CN101918341A - 含有作为主要成分的各向同性高结晶天然石墨和碳化硅或碳化锆的反射器用石墨及其制备 - Google Patents

含有作为主要成分的各向同性高结晶天然石墨和碳化硅或碳化锆的反射器用石墨及其制备 Download PDF

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CN101918341A
CN101918341A CN2008801251669A CN200880125166A CN101918341A CN 101918341 A CN101918341 A CN 101918341A CN 2008801251669 A CN2008801251669 A CN 2008801251669A CN 200880125166 A CN200880125166 A CN 200880125166A CN 101918341 A CN101918341 A CN 101918341A
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graphite
reflector
weight
reflector graphite
carbide
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米兰·赫罗瓦特
卡尔-海因茨·格罗斯
理查德·西曼
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ALD Vacuum Technologies GmbH
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Abstract

反射器用石墨，其特征在于，该石墨的主要成分包括核纯的天然石墨以及碳化硅和/或碳化锆。通过组合的热-冷-挤压使压制品成形且对压制品进行的热处理限制在小于2000℃的温度。

Description

含有作为主要成分的各向同性高结晶天然石墨和碳化硅或碳化锆的反射器用石墨及其制备
“反应堆用石墨”这个名称产生于1942年末，其时在石墨慢化核反应堆中实现了第一次核裂变。石墨在核技术中的意义主要在于它有利的核物理特性。其中包括它的低俘获截面、相对低的质量数和好的慢化剂效果和反射器效果。
在设计和建造气冷式核反应堆时，特别是设计和建造MK II高级气冷式反应堆(Mk II Advanced Gascooled Reactors，AGR)和氦气高温气冷式反应堆(HTGR)时，核纯(nuklearrein)的石墨成了最重要的反应堆材料之一。反应堆用石墨不仅用于制备燃料元件而且用作固定组件。固定组件主要指的是反射器(Reflektor)。反射器的作用是避免反应堆堆芯因迁移引起的中子损失。此处，堆芯被由石墨构成的顶部反射器、侧面反射器和底部反射器包围。用于构成反射器的反应堆用石墨称作反射器用石墨(Reflektorgraphit)。
对于反射器用石墨提出了以下一系列要求：
-石墨必须是核纯的。这是指，热中子的俘获截面应该小于4mbar；
-体积密度大；
-机械强度特性好；
-弹性模量低；
-热膨胀低；
-导热性好；
-针对水蒸汽和空气的耐腐蚀性好；
-快中子辐照时的稳定性好。
石墨在反应堆运行中暴露于高通量的快中子中。中子会引起石墨晶格的剧烈破坏。因为辐照引发的破坏对石墨的物理特性、机械特性和化学特性会产生强烈的负面影响，所以反应堆用石墨要求有满足反应堆设计标准的抗辐照性。
这些要求在反射器用石墨的情况下首先被提及，这是因为所述反射器用石墨的寿命应该至少有30年。在这个期间所述反射器用石墨接受大约3×1022n/cm2个E＞0.1MeV的高积分通量(integrale Fluenz)的快中子。
相关文献中提到，在用快中子辐照、温度高于1000℃和高中子剂量的条件下，石墨只有当符合如下条件时才能保持所需的尺寸稳定性和机械完整性多晶的石墨成型体具有高石墨化度和具有在复合体存在的单个石墨晶体的各向同性的取向。
辐照行为和其相应的结果描述于GA-Report(1970年3月)Engel，G.B.：“Irradiation Behaviour of Nuclear Graphites at Elevated Temperatures”和BNWL-1056Report Pacific Northwest-Laboratory Richland，Washington(1969)，Helm，J.W.中，等。
到目前为止用于制备反射器部件的反应堆用石墨仅由人造石墨(Elektrographit)制备。
图1表示生产反应堆用石墨的示意图。
多晶的石墨成型体包含纯的单质碳(elementar Kohlenstoff)。从原材料到石墨成品都要求是纯的且是可石墨化的。
石油焦和沥青焦满足所述要求，该石油焦和沥青焦源自原油残渣和煤焦油沥青的焦化。优选焦油或沥青用作含碳的液体粘结剂。人造石墨的制备基于多个步骤。
在图1中图示描述了制备步骤和其时间上的顺序。
首先对经煅烧的石油焦和沥青焦进行研磨，筛分，并且在与液态焦油粘结剂和液态沥青粘结剂混合之后通过热混捏(Heiβkneten)均化，接着通过管线挤压(Strangpressen)或空腔挤压(Gesenkpressen)制备新制(grün)的成型体。粘结剂含量较高，计大约30重量％。接着，在气热圆炉或油热圆炉里于气封条件下进行的烧成过程中，通过焦化粘结剂生成了固体产品，所谓的硬燃烧炭(Hartbrandkohle)。用焦炭粉(Kokspulver)包覆该新制的炭成型体从而防止在圆炉里被烧损。焦化之后，对炭成型体进行最多达3100℃的温度处理。
这个过程发生在大型的电阻加热的石墨炉里，其中，具有焦炭粗粒(Koksgrieβ)包覆层的用料(Einsatzgut)用作加热电阻。
石墨化通过在长度长至30m的长炉中发生短路而进行。该炉的电流消耗最多达80000安培。在石油焦或沥青焦中焦化形成的直径约0.5nm的石墨晶体(初级热解
Figure BPA00001184779000022
))在该石墨化过程中生长至约60nm。在粘结剂焦化时形成的焦炭桥
Figure BPA00001184779000023
称作二次热解
Figure BPA00001184779000024
石墨化是物理结晶过程。石墨化程度主要取决于温度、焦炭的可石墨化性和初级与二次形成的焦炭的比例。
测量显示，在优化所述的参数时，初级焦炭和二次焦炭的石墨化程度随着温度的提高而提高，但是达不到天然石墨的石墨化程度。
合成的反应堆用石墨的制备方法在如下公开文献中得以描述：H.Nickel：Reaktorchemische Probleme，Teil II，Kernforschungsanlage Jülich，Institut fürReaktorwerkstoffe(1968)。
对于反应堆用石墨而言，特别是对于后续几代的高温核反应堆(HTR)的反射器而言，额外要求提高的针对氧气和水蒸气的耐腐蚀性。假设发生如下事故反射器用石墨应该还是保持运行能力的，这些事故例如冷却完全失效和/或空气、水或水蒸气不可控制地进入堆芯。
本发明的目的如下，研制用于气冷式核反应堆、特别是用于高温反应堆的反应堆用石墨和其制备，这种反应堆用石墨适合用于制备组件，主要适合用于制备反射器部件，并且不用更换反应堆用石墨就能保证反应堆终生运行。
根据本发明，该目的通过如下方式解决：反应堆用石墨是核纯的，并且主要成分包含核纯的天然石墨以及碳化硅和/或碳化锆，其中碳化物是通过加热粘结剂焦炭或炭黑与SiO2和/或ZrO2在原位形成的。粘结剂焦化时形成的二次焦炭最后通过在真空中和在温度小于2000℃时进行加热转化成上述碳化物。本发明优选涉及反射器用石墨。
在本发明的范围内，“主要成分”表示反应堆石墨基本上由这些成分组成。在此，“基本上由......组成”指的是，没有添加给石墨但在本发明的处理后在石墨中存在附加物。然而根据本发明的石墨包含通常的杂质。这些杂质是在使用的材料中存在的并且不是为了达到确定的目的而加入的物质。
本发明反应堆用石墨的制备是基于以下原料成分：
-细磨的核纯的高结晶度天然石墨粉
-粉末形式的在高于3000℃石墨化的石油焦(也可以使用炭黑代替石墨化的石油焦)
-苯酚甲醛-粘结剂树脂
-SiO2粉末或ZrO2粉末和挤压辅料十八酸和辛醇
在图2中图示描述了方法步骤。
该方法优选为制备反射器用石墨的方法。在室温时通过与溶于甲醇中的粘结剂树脂混捏并接着进行干燥和研磨，生成了挤压粉(Preβpulver)。
根据本发明，挤压通过两个步骤得以实现：
在第一步中，在室温时对橡胶形式的球体在尽可能等压和在大于100MPa的较高压力下进行挤压。接着破碎这些球体从而产生平均直径约为1mm的颗粒。具有一定孔隙率的颗粒由大约1百万个空间上各向同性排列的初级石墨粒子构成。
在第二步中，各向同性的颗粒在粘结剂树脂的塑性温度范围内和约12MPa的较低压力下最后挤压成为石墨成型体(Graphitformteilen)。为了改善可挤压性，在颗粒晶粒(Granulatkorn)中引入润滑剂和排气剂。已证明十八酸是最好的润滑剂。
在最后挤压(fertigpressen)石墨成型体时颗粒松散物料(Granulatschüttung)中含有的空气被证明是非常有害的。在挤压过程中所述空气中的大部分挤入压制品中并且在那里最终被压进去。在焦化时该压缩空气的回弹量(Rückfederung)导致压制品结构的弱化
Figure BPA00001184779000041
在焦化之后石墨成型体显示与形成裂缝有关的趋向平均的性质梯度。为了避免这种性质梯度，根据本发明向晶粒颗粒中引入碳氢化合物，所述碳氢化合物的蒸汽气压从室温时非常低的数值升高至挤压温度时的大约1000hPa，并且在挤压时液化。以这样的方式，在加热期间位于钢模(Stahlmatrize)里的空气通过形成蒸汽得以排出，并且石墨成型体的压力负荷在热处理时大量地减少。这里辛醇证明是最好的排气剂。
加热至挤压温度在配置有油热加热装置的钢模中进行。加热装置包含温度仪器用于加热和冷却。该温度仪器提供给底板和模两个加热循环。在加热时向模上施加绝缘盖。
在加热至挤压温度后除去绝缘盖，并且将模和压制品移至挤压机里。挤压在两个可彼此独立操作的冲模之间进行。挤压机使得压缩时的压制品在负荷下相对于模上下移动。在这些条件下压制品达到几乎理论上可能的饱和密度，从而压制品整个长度上的密度梯度小到可以忽略。
在冷却之后所述模转移至推出设备，并且所述压制品在粘结剂树脂凝固但润滑剂还是液态的温度时以形状稳定的形式推出。
为了焦化苯酚甲醛粘结剂树脂，在惰性气体气氛(优选氩气)中将新制的石墨成型体在循环炉里加热至大约800℃。所述循环炉是圆柱形罩式炉(Haubenofen)。转数可调节的燃气通风机从上向下吸收保护气体氩气，又在导管和加热的炉外壳之间向上鼓吹保护气体氩气。在焦化期间形成的裂解产物在底部炉区中冷凝，并且在生成之后直接从焦化过程中去除。和已知的方法相比，这种加热和冷却循环是非常短的，为24小时。
最后，对焦化的石墨成型体在真空中(P＜10-2mbar)且在大约1900℃时在石墨电阻加热炉中进行加热。
根据本发明，在真空中灼烧时利用了粘结剂焦炭或炭黑的高化学亲和性。高亲和性主要是由于这两个成分中的占大部分的无定形结构。结果是，胶粘剂焦炭或炭黑在真空中灼烧时与SiO2或ZrO2选择性地反应转化成碳化硅(SiC)或碳化锆(ZrC)，其中SiO2或ZrO2是在制备挤压粉时混入的。这两个碳化物，SiC和ZrC，是试验过的反应堆用材料，具有立方晶体排列(晶系)并且因此是本征各向同性的。SiC和ZrC具有显著的高硬度、高机械强度和非常好的耐腐蚀性。由此反应堆用石墨的性质，例如密度、断裂载荷、耐腐蚀性和特别是在快中子辐照时的高稳定性，得到明显改善。
如下的实施例应当进一步阐述了反应堆用石墨和其制备的发明，而不是用来限制本发明。
实施例1
天然石墨、石墨化的石油焦、SiO2以及挤压辅料十八酸和辛醇的使用。
为了制备挤压粉，天然石墨粉与在3100℃时石墨化的石油焦粉以重量比4∶1进行干燥预混合。基于石墨成分，添加20重量％的溶于甲醇中的苯酚甲醛粘接剂树脂和SiO2悬浮液。以悬浮液存在的SiO2粉末的含量计83.4重量％，基于粘结剂树胶。室温时在混捏机中进行均化。在105℃时对混捏品进行干燥，然后进行研磨。接着混合入挤压粉1重量％的十八酸(润滑剂)和0.16重量％的辛醇(排气剂)。为了制备均匀的混合物，使十八酸熔融、添加辛醇和搅拌入一部分所使用的挤压粉(大约10重量％)，然后使其冷却。可研磨的物料在细化至颗粒大小为＜1mm之后，干燥地混入剩余的粉末里。
原料成分具有如下的性质：
-Kropfmühl公司的标志为FP的天然石墨：
·松密度0.4g/cm3
·晶粒密度(Korndichte)2.26g/cm3
·BET-表面2m2/g
·晶粒大小Lc＝100nm
·平均晶粒直径10～20μm
·灰分含量200ppm，和
·灰分杂质中的硼当量＜1ppm。
-Ringsdorff公司的标志为KRB＜0.1mm的石墨化石油焦：
·石墨化温度3000℃
·松密度0.65g/cm3
·晶粒密度2.2g/cm3
·BET-表面1.2m2/g
·晶粒大小Lc＝60nm
·平均晶粒直径30～40μm
·灰分含量10ppm，和
·灰分杂质中的硼当量＜1ppm。
-Bakelitte公司的标志为4911的线性酚醛树脂(Novolak)类型的苯酚甲醛树脂：
·缩聚剂(Kondensationsmittel)HCl
·分子量690
·软化点101℃
·pH值＝6
·酸值＝7.5
·游离苯酚0.12重量％
·焦炭产率50％
·在甲醇中的溶解性99.97重量％
·灰分含量160ppm和
·灰分杂质中的硼当量1ppm。为了提高分子量，在缩聚以后对树脂进行水蒸气蒸馏。
-SiO2粉末，具有1～5μm的平均晶粒直径和99.95％纯度的细磨的市购SiO2粉末。
将挤压粉填充进硅橡胶的橡胶模具中，且在钢模槽型腔(Stahlgesenk)中以100MPa半等压地挤压成直径约62mm的球体。为了接收挤压粉，橡胶模具(Gummiform)具有椭球状的空腔。
在灰分比例为1∶1.17时，椭球状空腔的体积计550cm3。破碎球体，并且使用0.314～3.14mm的颗粒筛分级分(Granulat-Siebfraktion)用于后续操作。
为了制备具有300mm直径和600mm高度的圆柱体，用颗粒逐层地填充装备有油热加热装置的钢模，并且加热压制品至180℃。挤压在12MPa的挤压压力时在两个可彼此独立操作的冲模之间进行。因此压制品可以在挤压负荷下在钢模里上下移动。在这些条件下，压制品达到几乎理论上的1.95g/cm3的饱和密度，并且因此在整个长度上没有密度梯度。为了焦化粘结剂，在氩气流中在18小时内将压制品加热至800℃，并最终在真空中(P＜10-2mbar)和在1950℃时灼烧(glühen)。其中根据本发明由苯酚甲醛树脂形成的二次焦炭与SiO2反应成SiC。
实施例2
除了用ZrO2粉末代替SiO2粉末和用灯黑代替石墨化的石油焦之外，所有的制备步骤在3000℃的粉末形式保持不变，如实施例1中所描述的。在此，用只有4重量％的炭黑代替16重量％的石油焦。使用的ToyoSoda公司的标志为TZ的ZrO2粉末具有大约1μm的平均晶粒直径和99.99％的纯度。
灯黑具有大约0.1μm的平均粒径、0.2nm的晶粒大小Lc、110ppm的灰分含量和＜1ppm的灰分杂质中的硼当量。
在热处理之后从石墨成型体中取出试样，并且对试样就以下性质进行研究：
-体积密度
-抗压强度
-抗拉强度
-动弹性模量
-比电阻
-线性热力学膨胀系数
-线性热力学膨胀系数的各向异性因子，以及
-耐腐蚀性
结果在下表中列出：
石墨组成        天然石墨        天然石墨
                石墨化的石油焦  炭黑
                            SiC    ZrC
-密度(g/cm3)                1.77   1.88
-抗压强度(MPa)
轴向                        48.3   51.7
径向                        49.8   53.2
-抗拉强度(MPa)
轴向                        10.6   11.8
径向                        11.8   12.7
-动弹性模量
(MPa×103)
轴向                        10.1   11.9
径向                        11.7   12.3
-线性热力学膨胀系数
(20-1000℃.10-6/K)
轴向                        3.8    3.7
径向                        4.5    4.6
-线性热力学膨胀系数的
各向异性因子                1.2    1.2
-在1000℃和1体积％的水
蒸气时的耐腐蚀性
(mg/cm2小时)                0.45   0.38
-热中子吸收截面(mbar)       4.2    3.9
从表格中可知，根据本发明研制的反应堆用石墨的特征为高密度、好的强度性质和好的导热/电性
Figure BPA00001184779000081
特别是好的各向同性的热力学膨胀系数、好的耐腐蚀性和低的热中子吸收截面。
因为在制备石墨挤压粉时单个的石墨晶粒均匀地包覆有溶于甲醇中的粘结剂树脂层，所以在接着进行树脂焦化时有利于形成微裂纹。
形成的微裂纹、主要原料成分天然石墨的高石墨化程度和石墨晶体的各向同性的空间排列是在高于要求的温度和高通量的快中子时保证石墨的高抗辐照性的前提条件。
为了制备合成的人造石墨需要大约2个月的非常长的时间周期。相比而言，根据本发明的反应堆用石墨需要的大约1个星期的时间花费几乎缩短了一个数量级(参见图1和图2)。
在烧制过程(粘结剂的焦化)时不需要用焦炭粉末包覆压制品。因为作为原料成分使用了石墨化程度尽可能高的石墨粉，所以不用进行高能量消耗的石墨化过程。
另外在石墨制备过程中，由结合剂形成的二次炭黑不需额外的消耗就转化成化学碳化物SiC和/或ZrC。
通过前文介绍的本发明的方法的特征，本发明的方法相比较目前已知的用来制备反应堆用石墨的方法更加经济。

Claims (18)

1.反射器用石墨，其特征在于，所述石墨的主要成分包括核纯的天然石墨以及碳化硅和/或碳化锆。
2.根据权利要求1所述的反射器用石墨，其特征在于，所述天然石墨的份额为大于50重量％。
3.根据权利要求1或2所述的反射器用石墨，其特征在于，所述碳化硅的份额在6重量％～14重量％、优选5重量％～10重量％的范围。
4.根据权利要求1～3中任一项所述的反射器用石墨，其特征在于，所述碳化硅的含量为10重量％。
5.根据权利要求1～4中任一项所述的反射器用石墨，其特征在于，所述碳化锆的含量在10重量％～30重量％、优选20重量％～25重量％的范围。
6.根据权利要求5所述的反射器用石墨，其特征在于，所述碳化锆的含量为20.1重量％。
7.根据权利要求1～6中任一项所述的反射器用石墨，其特征在于，硅-、锆-化合物通过氧化物的形式引入。
8.根据权利要求7所述的反射器用石墨，其特征在于，所述氧化硅和/或氧化锆悬浮在甲醇-苯酚甲醛树脂溶液中，并且所述悬浮液在室温时通过混捏机与石墨粉成分进行均化。
9.根据权利要求1～8中任一项所述的反射器用石墨，其特征在于，在橡胶模具里(50MPa～200MPa)各向同性地挤压挤压粉成球体，将所述球体破碎成包含大于1百万个各向同性排列的石墨粒子的颗粒，所述颗粒在低压下通过后续的热压挤压成石墨成型体。
10.根据权利要求9所述的反射器用石墨，其特征在于，在所述的晶粒颗粒中引入润滑剂和排气剂。
11.根据权利要求10所述的反射器用石墨，其特征在于，十八酸用作润滑剂。
12.根据权利要求11所述的反射器用石墨，其特征在于，十八酸的含量最多达5重量％，优选1～5重量％，优选1重量％。
13.根据权利要求10所述的反射器用石墨，其特征在于，引入碳氢化合物作为排气剂，所述碳氢化合物的蒸汽压从室温时很低的数值升高到挤压温度时的大约1000hPa，并且所述碳氢化合物在挤压时在适中的压力下液化。
14.根据权利要求13所述的反射器用石墨，其特征在于，辛醇用作碳氢化合物。
15.前述权利要求中一项或更多项所述的反射器用石墨的制备方法，其特征在于，所述压制品在最后挤压时于挤压负荷下在压模里上下移动。
16.根据权利要求15所述的反射器用石墨的制备方法，其特征在于，所述压制品在粘结剂凝固且润滑剂十八酸还是液态的温度时推出。
17.根据权利要求15或16所述的反射器用石墨的制备方法，其特征在于，所述压制品在气体循环炉里焦化。
18.根据权利要求15～17中一项或更多项所述的反射器用石墨的制备方法，其特征在于，在粘接剂树脂焦化时形成的二次焦炭在真空和低于2000℃的温度时通过灼烧转化成碳化硅和/或碳化锆。
CN2008801251669A 2007-11-20 2008-11-19 含有作为主要成分的各向同性高结晶天然石墨和碳化硅或碳化锆的反射器用石墨及其制备 Pending CN101918341A (zh)

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PCT/EP2008/065858 WO2009065865A2 (de) 2007-11-20 2008-11-19 Reflektorgraphit bestehend aus isotropem hochkristallinen naturgraphit als hauptkomponente und silizium- oder zirkoncarbid und seine herstellung

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