CN101687714B - 半导体陶瓷材料 - Google Patents

半导体陶瓷材料 Download PDF

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CN101687714B
CN101687714B CN2008800202491A CN200880020249A CN101687714B CN 101687714 B CN101687714 B CN 101687714B CN 2008800202491 A CN2008800202491 A CN 2008800202491A CN 200880020249 A CN200880020249 A CN 200880020249A CN 101687714 B CN101687714 B CN 101687714B
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胜勇人
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Murata Manufacturing Co Ltd
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Abstract

本发明提供不含Pb、居里点高、而且电阻率低的具有PTC特性的半导体陶瓷材料。该半导体陶瓷材料是以通式：ABO3表示的具有PTC特性的半导体陶瓷材料，A包括Ba、Ca、碱金属元素、Bi及稀土类元素，B包括Ti。相对于100摩尔份Ti，Ca的含量为5～20摩尔份，较好是12.5～17.5摩尔份。碱金属元素的含量/(Bi的含量+稀土类元素的含量)优选为1.0～1.06的范围。较好是还含有Mn，其含量相对于100摩尔份Ti为0.01～0.2摩尔份。

Description

半导体陶瓷材料
技术领域
本发明涉及具有PTC特性的半导体陶瓷材料。
背景技术
正特性热敏电阻所用的半导体陶瓷材料具有PTC特性，该PTC特性是指在升温过程中到达规定的温度(居里点)时电阻急剧升高的特性。具有该PTC特性的半导体陶瓷材料已知例如BaTiO3系材料。
此外，根据用途不同，加热器用热敏电阻等有时要求半导体陶瓷材料能够在更高的温度使用，也已知为了提高居里点而将BaTiO3系中的部分Ba用Pb置换得到的(Ba，Pb)TiO3系的半导体陶瓷材料。
但是，如上所述添加Pb来提高居里点时，例如像日本专利特开昭56-169301号公报(专利文献1)中作为现有技术的问题点所述的那样，电阻温度系数小，具有电压依赖性，因此需要以不含Pb的组成具有高居里点的正特性热敏电阻用的半导体陶瓷材料。
因此，最近，作为具有高居里点的正特性热敏电阻用半导体陶瓷材料，已知例如上述专利文献1或日本专利特开2005-255493号公报(专利文献2)所揭示的(Ba，Na，Bi，Ln)TiO3系的半导体陶瓷材料(Ln为稀土类元素)。
但是，存在居里点越高，室温下的电阻率越高的倾向，上述专利文献1或2所揭示的组成虽然居里点高达120℃以上，但电阻率也较高、达到70Ω·cm左右，用于加热器用热敏电阻时，需要具有更低的电阻值。
专利文献1：日本专利特开昭56-169301号公报
专利文献2：日本专利特开2005-255493号公报
发明内容
为此，本发明的目的是提供不含Pb、居里点高、而且电阻率低的具有PTC特性的半导体陶瓷材料。
本发明涉及以通式：ABO3表示、且具有PTC特性的半导体陶瓷材料，其特征在于，A包括Ba、Ca、碱金属元素、Bi及稀土类元素，B包括Ti，相对于100摩尔份Ti，Ca的含量为5～20摩尔份。
较好是相对于100摩尔份Ti，上述Ca的含量为12.5～17.5摩尔份。
碱金属元素、Bi及稀土类元素的含量以碱金属元素的含量/(Bi的含量+稀土类元素的含量)表示时，优选1.0～1.06的范围。
本发明的半导体陶瓷材料较好是还含有Mn，其含量相对于100摩尔份Ti为0.01～0.2摩尔份。
本发明的半导体陶瓷材料，由于以相对于100摩尔份Ti为5～20摩尔份的含量将A位(Ba位)的一部分用Ca置换，因此如后述的实验例所表明的，可以在保持居里点高的同时降低电阻率。因此，将本发明的半导体陶瓷材料应用于正特性热敏电阻时，可以流通大电流，从而可以得到大功率的PTC加热器。
如果Ca的含量在相对于100摩尔份Ti为12.5～17.5摩尔份这样进一步限定的范围内，则可以进一步降低电阻率。
如果使碱土类金属元素的含量/(Bi的含量+稀土元素的含量)在1.0～1.06的范围内，则可以在获得良好的载流子平衡的同时提高电阻变化率，从而使PTC特性提高。
此外，如果本发明的半导体陶瓷材料还含有相对于100摩尔份Ti为0.01～0.2摩尔份的作为受主的Mn，则可以形成晶界处的受主能级，提高电阻变化率，进一步提高PTC特性。
附图说明
图1为实验例1制得的评价用试样的由X射线衍射算出的陶瓷晶体的a轴和c轴的各轴长及轴比(c/a轴比)与Ca含量的关系图。
图2为对实验例1所得的烧结体试样的经化学腐蚀的研磨面的SEM像所显示出的陶瓷晶体的晶界进行描绘而制成的图。
具体实施方式
本发明的具有PTC特性的半导体陶瓷材料具有以通式：ABO3表示的组成。在ABO3中，A包括Ba、Ca、碱金属元素(Na、K、Li等)、Bi及作为施主的稀土类元素(La等)，B包括Ti。并且，相对于100摩尔份Ti，上述Ca的含量为2～20摩尔份，较好为12.5～17.5摩尔份。
一般，在BaTiO3系的半导体陶瓷材料中，为了使作为PTC热敏电阻的耐电压性提高，有时添加Ca。这是因为随着Ca的添加，微粒化得到促进，藉此使耐电压性提高。换而言之，由于随着Ca的添加，微粒化得到促进，因而存在电阻率上升的趋势。因此，像本发明这样通过添加Ca使电阻率降低的效果是不同于一般的。
已知本发明的半导体陶瓷材料的组成(Ba，Na，Bi，Ln)TiO3系(Na有时被其它碱金属元素置换。Ln为稀土类元素)原本粒子就呈微粒的状态，通过向其中添加Ca，使粒子生长(参照后述的实验例)。
此外，在具有PTC特性的半导体陶瓷材料中，通过陶瓷晶体的粒内的自发极化抵消晶界势垒而低电阻化。根据本发明可知，通过用Ca置换部分Ba，晶体的正方性(日文：正方晶度)提高、即晶体的轴比(c/a轴比)变大。其结果是，可以推测自发极化增大，晶界势垒被抵消，从而低电阻化。
在本发明的半导体陶瓷材料中，碱金属元素、Bi和作为施主的稀土类元素的含量以碱金属元素的含量/(Bi的含量+稀土类元素的含量)表示时，优选1.0～1.06的范围。在该范围内，载流子平衡良好，并且可以获得高电阻变化率，使PTC特性提高。
此外，本发明的半导体陶瓷材料较好是还含有相对于100摩尔份Ti为0.01～0.2摩尔份的作为受主的Mn。像这样添加Mn会形成晶界处的受主能级，提高电阻变化率，使PTC特性进一步提高。
SiO2容易与Bi或Na形成玻璃成分。因此，即使将主成分预烧后，再添加S iO2实施烧成，也容易在烧成时与预烧时未反应而残留的Na和Bi一起进入玻璃中。其后果是，有时会引起母晶的组成偏离，电阻上升。因此，添加SiO2时，较好是相对于100摩尔份Ti为0.2摩尔份以下。
以下，对为了确定本发明的范围、以及为了确认本发明的效果而实施的实验例进行说明。
实验例1
按照在烧成后形成表1所示的组成的条件，称量、调合作为原料的BaCO3、CaCO3、Na2CO3、Bi2O3、TiO2和半导体化剂La2O3各粉末。
然后，在调配好的混合粉末中加入乙醇系溶剂，用氧化锆球混合粉碎24小时，使溶剂干燥，用50号筛整粒。接着，将经过造粒的混合粉末在800～1000℃的温度范围内进行2小时热处理，获得预烧粉末。
接着，在该预烧粉末中加入作为有机粘合剂的乙酸乙烯酯、作为分散剂的聚羧酸铵和水，用氧化锆球混合粉碎处理16小时。使粉碎处理后的浆料干燥，用50号筛整粒，得到压制用的原料。
采用单轴压制机以1000kgf/cm2的压力将该原料成型为圆板状，制得圆板试样。接着，将该圆板试样在大气中脱脂后，在使最高温度为1250～1400℃的同时，在氮气气氛中进行2小时的烧成。藉此，将圆板试样制成直径12mm、厚2mm的试样。表1所示的组成是通过电感耦合等离子体发射光谱分析法(ICP-AES)分析求得的。
然后，将如上所述烧好的圆板试样的两主面用砂纸研磨，采用干式镀敷在研磨面上形成由Ni/Ni-Cu合金/Ag构成的电极，藉此制得评价用试样。接着，使用该评价用试样，评价表1所示的特性。
电阻率是在室温(25℃)下，向评价用试样施加0.1V的直流电压，用直流四端子法测定室温电阻值，计算该室温电阻值的每单位长度的电阻，作为电阻率。
PTC数量级(日文：PTC桁数)是指通过与上述室温电阻值测定时同样的方法测定25℃时的电阻值(R25)和250℃时的电阻值(R250)，将由ΔR＝log(R250/R25)的公式求出的电阻变化率ΔR作为PTC数量级。
Tc(居里点)是将上述电阻率达到2倍的点的温度定义为居里点。
表1
Figure G2008800202491D00051
由表1可知，相对于100摩尔份Ti的Ca含量为5～20摩尔份的范围内的试样4～9在保持Tc高达150℃以上的居里点的同时，得到了30Ω·cm以下的电阻率。Ca的含量在12.5～17.5摩尔份这样进一步限定的范围内的试样6～8得到了更低的电阻率。
与此相对，Ca的含量低于5摩尔份或超过20摩尔份的试样1～3和10～12显示出超过30Ω·cm的电阻率。
这样，在本发明的范围内的试样4～9得到低电阻率的原因可以作如下推断。
图1中，以与Ca含量的关系示出了由X射线衍射(XRD)算出的陶瓷晶体的a轴和c轴的各轴长及轴比(c/a轴比)。如图1所示，具有c/a轴比随着Ca含量的增加而增加的趋势。即，可推测陶瓷晶体具有更高的铁电性。因此可以认为，这对于依靠自发极化而低电阻化的具有PTC特性的半导体陶瓷材料是有利的，通过Ca的含有和其含量的增加而实现低电阻。此外，如表1所示，Ca含量超过20摩尔份时，电阻率再次上升，这是由于超过了Ca的固溶极限。
图2(a)、(b)、(c)和(d)是对Ca含量分别为0摩尔份、5摩尔份、20摩尔份和25摩尔份的试样1、4、9和11的各烧结体的经化学腐蚀的研磨面的SEM像所显示出的陶瓷晶体的晶界进行描绘而制成的图。
由图2可知，Ca含量增加到5摩尔份以上且不超过20摩尔份时，可见陶瓷晶体的晶粒生长，粒径进一步变大。基于此可以认为，通过使Ca含量在5～20摩尔份的范围，可以减少烧结体的每单位厚度的晶界数，因而有助于低电阻化。
在图2(d)所示的Ca含量为25摩尔份时虽然也可见一定的晶粒生长，但在晶界析出白色的多相(在图中以阴影线部分表示)。可以认为这是因为Ca在陶瓷晶体的晶界析出，结果导致电阻率上升。
实验例2
再准备Mn3O4粉末，按照烧成后形成表2所示的组成的条件加入所述预烧粉末中，除此之外，通过与实验例1同样的方法制作评价用试样，且进行同样的评价。
表2
Figure G2008800202491D00061
由表2可知，虽然可见由于添加Mn，电阻率有一些增高的趋势，但如试样23～25所示，通过使Mn添加量在相对于100摩尔份Ti为0.01～0.2摩尔份的范围内，可以在不会引起电阻率太大上升，并且不会使Tc(居里点)太大降低的前提下，使PTC数量级上升至4.0数量级以上。这样，PTC特性的提高可以认为是由于通过添加规定量的Mn，在陶瓷晶体的晶界形成了受主能级。
实验例3
除了如表3所示，在烧成后的组成中，对[Na/(Bi+La)]的比率作各种变更之外，用与实验例1同样的方法制作评价用试样，且进行同样的评价。
表3
Figure G2008800202491D00071
如表3所示，[Na/(Bi+La)]在1.0～1.06范围内的试样33～36与在该范围外的试样31、32及37相比，可以进一步增大PTC数量级，可以实现PTC特性的提高。
实验例4
再准备Mn3O4粉末和SiO2粉末，按照烧成后形成表4所示的组成的条件将它们加入所述预烧粉末中，除此之外，通过与实验例1同样的方法制作评价用试样，且进行同样的评价。
表4
Figure G2008800202491D00081
由表4可知，如试样56～62，SiO2的添加量相对于100摩尔份Ti为0摩尔份或0.2摩尔份以下时，电阻率低、PTC数量级大、Tc高。而相对于100摩尔份Ti添加了0.3摩尔份SiO2的试样63的电阻率增高、PTC数量级减小。

Claims (4)

1.一种半导体陶瓷材料，它是以通式ABO3表示、且具有PTC特性的半导体陶瓷材料，其中，A包括Ba、Ca、碱金属元素、Bi及稀土类元素，B包括Ti，并且相对于100摩尔份Ti，所述Ca的含量为5～20摩尔份；居里点Tc在150℃以上。
2.如权利要求1所述的半导体陶瓷材料，其特征在于，相对于100摩尔份Ti，所述Ca的含量为12.5～17.5摩尔份。
3.如权利要求1所述的半导体陶瓷材料，其特征在于，所述碱金属元素、所述Bi及所述稀土类元素的含量以碱金属元素的含量/（Bi的含量＋稀土类元素的含量）表示时，在1.0～1.06的范围内选择。
4.如权利要求1～3中任一项所述的半导体陶瓷材料，其特征在于，所述半导体陶瓷材料还含有Mn，其含量相对于100摩尔份Ti为0.01～0.2摩尔份。
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