In the X-ray diffraction chart when using Cukα rays of dielectric ceramics, the diffraction peaks of the (200) plane and (002) plane of barium titanate at room temperature Of half-width of 0.2 to 0.26 deg. Thus, a dielectric ceramic having a high dielectric constant can be obtained.

  Moreover, in the dielectric ceramic according to the present invention, when the coefficient of variation of the grain size of the crystal particles is 45% or less, a dielectric ceramic having a higher dielectric constant can be obtained.

  According to the multilayer ceramic capacitor of the present invention, a high-capacity multilayer ceramic capacitor can be obtained by applying the dielectric ceramic as the dielectric layer.

  The dielectric ceramic according to the present invention is composed of crystal particles mainly composed of barium titanate, and the average crystal particle size of the crystal particles is 30 to 100 nm.

  Further, the dielectric ceramic of the present invention is an X-ray diffraction chart when using the dielectric ceramic Cukα ray. Each of the diffraction peaks of the (200) plane and (002) plane of barium titanate at room temperature (25 ° C.). Of half-width of 0.2 to 0.26 deg. Is important.

  When the dielectric ceramic has the above-described crystal structure, there is an advantage that the relative dielectric constant can be 3500 even if the average crystal grain size of the crystal grains is 30 to 100 nm.

  As described above, the crystal particles constituting the dielectric ceramic of the present invention are extremely fine crystal particles having an average crystal grain size of 100 nm or less. In this case, the crystal grain size is increased because the relative dielectric constant can be increased. It is desirable that the coefficient of variation in diameter is 45% or less.

  Here, the coefficient of variation for the particle size is a value obtained by dividing the standard deviation of the particle size of the crystal particles by the average crystal particle size, and is an index representing the variation in the particle size. The average crystal grain size of the crystal particles is determined by polishing the cut surface of the dielectric porcelain and then performing etching from the contours of the crystal particles displayed in the scanning electron microscope (SEM) photograph. The area is obtained, the diameter is calculated when the circle is replaced with a circle having the same area, and the average value of about 100 crystal grains whose diameter is thus obtained is obtained. Moreover, a standard deviation is calculated | required from the data of the measured particle diameter of each crystal grain.

  The dielectric ceramic of the present invention preferably has a relative density of 99% or more, particularly 99.7% or more. When the relative density of the dielectric ceramic is high, there is an advantage that the relative permittivity can be further increased because there are few regions occupied by air such as air gaps.

  FIG. 1A is an X-ray diffraction pattern of the (200) plane and (002) plane when the dielectric ceramic of the present invention is measured at 25 ° C., and FIG. 1 (b) is the dielectric ceramic of the present invention. 2 is an X-ray diffraction pattern of (200) plane and (002) plane when measured at 150 ° C. FIG.

  In FIG. 1A, two peaks B and C drawn in the diffraction peak A indicate that the diffraction peaks on the (200) plane and the (002) plane are separated by a pseudo-Forked function. Separation of diffraction peaks uses a pseudo-Forked function to provide a constraint that the peak widths of the (200) plane and (002) plane are the same, and the diffraction intensity ratio of the (200) plane and (002) plane is ( The diffraction peak profile is fitted so that the relationship of the multiplicity of the (200) plane and the (002) plane is (2: 1). In this manner, the peaks can be separated into individual peaks B and C on the (200) plane and (002) plane.

  In this case, for separating the diffraction peak, it is preferable to select a function that provides an optimal fitting for the diffraction peak. In this case, in addition to the pseudo-Forked function (a combination of a Gaussian function and a Lorentz function), Any one of a Gaussian function, a Lorentz function, a Pearson-7 function, and the like can be used. Note that the arrows in diffraction peaks B and C in FIG. 1A and the arrow in diffraction peak D in FIG.

  The dielectric ceramic of the present invention has diffraction peaks B and C indicating the (200) plane and (002) plane of barium titanate, and has a tetragonal crystal structure at room temperature (25 ° C.). For example, as further clarified from the separation using the pseudo-Forked function, the half-value widths of the diffraction peaks B and C on the (200) plane and the (002) plane are 0.2 to 0.26 deg. It is important that the full width at half maximum is 0.21 to 0.25 deg. It is desirable to be in the range.

  On the other hand, in a dielectric ceramic mainly composed of barium titanate, the half width of each of the diffraction peaks of the (200) plane and the (002) plane is 0.2 deg. Is smaller, the crystallite diameter is increased and the diffraction peaks of the (200) plane and (002) plane are remarkably separated, resulting in crystal grains having high tetragonality, but the crystal grains are large. Therefore, it is difficult to achieve atomization.

  On the other hand, the half width of each of the diffraction peaks of the (200) plane and the (002) plane is 0.26 deg. Is larger than the crystallite diameter, the diffraction peaks of the (200) plane and the (002) plane cannot be separated, and the crystal phase has a cubic crystal structure. It becomes like this.

In the dielectric ceramic according to the present invention, d _{2 (200)} and d 2 ₍₀₀₂₎ are obtained from Bragg's equation from 2θ of the diffraction peak B of the (200) plane and the diffraction peak C of the (002) plane of barium titanate. The lattice constant ratio (c / a ratio) of the crystal structure obtained when the ratio (d ₍₀₀₂₎ / d ₍₂₀₀₎ ) is calculated is preferably 1.001 to 1.004. If the lattice constant ratio (c / a ratio) is 1.001 to 1.004 at room temperature (25 ° C.), it has a tetragonal crystal structure with a relatively small lattice strain. A high dielectric constant can be achieved.

  In addition, the dielectric ceramic of the present invention has a diffraction peak of (200) plane and (002) plane of barium titanate measured by X-ray diffraction at 150 ° C. which is a temperature region higher than the Curie temperature of barium titanate. Are not separated, the crystal structure is cubic, and the half-value width of the diffraction peak where the (200) plane and (002) plane of barium titanate overlap is 0.13 deg. The lattice constant ratio (c / a ratio) is 1.000.

  As described above, the dielectric ceramic of the present invention has a region higher than the Curie temperature because the diffraction peaks of the (200) plane and (002) plane of barium titanate at 150 ° C. are superimposed and appear as one diffraction peak. Shows a cubic system. On the other hand, in the temperature range lower than the Curie temperature, the diffraction peak width is wider than the (200) plane diffraction peak at 150 ° C. The crystal structure changes greatly between the high temperature side and the low temperature side.

  As in the dielectric ceramic according to the present invention shown in FIG. 1A, in the X-ray diffraction chart, a diffraction peak A having a tetragonal crystal with a peak separation between the (200) plane and the (002) plane is not clear. As shown in FIG. 1B, confirmation of the crystalline structure should be made by comparison with an X-ray diffraction chart measured at a temperature higher than the Curie temperature at which the dielectric ceramic undergoes phase transformation. Is possible.

  In this case, the diffraction peak A of the (200) plane and the (002) plane in the X-ray diffraction chart measured near room temperature lower than the Curie temperature is diffraction of the X-ray diffraction chart measured at a temperature higher than the Curie temperature. What changes in a direction larger than the width of the peak D is such that the crystal structure of the dielectric ceramic has a tetragonal crystal structure in a temperature region near room temperature with the Curie temperature as a boundary.

  As described above, in the X-ray diffraction chart, even if the dielectric ceramic is such that the peak separation between the (200) plane and the (002) plane cannot be clearly defined, the crystal is not observed at the temperature above and below the Curie temperature. By combining the evaluation of the structure and the separation of diffraction peaks measured at room temperature using an optimal function, it is possible to accurately evaluate that the crystal structure is tetragonal.

  Next, a method for manufacturing a dielectric ceramic according to the present invention will be described.

  A method for producing a dielectric material powder for forming the dielectric ceramic of the present invention will be described. First, a mixed powder composed of barium carbonate powder and titanium oxide powder is prepared.

  It is desirable that the purity of the barium carbonate powder is 99% or more, particularly 99.9% or more, and the purity of the titanium oxide powder is 99% or more, particularly 99.9% or more. By using barium carbonate powder and titanium oxide powder having a purity of at least 99%, the resulting barium titanate powder can also be highly purified.

  In this case, the barium carbonate powder has columnar crystals, but the specific surface area is preferably 10 to 100 m / g, and the long side size is preferably 150 nm or less. The average particle size of the titanium oxide powder is preferably 20 to 100 nm and the specific surface area is preferably 10 to 100 m / g. If the barium carbonate powder and the titanium oxide powder have a specific surface area and an average particle diameter in the above ranges, atomization is facilitated.

  Next, the mixed powder is mixed and pre-ground using a bead mill. The bead mill is advantageous in that the grinding can be performed in a short time and the contamination from the media ball due to the impact of the media ball or the like during the grinding can be reduced. The lining of the bead mill container and the pulverized balls are preferably zirconia having a purity of 99% or more.

  In the present invention, next, a step of heating the obtained mixed powder under a pressure lower than atmospheric pressure to obtain a calcined powder is incorporated.

In the present invention, the calcining conditions for obtaining the calcined powder are as follows. It is desirable to perform heating with Specifically, determining the difference between the weight of the value and weight of the value at 500 ° C. at 200 ° C. was divided by the temperature difference.

  Usually, when calcining by heating the raw material powder, the mixed powder is filled in a ceramic container, and the mixed powder in the ceramic container is heated in a state in which soaking is maintained. That is, the ceramic container is heated by being covered with a lid made of the same material.

  On the other hand, the dielectric powder used in the dielectric ceramic of the present invention has a temperature of 500 ° C. of the barium carbonate powder in the mixed powder rather than the thermal uniformity in the ceramic container filled with the mixed powder of the barium carbonate powder and the titanium oxide powder. Emphasis is placed on facilitating the breakdown of: As a method of promoting the decomposition of the barium carbonate powder in the mixed powder, for example, a method of opening the ceramic container without a lid or the like, or using a ceramic container with a vent hole on the upper side surface is used. Is preferred.

For example, when the mixed powder is heated in a temperature range of 500 ° C. or less under the condition of −3 mass% / ° C. (the ceramic container is covered and heated at 0.1 Pa), the heating temperature is directly mixed powder. Further, since the ceramic container is covered, the volatilization of carbon dioxide, which is a decomposition gas from barium carbonate, of the raw material powder at the low temperature of the heating condition is delayed. Therefore, by-products such as Ba ₂ TiO ₄ are formed, the barium titanate powder in the calcination stage is likely to aggregate, and the calcined barium titanate powder is likely to grow as the heating temperature is increased.

  On the other hand, the barium titanate powder having an average particle diameter of 100 nm or less is easily obtained in a temperature range of 500 ° C. or less under conditions with a weight loss of −5% by mass / ° C. or more. By using a calcined powder having such an average particle size, the crystal particles after firing can be controlled to an appropriate particle size (average particle size of 30 to 100 nm). The dielectric ceramic can be easily formed.

  In the method for producing a barium titanate powder of the present invention, a material having a high thermal conductivity and a low reactivity with the raw material powder is preferable as the ceramic container. For example, zirconia or carbon is preferable.

  So far, when barium titanate powder is synthesized by a solid phase method, heating under reduced pressure conditions has been performed. However, in the past, although the conditions for the set temperature and the set pressure were all determined, the thermal decomposition behavior of the raw material powder to be heated has not been controlled.

  Therefore, even when heated under reduced pressure conditions, if it is not considered to heat the raw material powder in a temperature range of 500 ° C. or less with a weight change of 5% by mass / ° C. or more, grain growth occurs during heating. It is easy to obtain barium titanate powder having an average crystal grain size of 100 nm or less, particularly 20 to 100 nm, as crystal particles after firing.

  Next, the manufacturing method of the dielectric ceramic of the present invention obtained by using the barium titanate powder will be described.

  The dielectric ceramic of the present invention is obtained by forming the above-mentioned barium titanate powder into a predetermined shape by adding an organic binder, and performing firing at a temperature of 800 ° C. or higher, for example, by a hot press method using a carbon mold. can get. The pressure is preferably 10 MPa or more. In addition, baking at normal pressure is also possible by adding an appropriate sintering aid.

  FIG. 2 is a schematic cross-sectional view showing an example of the multilayer ceramic capacitor of the present invention. In the multilayer ceramic capacitor of the present invention, an external electrode 12 is provided at the end of the capacitor body 10. The capacitor body 10 is configured by alternately laminating dielectric layers 13 and internal electrode layers 14. In FIG. 2, the laminated state of the dielectric layer 13 and the internal electrode layer 14 is shown in a simplified manner, but the laminated ceramic capacitor of the present invention has a laminated layer in which the dielectric layer 13 and the internal electrode layer 14 are several hundred layers. It is a body.

Here, it is important that the dielectric layer 13 is formed by the above-described dielectric ceramic of the present invention. The internal electrode layer 14 is preferably a base metal such as Ni or Cu in that the manufacturing cost can be suppressed even when the internal electrode layer 14 is made highly stacked, and in particular, Ni and Cu are simultaneously fired with the dielectric layer 13 constituting the capacitor of the present invention. Is more desirable. The average thickness of the internal electrode layer 14 is preferably 1 μm or less. In FIG. 2, the laminated state of the dielectric layer 13 and the internal electrode layer 14 is shown in a simplified manner, but the multilayer ceramic capacitor of the present invention has several hundreds of dielectric layers 13 and internal electrode layers 14. A laminated body extending to the layers is formed.

  Since the multilayer ceramic capacitor of the present invention uses a dielectric ceramic having a high dielectric constant, the dielectric layer can be made thinner than a conventional multilayer ceramic capacitor, and a multilayer capacitor having a higher capacity can be formed. .

When producing such a multilayer ceramic capacitor, a mixed powder obtained by adding an additive such as Y ₂ O ₃ to the above-mentioned barium titanate powder is formed into a green sheet and becomes an internal electrode layer. A conductor paste is prepared, printed on the surface of the green sheet, and then laminated to form a laminate. Thereafter, the laminate is fired at a predetermined temperature in an atmosphere matched to the internal electrode layer. Thereafter, a conductive paste is further printed on the end face of the fired laminated body and fired to form external electrodes, thereby obtaining a multilayer ceramic capacitor.

  First, the following barium carbonate powder and titanium oxide powder were prepared as raw material powders. The barium carbonate powder had a short side dimension of 20 nm on average, a long side dimension of 100 nm on average, and a purity of 99.9%. A titanium oxide powder having an average particle diameter of 20 nm and a purity of 99.9% was used. The composition was Ba / Ti = 1.

  Next, a mixed powder was prepared with a stirrer using the above barium carbonate powder and titanium oxide powder. Next, the prepared mixed powder was preliminarily pulverized with a bead mill using ion-exchanged water, and then dried in the atmosphere at a temperature of 120 ° C. In this case, zirconia having a purity of 99.9% was used for the mixing container and the ball.

  Next, this mixed powder was filled into a ceramic container (thickness 5 mm) made of stabilized zirconia (containing 3% CaO) having a purity of 99.5% or a carbon container having the same thickness, and the temperature and pressure shown in Table 1 were measured. A calcined powder was prepared by heating under conditions. In this case, the heating rate of the heating furnace was 300 ° C./min.

  Moreover, the thermocouple was set to the upper part of the raw material powder with which the ceramic container was filled, and the temperature of the raw material powder heated was measured.

  Moreover, the thermogravimetric change at the time of heating of raw material powder was previously evaluated using a thermogravimetric analyzer set to a pressure for preparing a mixed powder. At this time, in the thermogravimetric analysis, the analysis was performed with the ceramic container covered and without the lid. A 99.6% alumina container was used for thermogravimetric analysis. The measurement was obtained by dividing the difference between the weight change at 200 ° C. and the weight change at 500 ° C. by the temperature difference.

  The following evaluation was performed about the obtained barium titanate powder.

  About the obtained barium titanate powder, the average particle diameter and its variation coefficient were measured by electron microscope observation. The average particle diameter and coefficient of variation of the particle diameter of the barium titanate powder are obtained by subjecting the obtained barium titanate powder to image processing of the outline of the barium titanate powder projected in a scanning electron microscope (SEM) photograph. The diameter when the area was replaced with a circle having the same area was calculated and averaged. The coefficient of variation was determined from all the data on the particle size of 100 extracted barium titanate powders.

  Next, a pellet-shaped molded body having a diameter of 12 mm and a thickness of 1 mm was produced using barium titanate powder, and hot press (HP) firing was performed at 800 to 1000 ° C. for 2 hours. The firing temperature of each sample was a temperature at which the relative density could be 99% or more.

  For the dielectric ceramic, the average crystal grain size and the coefficient of variation of the grain size were determined. In this case, after polishing the fracture surface of the obtained sintered body, this is also image-processed the contours of the crystal particles reflected in the scanning electron microscope (SEM) photograph, the area of each crystal particle is obtained, the same The diameter when replaced with a circle with an area was calculated and averaged. The coefficient of variation was determined from all the data on the particle diameters of 100 extracted crystal grains.

  Next, the crystal structure of the dielectric ceramic obtained by the X-ray diffractometer was identified. The X-ray tube was Cukα, the scanning angle was an angle between the (200) plane and the (002) plane of barium titanate (2θ = 44 to 46 °), and measurement was performed at room temperature (25 ° C.) and 150 ° C. The number of measurements was 3 for each sample, and the average value was determined.

  Next, the relative density and relative dielectric constant of the dielectric ceramic were measured. The density was measured using the Archimedes method, and the ratio to the theoretical density was determined as a relative density. The theoretical density of barium titanate was 6.1.

  The relative dielectric constant is obtained by applying In—Ga metal to both main surfaces of the sintered body sample, measuring the capacitance using an LCR meter (HP 4192A), and calculating the relative dielectric constant from the thickness and surface area of the sample. Asked. The number of samples was 10 each. The capacitance was measured under the conditions of a frequency of 1 kHz and an AC voltage of 1 Vrms, and the value one minute after voltage application was read.

  In addition, the dependence of the dielectric constant on DC voltage was evaluated. In this case, a DC voltage up to 1000 V was applied to a sample thickness of 0.5 mm.

As comparative examples, as shown in Table 1, barium titanate powder and barium titanyl oxalate powder prepared by setting heating conditions with a weight change smaller than 5 mass% / ° C. in the air and in a temperature range of 500 ° C. or lower ( The same evaluation as that of the sample of the present invention was performed on the dielectric ceramics that were sintered bodies.

  As is apparent from the results of Tables 1 and 2, barium titanate is the main component, the average grain size of the crystal grains is 30 to 100 nm, and in the X-ray diffraction chart of the dielectric ceramic, The half width of each of the diffraction peaks of the (200) plane and the (002) plane is 0.2 to 0.26 deg. Sample no. In 2-8, the relative dielectric constant was 3500 or more. In particular, the relative dielectric constant was 3700 or more in the sample having a variation coefficient of the particle size of 45% or less.

On the other hand, the half width of each of the diffraction peaks of the (200) plane and (002) plane of barium titanate is 0.2 deg. Samples having a lower value than that of Sample No. with an average crystal grain size of 150 to 200 nm were used. 9 and 10, the relative dielectric constant was 3300 or less, and when the average crystal grain size of the crystal particles was smaller than 30 nm (Sample No. 1 had a cubic crystal structure and the relative dielectric constant was as low as 1700.

FIG. 1A is an X-ray diffraction pattern of the (200) plane and (002) plane when the dielectric ceramic of the present invention is measured at 25 ° C., and FIG. 1 (b) is the dielectric ceramic of the present invention. 2 is an X-ray diffraction pattern of (200) plane and (002) plane when measured at 150 ° C. FIG. It is a cross-sectional schematic diagram which shows the example of the multilayer ceramic capacitor of this invention.

Explanation of symbols

10 Capacitor body 12 External electrode 13 Dielectric layer 14 Internal electrode layer

Claims (3)

A mixed powder containing barium carbonate powder and titanium oxide powder is filled into a partially opened ceramic container, and is −5 mass% / ° C. in a temperature range of 500 ° C. or lower under a pressure lower than atmospheric pressure. It is composed of crystal particles mainly composed of barium titanate obtained by calcining the calcined powder of barium titanate obtained by heating with the above weight change, and the average crystal grain size of the crystal particles is 30 to 100 nm. In the X-ray diffraction chart when using Cukα of dielectric ceramic,
The full width at half maximum of the diffraction peaks of the (200) plane and (002) plane of the barium titanate at room temperature is 0.2 to 0.26 deg. A dielectric porcelain characterized by the above.   2. The dielectric ceramic according to claim 1, wherein a coefficient of variation of the grain size of the crystal particles is 45% or less.   A multilayer ceramic capacitor comprising a laminate of a dielectric layer comprising the dielectric ceramic according to claim 1 and an internal electrode layer.

Images