研究论文 正式出版 版本 2 Vol 9 (6) : 614-621 2018
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石英E1'心ESR信号强度的热活化研究
Study on the thermal activation of ESR signal intensity in quartz E1' center
: 2018 - 05 - 31
: 2018 - 07 - 27
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摘要&关键词
摘要:石英E1'心是一种重要的顺磁性缺陷,其热活化ESR信号强度的最大值有许多新的应用,但对于获取E1'心信号强度最大值的方法尚存不同的认识。为了进一步研究和探讨石英E1'心信号的增强机理及石英E1'心ESR信号强度最大值的获取方法,采用电子自旋共振(ESR)技术测量了两个冰碛样品在300℃等温加热前和加热后石英E1'心的ESR信号强度,结果表明:在室温下,石英E1'心信号强度随辐照剂量的增加而增强是由于伪E1'心的形成而造成的;而石英E1'心信号强度随辐照剂量增加而减弱可能是因为辐照使已增强的E1'心产生衰退。300℃的等温热退火实验(分为15 min和20 min)结果表明,获取E1'心ESR信号强度的最大值需要人工辐照,人工辐照的作用不仅是提供更多的空穴,而且可能更有利于氧空位向E1'心转化。
关键词:石英E1'心;ESR信号强度;伪E1'心;等温热退火实验
Abstract & Keywords
Abstract: Background, aim, and scope The E1' center in quartz is an important paramagnetic defect, and the maximum value of its thermal activation ESR signal intensity has many new applications. However, there are different views on the method for obtaining the maximum value of the E1' center intensity.Therefore, in order to further study and discuss the enhancement mechanism of the quartz E1' center and the condition for obtaining the maximum ESR signal intensity of the quartz E1' center, it is important to carry out the study on the thermal activation of ESR signal intensity in quartz E1' center. Materials and methods The Sample A was collected from the bottom part of the moraine in the Urumqi Valley of Xinjiang, China. And the Sample B was collected from the surface part of the moraine in Purog Kangri, Tibetan Plateau. The samples were irradiated by artificial gamma rays at different doses, and then the E1' center signal intensities of the samples were measured by electron spin resonance (ESR) technology before and after the 300℃ isothermal heating. ResultsAt room temperature, the intensity of the E1' signal of Sample A is enhanced with the increase of irradiation dose, while the intensity of the E1' signal of Sample B descends first and then rises gradually with the increase of radiation dose. After heating at 300℃ for 15 min, the E1' center signal intensities of Sample A increases first with the increase of radiation dose, and then saturates above 400 Gy. For another set of aliquots of Sample A which are heated at 300℃ for 20 min, the E1' center signal intensities are basically the same. For Sample B, the E1' center signal intensities of the unirradiated (0 Gy) aliquots are smaller than that of the irradiated aliquots, regardless of heating for 15 minor 20 min. DiscussionThe signal intensity of the E1' center is enhanced with the increase of irradiation dose at room temperature due to the formation of the counterfeit E1' center, while the intensity of E1' center decreases with the increase of radiation dose may due to the gamma ray irradiation cause the real E1' center to decay. For the one set of aliquots of Sample A which are heated at 20 min at 300℃, the signal intensity of the E1' center are basically the same indicate that the amount of the holes in the Sample A is sufficient, and the E1' signal intensities of the another set of aliquots of Sample A which are heated at 300℃ for 15min increase first with the increase of radiation dose and then saturate above 400 Gy show that the irradiation can facilitate the conversion of oxygen vacancies to E1' center. After the isothermal heating at 300℃ for 15 min or 20 min, the E1' center signal intensities of the unirradiated (0 Gy) aliquots of Sample B are smaller than that of the irradiated aliquots indicate that radiation can produce more holes, so that all oxygen vacancies can be transformed into E1' center. ConclusionsGamma irradiation would enhance the intensity of the counterfeit E1' center, but can reduce the real E1' center signal intensity. In order to obtain the maximum value of the E1' signal intensity, the pre-irradiation is needed and the effect of artificial irradiation is not only to provide more holes, but also to facilitate the conversion of oxygen vacancies to E1' center. Recommendations and perspectives The maximum value of the quartz E1' center signal intensity is not only related to irradiation, but also related to temperature. Therefore, in order to get the maximum value of E1' center signal intensity more accurately, more study on the thermal activation characteristics of the ESR signal intensity of quartz E1' center should be carried out.
Keywords: quartz E1' center; ESR signal intensity; counterfeit E1'; isothermal annealing experiment
石英晶体在受到高能光子、中子或电子的辐照作用下,Si-O-Si结构中的氧原子会偏离其原来的晶格位置,导致两个硅原子直接成键(即Si—Si键),形成氧原子空位的晶格缺陷——氧空位,当氧空位捕获一个空穴之后会形成具有顺磁性的缺陷中心,即E1'心(Silsbee,1961;Feigl et al,1974;Weil,1984;Preusser F et al,2009)。然而,与杂质心不同的是,在加热的过程中E1'心的信号强度会增强(Weeks and Nelson,1960;Toyoda and Ikeya,1991)。热处理后的石英E1'心信号强度的最大值(或饱和值)被认为能够指示石英中氧空位的数量(Toyoda and Ikeya,1991;Toyoda et al,1992;Toyoda and Hattori,2000),而且有许多新的应用:对花岗岩和铀矿石进行定年(Toyoda,2005),评估石器热处理的温度(Toyoda,1992;Toyoda,2005;)和评估古地温(业渝光等,2000),及对沉积物的物源进行研究(Naruse et al,1997;Ono et al,1998;Toyoda and Naruse,2002;Nagashima et al,2007,2011,2013;Sun et al,2007,2008,2013;Tada,2012;Yamamoto et al,2013;Saito et al,2017)。
但是,前人关于获取石英E1'心ESR信号强度最大值的方法尚存不同的认识。Toyoda and Ikeya(1991)及Toyoda et al(1992)认为经过200 Gy以上γ辐照后的石英E1'心热活化强度达到最大值,提出应对样品先进200 Gy以上的γ辐照后再在300℃下加热15 min;而陈继镛等(1991)认为获取石英E1'心信号强度最大值的条件是在300℃下加热 20 min。因此,进一步研究和探讨石英E1'心的增强机理及石英E1'心ESR信号强度最大值的获取方法具有重要的理论和实际意义。本文通过对两个冰碛物样品进行了不同剂量的人工γ辐照,并在300℃等温加热前和加热后运用ESR技术对样品的E1'心信号强度进行了测量,来研究石英E1'心的增强机理,以及为石英E1'心热活化ESR信号强度最大值的获取方法提供基础性的实验数据和一定的理论参考,从而促进石英E1'心在实践中更好地应用和发展。
1   采样和前处理
1.1   采样
本次的研究采用了两个冰碛物样品,样品A采自于中国新疆乌鲁木齐河谷内冰碛物的底层部分(43°07′25.3″N,86°52′35.7″E),海拔3407 m。样品B采自于青藏高原普若岗日冰碛物的表层部分(33°54′59.6″N,89°16′20.3″E),海拔5447 m。
1.2   前处理
首先从野外采集的样品中筛取出0.125—0.25mm粒级的组分,用清水冲洗干净。然后用H2O2去除有机质,水洗干净后用HCl去除碳酸盐,水洗后用硅钨酸溶液去除重矿物和长石。然后再用HF蚀刻去除剩余的长石及石英表层受到α辐照的部分,然后用蒸馏水冲洗至中性,在50℃恒温箱内烘干,烘干后的样品通过磁选除去磁性矿物。关于样品前处理的更多细节可以参考Yi et al(2016)。最后,将提纯后的石英样品等分为11等份,每份300 mg。除了保留1份自然样品外,其余样品送到北京大学化学系用60Co进行200—2000 Gy的γ辐照。
2   实验方法
将样品A和样品B的自然样品和辐照后的样品再等分为2份,即0-1、0-2;2-1、2-2;4-1、4-2……20-1、20-2,每份150 mg,装入样品管。在进行加热试验之前首先将每一份样品的E1'心信号强度进行ESR测量,然后再进行等温加热试验。加热设备采用马弗炉,待温度上升到300℃后再快速放入样品。其中一组样品(0-1、2-1、4-1......20-1)在300℃下加热20 min,另外一组(0-2、2-2、4-2......20-2)在300℃下加热15 min。另外,为了排除样品管对样品信号的影响,在等温加热前后对样品管进行了ESR测量。
ESR信号测量是在清华大学化学系测试与分析中心进行的,ESR波谱仪微波频率为X波段,仪器型号为JEOL JES-FA-200,并带有锰标。测量条件为:室温,微波功率0.1 mW,调制幅度为0.1 mT,扫场范围为10 mT,扫描时间1 min,时间常数为0.1 s,调制频率为100 kHz。将g=2.001处的ESR信号强度的峰-峰高作为E1'心的信号强度。每份样品通过在谐振腔内每次旋转120°,一共测量E1'心信号强度3次后取平均值。为了消除仪器和其它因素在样品信号测量过程中的影响,将样品的信号强度与锰标的信号强度的比值作为样品的最终信号强度。
3   结果和讨论
3.1   样品管加热前后的ESR测量结果
图1显示的是带有低噪音信号的平坦的ESR信号一次微分曲线。最左边和最右边的峰是锰标的第三和第四信号峰,曲线上没有其它明显的峰,这表明样品管在加热前后不会对样品的信号产生影响。


图1   样品管测试结果图1 样品管测试结果
Fig.1 Test results of the sample tube
a:加热前,b:在300℃下加热15 min,c:在300℃下加热20 min。a: before heating, b: after heating at 300℃ for 15 min, c: after heating at 300℃ for 20 min.
3.2   样品A在等温加热前后的E1'心信号强度
图2显示的是样品A在等温加热前的E1'心信号强度随辐照剂量的变化情况,图2表明在常温下样品A的E1'心信号强度随着辐照剂量的增加而增强,但根据前人的报导(Toyoda and Schwarcz,1997),这种增强不是真正的E1'心信号增强,而是由于形成了伪E1'心(另一种与氧空位有关的点缺陷,与真E1'心具有相似的微波功率相关性,该心的信号形状为单峰,热稳定性较差,在加热到170℃左右时就会大幅衰退)。在样品A的ESR测量过程中,同样有这种现象:E1'心信号的两个峰随着辐照剂量的增加逐渐变成一个峰(图3所示),而且这种现象在样品B加热前的ESR测量过程中表现地更加明显(图6)。在加热之后,伪E1'心的确消失了,辐照样品的E1'心的信号都恢复成了两个峰的形状。


图2   加热前样品A的E1'心信号强度随辐照剂量的变化
Fig.2 Intensity change of the E1' center of Sample A with radiation dose before heating


图3   加热前样品A的E1'心信号形状随辐照剂量的的变化
Fig.3 Change in signal shape of the E1' center of Sample A with radiation dose before heating
在300℃进行等温加热后样品A的E1'心ESR信号强度变化如图4所示,图4表明:对于在300℃下加热15 min后的不同辐照剂量的一组等份样品(即0-2、2-2、4-2……20-2),其E1'心的信号强度先随着辐照剂量的增加而增强,并在400 Gy以上达到饱和,这一结果与Toyoda and Hattori(2000)报导的是一致的。


图4   在300℃下分别加热15 min和20 min后,样品A的E1'心强度随辐照剂量的变化
Fig.4 Change of the E1' center intensity of Sample A with radiation dose after heating at 300℃ for 15 min and 20 min respectively
对于另一组在300℃下加热20 min的不同辐照剂量的等份样品(即0-1、2-1、4-1……20-1),它们的E1'心ESR信号强度全部基本一致。按照前人对E1'心信号强度在加热过程中增强的解释(Jani et al,1983;Toyoda and Hattori,2000):在加热过程中,含有两个电子的中性氧空位能够捕获从空穴心如Al心释放出来的空穴,其中一个电子与空穴结合,从而使氧空位剩下一个未配对电子,即形成具有顺磁性的E1'心。则对于在300℃下加热20 min后的不同辐照剂量的等份样品,它们的E1'心信号强度基本一致说明样品A中已有的空穴数量是足够的,这样通过加热可以使氧空位全部转化成E1'心,从而E1'心的信号强度才最终基本相同。因此对于在300℃下加热15 min的一组等份样品来说,其E1'心的信号强度先随着辐照剂量的增加而增强,出现这种现象可能不是由于空穴数量不够而造成的,可能是因为加热时间不足导致0 Gy和400 Gy的等份样品的氧空位未能全部地充分地与空穴结合,从而造成E1'心的信号强度相对较低。因此,对于样品A来说,在300℃下加热20 min可能是合适的。
然而对于辐照400 Gy以上的等份样品A,它们加热15 min后的E1'心的信号强度和加热20 min后的强度一致,则说明辐照400 Gy以上的样品在加热15 min后氧空位已经全部转变成E1'心了,这样E1'心强度才会一致,这表明辐照除了有提供更多的空穴的作用之外,还有其它作用,即辐照可能使氧空位更容易向E1'心转化。辐照可能使样品内部的微观粒子的能量状态发生了变化,从而在加热过程中大剂量辐照过的样品相对于未辐照和辐照小剂量的样品,其氧空位更容易转化成E1'心,因此E1'心的信号强度会提前达到峰值。
3.3   样品B在等温加热前后的E1'心信号强度
图5显示的是样品B在加热前E1'心ESR信号强度随辐照剂量的变化情况,图5表明样品B的E1'心ESR信号强度随辐照剂量的增加先减小后逐渐上升,Toyoda and Schwarcz(1997)曾报导过类似的结果。


图5   加热前样品B的E1'心信号强度随辐照剂量的变化
Fig.5 Change of the E1' center intensity of Sample B with radiation dose before heating
由于样品B是采自于普若岗日冰碛物的表层部分,根据前人的报导,光照也能使E1'心信号增强(金嗣炤等,1991;赵兴田等,1991;Toyoda et al,2000),因此该样品的E1'心信号强度可能在自然界受到光照的影响已经增强,不过该样品加热后的结果(图7)显示加热后E1'心的信号强度变得更强,说明光照未能使该样品的氧空位全部转化成E1'心,可能只是使样品表面部分的氧空位转化成E1'心。而E1'心信号强度先随着人工辐照下降可能是由于辐照使得样品表面部分的E1'心发生了衰退。在600 Gy后E1'心信号强度又随着辐照剂量的增加而增强是因为生成了伪E1'心,如图6所示。


图6   样品B中比较明显的伪E1'心变化
Fig.6 Clearer changes in the counterfeit E1' signal of Sample B
图6比较清楚地展示了伪E1'心信号随辐照剂量的变化过程,当真E1'心的信号强度随辐照剂量的增加而降低时,伪E1'心的信号强度增强,并逐渐覆盖真E1'心信号。在600 Gy以后,伪E1'心的信号开始占主导,两个峰变成了单峰,并伴随着峰的增宽。在等温热处理之后,该伪E1'心信号消失,样品E1'心的信号重新恢复成双峰形态。
在300℃进行等温热处理之后,样品B的E1'心信号强度变化情况如图7所示。图7表明,对于样品B的两组不同辐照剂量的等份样品,不论在300℃下加热15 min或是20 min,未经辐照(0 Gy)的样品的E1'心信号强度要小于经过辐照的样品E1'心信号强度,辐照200 Gy以上的样品的E1'心信号强度基本一样,这说明经过辐照产生了更多的空穴从而使得氧空位全部转化为E1'心。因此,该实验结果表明,为了使氧空位全部转化成E1'心,辐照是需要的。


图7   在300℃下分别加热15 min和20 min后,样品B的E1'心信号强度随辐照剂量的变化
Fig.7 The change of the E1' center intensity of Sample B with radiation dose after heating at 300℃ for 15min and 20min respectively
4   结论
γ辐照会使伪E1'心的信号增强,却能使真E1'心的信号强度降低。为了使氧空位全部转化为E1'心,在加热前需要进行预辐照,辐照的作用不仅是提供更多的空穴,而且可能使氧空位更容易向E1'心转化。
致谢:
感谢清华大学化学系测试与分析中心梁茜茜老师的帮助和审稿人的宝贵建议。
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稿件与作者信息
李淼1
LI Miao1
易朝路1,2*
YI Chaolu1,2*
YI Chaolu, E-mail: clyi@itpcas.ac.cn
毕伟力1
BI Weili1
杨**3
YANG Haijun3
邱登峰4
QIU Dengfeng4
国家自然科学基金项目(41271018,40971017)
National Natural Science Foundation of China (41271018, 40971017)
出版历史
出版时间: 2018年7月27日 (版本2
参考文献列表中查看
地球环境学报
Journal of Earth Environment