1.Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, China 2.Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 10774123) and the Fundamental Research Funds for the Central Universities, China (Grant No. 2682018ZT29)
Received Date:02 February 2021
Accepted Date:17 March 2021
Available Online:07 June 2021
Published Online:05 August 2021
Abstract:Amorphous selenium (Se) can be easily prepared by quenching the melt, which indicates that the Se possesses the good glass-forming ability. However, crystallization occurs after rapidly compressing the melt within about 20 ms. In this work, we investigate the mechanism of rapid compression-induced crystallization from Se melt. Compressing Se melt experiments are carried out at the following temperatures: 513, 523 and 533 K. The melt is rapidly compressed under 2.4 GPa for about 20 ms. Different holding times, i.e. 0, 30, 60 min after solidification are adopted. The samples are quenched to room temperature and then unloaded to ambient pressure. The X-ray diffraction analysis of the recovered sample indicates that the crystallization product is the t-Se. It is found that with the prolongation of holding time, the grain size increases due to the continuous aggregation growth of crystal grains. By comparing with the isothermal crystallization products of amorphous Se and ultrafine Se powder, it is suggested that the rapid compression-induced solidification product should be t-Se crystalline. The speculation that the solidification product is amorphous Se and it crystallizes in the cooling process does not hold true. The amorphous Se cannot be prepared through the rapid compression process on a millisecond scale. It is related to the thermal stability of amorphous Se under high pressure. It is reported that the dependence of crystallization temperature Tx on pressure i.e. dTx/dP for amorphous Se is about 40–50 K/GPa in a range of 0.1 MPa–1 GPa. However, the Tx of amorphous Se is almost constant in a range of 2–6 GPa. It means that the thermal stability of amorphous Se against crystallization does not increase with increasing pressure after 2 GPa. In this work, the temperature of 513–533 K in the experiments is higher than the Tx of amorphous Se. Therefore, the t-Se crystal is the stable phase and amorphous Se is unstable. The Se melt tends to crystallize in the supercooled liquid state after rapid compression. It is interesting to investigate the mechanism of dTx/dP curve discontinuous change at around 2 GPa in the future. Both the Se melt after rapid compression and the amorphous Se before crystallization are in supercooled liquid state. We speculate that high pressure may result in the microstructure transition in supercooled liquid state Se. Keywords:high pressure/ selenium/ crystallization/ compression-induced solidification
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3.实验结果与分析图3是快速压致凝固实验回收的9个样品XRD图谱, 图3(a)是513 K温度下快压、高压下分别保温0, 30, 60 min回收的样品XRD谱. 图3(b)和图3(c)分别是在523 K和533 K温度下快压并保温不同时间回收的样品XRD谱. 尖锐的衍射峰表明回收样品均为晶体相, 与标准谱对比发现这9个样品均为六方晶硒(t-Se, 空间群为P3121). 由于实验过程中样品包裹在h-BN传压介质中, 图中2θ = 26.7°峰是h-BN的(002)晶面衍射峰. 与图中初始微米粉末样品的XRD谱相比, 快压回收样品的半峰宽变窄, 说明晶体结晶性好, 晶体颗粒较大. 从图3(a)可以看到在相同实验温度下, 随着保温时间的延长, (100), (110), (012), (111), (201)等晶面的衍射峰相对强度明显增强, 说明多晶粉末样品中的晶体颗粒变大, 上述晶面的数量增加、衍射光强增高. 图3(b)和图3(c)中, 相比于高压下不保温的样品, 保温后回收的样品的上述晶面的相对强度也有不同程度的增加. 图 3 不同温度下快速压致凝固样品的XRD图谱(给出了初始微米粉末样品的XRD图谱作为对比) (a) 513 K; (b) 523 K; (c) 533 K Figure3. XRD patterns of Se samples, which are rapidly solidified from melt at different temperatures of (a) 513 K; (b) 523 K; (c) 533 K. As comparison, the XRD pattern of μm scale Se powder is also displayed.
硒的熔点随压力的升高而升高, 2.4 GPa压力下熔点约为827 K[23], 因此快速增压后熔体进入固相区而凝固了. 本实验中关于晶体硒的形成机理, 有两种可能: 一是熔体快压凝固的结构为非晶硒, 非晶硒结晶形成晶体硒; 二是熔体快压凝固的结构为晶体硒. 为此我们对比了9个快压回收样品的微观形貌, 如图4所示. 在513 K实验温度下, 快压后不保温的Sample A1为数百纳米的晶体小颗粒; 在快压后保温30 min的Sample A2中可以看到小晶体颗粒聚集生长, 晶粒之间连接生长; 高压下保温60 min的Sample A3中可看到小的晶体颗粒已聚集生长成大块的晶粒, 然而部分表面还是可以观察到小晶体颗粒的轮廓. 在523 K和533 K实验温度下, 随着保温时间的延长, 也观察到晶粒聚集生长、变大的现象. 晶粒变大, 相应地衍射晶面的数量增加, 与XRD的结果一致. 如果快压凝固的结构为非晶硒, 非晶硒结晶成核后, 非晶硒中的硒原子通过扩散进入晶格, 在相同温度下不同的保温时间使晶粒生长的时间不同, 最终会观察到不同大小的完整晶粒. 然而在本实验中, 延长保温时间明显观察到晶界处晶体聚集生长, 晶体生长的驱动力是降低总界面能, 因此我们推测硒熔体快压凝固的结构为晶体硒, 保温过程中这些晶体发生聚集生长. 图 4 不同温度下快速压致凝固样品的SEM图谱 Figure4. SEM pictures of Se samples, which are rapidly solidified from melt at different temperatures.
升高结晶温度通常会提高晶核生长速率, 导致晶体粒径增大. 图4中对比Sample A1, Sample B1, Sample C1发现提高实验温度, 压致凝固形成的晶体硒颗粒变大. 本文还开展了相同压力过程及温度条件下非晶硒、晶体硒等温结晶的对比实验. 首先对非晶硒初始样品进行了分析, 图5(a)中两个XRD谱都只包含宽而弥散的非晶峰, 表明硒熔体急冷制备的样品为非晶硒, 以及常温快压后回收的样品仍为非晶态结构, 即快压过程不能引起非晶硒结晶. 图5(b)和图6分别是非晶硒快压后分别在513, 523, 533 K保温30 min回收的3个样品XRD谱和SEM图, 表明非晶硒发生了结晶, 晶体颗粒的尺寸随着实验温度的升高而增大. 图7和图8是超细硒粉快压后分别在513, 523, 533 K保温30 min回收的3个样品XRD谱和SEM图. 从SEM图可以看到小晶体颗粒间发生了聚集生长. 推测快压作用下晶界处发生了塑性变形, 升高温度后硒原子振动加剧, 晶界处不同晶粒的硒原子发生键合. 随着实验温度的提高, 小的晶体颗粒聚结成大块晶粒, 但表面仍可以观察到小晶体颗粒的轮廓. 对比非晶硒结晶及晶体硒结晶的产物形貌, 认为非晶硒结晶的产物为完整晶粒, 晶粒大小与结晶温度及结晶时间有关, 在硒晶体结晶的产物中则明显观察到小晶粒间聚集生长. 这个结论支持了前述硒熔体快压凝固结构为晶体硒、延长保温时间晶体硒发生聚集生长的推测. 图 5 (a) 非晶硒样品的XRD图谱, 包括常压制备的非晶硒XRD图谱、非晶硒常温快压后回收样品的XRD图谱; (b)在513, 523, 533 K温度下非晶硒等温结晶样品的XRD图谱 Figure5. (a) XRD patterns of amorphous selenium (a-Se) sample and the compressed a-Se which is recovered after rapidly compressed at room temperature; (b) XRD patterns of Sample I, Sample II, Sample III, which are the isothermal crystallization products of a-Se at 513, 523, and 533 K, respectively.
图 6 在513, 523, 533 K温度下非晶硒等温结晶样品的SEM图谱 Figure6. SEM pictures of Sample I, Sample II, Sample III. The temperatures of 513, 523, and 533 K are the isothermal crystallization temperatures of a-Se.
图 7 在513, 523, 533 K温度下超细硒粉等温结晶样品的XRD图谱 Figure7. XRD patterns of Sample 1, Sample 2, Sample 3, which are the isothermal crystallization products of ultrafine Se powder at 513, 523, and 533 K, respectively.
图 8 (a)超细硒微粉的SEM图; (b)不同温度下超细硒粉等温结晶Sample 1, Sample 2, Sample 3的SEM图 Figure8. (a) SEM picture of ultrafine Se powder; (b) SEM pictures of Sample 1, Sample 2, Sample 3. The temperatures of 513 K, 523 K and 533 K are the isothermal crystallization temperatures of ultrafine Se powder.
图9是压致凝固得到的晶体硒、非晶硒等温结晶得到的晶体硒的XRD谱及精修结果. 拟合的Sample A1, Sample B1, Sample C1的晶胞参数分别为: a = 4.3598 ?, b = 4.3598 ?, c = 4.9518 ?; a = 4.3607 ?, b = 4.3607 ?, c = 4.9537 ?; a = 4.3570 ?, b = 4.3570 ?, c = 4.9517 ?. 拟合的样品Sample I, Sample II, Sample III的晶胞参数分别为: a = 4.3565 ?, b = 4.3565 ?, c = 4.9547 ?; a = 4.3617 ?, b = 4.3617 ?, c = 4.9562 ?; a = 4.3545 ?, b = 4.3545 ?, c = 4.9575 ?. 表1列出了它们主要的衍射峰信息, 同时也列出了初始微米粉末样品的峰信息作为对比. 6个高压回收样品的衍射峰半峰宽明显小于微米粉末样品, 说明它们结晶良好, 晶粒较大. 6个样品的衍射峰位及晶胞参数与初始微米粉末的衍射峰位及t-Se硒标准谱的晶胞参数符合得很好, 说明压致凝固得到的硒晶体、非晶硒结晶得到的硒晶体, 与常压制备的硒晶体相比无明显结构不同或残余应力引起的形变. 图 9 Sample A1, Sample B1, Sample C1, Sample I, Sample II, Sample III衍射谱的精修结果, 图中黑色点表示衍射实验数据, 红色曲线为计算的衍射峰, 蓝色曲线为实验数据与计算数据的偏差, 紫色的短线表示t-Se相衍射峰的位置 Figure9. XRD patterns of Sample A1, Sample B1, Sample C1, Sample I, Sample II, Sample III. Symbols: experimental data (black dots), calculated diffraction pattern (red line), residuals of the refinement (blue solid line), and peak positions of t-Se (purple vertical bar).
(100)
(101)
(110)
(012)
(111)
I/%
d/nm
FWHM/(°)
I/%
d/nm
FWHM/(°)
I/%
d/nm
FWHM/(°)
I/%
d/nm
FWHM/(°)
I/%
d/nm
FWHM/(°)
μm powder
43.7
3.795
0.446
100
3.013
0.353
13.9
2.187
0.593
30.4
2.074
0.453
19.2
2.002
0.592
Sample A1
25.3
3.793
0.316
100
3.010
0.187
8.6
2.186
0.474
15.3
2.077
0.292
13.4
2.001
0.395
Sample B1
24.8
3.779
0.251
100
3.004
0.166
7.7
2.182
0.301
24.6
2.072
0.215
9.5
1.997
0.323
Sample C1
34.7
3.785
0.231
100
3.010
0.182
10.5
2.184
0.292
38.5
2.075
0.243
13.5
1.999
0.323
Sample I
39.5
3.785
0.290
100
3.010
0.218
11.6
2.185
0.447
20.7
2.077
0.349
14.1
2.000
0.435
Sample II
51.7
3.789
0.324
100
3.007
0.238
10.4
2.185
0.521
19.7
2.077
0.349
12.2
1.999
0.489
Sample III
33.2
3.789
0.326
100
3.007
0.244
10.0
2.183
0.500
24.4
2.078
0.376
13.3
2.000
0.487
表1Sample A1, Sample B1, Sample C1, Sample I, Sample II, Sample III的XRD谱中部分衍射晶面的信息, 包括衍射峰的相对强度I、晶面间距d、半峰宽FWHM Table1.Diffraction peaks parameters of Sample A1, Sample B1, Sample C1, Sample I, Sample II, Sample III, including the relative peak intensity (I), interplanar distance (d) and peak width at half-height (FWHM).
如引言中所述, 硒熔体以低的冷却速率凝固即可形成非晶硒, 例如放入室温下的水中冷却[8], 说明硒非晶形成能力较强. 然而本文中, 毫秒时间内快速凝固没有得到非晶硒. 以523 K的实验温度为例, 2.4 GPa下硒的熔点约为827 K, 快压后硒的过冷度约为304 K, 按增压时间20 ms推算冷却速率约为$ 1.52\times {10}^{4} $ K/s. 为了探讨未得到非晶硒的可能原因, 图10中将本文的实验条件与高压下非晶硒晶化温度、硒熔化温度进行比较. Ye和Lu等[24]测得硒的晶化温度Tx随压力的升高而升高, 在0—1 GPa范围内dTx/dP约为40—50 K/GPa, 晶化激活能Ex由常压下的(69$ \pm $5) kJ/mol升高到(167$ \pm $5) kJ/mol, 认为高压会抑制晶核的形成[24]. 如图10的内插图所示, 如果压力高于1 GPa时非晶硒晶化温度随压力的变化延续dTx/dP约为40—50 K/GPa的趋势, 则本文的实验条件将低于高压下非晶硒的晶化温度, 该条件下非晶硒可以作为亚稳相存在. 然而, 如图10所示, He等[23]测得非晶硒的晶化温度Tx随压力的变化趋势在2 GPa前后明显不同, 2 GPa以后Tx几乎不随压力变化. 压力高于2 GPa后, 非晶硒的晶化激活能几乎不变, 当温度升高时非晶硒的能量增加, 在较低温度下(高于500 K)就可以跨越能量势垒发生结晶. 从图10可以看到, 本文的实验条件高于非晶硒的晶化温度, 该条件下非晶硒为不稳定相. 非晶硒在晶化发生前处于过冷液体状态, 无论是熔体结晶还是过冷液体结晶, 都包括成核和晶核长大的过程. 本文的实验条件下晶体硒是稳定相, 非晶硒为不稳定相, 硒熔体凝固时极易发生结晶, 因此即使毫秒时间内的快速凝固也无法得到非晶硒. 但是毫秒时间内的快压凝固可以抑制晶体长大, 因此熔体快速压致凝固法可以用于制备纳米硒[22]. 图 10 非晶硒的晶化起始温度Tx和晶化产物t-Se的熔化温度Tm随压力的变化关系, 其中, Ye和Lu[24]的数据测量实验的升温速率为8.7 K/min, He等[23]的数据测量实验的升温速率为8.6 K/min, 内插图清楚地显示了400—560 K温度范围内的关系曲线 Figure10. Onset crystallization temperature (Tx) of a-Se and the melting temperature (Tm) of a-Se crystallization product i.e. t-Se as a function of the applied pressure. Data from Ye and Lu[24] was measured under the heating rate of 8.7 K/min. Data from He et al.[23] was measured under the heating rate of 8.6 K/min. The pressure and temperature conditions in this work are shown. The inset figure displays clearly the data in the range of 400–560 K.