矿物及固体绝缘材料电阻率测量的小型电极【中文6350字】【PDF+中文WORD】

矿物及固体绝缘材料电阻率测量的小型电极【中文6350字】【PDF+中文WORD】,中文6350字,PDF+中文WORD,矿物,固体,绝缘材料,电阻率,测量,小型,电极,中文,6350,PDF,WORD 中国科学: 技术科学 2011 年第 41 卷第 7 期: 890 -895 【中文6350字】 矿物及固体绝缘材料电阻率测量的小型电极实验装置与应用 汪灵①②*, 罗柯①, 李自强①, 关淞云①, 葛伟①, 张浚源① 成都理工大学材料与化学化工学院, 成都 610059，中国; 成都理工大学金刚石薄膜实验室, 成都 610059，中国 收稿日期: 2010-10-20； 接受日期: 2011-01-07；在线发布日期：2011-02-03 目前还没有一种矿物及固体绝缘材料小块样品电阻率测量的有效方法。为此，根据国家标准GB/T1410-2006 和数字高阻计特点,研制了一种与通用高阻计配套使用的小型电极实验装置, 将试样直径由标准电极的100 mm 减小到18 mm,试样面积减少了30.86 倍；该装置采用2个直径60 mm×高20 mm的绝缘基座对三电极系统进行支撑和精确定位,以实现装置结构的精准性和测量结果的可靠性,其关键技术参数是高压电极和测量电极的直径分别为18 和14.6 mm,保护电极内径和外径分别为16和18 mm,保护电极与测量电极间隙尺寸为0.6 mm,适用于直径φ=18 mm的矿物及固体绝缘材料平板试样电阻率测量；体积电阻率和表面电阻率验证实验结果表明,采用小型电极实验装置与标准电极测量结果一致。 1、 前言 电阻率是表征材料电学性能的重要参数[1, 2],其常用测量方法是四探针法和三电极法[3]四探针法主要用于半导体和导体材料电阻率的测量,例如:王亚平等人[4]以四探针法为基础,发展了一种高精度四线交流电阻测试设备,能够原位监测Ni80P20, FeZr2 和Fe86B14 非晶合金的晶化动力学过程。李冠雄等人[5]采用高真空电子束蒸发方法制备半导体材料Si为过渡层的Co/Cu/Co 三明治膜过程中,利用四探针法研究了三明治膜的巨磁电阻效应及磁各向异性与Si过渡层的关系。周西松等人[6]利用四探针法研究了Sn(Pb)Te-Bi2Te3系热电材料的电导率随温度的变化规律,结果表明常温下样品的电导率最大, 之后随温度升高明显降低。Ozols等人[7]使用四探针法研究了在不同聚合物含量和有无铁镍合金粉末涂层的条件下软磁化合物的电阻率。Tang 等人[8]利用四探针法研究了La1−xSrxMnO3 系列混合物在不同温度下的电阻曲线。而三电极法则主要用于绝缘材料电阻率的测量, 例如: Vila 等人[9]采用三电极法研究了电子照射对聚乙烯与聚酰亚胺胶带体积电阻的影响。Gonon等人[10]使用三电极系统研究了环氧复合材料电阻率随含水量的变化趋势。绝缘材料是指当电压施加在材料两点之间或其内部时, 只产生极小甚至可以忽略不计的微弱电流[11]。 目前,绝缘材料电阻率测量方法(三电极法)已有相应的美国标准ASTM D 257-1999[12]和国家标准GB/T1410-2006(与国标IEC60093-1980 等效)[13]。其方法是：将样品加工成直径为=100 mm 左右、厚h=1~3 mm的标准尺寸，然后利用高阻计进行测量。非金属矿物通常具有优良的绝缘性能,但是，由于样品加工困难等原因，目前还没有一种测量矿物电阻率的有效方法,难以对非金属矿物绝缘性能进行表征和研究。其主要原因是:许多矿物的解理发育,或本身存在裂纹与缺陷,在加工过程中容易开裂,难以获得如此大的标准尺寸样品。另外,对于其他固体绝缘材料,在一些情况下要加工或获取标准尺寸的样品也是非常困难的。 近年来，非金属矿物因其优良的电气性能和低廉的价格被越来越多的应用于绝缘材料中，例如：2005年我国的矿物填料在塑料和橡胶中的用量已分别达到375×104 和120×104 t，成为我国矿物材料产业的重要组成部分[14]。因此，研究一种适用于矿物及固体绝缘材料小块样品电阻率测量的实验装置和方法，不仅对矿物绝缘性能的表征和研究具有不可替代的作用，而且由于实验样品尺寸的大幅减少，将使矿物粉体及其他粉体材料电阻率测量成为可能，这对于矿物资源开发利用以及新型绝缘材料研究与应用都具有十分重要的意义。 本工作根据中国的标准GB/T1410-2006 和数字高阻计特点，研制出一种适用于矿物及固体绝缘材料小块样品(直径φ=18 mm)电阻率测量的小型电极实验装置，并与通用高阻计配套使用，对一些非金属矿物及固体绝缘材料的体积电阻率和表面电阻率进行较系统测量， 通过与标准电极(样品直径φ=100 mm)测试结果以及这些材料已知数据进行比较分析，获得了一致的结果。 2、 固体绝缘材料电阻率测量原理 根据国家标准GB/T1410-2006, 固体绝缘材料体积电阻和表面电阻率采用高阻仪表进行测量。高阻仪表由数据测量系统、三电极系统和金属屏蔽箱组成。试样直径大小由测量电极、保护电极和高压电极所组成的三电极系统尺寸大小共同决定。图1是适用于直径φ=100 mm 固体绝缘材料电阻率测量的三电极系统工作原理示意图，为了消除外来电磁干扰所产生的影响，三电极系统应放置于金属屏蔽箱中, 其测量原理如下。 测量体积电阻时(图1(a))， 测量电极1-1#通过导线1-2#与高阻仪表的测量端相连，高压电极3-1#通过导线3-2#与高阻仪表的高压端相连，保护电极2-1#则通过导线2-2#与高阻仪表的接地端相连，电流按图1(a)箭头所示方向穿过测试样品，被测样品0#的体积电阻(Rv)由高阻仪表可直接读取。 图1 适用于直径=100 mm 固体绝缘材料电阻率测量的三电极系统工作原理示意图 0#－试样; 1-1#－测量电极; 2-1#－保护电极; 3-1#－高压电极; 1-2#, 2-2#, 3-2#－导线(a) 体积电阻测量原理; (b) 表面电阻测量原理 根据体积电阻(Rv)测试结果和国家标准GB/T1410-2006 计算公式如下, 可得到被测样品的体积电阻率（ρv） ρv= RvAeh (1) 式中，v 为体积电阻率(Ω·cm)；h为样品厚度(cm) ；Rv为体积电阻(Ω), 由高阻计直接测试得到；Ae为被保护电极的有效面积，由电极尺寸决定，其计算公式为： Ae=ππd1+g24 （2） 式中， d1 为测量电极(图1,1-1#)直径(cm)，g为测量电极与保护电极的间隙(cm), πp=3.1416。对于直径φ=100 mm 的标准电极, Ae=21.237 cm2；对于直径φ=18mm 自制小型电极, Ae=1.863 cm2。 测量表面电阻率时(图1(b)),测量电极1-1#通过导线1-2#与高阻仪表的测量端相连，保护电极 2-1#通过导线2-2#与高阻仪表的高压端相连,高压电极 3-1#则通过导线3-2#与高阻仪表的接地端相连， 电流按图1(b)箭头所示方向从测试样品表面通过, 被测样品0#的表面电阻率(ρs)由高阻仪表根据表面电阻(Rs)测量数据,经如下公式自动换算得到[15]。 ρs=Rs2πlnd2d1 （3） 式中,ρs 为表面电阻率; Rs 为表面电阻(Ω)；d1 为测量电极直径(cm)；d2 为保护电极(图1, 2-1#)内径(cm)。 3 、 小型电极实验装置的研制 图2是一种适用于直径φ=18 mm 矿物及固体绝缘材料电阻率测量的小型电极实验装置结构图，其测量原理与标准电极系统(图1) 完全相同。由于该装置的三电极尺寸较小，其研制的关键是三电极尺寸大小等重要技术参数确定及其精确定位。 3.1 小型电极系统的关键技术参数 三电极系统是整个装置的核心，如图2 所示,其技术关键是保护电极的内外径、测量电极的直径以及保护电极与测量电极之间的间隙尺寸大小等关键技术参数的确定。 （i）保护电极的内外径尺寸。样品直径的大小直接决定三电极的尺寸大小。样品直径过小, 三电极尺寸将相应减小, 并导致保护电极与测量电极之间的间隙尺寸g 过小,从而影响整个装置的使用安全性;相反, 若样品尺寸过大,又使得实验装置失去其小型化意义。经过多次试验,最终确定样品直径φ=18 mm,其面积为254.34 mm2。而φ=100 mm 的标准样品, 其面积为7850 mm2,与之相比,小型电极样品面积减少了30.86 倍,使矿物及固体绝缘材料小块样品电阻率测量成为可能。 在绝缘电阻测量时,为了抵消表面或体积效应引起的误差, 保护电极2-1#外径d3和高压电极3-1#直径d4应与样品0#直径d0相同, 即d3=d4= d0=18 mm。另外,由于电极尺寸较小,如果保护电极2-1#厚度过大, 将导致与高压电极3-1#之间隙尺寸过小而降低系统的安全性,并将大大增大加工难度。因此,综合各方面因素，保护电极2-1#的最小厚度为1 mm,则保护电极2-1#内径d2=16 mm。 （ii） 测量电极直径尺寸。由公式(3)可知, d2/d1 为定值, 表面电阻率与d2/d1 比值有关, 而与试样大小无关, 因此,可由高阻计直接读取. 由于标准电极d2/d1=54 cm/50 cm=1.08, 此常数不可更改, 那么小型电极d2/d1 也应等于1.08。由于小型电极的保护电极2-1#内径d2=16 mm,那么可确定测量电极1-1#直径d1 = 16 mm/1.08=14.8 mm。 （iii） 保护电极与测量电极之间的间隙尺寸。由于保护电极2-1#内径d2=16 mm,测量电极1-1#直径d1=14.8 mm,并由于二者之间的间隙距离g= d2-d1/2,那么,可确定间隙尺寸 g=(16 mm-14.8 mm) / 2 =0.6 mm 需要说明的是,由于高阻计最高工作电压通常为1 kV,而空气的直流击穿强度为33 kV/mm[16],在最高工作电压下,临界击穿间歇g=1 kV/(33 kV/mm)≈0.03 mm。也就是说,一般情况下,只要间歇尺寸g> 0.03 mm, 就能保证不被击穿。但是,在使用过程中,由于样品表面杂质和空气中悬浮颗粒可能落入间隙中,如果g 过小,就容易被击穿, 难以保证设备安全. 由于该装置g=0.6 mm,大于临界击穿间歇近20 倍, 能够保证电极系统的安全使用。 图2 适用于直径=18 mm 矿物及固体绝缘材料电阻率测量的小型电极实验装置结构图 0#, d0－测试样品及其直径; 1-1#, d1－测量电极及其直径; 2-1#, d2 , d3－保护电极及其内外径; 3-1#, d4－高压电极及其直径;2-2#, 3-2#－可调导体螺杆; 5-1#－上绝缘基座; 5-2#－下绝缘基座; 6-1#－不锈钢固定螺栓; M4, M6－螺杆螺纹直径; 其余数字为相关零件的尺寸大小(单位: mm) 3.2 小型电极系统的精确定位 如图2 所示,该装置采用直径60 mm×高20 mm的上绝缘基座5-1#和下绝缘基座5-2#对三电极系统进行支撑和精确定位, 以实现装置结构的精准性和测量结果的可靠性。因为, 保护电极2-1#与测量电极1-1#的间隙距离只有0.6 mm, 如果三电极系统不能精确定位, 将难以获得可靠测量数据, 并容易出现短路, 使仪器遭到破坏. 为了便于测量, 保护电极2-1#和高压电极3-1#分别通过导体螺杆2-2#和3-2#与高阻计测量系统实现联接, 而测量电极1-1#则通过适当加长与测量端口相连。 同时,为了便于样品精确放置在固定位置, 保护电极2-1#和测量电极1-1#相对基座5-1#向下伸出1mm,而高压电极3-1#相对基座5-2#向内凹陷1 mm,从而形成φ=18.5 mm×1 mm样品放置凹槽。 另外, 为了使高压电极3-1#与测量电极1-1#吻合,采用不锈钢固定螺栓6-1#将基座5-1#和5-2#进行固定和精确定位；为了便于样品安放和取出方便,使基座5-1#能够相对于基座5-2#沿Z 轴和XY 平面内360°活动和转动。 3.3 小型电极实验装置的材料选择 如图2 所示，小型电极实验装置材料主要包括电极材料和绝缘基座材料。 (i) 电极材料。电极材料应选取能与试样紧密接触的材料,而且不会因施加外电极引进杂质而造成测量误差,还要保证测量使用的方便、安全等。常用的电极材料有退火铝箔、喷镀金属层、导电粉末、烧银、导电橡胶、黄铜和水银电极等[15]。而从小型电极实验装置的结构特点来看不仅要求电极和导体螺杆有较高的导电性能,而且要有足够的机械强度以便于实际加工和与固定基座相配合, 另外考虑价格、使用难易程度、重复使用性后决定选用固体导电金属作为电极材料, 可选用的材料有: 紫铜、银铜、不锈钢等。 本实例选用紫铜作为电极和导体螺杆材料,因为紫铜具有良好的导电性能(20℃时,电阻率仅为1.69×10-2Ω.mm2m),并具有一定的机械强度和良好的耐腐蚀性, 易于焊接、加工等优点[16]。 (ii) 绝缘基座材料: 由于PC68高阻计所测的绝缘电阻极高, 最高可达1×1017Ω, 根据公式(1)和(2)换算成h=3 mm 的绝缘材料的体积电阻率最高可达7.08×1018Ω.cm, 和表面电阻率最高可达1×1017。如果固定基座绝缘电阻率相对较小会对测试结果产生较大的误差. 因此,固定基座首先要求有极高的电阻率(ρv >1017Ω.cm),以避免对测试结果产生较大的影响；同时,要求材料有较强的机械强度,以起支撑固定作用。据此,可选用的材料有聚四氟乙烯(F-4)、四氟乙烯和乙烯共聚物(F-40)、聚三氟氯乙烯(F-3)等。 本实例选用聚四氟乙烯(F-4)作为绝缘基座,其分子式为[17] —( CF2-CF2)— n, 化学稳定性较好,长期工作温度 250℃,分解温度为 415℃,电气性能优良(体积电阻率ρv >1017Ω.cm), 相对介电常数(εγ=2.0)和介质损耗角正切(tgσ<2×10-4)在已知固体绝缘材料中是最低的, 机械强度也较高(抗张强度s =1370~3000 N/cm2)[16]。 4 验证实验 4.1实验样品与加工 实验样品有两类，一类是固体非金属矿物，主要有： 微晶白云母，四川鑫炬矿业资源开发股份有限公司生产；白云母片,四川丹巴云母厂提供。另一类是固体绝缘材料,主要有: 800目微晶白云母绝缘灌注胶片,本课题组研制；环氧酚醛玻璃布板(3240)，四川东方绝缘材料股份有限公司生产；硅橡胶，四川东方绝缘材料股份有限公司生产；金云母软板,成都兴东方电工材料研究有限公司生产，醇酸柔软云母板(5131B)，成都兴东方电工材料研究有限公司生产；舒氏PVC 电气胶带, 舒氏集团生产。 将同一样品分别加工成直径φ=100 mm和φ=18mm,厚度h=0.3~3 mm 的圆片。但舒氏PVC 电气胶带除外,其制备方法是: 将胶带剪成数条使其呈米字型层层紧密平铺, 直至平铺成边长 a>100 mm, 厚h=3 mm 的正方板, 然后分别剪成直径φ=100 mm和φ=18 mm 圆片。另外,由于体积电阻和表面电阻对材料表面污秽和水膜等比较敏感, 需进行清洁与烘干处理,其方法是:用沾有无水乙醇的脱脂棉擦拭每个样品表面,然后用蒸馏水冲洗,再将清洗后的样品放入电热恒温鼓风干燥箱中,在控温110℃的条件下烘干24 h,取出后分别装入密封袋中待测。 4.2电阻率的测试方法 采用上海精密科学仪器有限公司生产的PC68 型数字高阻计(工作电压220 V,电压误差±3%,测量范围1×103~1×1017 ), 并分别使用标准电极和小型电极对标准样品和小块样品的体积电阻和表面电阻率进行测试分析。测试条件: 温度 t=15℃,对湿度 RH=62%, 施加电压U=500 V。所有测量均重复三次,分别取其平均值作为最终结果。 4.3测量结果与分析 表1和2是矿物及固体绝缘材料标准样品与小块样品体积电阻率(Ω.cm)和表面电阻率(Ω)测量结果。可以看出,采用标准电极对φ=100 mm 标准样品测试结果与同一样品的已知的标准值基本相同, 说明该仪器的测试结果符合测量要求；同时，与采用小型电极对φ=18 mm 的小块样品的测量结果也基本相同,说明小型电极也能够比较准确地测量矿物及其他固体绝缘材料小块样品的电阻率。需要说明的是,表1和2中测试结果并不是绝对相同,其原因是电阻率测试结果还与测试温度、湿度、样品烘干时间等因素有关,而且仪器误差和各个材料的不均匀性等对测试结果也有一定的影响。总的来看, 其测试数据变化在仪器的正常误差范围之内。以上结果说明,采用小型电极实验装置测量电阻率是有效的,适用于直径φ=18 mm 矿物及固体绝缘材料小块样品体积电阻率和表面电阻率的测量。 5 结论 (ⅰ) 研制了一种能够与通用高阻计配套使用, 适用于矿物及固体绝缘材料小块样品电阻率测量的小型电极实验装置,将试样直径由标准电极的100 mm 减小到18 mm,样品面积减少了30.86 倍。 (ⅱ) 该装置采用2个直径60 mm×高20 mm 的绝缘基座对对测量电极、保护电极、高压电极所组成的三电极系统进行支撑和精确定位, 以实现装置结构的精准性和测量结果的可靠性,其关键技术参数是高压电极和测量电极直径分别为18 和14.6 mm,保护电极内径和外径分别为16 和18 mm,保护电极与测量电极间隙尺寸为0.6 mm。 (ⅲ) 验证实验结果表明,小型电极实验装置与通用高阻计配套使用, 能够对直径φ=18 mm矿物及固体绝缘材料平板试样的体积电阻率和表面电阻率进行测量, 其结果与采用标准电极测量结果一致。 表1 矿物及固体绝缘材料标准样品与小块样品体积电阻率(Ω.cm)测量结果 表2 矿物及固体绝缘材料标准样品与小块样品的表面电阻率(Ω.cm)测量结果 参考文献 1 刘其昶. 电气绝缘结构设计原理-中册-绝缘结构总论. 北京: 机械工业出版社, 1988. 137 2 邱成军, 王元化, 王义杰. 材料物理性能. 哈尔滨: 哈尔滨工业大学出版社, 2003. 47–116 3 关振铎, 张太中, 焦金生. 无机材料物理性能. 北京: 清华大学出版社, 1992. 207–212 4 王亚平, 卢柯. 非晶态合金晶化过程的高精度电阻监测研究. 中国科学E 辑: 技术科学, 2000, 20: 193–199 5 李冠雄, 沈鸿烈, 沈勤我, 等. Si 过渡层对Co/Cu/Co 三明治膜巨磁电阻效应的影响. 中国科学E 辑: 技术科学, 2000, 30: 15–21 6 周西松, 邓元, 韦国丹, 等. 溶剂热法合成系热电材料Sn(Pb)Te-Bi2Te3系热电材料及其性能研究. 中国科学E辑: 技术科学, 2003, 33: 217–221 7 Ozols A, Pagnola M, García D I, et al. Electroless coating of Permalloy powder and DC-resistivity of alloy composites. Surf Coat Tech,2006, 200: 6821–6825 8 Tang G C, Yu Y, Chen W, et al. The electrical resistivity and thermal infrared properties of La1−xSrxMnO3 compounds. J Alloy Compd, 2008,461: 486–489 9 Vila F, Sessler G M, Sykj H. The influence of electron-beam irradiation on the volume resistivity of polyethylene and kapton. J Electrostat,2005, 63: 749–754 10 Gonon P, Hong T P, Lesaint O, et al. Influence of high levels of water absorption on the resistivity and dielectric permittivity of epoxy composites. Polym Test, 2005, 24: 799–804 11 ASTM D 1711-2002. Standard Terminology Relating to Electrical Insulation. American Society for Testing and Materials, Philadelphia,2002 12 ASTM D 257-1999. 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American Society for Testing andMaterials, Philadelphia, 1999 13 GB/T 1410-2006, 固体绝缘材料体积电阻率和表面电阻率试验方法. 中国国家标准化管理委员会, 北京, 2006 14 袁继祖. 非金属矿物填料与加工技术. 北京: 化学工业出版社, 2006. 8 15 伍洪标. 无机非金属材料实验. 北京: 化学工业出版社, 2002. 341–343 16 李正吾. 新电工手册. 合肥: 安徽科技出版社, 2000. 1794–1885 17 广州电器研究所上海试验站译. 电工材料手册. 北京: 中国工业出版社, 1963. 76 18 矿产资源综合利用编委会. 矿产资源综合利用手册. 北京: 科学出版社, 2000. 599 19 赵燕玲. 微晶白云母在绝缘灌注胶功能复合材料中的应用基础研究. 硕士学位论文. 成都: 成都理工大学, 2007. 37 SCIENCE CHINA Technological Sciences Science China Press and Springer-Verlag Berlin Heidelberg 2011 *Corresponding author(email:)RESEARCH PAPER April 2011 Vol.54 No.4:819825 doi:10.1007/s11431-011-4302-7 Design and application of a small electrode experimental installation for resistivity measurement of mineral and solid insulating material WANG Ling1,2*,LUO Ke1,LI ZiQiang1,GUAN SongYun1,GE Wei1&ZHANG JunYuan1 1 College of Materials and Chemical&Chemistry Engineering,Chengdu University of Technology,Chengdu 610059,China;2 Key Laboratory of Diamond Film,Chengdu University of Technology,Chengdu 610059,China Received October 20,2010;accepted January 7,2011;published online February 3,2011 There has not been an effective method to measure the resistivity of small-size sample of mineral and solid insulating material until now.According to the Chinese National Standard(GB/T1410-2006)and features of digital high resistance meter,a small electrode experimental installation was developed;it can work with current high resistance meter;the sample decreases to 18 mm from standard size 100 mm in diameter and reduces by 30.86 times in area.A three-electrode system is supported and pre-cisely positioned by two insulating bases whose diameter is 60 mm and height is 20 mm,which ensures accuracy of device structure and reliability of measuring results.The key technological parameters are as follows:diameter of high voltage elec-trode is 18mm;diameter of measuring electrode is 14.6 mm;internal diameter and external diameter of guard electrode are 16 and 18 mm,respectively;the gap between guard electrode and measuring electrode is set at 0.6 mm.These parameters are ad-equate for the measurement of flat specimen of mineral and solid insulating material whose diameter is 18 mm.According to the confirmatory experiment on the volume resistivity and surface resistivity,the measuring results are almost the same,using a small electrode experimental installation and a standard electrode.resistivity,insulating property,insulating material,mineral physics,material physics Citation:Wang L,Luo K,Li Z Q,et al.Design and application of a small electrode experimental installation for resistivity measurement of mineral and solid insulating material.Sci China Tech Sci,2011,54:819825,doi:10.1007/s11431-011-4302-7 1 Introduction The resistivity of materials is an important parameter to char-acterize its electrical properties 1,2,and the common meas-urement methods are four-point probe method and three-electrode method 3.The four-point probe method is mainly used in measuring the resistivity of semiconductor and con-ductive materials.For example,Wang et al.4 used this method to develop an accurate four-line AC electrical resis-tance measurement(ERM)apparatus which can monitor in situ the crystallization kinetics of amorphous alloys-Ni80P20,FeZr2 and Fe86B14.Li et al.5 fabricated a series of Co/Cu/Co sandwiches with a semiconductor Si buffer layer using high vacuum electron-beam evaporation method;they studied the dependence of GMR effect and magnetic anisotropy of the sandwiches on the Si buffer layer.Zhou et al.6 studied the change law between temperature and electrical conductivity of Sn(Pb)Te-Bi2Te3 compounds;they concluded that as the temperature goes up,the con-ductivity of the samples decreases rapidly,and its conduc-tivity reaches the max under usual temperature.Ozols et al.7 studied electrical resistivity of SMC with different polymer contents and the SMC with permalloy powders uncoated or coated.Tang et al.8 investigated the tem-perature dependence on resistivity of La1xSrxMnO3 com-820 Wang L,et al.Sci China Tech Sci April(2011)Vol.54 No.4 pounds.The three-electrode method is mainly used for re-sistivity of insulating material.Vila et al.9 researched influence of electron-beam irradiation on the volume resis-tivity of polyethylene and kapton by using this method.Gonon et al.10 studied variation tendency between water content and resistivity of epoxy composites.Insulating material,or dielectric,is a material in which a voltage applied across two points on or within the material produces a small and sometimes negligible current 11.Now,the American National Standard D 257-1999 12 and Chinese National Standard GB/T1410-2006 13 have set rules on the measuring methods(three-electrode method)for resistivity of insulating material.The method is to proc-ess the sample into standard-size wafer(=100 mm,h=13 mm),then measure it with a high resistance meter.Non-metallic minerals play an irreplaceable role in the in-sulating material because of their excellent insulating prop-erty.But there is no effective measurement method for measuring the resistivity of mineral due to difficulties in sample processing or other reasons at present.As a result,it is difficult to study the insulating property of non-metallic mineral.The primary cause is the sample is easy to crack during processing because there may be cleavage,crack or defect in minerals.So it is very difficult to obtain such a large-size standard sample.In addition,for other solid insu-lating material,it is not convenient to process or obtain standard samples in some cases.In recent years,non-metallic mineral is increasingly used in insulating material because of its excellent electrical property and low price.For example,in China the amount of this material used as filler in plastics and rubber reached 375 104 and 120 104 t,respectively in 2005,which indi-cated that it has become a very important part of mineral materials 14.Therefore,to research and develop a kind of experimental installation for measuring resistivity of small-size samples of minerals and solid insulating materi-als can not only study mineral insulating property but also make it possible to measure resistivity of mineral powder and other powder materials1)because the sample size is re-duced largely;thus it is of great significance to exploit min-eral resources and to research and produce new type of mineral materials and insulating materials.In this study,according to the Chinese National Standard(GB/T1410-2006)and the features of high resistance meter,a small electrode experimental installation was developed,which can work with high resistance meter and measure the resistivity of small-size sample(diameter=18 mm)of mineral and solid insulating material.This intallation can measure the volume resistivity and surface resistivity of some non-metallic mineral and solid insulating material sys-temically,and the results of measurement are consistent with their known data and the data measured by the stan-dard electrode(sample diameter=100 mm).2 Resistivity measurement principles of solid insulating material According to the Chinese National Standard GB/T1410-2006,volume resistivity and surface resistivity of the solid insulating material are measured by high resistance meter.The high resistance meter consists of measurement system,three-electrode system and metal shielded box.The diameter of a sample depends on the size of measur-ing electrode,guard electrode and high voltage electrode.Figure 1 is a schematic diagram of a three-electrode sys-tem which is fit for measuring solid insulating material whose diameter is 100 mm.In order to avoid electromag-netic interference,the three-electrode system should be put in the metal shielded box.The measurement principle is as follows.When the volume resistivity of sample is measured(Figure 1(a),the measuring electrode(1-1#)is linked with the measuring junction by the wire(1-2#),high voltage Figure 1 Schematic diagram of the three-electrode method for measuring resistivity of solid insulating material(diameter=100 mm).0#,Sample;1-1#,Measuring electrode;2-1#,Guard electrode;3-1#,High voltage electrode;1-2#,2-2#,3-2#,Wires.(a)Measurement principle of volume resistivity;(b)measurement principle of surface resistivity.1)Wang L,Li Z Q,Luo K,et al.Study on the measurement method for resistivity of mineral powderTaking micro-crystal muscovite for example.Powder Technol,2011(to appear).Wang L,et al.Sci China Tech Sci April(2011)Vol.54 No.4 821 electrode(3-1#)with high voltage terminal by the wire(3-2#)and guard electrode(2-1#)with the grounded junc-tion by the wire(2-2#).The electric current goes through the sample just in the direction of arrowhead(Figure 1(a).Then the volume resistance(Rv)of sample 0#is measured directly by the high resistance meter.According to the Chinese National Standard GB/T1410-2006,the volume resistivity(v)can be calculated by .vVAeRh(1)In the formula,v is the volume resistivity(cm);h,the thickness of the sample(cm);Rv,the volume resistance(),which is measured directly by the high resistance meter;Ae,the effective area of the guard electrode,which is deter-mined by the size of electrode and calculated by the for-mula:21().4dgAe(2)In the formula,d1 is the diameter(cm)of the measuring electrode(Figure 1,1-1#);g,the gap between the measuring electrode and guard electrode(cm);=3.1416.For the standard measuring electrode with diameter=100 mm,Ae=21.237 cm2,and for the self-made small measuring electrode with diameter=18 mm,Ae=1.863 cm2.When the surface resistivity of the sample is measured(Figure 1(b),measuring electrode(1-1#)is linked with the measuring junction by the wire(1-2#),guard electrode(2-1#)with high voltage terminal by the wire(2-2#)and high voltage electrode(3-1#)with the grounded junction by the wire(3-2#).The electric current goes through the sam-ple just in the arrowhead direction(Figure 1(b).The sur-face resistivity(S)of sample 0#can be calculated auto-matically by eq.(3)using the surface resistance(Rs)meas-ured from high resistance meter 15.212.lnsSRdd(3)In the formula,s is surface resistivity();Rs,the surface resistance();d1,the diameter(cm)of the measuring elec-trode;d2,the internal diameter(cm)of the guard electrode.3 Development of the small electrode exper-imental installation Figure 2 is a structural diagram about the small electrode experimental installation used to measure resistivity of mineral and solid insulting material.The measurement prin-ciple is the same as that of the standard electrode system(Figure 1).Because the size of three-electrode of the ex-perimental installation is small,it is important to confirm the size and exact position of the three-electrode and other key technical parameters in this study.3.1 Key technical parameters of the small measuring electrode system The core of whole device is the three-electrode system(Figure 2).And the key technology is to confirm the inter-nal diameter and external diameter of the guard electrode,the diameter of measuring electrode,the gap between guard electrode and measuring electrode,etc.(i)Internal diameter and external diameter of guard elec-trode.The size of the three-electrode is directly determined by the diameter of the sample.If the sample size is under-size,the size of three-electrode will be reduced accordingly.At last,the gap between guard electrode and measuring electrode will become undersized,which will affect the safety in utilization;on the contrary,if the sample diameter were oversized,the installation would not be designed to be a small one.After several experiments,the optimum di-ameter of sample is=18 mm,and its area is 254.34 mm2.Compared with the standard sample(=100 mm,area is 7850 mm2),the area is reduced by 30.86 times,so it is possible to measure the resistivity of small-size sample of minerals and solid insulating materials.In the process of insulation resistance measurement,in order to offset error caused by surface and volume effects,the external diameter d3 of guard electrode(2-1#)and di-ameter d4 of high voltage electrode(3-1#)should be the same as the diameter d0 of sample 0#,d3=d4=d0=18 mm.In addition,due to the small size of the electrode,if guard electrode(2-1#)was too thick,it would lead to decrease in the gap between 1-1#and 2-1#,which might reduce the system security and increase the difficulty in processing.Considering all the factors,the minimum thickness of the guard electrode(2-1#)is 1mm,and the internal diameter of guard electrode(2-1#)is d2=16 mm.(ii)The diameter of the measuring electrode.According to eq.(3),d2/d1 is a fixed value,which is related to surface resistivity,and has nothing to do with the size of the sample.Hence,the value can be obtained by high resistance meter directly.The value of the standard electrode,d2/d1=54 cm/50 cm=1.08,is constant which also applies to the small electrode.Therefore,the diameter of the measuring electrode can be calculated as d1=16 mm/1.08=14.8 mm when the internal diameter of guard electrode(2-1#)is d2=16 mm.(iii)The gap between the guard electrode and the measuring electrode.As the internal diameter of guard electrode is d2=16 mm,the diameter of measuring elec-trode is d1=14.8 mm and the gap is 21/2,gdd the gap is(16 mm 14.8 mm)/20.6 mm.g 822 Wang L,et al.Sci China Tech Sci April(2011)Vol.54 No.4 Figure 2 Structural drawing of small electrode resistivity measurement installation on the resistivity measurements of mineral and solid insulating material(diameter=18 mm).0#,d0,Sample and its diameter;1-1#,d1,Measuring electrode and its diameter;2-1#,d2,d3,Guard electrode,its internal diameter and external diameter;3-1#,d4,High voltage electrode and its diameter;2-2#,3-2#,Rotatable conductive screw;5-1#,up-insulating base;5-2#,down-insulating base;6-1#,Stainless steel mounting bolts;M4,M6,diameter of thread;The remaining number is the size of relevant parts(unit:mm).Attention should be paid to that the maximum working voltage of the high resistance meter is 1 kV,and DC break-down strength is 33 kv/mm in air.Under that circumstance,the critical breakdown gap is g=1 kV/33 kV/mm0.03 mm.That is to say,it usually will not be breakdown as long as g0.03 mm.However,the impurities on the sample surface and suspended particles in the air might fall into the gap when the installation is used;if g is undersize,the gap would be breakdown easily.In our installation,g is 0.6 mm which is 20 times larger than the critical breakdown gap,which can ensure the system security.3.2 Accurate location of small measuring electrode system As shown in Figure 2,the installation includes two bases,the up-insulating base and down-insulating base both with diameter 60 mm and height 20 mm.They can support and locate precisely the three-electrode system to ensure accu-racy of the device structure and reliability of the measure-ment results.Because the gap between the guard electrode(2-1#)and the measuring electrode(1-1#)is only 0.6 mm,if a three-electrode system could not be accurately positioned,it would be difficult to get secure reliable measurement data and be prone to a short circuit and damage the device.In order to facilitate measurement,the guard electrode(2-1#)and the high voltage electrode(3-1#)connect with the measurement system of the high resistance meter by the conductive screws(2-2#,3-2#),respectively.Measuring electrode(1-1#),after an appropriate extension(1-2#),can be connected to the measurement system of the high resis-tance meter.Meanwhile,in order to place sample accurately and conveniently in a fixed position,both the guard electrode(2-1#)and the measuring electrode(1-1#)should outstretch 1 mm more than the insulating base(5-1#),and the high voltage electrode(3-1#)hollow 1mm more than the insu-lating base(5-2#),then a groove(=18.5 mm1 mm)is formed for the sample.In addition,in order to match the high voltage electrode(3-1#)with the measuring electrode(1-1#),stainless steel mounting bolts(6-1#)are used to fix and position the insu-Wang L,et al.Sci China Tech Sci April(2011)Vol.54 No.4 823 lating base(5-1#,5-2#)precisely;in order to facilitate an easy placement and removal of sample,the insulating base(5-1#),relative to the insulating base(5-2#),can rotate at 360 degrees in XY plane and along Z-axis.3.3 Materials selection of small electrode experimental installation As shown in Figure 2,materials of small electrode experi-mental installation mainly include the electrode materials and insulating base materials.(i)Electrode materials.The electrodes for insulating ma-terials should be of a material that is readily applied,allows intimate contact with the specimen surface,and introduces no appreciable error because of electrode resistance or con-tamination of the specimen.Commonly used electrode ma-terials are annealed aluminum foil,spraying metal layer,conductive powder,burned silver,conductive rubber,brass and mercury electrodes,etc 15.The structure characteris-tics of the small electrode experimental installation require the electrodes and conductive screws not only to have good conductivity but also to have sufficient mechanical strength for actual processing and matching with the fixed base.Moreover,in relation to the price,application and reusabil-ity,the final option for the electrode material is solid con-ductive metals,including red copper,silver-copper alloys,stainless steel and so on.In this study,the electrode and conductor screw are made of red copper which has good conductivity(when the temper-ature is 20C,the resistance rate is only 1.69102 mm2/m),a certain mechanical strength plus good corrosion resistance,and is easy for welding and processing 16.(ii)Insulating base materials:Because insulation resis-tance measured by the PC68 digital high resistance meter is very high,reaching as high as 11017,according to eqs.(1)and(2),the relevant volume resistivity of insulating mate-rial(h=3 mm)could reach as high as 7.081018 cm and surface resistivity 11017.If insulation resistivity of the fixed base is relatively low,the measuring results will have a greater error.Therefore,it is required that the fixed base should possess extremely high insulation resistivity(v1017 cm)to avoid a greater impact on the measuring results;simultaneously,the insulating base materials should have strong mechanical strength to conduct support and fixment.Accordingly,the selections for materials are PTFE(F-4),tetrafluoroethylene-ethylene copolymer(F-40),PCTFE(F-3),etc.In this study,PTFE(F-4)is used as the insulating base material;its molecular formula 17 is(CF2CF2)n;it has a good chemical stability and excellent electrical properties(volume resistivity v1017 cm);its long-term working temperature is 250C,decomposition temperature is 415C;its relative dielectric constant(r=2.0)and dielectric loss angle tangent(tg1012 Shus PVC electrical tape 1.261014 2.041014 Table 2 The measuring data of surface resistivity()of small-size sample and standard sample Sample Standard sample (=100 mm)Small-size sample (=18 mm)Standard value of insulation Micro-crystal muscovite 4.391010 2.161010 Isinglass(Perpendicular to the 001)18 4.331011 2.741011 10111012 800-mesh micro-crystal muscovite insulating pouring sealant 19 1.421014 3.741014 10131014 Epoxy phenolic glass cloth rigid laminated sheet(3240)16 2.371014 4.351014 10131014 Silastic 1.191014 1.081014 Soft phlogopite plates 16 5.521010 6.031010 10101011 Alkyd soft mica plate(5131B)6.321013 5.891013 Shus PVC electrical tape 4.131013 3.881013 Wang L,et al.Sci China Tech Sci April(2011)Vol.54 No.4 825 This work was supported by the National Natural Science Foundation of China(Grant No.50974025),the National Key Technologies R&D Pro-gram of China(Grant No.2004BA810B02),the Applied Foundation of Basic Research in Sichuan Province(Grant No.07JY029-029),the Spe-cialized Research Fund for the Doctoral Program of Higher Education of China(Grant No.20095122110015)and the Scientific Research Founda-tion of the Education Ministry for Returned Chinese Scholars,China(Grant No.2010-32).1 Liu Q C.Electrical Insulation Design Principles-book2-insulation structure pandect(in Chinese).Beijing:Machinery Industry Press,1988.137 2 Qiu C J,Wang Y H,Wang Y J.Physical Properties of Materials(in Chinese).Harbin:Harbin Institute of Technology press,2003.47-116 3 Guan Z D,Zhang T Z,Jiao J S.Physical Properties of Inorganic Ma-terials(in Chinese).Beijing:Tsingh