CN104205266A - 层叠陶瓷电容器及其制造方法 - Google Patents

层叠陶瓷电容器及其制造方法 Download PDF

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CN104205266A
CN104205266A CN201280072165.9A CN201280072165A CN104205266A CN 104205266 A CN104205266 A CN 104205266A CN 201280072165 A CN201280072165 A CN 201280072165A CN 104205266 A CN104205266 A CN 104205266A
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dielectric
ceramic capacitor
dielectric layer
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laminated ceramic
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森田浩一郎
志村哲生
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Taiyo Yuden Co Ltd
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Abstract

在层叠陶瓷电容器中，通过焙烧使电介质层的电介质颗粒尽量均匀地成长，保持表面的平坦性，且以某个程度的粒径确保介电常数，由此，兼顾实现大容量和提高可靠性。构成电介质层的电介质颗粒的平均粒径差异nd小于4，该平均粒径差异nd为用与累计存在率90％相应的粒径D90除以与累计存在率10％相应的粒径D10所得到的值、即：D90/D10。

Description

层叠陶瓷电容器及其制造方法
技术领域
本发明涉及通过电介质层的高密度层叠来实现小型大容量化的层叠陶瓷电容器(MLCC)及其制造方法。
背景技术
随着移动电话等数字电子仪器的小型化以及薄型化，在安装在电子电路基板上的层叠陶瓷电容器(MLCC：Multi-Layer ceramiccapacitor)中，所要求的芯片尺寸的小型化以及大容量化的需求逐年增大。一般来讲，存在如果缩小电容器的尺寸，则与电介质层相对的内部电极的面积必然会变小，从而静电电容减少的关系。因此，为了实现芯片尺寸的小型化并确保电容器的容量，必须需要将内部电极间的电介质层做薄，且多层层叠电介质层的高密度层叠化技术。
在层叠陶瓷电容器中，为了将内部的电介质层做薄使其高密度化，尽可能将作为电介质的主要成分的例如BaTiO3(钛酸钡)的粒径做成微细。但是，如果在电介质层薄层化的同时缩小电介质的粒径，则根据其尺寸效果，介电常数就会下降，作为电容器整体无法获得足够的容量。因此，在陶瓷电容器的高密度层叠中采用以下这种技术：通过焙烧电介质的微粉末并使粒径生长，防止介电常数下降以确保容量。
例如，根据专利文献1，在将电介质层形成1μm左右的薄层时，作为原料粉末，将用Ca置换BaTiO3的一部分后的Ba1-XCaXTiO3(也称作“BCT”)调制成0.1～0.2μm的粒径，使其颗粒生长直至电介质颗粒的直径(晶粒粒径)变成0.35～0.65μm，由此得到6000以上的介电常数。一般情况下，为了抑制因焙烧所引起的电介质的还原，添加Mg作为受主元素。在专利文献1中，包含在电介质中的MgO的量相对于100摩尔的Ba1-XCaXTiO3为0.05～3.0摩尔。
另外，为了在1μm以下的电介质层中确保充分的电气绝缘性，电介质颗粒的粒径优选200nm以下。其理由在于，粒径越小，阻止静电场中的氧缺陷的移动(电场迁移)的晶界越增加。另外，反之，如果粒径越大，电介质层表面的颗粒与颗粒的缝隙越深，内部电极膏渗入该缝隙中，容易形成朝向电介质层内部的电极的凸部。能够将层叠陶瓷电容器视作与仅以层叠数并列连接构成各电介质层的各个电容器的电路等效，因此，如果电场仅集中在电介质层的一层发生短路，则电容器整体也会变成导通状态。像这样，电场集中于在颗粒间的缝隙中所产生的电极的凸部，成为电容器的绝缘劣化和耐电压下降的主要原因，因此，优选电介质层和内部电极接触的界面同样平坦。
例如，在专利文献2中公开了一种电介质陶瓷组成物，其主成分为(Ba1-XCaX)m(Ti1-YZrY)O3(也称作“BCTZ”)，焙烧后的平均结晶粒径为0.15～0.51μm，从电介质颗粒的对应于100％粒径中减去对应于50％粒径所得到的差的粒度分布为0.3～0.9μm。所公开的电介质陶瓷组成物的介电常数是1651以下。
现有技术文献
专利文献
专利文献1：日本特开2010－180124号公报
专利文献2：日本特开2006－282483号公报
发明内容
发明所要解决的课题
像这样在层叠陶瓷电容器中，为了得到良好的电气绝缘性、耐电压特性以及寿命等，优选将电介质颗粒的粒径尽量做成微细，将电介质层的表面保持平坦，并且不在内部电极中产生凹凸。但是，另一方面，如果电介质层的粒径太过细微，则根据上述的尺寸效果，就会相反地产生介电常数下降这样的问题。
本发明就是为了解决上述课题而完成，其目的在于，提供一种层叠陶瓷电容器及其制造方法，通过焙烧使电介质层的电介质颗粒尽可能地均匀生长以保持表面的平坦性，且按照一定程度的粒径确保介电常数，由此，兼顾实现大容量和提高可靠性两者。
用于解决课题的技术手段
本发明是层叠陶瓷电容器，其通过电介质层与内部电极层交替层叠而成，构成所述电介质层的电介质颗粒的平均粒径差异nd小于4，所述平均粒径差异nd是与累计存在率90％相应的粒径D90除以与累计存在率10％相应的粒径D10所得的值，即：D90/D10。
另外，优选所述电介质层中不含Mg，但是也可以相对于100摩尔的BaTiO3含有0.03摩尔以下的Mg。
另外，也可以优选所述电介质层中相对于100摩尔的BaTiO3含有0.01摩尔以上且0.03摩尔以下的Mg。
另外，优选所述电介质颗粒的平均粒径大于300nm且小于1000nm。
另外，本发明是一种层叠陶瓷电容器的制造方法，该方法包括：调制平均粒径为200nm以下、优选80nm以上的电介质原料粉末的工序；和焙烧所述电介质原料粉末以使构成所述电介质层的电介质颗粒的平均粒径差异nd小于4，并且所述电介质颗粒的平均粒径大于300nm且小于1000nm的工序。
发明效果
根据本发明，以使电介质层的电介质颗粒的平均粒径差异nd为小于4的值的方式使颗粒均匀地生长，从而能够一定程度地使电介质层的表面变得平坦。由此，能够获得较高的寿命特性。与此同时，能够确保电介质颗粒的粒径，获得高介电常数。因此，能够兼顾实现层叠陶瓷电容器中的大容量与提高可靠性两者。
附图说明
图1是本发明的实施方式的层叠陶瓷电容器的概略纵截面图。
图2是用于说明“晶粒粒径”所表示的电介质层的截面图。
图3是用于说明“平均粒径差异nd”所表示的电介质颗粒的粒径与累计存在率的关系的图。
具体实施方式
下面，说明本发明的一个实施方式的层叠陶瓷电容器。图1是层叠陶瓷电容器1的概略纵截面图。层叠陶瓷电容器1大致包括：具有按照规格确定的芯片尺寸和形状(例如1.0×0.5×0.5mm的长方体)的陶瓷烧结体10；和形成于陶瓷烧结体10两侧的一对外部电极20。陶瓷烧结体10包括：例如以BaTiO3(钛酸钡)为主成分，在内部交替层叠电介质层12与内部电极13而成的层叠体11；和作为层叠方向上下的最外层形成的覆盖层15。
层叠体11根据静电容量和所要求的耐压等的规格，具有被两个内部电极层13夹着的电介质层12的一层厚度为1μm以下，且整体的层叠数为数百层的高密度多层构造。形成于层叠体11的最外层部分的覆盖层15保护电介质层12以及内部电极层13不受外部的湿气和污染物等的污染，防止它们随时间而劣化。
层叠陶瓷电容器1例如按照以下的方式制造。首先，将以BaTiO3为主成分的粒径为200nm以下的原料粉末与添加化合物一起进行湿式混合，并且进行干燥、粉碎，调制电介质原料粉末。此处，在电介质原料粉末中所混合的MgO的量为，相对于100摩尔的BaTiO3，Mg的含量可以是0.01摩尔以上且0.03摩尔以下。也可以不添加MgO，Mg的含量为0。
利用聚乙烯醇缩醛树脂和有机溶剂对所调制的电介质原料粉末湿式混合，例如采用刀刮法涂覆1μm以下的带状的电介质生片并将其干燥。接着，在电介质生片的表面通过将包含有机粘合剂的导电膏丝网印刷，从而配置内部电极层13的图案。此外，导电膏中，作为金属粉末例如优选使用Ni。另外，作为共材也可以均匀地分散粒径为50nm以下的BaTiO3。
然后，例如将冲裁为15cm×15cm大小且一致的电介质生片按照内部电极层13相互错开的方式层叠规定层数。在所层叠的电介质生片的上下压接作为覆盖层15的覆盖片，切割成规定芯片尺寸(例如1.0×0.5mm)。然后，将作为外部电极20的导电膏涂覆在层叠体的两侧并使其干燥。由此，得到层叠陶瓷电容器1的成型体。
在大约350℃的N2气体气氛中对由此获得的成型体进行脱粘合剂，然后，在N2、H2、H2O的混合气体(氧分压约为1.0×10-11MPa)中、在1220～1280℃下焙烧10分钟～2小时。焙烧后，在大约1000℃的N2气体气氛中进行大约1小时的电介质的氧化处理，从而得到使构成电介质层的电介质颗粒生长为所希望的晶粒粒径(焙烧后的电介质颗粒的直径)的层叠陶瓷电容器1。
图2是示意地表示层叠陶瓷电容器1的电介质层12的截面的图。在本说明书中将“晶粒粒径”Dg定义为，与内部电极层平行的方向(即，与电场方向正交的方向)上的焙烧后的电介质颗粒(晶粒)的最大长度Dgs的平均。即，参照图2，晶粒粒径Dg是通过用取样的电介质颗粒的最大长度Dgs的总和除以其取样数而求得的。此外，对于测定晶粒粒径Dg的电介质颗粒的取样，取样数为500个以上，在一处的观察部位(例如一张通过SEM放大2000倍时的图片)有500个以上的情况下，对于其中的所有电介质颗粒进行取样，在小于500个的情况下，在多处进行观察(拍摄)，使其变成500个以上。另外，图3是为了说明电介质颗粒的平均粒径差异nd，表示与将在取样的电介质层12中所观察的焙烧后的电介质颗粒的粒径按照从小到大的顺序累计其数量而得到的累计存在率的关系的图。此处，用与累计存在率90％相应的晶粒粒径D90除以与累计存在率10％相应的晶粒粒径D10所得的值、即按照以下的公式(1)来定义平均粒径差异nd。
nd＝D90/D10…式(1)
根据式(1)能够评价，平均粒径差异nd越小电介质颗粒的粒径越均匀，电介质层12的表面平坦。此外，粒径越均匀，nd越接近1。
构成本实施方式的层叠陶瓷电容器1的电介质层12的电介质颗粒优选具有平均粒径差异nd的值小于4的均匀性。通过均匀地焙烧电介质颗粒，能够使电介质层12的表面变得平坦，减少在与内部电极层13的边界的凹凸。由此，能够防止电极凸部的电场集中，获得高可靠性。另外，优选电介质层12的颗粒的晶粒粒径大于300nm小于1000nm(1μm)。像这样，即使是1μm以下的电介质层12，也能确保较大的粒径，获得高介电常数。
根据本实施方式的层叠陶瓷电容器1，按照以上的条件使电介质颗粒生长，由此，在5000以上的介电常数(相对介电常数)、150℃、8.5V/μm的条件下的加速寿命试验中，能够得到25小时以上的高寿命特性。
实施例
下面，说明本发明的层叠陶瓷电容器(以下称作“MLCC”)的实施例。
＜MLCC的制作＞
(1)电介质原料粉末的调制
首先，作为电介质的原料粉末，称量平均粒径为80～280nm的高纯度BaTiO3粉末、和相对于100摩尔的BaTiO3为0.5摩尔的HoO3/2、0.5摩尔的SiO2、0.1摩尔的MnCO3(通过焙烧分离CO2变成MnO)、0.1摩尔的V2O5、0.1摩尔的ZrO2、0～0.04摩尔的MgO的各化合物，准备用作电介质的原料粉末。原料粉末的平均粒径是通过用SEM观察钛酸钡的粉末样品，样品数为500，取其中值粒径而求出的。接着，用水湿式混合表1～3所示的各试样的原料粉末，进行干燥、干式粉碎，调制电介质原料粉末。覆盖层用的电介质原料粉末也按照同样的组成化合物进行准备。
(2)MLCC成型体的制作
利用聚乙烯醇缩醛树脂和有机溶剂湿式混合所调制的电介质原料粉末，采用刀刮法涂覆1.0μm厚的陶瓷生片并将其干燥。对于覆盖层用的陶瓷覆盖片，厚度采用10μm。在作为电介质层的生片上按照规定的图案丝网印刷Ni导电膏，由此配置内部电极。层叠101该配置有电极图案的生片，在电介质层的层叠数n达到100后，在该层叠体的上下，分别在单侧各压接20个10μm厚的覆盖片，然后，将其切割成1.0×0.5mm。然后，将作为外部电极的Ni导电膏涂覆在层叠体的两侧并使其干燥，从而得到MLCC成型体的试样。
(3)MLCC成型体的焙烧
在N2气体气氛中在350℃下对MLCC成型体的试样进行脱粘合剂。然后，在N2、H2、H2O的混合气体(氧分压约为1.0×10-11MPa)中、在1220～1280℃下焙烧10分钟～2小时。适当调整焙烧的温度和时间以得到目标晶粒粒径。焙烧后，在N2气体气氛中在1000℃下进行1小时的电介质的氧化处理。
＜MLCC的评价方法＞
(1)晶粒粒径的评价方法
通过研磨MLCC的一部分截面进行抽取，根据用扫描式电子显微镜(SEM)拍摄电介质层的截面的图片，测定电介质颗粒的晶粒粒径。此处，对于取样的500个电介质颗粒，根据SEM图片通过图像解析测定最大长度Dgs，将它们的平均值作为晶粒粒径Dg进行评价。此外，为了清晰地拍摄SEM图片中的晶界的边界线，预先在与焙烧工艺相同的气体气氛(N2、H2、H2O的混合气体)中，在1180℃下进行5分钟的热处理，实施颗粒界面的热蚀刻。
(2)粒径差异的评价方法
根据从利用扫描式电子显微镜(SEM)拍摄的电介质层的截面图片取样的500个以上的电介质颗粒的晶粒粒径的数据所求出的累计存在率特性(参照图3)，基于该累计存在率特性利用上述式(1)计算出平均粒径差异nd。此外，对于样品采用500个以上，在一张截面图片中有500个以上的情况下，对其中的所有电介质颗粒取样，在未满500个的情况下，从其他部分的截面图片取样，采用500个以上。
(3)介电常数的评价方法
将焙烧后进行了氧化处理的MLCC在150℃恒温槽内静置1小时，然后在室温25℃下静置24小时，在条件齐备后，使用阻抗分析仪测定静电容量Cm。用于测定的电压施加条件为1kHz、1.0Vrms。根据所测定的静电容量Cm，使用下述式(2)求出介电常数(相对介电常数)ε。
Cm＝ε×ε0×n×S/t…式(2)
此处，ε0是真空的介电常数，n、S、t分别是电介质层的层叠数、内部电极层的面积、电介质层的一层的厚度。
(4)寿命特性的评价方法
将焙烧后进行了氧化处理的MLCC在150℃恒温槽内静置1小时，条件齐备后，继续放置在恒温槽内温度为150℃、电场强度为8.5V/μm(相对于电介质层厚为0.7μm，DC电压为6V)的环境下。将直至MLCC的漏电流变成初始的100倍的时间的平均值定义为加速寿命时间，根据该时间评价寿命特性。
＜MLCC的评价结果＞
参照表1～表3说明对于根据以上的条件所制作的MLCC的电介质层的评价结果。
(1)试样No.1～12
[表1]
试样No.1～12是，焙烧前的BaTiO3的原料粒径为110nm，以通过焙烧晶粒粒径达到300nm程度为目标使其生长的例子。相对于100摩尔的BaTiO3，以0～0.04摩尔的范围添加Mg(镁)元素量。
在试样No.1～9中，在Mg的含量为0.03摩尔以下的条件下，介电常数(相对介电常数)ε＞5000，且达到25小时以上的加速寿命。另外，还确认了电介质颗粒的平均粒径差异nd小于4的均匀性。在Mg量为0.04摩尔的试样No.10～12中，介电常数ε小于5000，加速寿命小于25小时，平均粒径差异nd大于4。
(2)试样No.13～17
[表2]
试样No.13～17是，焙烧前的BaTiO3的原料粒径为80nm～280nm的各个样品，是未添加Mg时的例子。确认任意一个样品都是4以下的平均粒径差异nd。
在BaTiO3的原料粒径为80nm～200nm的试样No.13～15中，介电常数ε＞5000，且达到25小时以上的加速寿命。在原料粒径为250～280nm的试样No.16、17中，加速寿命比25小时短。
(3)试样No.18～26
[表3]
试样No.18～21是BaTiO3的原料粒径为110nm，试样No.22～26是BaTiO3的原料粒径为200nm，以晶粒粒径达到300nm～1000nm为目标使其生长时的例子。未添加Mg。
在焙烧后的晶粒粒径大于300nm且小于1000nm的试样No.1、19、20、23～25中，介电常数ε＞5000，且达到25小时以上的加速寿命。另外，还确认了电介质颗粒的平均粒径差异nd小于4的均匀性。
在试样No.18、22中，焙烧后的晶粒粒径小于300nm，介电常数小于5000。在试样No.21、26中，晶粒粒径大于1000nm，平均粒径差异nd大于4，以及加速寿命小于25小时。
符号说明
1  层叠陶瓷电容器
10 陶瓷烧结体
11 层叠体
12 电介质层
13 内部电极层
15 覆盖层
20 外部电极

Claims (7)

1.一种层叠陶瓷电容器，其通过电介质层与内部电极层交替层叠而成，该层叠陶瓷电容器的特征在于：
构成所述电介质层的电介质颗粒的平均粒径差异nd小于4，所述平均粒径差异nd是与累计存在率90％相应的粒径D90除以与累计存在率10％相应的粒径D10所得的值，即：D90/D10。
2.如权利要求1所述的层叠陶瓷电容器，其特征在于：
所述电介质层不含Mg。
3.如权利要求1所述的层叠陶瓷电容器，其特征在于：
所述电介质层中相对于100摩尔的BaTiO3含有0.03摩尔以下的Mg。
4.如权利要求3所述的层叠陶瓷电容器，其特征在于：
所述电介质层中相对于100摩尔的BaTiO3含有0.01摩尔以上且0.03摩尔以下的Mg。
5.如权利要求1～4中任一项所述的层叠陶瓷电容器，其特征在于：
所述电介质颗粒的平均粒径大于300nm且小于1000nm。
6.一种层叠陶瓷电容器的制造方法，其特征在于，包括：
调制平均粒径为200nm以下的电介质原料粉末的工序；和
焙烧所述电介质原料粉末以使构成所述电介质层的电介质颗粒的平均粒径差异nd小于4，并且所述电介质颗粒的平均粒径大于300nm且小于1000nm的工序，所述平均粒径差异nd是与累计存在率90％相应的粒径D90除以与累计存在率10％相应的粒径D10所得的值，即：D90/D10。
7.如权利要求6所述的层叠陶瓷电容器的制造方法，其特征在于：所述电介质原料粉末的平均粒径为80nm以上且200nm以下。
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