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[Ca24Al28O64]4+(4e–) eletride, as the first room-temperature stable inorganic electride, has attracted intensive attention because of its fascinating chemical, electrical, optical, and magnetic properties. However, it usually needs synthesizing through a complicated multistep process involving high temperature (e.g., 1350 °C), severe reduction (e.g., 700–1300 ℃ for up to 240 h in Ca or Ti metal vapor atmosphere) and post-purification. Owing to the H2O sensitivity of mayenite, the post-purification is quite troublesome once impurities are introduced. High-density, loosely bound encaged electrons with a low work function make it promise to possess practical applications. Therefore the facile method of massively producing the high-quality C12A7:e– with high Ne is extremely desired. In this work, C12A7:e– bulks are for the first time synthesized by simple spark plasma sintering process directly from a mixture of C12A7, CA and Ca powders under milder conditions (e.g., sintered at 1070 ℃ for 10 min in a vacuum). The obtained electride, which exhibits a relative density of 99%, an electron concentration of ~2.3×1021 cm–3 and an obvious absorption peak at 2.5 eV, is obtained via SPS process at 1100 ℃ for 10 min. Electronic structure is also investigated by electron paramagnetic resonance. The occurrence of Dysonian characteristic, a typical feature of good electronic conductors, strongly suggests that the electrons are trapped in mayenite cavities. Furthermore, the obtained C12A7:e– exhibits good sinterabilty on a crystal scale of 5–40 μm. Thermionic emission test results show that the thermionic emission begins to occur at 700 K and a large current density of 1.75 A/cm2 is obtained in the electron thermal emission from a flat surface of the polycrystalline C12A7:e– with an effective work function of 2.09 eV for a temperature of 1373 K with an applied electric field of ~35000 V/cm in a vacuum. Owing to no external reductant is needed, this developed route exhibits notable superiority over the conventional reduction method for phase-pure C12A7:e–. Therefore, these results not only suggest a novel precursor for fabricating mayenite electride but also make it possible to produce efficiently the electride in large volume.
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Keywords:
- C12A7:e– /
- calciothermic reaction /
- thermionic emission
1. 引 言
钙铝石电子化合物[Ca24Al28O64]4+(e–)4(C12A7:e–)是第一种可以在空气中稳定存在的无机电子化合物, 它是其前驱体七铝酸十二钙12CaO·7Al2O3(C12A7:O2–)的稳定贫氧形式[1,2], 其单胞由12个三维互连的亚纳米笼腔和束缚于笼腔内部的4个e–组成[3]. 这些笼腔通过Ca-O-Al-O-Al-O结构共面连接[4], 笼腔电子可以跳跃到近邻笼腔或次近邻笼腔形成跳跃导电或变程跳跃导电即类似于小极化子[5], 这极大地增强了其电传导能力(1500 S/cm)[6]. 同时足够大的笼腔密度(6 × 1021 cm–3)和阴离子电子密度(2.33 × 1021 cm–3)在框架导带(FCB)底部形成一个被部分占满的笼腔导带(CCB), 提高了其费米能级的位置[7—9]. 类似于碱金属氧化物(CaO, MgO)中束缚于氧空位中的电子, 这些定域态的电子形成一种类F心结构[10—12]. 然而每个笼腔仅拥有1/3个正电荷, 远远低于典型F心周围原子所提供的电荷量[13], 同时笼腔内径较大(0.5 nm), 且随着电子浓度提高而弛豫增大, 即正电框架提供的马德隆势与典型F心相比是相当小的, 这就导致了电子处于弱束缚态易发生能级跃迁或者极化跳跃[14—17]. C12A7:e–的这种独特的晶体结构与电子结构使其具备了相当低的本征逸出功2.4 eV, 这个数值远低于ReB6和半导体氧化物, 与碱金属Na相当[18], 但它却具有良好的化学惰性. 较高的费米能级位置、较小的逸出功与良好的稳定性使C12A7:e–在低温催化与电子发射领域有着良好的应用前景.
显然, 简单高效的制备高质量的C12A7:e–极具研究价值. 目前, C12A7:e–的制备方法主要包括: 高温金属蒸汽还原(e.g. Ca, Ti, V)[19,20]、碳还原[21]、氢气氛处理结合UV辐照[22]、活性气氛还原(CO)[23]. 但是这些制备工艺都需要繁琐的步骤、苛刻的工艺条件与较长的制备周期. 例如: 金属蒸汽处理虽然可以得到接近理论值的电子浓度, 但需要在高真空(10–5 Pa)环境下封管退火数十个小时; 碳还原需要预结晶或者玻璃化过程, 且得到的电子浓度偏低(1018 cm–3); 氢气氛处理结合UV辐照和CO还原也都存在电子浓度低(1019 cm–3)的问题. 金属蒸汽还原效果较好, 但会给C12A7:e–表面带来金属氧化层, 由于C12A7对水溶剂敏感[24], 因此存在后净化困难的问题以致难以获得干净的表面. 此外, 以上所有方法都需要引入外来的还原剂, 这将不可避免地引入杂质; 同时这些工艺都对前驱体C12A7的尺寸有着严格的限制, 因此无法实现大规模生产, 实际应用受阻. 本文以CaCO3, Al2O3和Ca为原料, 在不引入外来元素的条件下利用原位钙热反应在放电等离子烧结(spark plasma sintering, SPS)系统里直接合成了致密的多晶C12A7:e–块体, 电子浓度接近理论值, 制备周期大幅度缩短(10 min), 同时对其电子结构与热电子发射性能做了系统的研究.
2. 实验方法
以11∶7的化学计量比将CaCO3 (99.99 wt.%)和α-Al2O3 (99.99 wt.%)粉末高能球磨20 min, 将混合均匀的粉末置于高温炉中, 在1200 ℃下煅烧12 h, 得到化学计量比为4∶7 的C12A7+CA的两相混合物前驱体, 反应过程如下:
11CaCO3+7Al2O3=0.8Ca12Al14O33+1.4CaAl2O4+11CO2. (1) 将上述前驱体破碎研磨, 装入真空石英管退火处理, 以去除吸附的超氧离子, 真空度为10–5 Pa. 然后从手套箱中取出, 加入标准计量比的CaH2 (纯度为99.9 wt.%)粉末, 在氩保护气氛下球磨后装入ϕ20 mm的石墨模具中, 放入SPS系统中进行烧结. 烧结温度为1100 ℃, 保温时间为10 min. 烧结过程中发生的反应如下:
0.8Ca12A114O33+1.4CaA12O4+CaH2=Ca12A114O32:2e−+H2. (2) 烧结参数如下: 腔体真空度低于8 Pa, 烧结压强50 MPa, 烧结温度1000 ℃, 升温速率80 ℃/min, 保温时间10 min, 随炉冷却. 在烧结的同时发生原位钙热反应, 直接得到致密的C12A7:e–多晶体.
使用X射线衍射仪(XRD, DMAX-IIIB)分析样品的物相, 借助扫描电子显微镜(SEM, FEI-NANOSEM 200)观察试样微结构, 利用阿基米德排水法计算样品的相对密度, 利用电子顺磁共振谱仪(Bruker E500 X-band)分析电子结构, 利用紫外分光光度计评价电子浓度. 采用电子发射测试平台评价热电子发射性能, 测试条件: 真空度为6 × 10–5 Pa, 测试温度为973—1373 K, 引出电场为35000 V/cm, 样品尺寸为1 mm × 1 mm × 3 mm, 对发射面进行机械抛光以及后净化处理已得到洁净的平面. 从I-V特性曲线中外推出零场发射电流密度, 从而计算得到有效理查生逸出功以及发射常数.
3. 结果与讨论
3.1 物相组成及微结构
图1为前驱体与烧结产物的XRD与对应的实物照片. 从图中可以看出前驱体为C12A7与CA两相共存. 加入化学计量比的CaH2粉末烧结后, CA相消失, 表现为C12A7单相, 无杂相出现, 这表明SPS工艺有助于快速形成C12A7结构, 且不会引入杂相. 前驱体在烧结前为暗黄色, 烧结后转变为亮黑色, 颜色的变化是由于笼腔电子吸收波长为2.5 eV左右的光子能量从1s基态跃迁到1p激发态造成的[25], 这表明在烧结过程中既引入了未配对电子又形成了C12A7纯相. 此外, 借助X射线光谱分析仪(XRF)分析了所制备样品的元素比例, 得到的结果为Ca∶Al∶O = 12.0∶14.1 ± 0.1∶31.7 ± 0.2, 可以看出Ca/Al的值完美地符合理论值, 显然O元素的含量被低估了, 这是由于XRF对轻元素的评价不准造成的. 以上结果初步说明了C12A7:e–电子化合物的形成.
图 1 C12A7+CA混合前驱体烧结前后XRD与实物照片Fig. 1. Powder X-ray diffraction patterns of precursor before and after SPS process, insets are digital pictures of the precursor and the obtained electride.更进一步, 使用SEM对抛光热腐蚀处理后的样品进行了微结构分析. 如图2所示, 晶粒尺寸大小分布范围为5—40 µm, 且晶界清晰可见, 有明显的三叉晶界存在, 这可能对相变形核及晶粒的长大有着显著的影响. 此外, 所有的晶粒被致密地烧结在一起, 在晶界及其邻近区域处没有气孔、微裂纹或者第二相出现, 也没有非晶或者微结构缺陷在相界面处出现. 用阿基米德排水法测试了样品的相对密度为99.2%, 较高的密度也与得到的微结构结果是完全一致的. 这些结果说明: C12A7+CA前驱体可烧结性良好, SPS工艺是制备高质量致密C12A7:e–的有效途径.
3.2 电子结构与电子浓度
通过XRD, XRF与样品的颜色初步确定了SPS工艺合成了C12A7:e–. 进一步使用电子顺磁共振(EPR)分析样品的电子结构, 通过紫外分光光度计准确的评价样品电子浓度. 图3(a)为C12A7:e–的EPR一次微分吸收谱, 只有自旋可以任意取向的未配对电子才能吸收磁场能量, 出现共振吸收. 可以看出, 样品在磁场强度为352 mT左右出现强烈的吸收峰, 线型为洛伦兹型, 这表明样品内部存在高密度的未配对电子, 这和C12A7:e–中笼腔结构限域的电子的属性是一致的. 吸收峰的最大值(A)与最小值(B)的比值为2.69, 这个比值的意义在于可以对顺磁性杂质(如未配对电子)的分布情况做出判断. 根据Dyson理论, 当比值接近2.7时, 说明未配对电子均匀分布在整个块体内[26]. 即该EPR曲线呈Dyson特性, 说明烧结制得的样品是一个良好的电子导体. 对吸收曲线进行积分可以得到自旋浓度为6.3 × 1019 cm–3, 这个值几乎与用Ca蒸汽还原制得的C12A7:e–中观察到的自旋浓度相同. 以上结果充分地证明了SPS工艺成功地合成了C12A7:e–. 由于电子浓度超过一定值时笼腔电子会发生反平行自旋耦合, 生成饱和的抗磁性电子对, 因此无法用EPR准确地评价电子浓度[27].
图 3 (a) 样品的顺磁共振图谱; (b) 样品的紫外吸收光谱Fig. 3. (a) EPR spectra of the sintered C12A7 bulk; (b) optical absorption spectra of the sintered C12A7:e– powders.通过紫外吸收光谱准确的计算了样品中的电子浓度, 图3(b)为样品的紫外吸收光谱, 可以看到, 前驱体C12A7+CA无明显的吸收现象, 这是因为C12A7的带隙宽度为7 eV, 在该波长范围内无法提供这样高的能量. 而C12A7:e–在2.5 eV左右出现明显的吸收峰, 该吸收峰对应于笼腔电子吸收光子能量由1S基态跃迁至1P激发态(intra-cage transition), 该吸收峰反映了CCB到FCB的能量差, 可以通过该吸收峰值的横坐标计算样品的电子浓度. 具体利用以下等式进行计算:
Ne=[−(ESP−E0SP)/0.119]0.782, (3) 这里, Ne为电子浓度; ESP为电子从1s态到1p态所需要的的能量;
E0SP=2.83eV ,E0SP 对应于电子浓度Ne = 1.0 × 1021 cm–3时跃迁所需能量, 其值为2.83 eV[28]. 计算可得, 样品的电子浓度为2.3 × 1021 cm–3,基本达到理论值(2.33 × 1021 cm–3).3.3 热电子发射性能
我们对制备的C12A7:e–的热电子发射性能进行了测试, Schottky效应对热发射电流密度的影响可以用如下Richardson等式进行描述[29]:
Js=AT2e−11605φ/Te4.4√E/T, (4) 其中, Js (A/cm2)是发射电流密度的饱和值, φ(eV)是逸出功, A (A·cm–2·K–2)是有效Richardson常数, T (K)是阴极温度, E (V/cm)是引出电场强度.
如图4(a)所示, 当阴极温度分别为为973, 1073, 1173, 1273和1373 K, 引出电场为35000 V/cm时, 最大热发射电流密度为0.39, 0.80, 0.92, 1.48和1.75 A/cm2. C12A7:e–由于具有较低的逸出功而广受关注, 可以通过肖特基外延法对本实验制备的C12A7:e–的逸出功进行计算. 当引出电场E为0时, 可以得到零场发射电流密度如下 [30]:
图 4 (a) 不同温度下发射电流密度随电场强度的变化; (b) 零场电流密度的拟合直线; (c) Richardson直线; (d) 发射稳定性曲线Fig. 4. (a) Emission current density as a function of electric field at various in the range of 973 to 1373 K; (b) Schottky plots at various temperatures, fitting of the curves result in zero field emission current density at each temperature; (c) Richardson plot of the sample; (d) scatter plot of the emission current density versus time.J0=AT2e−11605φ/T. (5) 将图4(a)中饱和热发射电流密度区域外推到x轴, 直线在y轴上的截距即为LnJ0. 即将(5)式代入(4)式[31,32]得
lnJs=lnJ0+4.4√ET. (6) 如图4(b)所示, C12A7:e–在973, 1073, 1173, 1273和1373 K下零场发射电流密度分别为0.09, 0.17, 0.49, 0.89和1.15 A/cm2. 对等式(5)两边同时除以T2并取自然对数可以得到逸出功与有效发射常数分离的直线方程, 从而可以计算得到C12A7:e–的有效逸出功与发射常数.
lnJ0T2=−11605φT+lnA. (7) 图4(c)是lnJ0/T2 的位置随着1/T变化的分布图, 可以看出这些点近似分布在一条直线上, 符合(7)式的线性特征. 同时, 这条直线的斜率对应着–11605φ, 简单计算即可得到有效逸出功为2.09 eV; 直线的斜率对应着lnA, 计算可得有效发射常数为1.03 A·cm–2·K–2. 本实验得到的逸出功(2.09 eV)略低于文献[33—36]通过热场发射得到的逸出功2.14 eV. 图4(d)是在阴极温度为1373 K, 引出电场为35000 V/cm的条件下得到的发射电流密度随时间的变化关系. 在10 h的稳定性测试中, C12A7:e–的热发射电流密度保持在1.7 A/cm2左右, 未出现明显衰减. 较低的逸出功与较好的稳定性表明C12A7:e–在热发射领域有着较高的应用潜力.
4. 结 论
以C12A7+CA为前驱体、以CaH2为原位还原剂, 通过原位钙热反应与SPS工艺相结合的方法, 在烧结温度为1100 ℃、保温时间为10 min的工艺条件下成功制备了高电子浓度(2.3 × 1021 cm–3)且致密的C12A7:e–多晶体. C12A7:e–的制备周期由数十个小时大幅度缩短至10 min, 且不需要外来还原剂, 避免了杂质的引入, 也不需要后净化处理, 与传统还原的方法相比具有显著的优势. 热电子发射测试结果表明, C12A7:e–在温度为1373 K、引出电场为35000 V/cm的条件下, 最大热发射电流密度为1.75 A/cm, 零场电流密度为1.15 A/cm2, 有效逸出功仅为2.09 eV, 且在10 h热发射测试中稳定性良好, 这表明C12A7:e–是一种极具发展潜力的阴极材料. 此外, C12A7:e–表面笼腔结构的缺失导致CCB的缺失, 这极大地增加了逸出功, 优化C12A7:e–的表面状态(如修复表面笼腔结构)将有望进一步降低有效逸出功、提高电子发射性能.
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图 1 C12A7+CA混合前驱体烧结前后XRD与实物照片
Figure 1. Powder X-ray diffraction patterns of precursor before and after SPS process, insets are digital pictures of the precursor and the obtained electride.
图 2 C12A7:e–扫描照片
Figure 2. SEM images of the sintered C12A7:e– ceramic.
图 3 (a) 样品的顺磁共振图谱; (b) 样品的紫外吸收光谱
Figure 3. (a) EPR spectra of the sintered C12A7 bulk; (b) optical absorption spectra of the sintered C12A7:e– powders.
图 4 (a) 不同温度下发射电流密度随电场强度的变化; (b) 零场电流密度的拟合直线; (c) Richardson直线; (d) 发射稳定性曲线
Figure 4. (a) Emission current density as a function of electric field at various in the range of 973 to 1373 K; (b) Schottky plots at various temperatures, fitting of the curves result in zero field emission current density at each temperature; (c) Richardson plot of the sample; (d) scatter plot of the emission current density versus time.
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