CN1659114A - 低热膨胀铝酸钙制品的制造 - Google Patents

低热膨胀铝酸钙制品的制造 Download PDF

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CN1659114A
CN1659114A CN038131218A CN03813121A CN1659114A CN 1659114 A CN1659114 A CN 1659114A CN 038131218 A CN038131218 A CN 038131218A CN 03813121 A CN03813121 A CN 03813121A CN 1659114 A CN1659114 A CN 1659114A
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calcium aluminate
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D·M·比尔
S·G·拉克瓦尼
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Abstract

揭示了组合物、制品以及制造这类制品的方法。所述组合物、制品与方法包括具有低热膨胀系数的铝酸钙材料。

Description

低热膨胀铝酸钙制品的制造
发明领域
本发明涉及铝酸钙组合物，由这些组合物制成的制品，以及制造这类制品的方法。更具体地说本发明涉及低热膨胀性的铝酸钙组合物与制品，以及制造低膨胀制品的方法。
发明背景
低热膨胀陶瓷体在各种各样应用中是理想的。例如，低膨胀体已被用作流体过滤器，特别用作柴油机内颗粒过滤器，以及用作催化转化器的基材，这种应用例子在现有技术中已知为蜂窝状载体。此外，低膨胀体在抗热冲击性和最终使用温度均很高的应用是理想的。高温度梯度条件下使用的基材是这方面应用的例子。例如，蜂窝状与多孔载体结构需经受苛刻的环境，要求具备高抗热冲击性、低热膨胀性、以及高抗机械冲击性等。要达到在使用环境中在相当长一段时间内维持这些性质，因而淘汰了许多有潜质用途的耐火材料。
堇青石基材，通常呈蜂窝体形式，长期来被优选用作载体，负载汽车催化转化器的催化活性组分，部分是由于其高抗热冲击性。抗热冲击性与热膨胀系数呈反比关系。即低热膨胀的蜂窝体具有良好的抗热冲击性，能承受应用中遭遇大范围的温度波动。制造商们持续研究，以优化堇青石基材的特性，加强其作为催化剂载体的用途。特别是，制造商们继续努力优化堇青石基材的抗热冲击性与其它性质。
某类催化剂的其它特性是指净化发动机运行期间产生的尾气的能力和将运行期间产生的一氧化碳、烃类化合物与氮氧化物(NOx)转化成对环境危害较轻的气体的能力。一些催化剂体系利用催化剂载体所包含的碱金属来储存氮氧化物，这类催化剂在现有技术中被称之为NOx吸收剂。现有的催化剂与净化体系的缺点在于储存NOx用的催化剂载体包含的大多数碱金属在有利于NOx吸收剂的操作温度范围内容易与堇青石反应。例如，钾是广泛使用的碱吸收剂材料，易与堇青石反应，将钾从高表面积基面涂层提取出来，阻止其发挥吸收剂的功能。而且，钾与堇青石反应会形成热膨胀系数(CTE)相对较高的物相，使得基材与催化剂体系的抗热冲击性更差。
需要提供另一种能在高温下使用的低CTE材料。要求提供的材料具有低CTE与优异的抗热冲击性。
发明概述
本发明一个实施方式涉及一种陶瓷制品，由包含CaAl4O7主相和CaAl2O4 -次相的铝酸钙组成，该制品在25-800℃温度范围具有小于约25×10-7/℃的热膨胀性。其它实施方式涉及的铝酸钙制品在25-800℃温度范围具有小于约20×10-7/℃的热膨胀性。还有一些实施方式涉及的铝酸钙制品在25-800℃温度范围具有小于约15×10-7/℃的热膨胀性。其它实施方式涉及的铝酸钙制品具有在25-800℃温度范围小于约10×10-7℃的热膨胀性。还有的实施方式涉及在25-800℃温度范围具有小于约5×10-7/℃热膨胀性的铝酸钙制品。在某些实施方式中，所述制品含有微裂纹网，且包含平均粒径在10-100μm之间的颗粒。本发明制品具有广泛用途，包括制造高温下使用的蜂窝状基材与吸收NOx用的蜂窝状基材，但不限于此。
本发明其它实施方式涉及制造呈现低热膨胀性的铝酸钙制品的方法。在一个实施方式中，所述方法包括将CaO与Al2O3粉末原料混合在一起，形成有形制品，将所述制品加热到超过1500℃，形成含有21.6-30重量％的CaO的制品。在一些实施方式中，制品在某一温度下焙烧，使焙烧后的制品中存在微裂纹网与粒度在焙烧10-100μm的颗粒。
应理解，上面一般性描述与下面详细描述是示例性的，旨在对声明的本发明提供更详细的说明。
附图简要说明
图1是CaO-Al2O3的相图。
详细说明
在叙述本发明几个示例性实施方式之前，应理解，本发明不限于下面叙述中描述的构造与方法步骤的细节。本发明能以其它方式实施或以各种方式执行。
本发明各种实施方式提供了材料、方法与具有低热膨胀系数的制品。已发现，要获得这种低热膨胀性，通过改变原材料组合与焙烧程序就可得到要求的性质。通过控制焙烧后制品的最终化学计量，原料类型适当组合，以及在制造陶瓷制品中采用的焙烧程序，可以降低制品的热膨胀。
制造本发明制品的一般方法包括混合适当的批料，较好是平均粒径小于150μm的原料。在某些实施方式中，起始原料粉末的平均粒径小于50μm，在另一个实施方式中，平均粒径小于约15μm。将混合后的粉料进行掺混，然后通过挤出或其它合适方法成形为生坯件，如蜂窝体。随后，将生坯件烧结成硬质多孔结构。各种润滑剂与有机粘接剂如甲基纤维素在混料阶段加入到批料中，进行粘度控制和提供焙烧前的强度，并在焙烧后提供多孔结构。孔隙率还可以由原料和焙烧温度控制。较高的焙烧温度得到孔隙率较低的结构。在某些实施方式中，焙烧温度高于1450℃，在其它实施方式中，焙烧温度高于1550℃。在其它实施方式中，制品在高于1550℃的焙烧温度下焙烧，一些实施方式中，制品在至少约1600℃下焙烧。从下面实施例可见，实际焙烧温度取决于陶瓷体的化学计量。
本发明另一个实施方式涉及可用于制造催化剂载体的低热膨胀铝酸钙材料。铝酸钙是相对不活泼的耐火材料，通常其热膨胀系数在25℃-800℃温度范围时为40-60×10-7/℃。尽管在一些应用场合下，此范围的热膨胀是可以被接受的。但在有些应用场合下此范围是被禁用的，因为热冲击条件与热应力对整体结构形式的制品来说太严格了。
因此，低CTE对于制造在某些热冲击条件下更具抵抗失效的整体结构的制品来说是所希望的。专利申请人惊奇地发现，可以制造比由这类材料制成的制品通常呈现的CTE低得多的铝酸钙制品。改变氧化钙与氧化铝的比率和所述制品的焙烧温度，提供的铝酸钙化合物具有出奇低的CTE值。通过混合21.6-30重量％的CaO原料粉末与余量的氧化铝原料粉末，将新的混合物制成制品，再在超过1500℃的温度下焙烧，在某些实施方式中，温度最佳在约1600℃。然而，实际焙烧温度将取决于坯体的组成。CaO原料粉末的例子包括碳酸钙、草酸钙、氟化钙与氢氧化钙粉料。尽管可以使用纯净态的氧化钙，但本技术领域的技术人员认为，纯氧化钙与水反应活性高，因此很难处理与储存。如果碳酸钙用作氧化钙来源粉末，则用于达到合适的CaO重量百分含量的粉末重量必须向上调整1.785倍，此值反映等当于CaO所需的重量。按照本发明某些实施方式，被制成的制品在25-800℃温度范围内，CTE值低至约-1×10-7/℃。
按照本发明一个特定的实施方式，提供了净化尾气的催化剂系统的基材，特别可用于提高NOx的吸收效率。在某些实施方式中，载体在低于1000℃具有抗碱金属迁移的能力，以及在25-800℃温度范围低于25×10-7/℃的热膨胀系数。在有些实施方式中，热膨胀系数在25-800℃温度范围低于20×10-7/℃。在某些实施方式中，在25-800℃温度范围，热膨胀系数低于15×10-7/℃。在其它实施方式中，在25-800℃温度范围，CTE低于10×10-7/℃。
不想以任何方式限制本发明，通过下面实施例更为完整叙述本发明。
实施例
表I以重量％列出为适当混合生成铝酸钙陶瓷体的无机粉末批料。所述无机组合物由平均粒径为0.5-20μm的碳酸钙和氧化铝原料粉末与纤维素醚粘合剂一起干混形成，所述纤维素醚者是甲基纤维素，或是其它纤维素醚的衍生物，占无机组分重量的2-6％。然后将水加入到混合批料，加水量占无机组分重量的15-25重量％。最终形成的批料进一步混合，形成增塑的陶瓷批料混合物。使用柱塞式挤出机，将所有照此制备的批料通过蜂窝状模，挤出成蜂窝陶瓷结构。将该蜂窝体干燥过夜，并在1320-1600℃温度焙烧，烧所述部件，生成要求的化学计量量的铝酸钙蜂窝结构。在1320℃焙烧的组合物在最高温度下保温24小时，而所有其它样品保温8-24小时。下面列出的组合物是基于CaO与Al2O3，但用碳酸钙作为CaO的来源材料。
含有50％与40％CaO的组合物在不高于1320℃温度下焙烧，因为在高于这个温度焙烧，这种坯体会熔化。约含大于21.6％CaO的组合物在不高于1600℃下焙烧，因为高于这个温度焙烧，会引起坯体的熔化。
                                                  表1
    CaO(重量％)     Al2O3(重量％)     CaAl4O7(重量％)     1320℃     1400℃     1450℃     1500℃     1550℃     1575℃     1600℃
    50     50     0     47
    40     60     0     41
    32     68     25     63     65     50
    29.2     70.8     45     45     48     43.3
    27.8     72.2     55     54     28.4     30.7
    26.4     73.6     65     52.3     29.9     21.6
    25.0     75.0     75     44.6     17.2     13.9
    24.8     75.2     77     51.0     53.0     16.3
    23.7     76.3     85     24.7     12.2     5.6     1.9
    22.3     77.7     95     24.1     1.0     -1.0     -3.3
    21.6     78.4     100     51     38
    19     81     92     52     51     50
    10     90     11     71     73     72
    5     95     0     73     78
专利申请人测得改变起始原料的粒度对所测试的组合物与焙烧温度下制成的坯体的最终CTE的影响可以忽略。表中结果表明含有27.8-21.6重量％的CaO坯体获得CTE低于25×10-7/℃的铝酸钙坯体。CaO含量更高的坯体需要在更高的焙烧温度下焙烧方能获得低CTE值。与其它例子相比，相对于文献报道的铝酸钙的典型CTE值，在25-800℃温度范围通常大于40×10-7/℃，在此范围的所述组合物呈现极低的CTE值。因此，起始CaO含量在20-30重量％和70-80重量％的氧化铝并在超过1500℃温度下焙烧的组合物具有低CTE值，即在25-800℃温度范围其CTE值小于25×10-7/℃。这些低CTE材料用于可能遭遇热冲击条件的应用，因为较低的CTE值可改善材料的抗热冲击性。这些材料特别适合于制造用于汽车中使用的催化剂基材。
尽管本发明不愿受具体理论的限制，但认为在此所述的新颖铝酸钙坯体经焙烧后在冷却中形成微裂纹网。所述低CTE值坯体由CaAl4O7主相(Grossite或二铝酸钙或CA2)和CaAlO4次相(单铝酸钙或CA)构成。微裂纹被认为是由于CaAl4O7晶胞的热膨胀各向异性所造成的。这种热膨胀各向异性造成不同取向晶粒之间局部不匹配应力，如果所述应力足够大，有可能在陶瓷微结构中产生微裂纹。较高焙烧温度可能使颗粒生长到大于形成微裂纹所要求的临界平均粒度。可以认为所述微裂纹网对降低宏观的热膨胀系数是有效的，因为在加热过程中，所述微裂纹弥合了。通过调节周围晶粒的正热膨胀来达到这种微裂纹的弥合。因为正的热膨胀用来弥合微裂纹，它的作用是降低多晶样品的总的热膨胀。观察到的这种热膨胀系数降低一直持续至裂纹充分弥合，至此，所观察到的CTE值恢复到该晶胞三个晶轴中线性膨胀的平均值。冷却中该过程相反，由于局部相邻晶粒排列紊乱产生的断裂能又致使裂纹重新开裂。
查看CaO-Al2O3相图(如图1所示)，可以发现处于CA+CA2相区的组合物(约含64.5-78.4％Al2O3)在接近1600℃(或低于1600℃，取决于杂质含量)焙烧时形成液相。氧化铝含量在此范围的本发明组合物，有可能在高焙烧温度下液相生成，导致液相烧结。这种液相可能支持形成微裂纹所必需的大晶粒生长。
当CA+CA2相区中的组合物在足够高温度下焙烧时形成液相，且稳定相为CA2+液相。在指定温度下生成液相的量随原料组合物中氧化铝量的降低而增加，由杠杆规则定义。因此，在此相区的组合物中氧化铝含量可增加至高达几近78.4％，固相线温度之上出现的液相量降低。可以认为，从加工观点来看，优选获得充分晶粒生长所需的最少量液相。这是因为大量液相会造成部件明显变形，以及粘结到焙烧陶瓷制品用的耐火架子(setter)上。
因此，为了获得富含CA2组合物的低热膨胀系数，坯体应包含细小微裂纹网，它能在受热时因裂纹弥合而起到缓和整体膨胀的作用。相反，在冷却的时候，所述裂纹重新开裂造成热膨胀下降，因为与冷却时没有裂纹开裂的坯体相比，裂纹的开裂降低了热膨胀系数。在样品的SEM照片上还可以发现，要求CA2相一定的晶粒粒度来产生足够的断裂能，以在邻近排列紊乱晶粒中产生微断裂构造。可以认为这样的晶粒粒度在10-100μm之间。
在不偏离本发明的范围和精神的条件下，显而易见，本领域普通技术人员可以对本发明作各种改进和变动。因此，本发明包括所有这些修改和变化，只要它们均在所附的权利要求和其等价内容的范围内。

Claims (10)

1.一种陶瓷制品，由包含CaAl4O7主相和CaAl2O4次相的铝酸钙组成，其热膨胀性在25℃-800℃温度范围小于约25×10-7/℃。
2.如权利要求1所述的制品，其特征在于所述制品的热膨胀在25℃-800℃温度范围小于约15×10-7/℃。
3.如权利要求1所述的制品，其特征在于所述制品的热膨胀在25℃-800℃温度范围小于约10×10-7/℃。
4.如权利要求1所述的陶瓷制品，其特征在于所述制品的热膨胀在25℃-800℃温度范围小于约5×10-7/℃。
5.如权利要求1所述的陶瓷制品，其特征在于所述制品含有微裂纹网。
6.如权利要求5所述的陶瓷制品，其特征在于所述制品含有平均粒度在10-100μm的颗粒。
7.如权利要求6所述的陶瓷制品，其特征在于所述制品是用于尾气净化的催化剂。
8.一种制造铝酸钙制品的方法，所述制品的热膨胀在25℃-800℃温度范围小于20×10-7/℃，所述方法包括混合CaO源和Al2O3粉末原料，包含原料粉末的所述混合物制成含有21.6-30重量％CaO和余量为Al2O3的最终相，由所述粉料制成成形制品，并在超过1450℃温度下加热。
9.如权利要求8所述的方法，其特征在于，由包含粉末原料的所述混合物制成含有21.6-22重量％CaO和余量为Al2O3的粉末原料的最终相，以及所述制品在超过1550℃温度下进行焙烧。
10.如权利要求8所述的方法，其特征在于，由包含粉末原料的所述混合物制成含有21.6-22重量％CaO和余量为Al2O3粉末原料的最终相，以及所述制品在1450℃-1600℃温度下进行焙烧。
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