Measurement of the Melting Point of Hexagonal Boron Nitride under Pressures below 5 GPa
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摘要: 设计并组装了一套瞬间放电装置,实现了样品在DS 6×14 MN国产铰链式六面顶大腔体压机下的高压瞬间放电加热。结合大腔体静高压加载技术以及瞬间放电加热技术,利用熔体凝固过程中晶体经历形核和生长2个阶段的特征,判断晶体的熔化情况。对h-BN粉晶进行了高压环境下的瞬间放电加热处理,以研究高压下h-BN的熔化行为。采用扫描电子显微镜对高压条件下经历瞬间放电加热处理的样品进行微观形貌分析,判断h-BN晶体的熔化情况,确定了在3.4和4.3 GPa压力下h-BN的熔点分别为(
4251 ±150) K和(4531 ±200) K。这些发现有利于h-BN的应用探索以及现有氮化硼高温高压相图的修正。Abstract: In this study, a flash-heating device was designed and assembled to achieve instantaneous discharge heating of samples under high pressure in the DS 6×14 MN domestic hinged six-anvil large chamber press. By combining large chamber static high pressure and instantaneous discharge heating technologies, the melting state of crystals has been determined by the nucleation and growth characteristics during solidification. The melting behavior of h-BN powder crystal under high pressure was studied by instantaneous discharge heating treatment. Using scanning electron microscopy (SEM), the microstructures of samples obtained by high-pressure instantaneous discharge heating treatment was analyzed in order to assess the melting state of h-BN crystals. It was determined that the melting points of h-BN under 3.4 and 4.3 GPa are (4251 ±150) K and (4531 ±200) K, respectively. These results are beneficial for exploring the applications of h-BN and revising the existed temperature-pressure phase diagram of boron nitride. -
氮化硼(BN)是一种具有多种晶体相的物质,因其优异的性能而备受关注。BN主要包括六方氮化硼(h-BN)和立方氮化硼(c-BN)。h-BN 是一种具有层状结构的高温绝缘材料,具有优异的化学稳定性和电绝缘性,被广泛应用于多个领域,如半导体、电子设备、高温陶瓷器件[1–8]等。当前,生产h-BN的常用方法是以硼酸作为硼源,三聚氰胺作为氮源,在氮气保护氛围中将混合物加热并高温煅烧,促使反应物分解并生成BN[9]。h-BN的晶体结构为六方晶系,空间群为P63/mmc,每层中硼和氮原子交替排列,并与相邻层形成逆向排列,层间通过较弱的范德华力相互作用,容易相对滑动。c-BN是一种超硬材料,在工业大规模生产的超硬材料中,其硬度仅次于金刚石,被广泛应用于研磨和切割工具[10–11]。c-BN可以通过h-BN在高温高压条件下合成,合成过程中通常需要催化剂,如碱金属或碱土金属的化合物,以降低相变所需的温度和压力。c-BN与h-BN之间的转变属于固态扩散型相变,通常需要高温高压条件促使原子重新排列。在此相变过程中,h-BN的层状结构被破坏,B和N原子重新排列,形成BN的立方晶格结构[12]。
h-BN中的B―N键键能较大,导致h-BN的熔点较高,且在常压或较低压力下到达熔化温度之前容易发生分解、汽化或氧化,常规的方法难以直接测量其熔点,因此,目前缺乏测量h-BN熔点的实验数据。早在1963年,Bundy等[13]估计,在4 GPa的压力下,h-BN的熔点约为
3500 K。Solozhenko 等[14]的计算结果显示,h-BN的熔化曲线斜率在常压至3.4 GPa时为正,在更高的压力下为负,且三相点的温度和压力分别为(3480 ±10) K和(5.9±0.1) GPa。此外,Liang等[15]采用瞬间放电方法测量了15 GPa下c-BN的熔点,约为(5689 ± 411) K。2021年,Zhang等[16]通过测量c-BN的再结晶温度,依据再结晶温度与熔化温度之间的经验关系,推算出c-BN在8~16 GPa压力范围内的熔化曲线斜率,约为125 K/GPa。高压可以抑制h-BN的分解,利用合适的高压组装能够尽量避免其在高温下与环境物质反应或氧化。然而,在高压条件下加热使样品温度超过3000 K并对温度标定一直是一个挑战,这也导致了难以在高压下测量h-BN的熔点。具体来说,测量h-BN在高压下的熔化温度主要存在3方面的困难:(1) 如何在高压腔中产生3000 K以上的高温,(2) 如何标定高压腔中3000 K以上的温度,(3) 如何判定样品在高温高压下是否熔化。本研究将设计并组装一套瞬间放电装置,以期在DS 6×14 MN国产铰链式六面顶大腔体压机上实现高压腔内的瞬间放电加热。通过分析淬火样品的微观结构变化,判定物质是否在高压及瞬态高温下熔化。当温度低于晶体在此压力下的熔点时,晶态样品会发生晶粒长大现象;当温度高于晶体在此压力下的熔点时,晶粒经历快速熔化-凝固并重新结晶,形成均匀且细小的晶粒结构[17]。晶体熔化再结晶过程中会经历形核和生长2个阶段。如果结晶时间极短,冷却速率大于晶体成核生长速率时,可能会阻断晶粒的生长过程,从而在熔化后的晶体表面形成细小的纳米晶核。本研究将通过对3种熔点随压力变化规律已知的晶体绝缘材料进行高压环境下的瞬间放电加热实验,根据晶体表面形貌的变化,对瞬间放电电压与温度之间的关系进行标定;然后,采用高压腔瞬间放电加热技术,使h-BN晶体表面瞬间达到极高的温度,观察快速液-固转变导致的晶粒细化现象;采用扫描电子显微镜(scanning electron microscope,SEM)分析处理后的h-BN晶体的表面形貌,以观察是否生成纳米晶粒;通过分析熔化后的形核和生长阶段样品的微观形貌,测量h-BN在5 GPa以下的熔点。
1. 实 验
1.1 瞬间放电装置设计
瞬间放电装置是一种利用电容器在短时间内释放存储电能的系统[15]。为满足实验需求,自行搭建了一套瞬间放电装置,详细电路如图1所示,其中:VCC(volt current condenser)表示供电电压,GND(ground)表示接地端,Q1和Q2为三极管,C1和C2为电容,R1和R2为电阻,S1~S6为开关。为了确保实验安全、顺利地进行,装置中的所有配件均采用了冗余设计。主要配件包括:用于控制充电电路的MTC 25 A、1.6 kV可控硅,用于控制放电电路的MTC 300 A、1.6 kV 可控硅,用于储能的2个500 V、15 mF储能电容,以及用于防止电流倒灌互充的MD 200 A、1.6 kV整流二极管。此外,采用长达10 m的10 AWG规格的导线,电路中还配备了水泥电阻以保护电路。
图 1 瞬间放电装置内部电路 (a) 以及组装连接 (b) 示意图(使用10 AWG粗导线连接,并联的电容分为上级电容和下级电容,最大输出电压为1 kV,不能同时充放电)Figure 1. Schematic diagrams of the internal circuit (a) and connection assembly (b) of the flash-heating device (The capacitors are arranged in parallel by using 10 AWG thick wires and divided into upper-level and lower-level. The maximum output voltage is 1 kV. Simultaneous charging and discharging are not permissible.)电容器是瞬间放电装置的核心部件。在充电阶段,电容器的一个极板积累正电荷,另一个极板积累等量的负电荷,以储存电能,其能量表达式为
(1) 式中:E为能量,C为电容,U为电压。在放电阶段,电容器通过快速操作开关接入负载电路,电容器中积累的电荷迅速通过负载,从而释放储存的能量。在瞬间放电过程中,电容器储存的电能转换为加热体表面的热能。根据能量守恒和电功率公式,通过控制充电阶段电容两端的电压来调节储存的能量大小,从而有效控制产生的热量,进而调控温度。
使用瞬间放电装置时,为了实现高电压、大电流的放电效果,需用导线直接连接压机内的高压组装,确保瞬间放电能够直接作用于实验样品。
1.2 实验设计与测试
在实验的准备阶段,对初始h-BN粉末进行预处理。使用高温真空炉对粉末材料进行真空热处理,以去除粉末中残留的气体杂质,如水蒸气、氧气以及其他可能导致材料氧化的有害气体[18–19]。
粒径较小的粉晶具有较高的表面活化能,相比粒径较大的粉晶,更易发生熔化。因此,应选择粒径尽可能小的粉晶作为前驱体,以降低熔化过程中的相变潜热对温度测量带来的影响。判断晶粒熔化的方式是通过SEM观察实验后晶粒表面是否有大小均匀且尺寸约200 nm的晶粒出现,故不能选择纳米尺度的粉晶作为前驱体。综合考虑以上因素,采用纯净的、粒径为1~2 µm的h-BN粉末作为初始材料。经过预压成型及组装后,将组装放入DS 6×14 MN国产铰链式六面顶压机中。为了确保放电过程中导线能够承受大电流,防止电线过热和损坏,使用直径为3 mm的高导电率铜线(10 AWG)连接六面顶压机与瞬间放电装置。高温高压实验前,先使用小电压进行初步放电,以测试电路的完整性,确保实验的安全性。实验中,当压力分别达到3.4和4.3 GPa时,维持压力稳定。随后,使用瞬间放电装置,根据预先标定的瞬间放电电压-温度曲线设定目标电压,对样品进行瞬间放电实验。放电结束后,迅速关闭瞬间放电装置,逐步卸载压力,取出样品后,进行后续的结构和化学组成检测分析。
使用JSM-IT500HR型场发射扫描电子显微镜(field emission scanning electron microscope,FESEM)观察样品表面的微观结构。通过FESEM高倍率成像,检查样品是否出现纳米晶粒,这通常表明材料经历了高温熔化,并在随后的快速凝固过程中形成了超细晶粒。
2. 结果与讨论
2.1 瞬间放电电压与温度的关系
传统的大腔体压机加热方法的加热极限是
3000 K左右,该温度低于高压下h-BN的熔化温度[20]。瞬间放电法与传统的大腔体压机加热方式有所不同:前者是通过电容储能并瞬间放电的加热方式,只在样品与加热体紧密接触的局部区域产生大量的热量,并以极快的速度恢复到常温状态[15],使用常规的测温方式无法测得样品温度;后者是采用直流电通过电阻加热体间接加热样品,根据实验需要增大功率,通过热电偶原位测量样品温度。在晶体熔化再结晶过程中,理论上包括形核和生长2个阶段。如果温度变化极快,则晶体在形核之后没有足够长的时间生长,致使晶粒细化。为了实现这一过程,采用瞬间放电装置,通过迅速释放大量能量至加热体,使h-BN材料熔化并重新结晶。通过SEM观察,以样品与放电加热界面是否出现大小均匀且尺寸约200 nm的细小晶粒作为判据,判断是否出现细晶现象,进而检验晶体是否经历了快速熔化再结晶过程。
为了建立电容瞬间放电电压与样品温度之间的关系,即温度标定,需要选择在一定压力下熔化温度已知的材料作为标温物质。遴选标温物质时需要综合考虑以下关键因素。首先,由于熔化状态的确认依赖于瞬间放电加热后样品是否出现晶粒细化现象的观察,因此,标温物质必须具有晶体结构。其次,实验的目标物质h-BN为绝缘体,为避免电容瞬间释放的能量流向导体,标温物质也应为绝缘体。综合以上要求,选择4种具有明确熔点且熔点随压力变化曲线已知的绝缘材料:氯化钠(NaCl)、氯化钾(KCl)、氧化铝(Al2O3)和氧化镁(MgO),其在常压下的熔点分别为
1075 、1043 、2350 、3073 K。这些物质不仅满足作为标温物质的基本要求,还具有晶体结构稳定、高压下熔点已知的特点,是确定电容放电电压与样品温度关系的理想选择。通过使用这些标温物质,可以准确测定不同放电电压条件下样品达到的温度,进而控制和预测h-BN等其他绝缘体材料的加热效果[21–24]。为了确立瞬间放电电压与样品温度的关系,将4种绝缘体材料(NaCl、KCl、Al2O3、MgO)放入高压组装的样品腔中,将压力调至(3.4±0.2) GPa,分别进行瞬间放电实验。以NaCl为例,首先使用300 V的较高电压进行放电,实验后观察到NaCl样品表现出明显的晶粒细化现象,如图2所示,以此作为熔化后晶粒细化的判定依据。随后,进行多次实验,并逐次降低瞬间放电电压至70、35、30 V,实验后仍然可以观察到晶粒细化现象。当电压降至25 V时,晶粒细化现象不再出现,表明此时样品未达到熔化温度。因此,可推断在(3.4±0.2) GPa的条件下,NaCl的熔化瞬间放电电压约为(27.5±2.5) V。同时,在3.4 GPa下NaCl的熔点约为
1582 K,表明27.5 V的电压足以使样品腔内的NaCl达到甚至超过1582 K的瞬间温度,从而导致其熔化[23]。图 2 在(3.4±0.2) GPa压力下NaCl经过不同电压瞬间放电后样品的SEM图像(红圈部分为出现细晶的位置, 300 V时有非常明显的细晶现象。70、35、30 V时均出现细晶现象,说明在此电压下温度达到了NaCl的熔点。当电压降至25 V时,在SEM图像中未发现晶粒细化现象,只观察到晶粒因挤压作用而破碎,说明NaCl在3.4 GPa下的熔化电压在25~30 V之间,约为(27.5±2.5) V,此压力下NaCl的熔点为1582 K,即 27.5 V的电压使样品腔的瞬间温度达到约1582 K。)Figure 2. SEM images of NaCl samples after flash-heating with different voltages under (3.4±0.2) GPa (The red circled areas indicate the locations where fine grains are observed, with a distinct grain refining at 300 V. Multiple experiments are conducted, sequentially reducing the voltage to 70, 35, and 30 V, all of which resulted in the appearance of fine grains, demonstrating that at such voltages, the temperature is above the melting point of NaCl. When the voltage reduced to 25 V, no fine-grained areas can be identified in the SEM image; instead, only fragmented grains due to pressure are observed. This suggests that the melting voltage of NaCl under 3.4 GPa is between 25 and 30 V, approximately (27.5±2.5) V. At this pressure, the melting point of NaCl is1582 K, suggesting that a voltage of 27.5 V induces an instantaneous temperature of approximately1582 K in the sample chamber.)重复上述实验,如图3所示,在21 V未发现KCl的晶粒细化,而在25 V发现晶粒细化,得到在(3.4±0.2) GPa压力下KCl的熔化瞬间放电电压为(23±2) V,对应的温度为
1416 K[21];如图4所示,在46 V未发现Al2O3的晶粒细化,而在51 V发现了Al2O3的晶粒细化,据此确定Al2O3的熔化瞬间放电电压为(48.5±2.5) V,对应的温度为2701 K[22];如图5所示,在65 V未发现MgO的晶粒细化,而在70 V发现了MgO的晶粒细化现象,因此,MgO的熔化瞬间放电电压为(67.5±2.5) V,对应的温度为3419 K[24]。Liang等[15]通过实验测得15 GPa下c-BN和金刚石的熔点分别为(5689 ±411) K和(5968 ±457) K。根据式(1),能量的大小仅与电压及电容容量有关,且电压为主要影响因素。本实验所选用的电容容量与Liang等[15]的研究相近,因此,结合Liang等[15]的实验数据,拟合得到高压腔内温度与瞬间放电电压之间的关系曲线,如图6所示。图 3 3.4 GPa下KCl瞬间放电后的SEM图像(21 V时未发现晶粒细化现象,而在25 V时发现红圈处的晶粒细化现象,说明KCl在3.4 GPa压力下的熔化电压在21~25 V之间,约为(23±2) V,此压力下KCl的熔点为1416 K,即 23 V的电压使样品腔的瞬间温度达到约1416 K。)Figure 3. SEM images of KCl samples after flash-heating under 3.4 GPa (No grain refinement was observed at 21 V, but appeared at 25 V within the red-circled areas. It indicates that the melting voltage of KCl under 3.4 GPa is between 21 and 25 V, approximately (23±2) V. At this pressure, the melting point of KCl is1416 K, suggesting that a voltage of 23 V induces an instantaneous temperature of approximately1416 K in the sample chamber.)图 4 3.4 GPa下Al2O3瞬间放电后的SEM图像(46 V时未发现晶粒细化现象,而在51 V时发现红圈处的晶粒细化现象,说明Al2O3在3.4 GPa下的熔化电压在46~51 V之间,约为(48.5±2.5) V,此压力下Al2O3的熔点为2701 K,即 48.5 V的电压使样品腔的瞬间温度达到约2701 K。)Figure 4. SEM images of Al2O3 samples after flash-heating under 3.4 GPa (No grain refinement was observed at 46 V, but appeared at 51 V within the red-circled areas. It indicates that the melting voltage of Al2O3 under 3.4 GPa is between 46 and 51 V, approximately (48.5±2.5) V. At this pressure, the melting point of Al2O3 is2701 K, suggesting that a voltage of 48.5 V induces an instantaneous temperature of approximately2701 K in the sample chamber.)图 5 3.4 GPa下MgO瞬间放电后的SEM图像(65 V时未发现晶粒细化现象,而70 V时发现红圈处的晶粒细化现象,说明MgO在3.4 GPa下的熔化电压在65~70 V之间,约为(67.5±2.5) V,此压力下MgO的熔点为3419 K,即 67.5 V的电压使样品腔的瞬间温度达到约3419 K。)Figure 5. SEM images of MgO samples after flash-heating under 3.4 GPa (No grain refinement was observed at 65 V, but appeared at 70 V within the red-circled areas. It indicates that the melting voltage of MgO under 3.4 GPa is between 65 and 70 V, approximately (67.5±2.5) V. At this pressure, the melting point of MgO is3419 K, suggesting that a voltage of 67.5 V induces an instantaneous temperature of approximately3419 K in the sample chamber.)2.2 基于瞬间放电法对h-BN熔点的测量
将纯净的h-BN粉末放置在高压腔内,调整压力至(3.4±0.2) GPa,进行瞬间放电实验。实验开始时,将电压直接提高至300 V,放电后卸压。从图7(b)可以明显地看到h-BN出现晶粒细化现象,将该结果作为后续样品的参考。在随后的实验中,将电压从0 V逐步升至90 V,未观察到类似的晶粒细化现象。当电压进一步升至96 V时,才开始观察到图7(c)中红圈标记的晶粒细化现象。因此,推断h-BN熔化对应的电压约为(93±3) V,根据图6所示的瞬间放电电压-温度曲线,推断对应的熔化温度为(
4251 ±150) K。图 7 3.4 GPa下h-BN经过不同电压瞬间放电后样品的SEM图像(红圈部分为细晶出现的位置。300 V时,有非常明显的晶粒细化现象;96 V时,仍能观察到晶粒细化现象;而90 V时,未见晶粒细化区域,说明在该压力下h-BN熔化,对应的电压约为(93±3) V,对应的熔化温度为(4251 ±150) K。)Figure 7. SEM images of h-BN samples after flash-heating at different voltages under 3.4 GPa (The red circled areas indicate the locations where fine grains are observed, with a distinct grain refining at 300 V. Grain refinement is still observable at 96 V, but cannot be observed at 90 V. It indicates that the melting voltage of h-BN under this pressure is approximately (93±3) V. The corresponding melting temperature is approximately (4251 ±150) K.)调整压力至(4.3±0.2) GPa,对h-BN样品进行重复的瞬间放电实验。结果显示:97 V的电压下未观察到晶粒细化现象;当电压增至105 V时,如图8(c)所示,出现明显的h-BN晶粒细化现象。基于以上实验数据,可以估算出在该压力下h-BN熔化时对应的电压约为(101±4) V。根据图6所示的瞬间放电电压-温度曲线,该电压对应的熔化温度为(
4531 ±200) K。图 8 4.3 GPa下h-BN经瞬间放电后样品的SEM图像(97 V时,未发现晶粒细化现象,而105 V时,红圈处出现晶粒细化现象,说明在该压力下,h-BN的熔化电压约为(101±4) V,对应的熔化温度为(4531 ±200) K。)Figure 8. SEM images of h-BN samples after flash-heating at 4.3 GPa (No grain refinement is observed at 97 V, but appeared at 105 V within the red-circled areas. It suggests that the melting voltage of h-BN under this pressure is approximately (101±4) V. The corresponding melting temperature is approximately (4531 ±200) K.)熔点是指纯物质从固态转变为液态的温度,是物质基本热力学性质之一。过热是指物质的实际温度超过其熔点却未熔化的现象,在高压环境下较为常见。高压能够提升物质的热稳定性,致使物质在不发生相变的情况下达到更高温度。在瞬间放电过程中,由于加热速度极快,系统可能没有足够时间达到热平衡,从而导致局部过热,可能增加温度测量的不准确性,因此,本研究适当增大了误差范围。此外,在高温条件下,h-BN不仅仅会熔化,还可能发生化学分解。h-BN分解会生成硼和氮气,从而形成气泡,干扰熔点测量。如果分解反应是放热的,还可能导致样品局部温度进一步升高,从而误导熔点测量结果。然而,高压环境会抑制h-BN的化学分解。在本研究中,SEM图像并未显示出气泡或气孔等BN分解迹象。由此得出,在3.4和4.3 GPa的高压下h-BN的熔点分别为(
4251 ±150) K和(4531 ±200) K。将本研究得到的结果与Zhang等[16]的c-BN熔化曲线数据进行延伸和比较,如图9所示,其中:蓝色区域为c-BN熔化曲线可能存在的区间,绿色区域为本实验得到的h-BN熔化曲线可能存在的区间,中间红色区域为三相点可能存在的范围。可以看出,h-BN的熔化温度随压力的升高而升高,且熔化曲线的斜率逐渐增大,但始终保持正值。这可能是因为在压力作用下,h-BN的层状晶体结构变得更加紧密,所以在熔化过程中需要更多的能量(即更高的温度)来克服层内的强共价键。这与Solozhenko 等[14]预测的h-BN熔化曲线斜率先正后负的变化趋势不同。
图 9 实验得到的c-BN的熔化数据与Zhang等[16]得到的数据的延伸和比较(蓝色区域为c-BN熔化曲线可能存在的区间,绿色区域为本实验得出的h-BN熔化曲线可能存在的区间,中间红色区域为三相点可能存在的范围。h-BN的熔化温度随压力的升高而升高,且斜率逐渐增大,但不为负值,与Solozhenko 等[14] 预测的结果不同。)Figure 9. Extension and comparison of the results obtained in this study with data of Zhang et al.[16] on the melting curve of c-BN (The blue region indicates the possible range of the c-BN melting curve, the green region represents the possible range of the h-BN melting curve obtained from this study, and the middle red region indicates the possible range of the triple point. The melting point of h-BN is expected to increase with pressure, with the slope gradually steeper but not negative. This differs from the predictions made by Solozhenko et al.[14].)3. 结 论
采用瞬间放电法测量了3.4和4.3 GPa压力下h-BN的熔点,分别为(
4251 ±150) K和(4531 ±200) K,推测h-BN的熔点随压力的升高而升高,且斜率逐渐增大,但不为负值。本方法的核心是利用SEM观察瞬间放电加热淬火后的样品,通过分析晶粒细化现象来判断材料是否经历了熔化。在高压高温条件下,晶粒尺寸和形态的变化是判断熔化现象的直接证据,晶粒细化通常表明材料经历了由固态转变为液态的过程。基于微观结构观察的方法可能受样品初始状态、放电过程中温度分布不均以及SEM图像解析度等因素的影响,导致存在一定程度的实验误差。尽管如此,鉴于BN相图中h-BN熔化曲线的实验数据相对稀缺,通过晶粒细化现象来评估熔化仍为理解BN在高压条件下的行为提供了有价值的信息。 -
图 1 瞬间放电装置内部电路 (a) 以及组装连接 (b) 示意图(使用10 AWG粗导线连接,并联的电容分为上级电容和下级电容,最大输出电压为1 kV,不能同时充放电)
Figure 1. Schematic diagrams of the internal circuit (a) and connection assembly (b) of the flash-heating device (The capacitors are arranged in parallel by using 10 AWG thick wires and divided into upper-level and lower-level. The maximum output voltage is 1 kV. Simultaneous charging and discharging are not permissible.)
图 2 在(3.4±0.2) GPa压力下NaCl经过不同电压瞬间放电后样品的SEM图像(红圈部分为出现细晶的位置, 300 V时有非常明显的细晶现象。70、35、30 V时均出现细晶现象,说明在此电压下温度达到了NaCl的熔点。当电压降至25 V时,在SEM图像中未发现晶粒细化现象,只观察到晶粒因挤压作用而破碎,说明NaCl在3.4 GPa下的熔化电压在25~30 V之间,约为(27.5±2.5) V,此压力下NaCl的熔点为
1582 K,即 27.5 V的电压使样品腔的瞬间温度达到约1582 K。)Figure 2. SEM images of NaCl samples after flash-heating with different voltages under (3.4±0.2) GPa (The red circled areas indicate the locations where fine grains are observed, with a distinct grain refining at 300 V. Multiple experiments are conducted, sequentially reducing the voltage to 70, 35, and 30 V, all of which resulted in the appearance of fine grains, demonstrating that at such voltages, the temperature is above the melting point of NaCl. When the voltage reduced to 25 V, no fine-grained areas can be identified in the SEM image; instead, only fragmented grains due to pressure are observed. This suggests that the melting voltage of NaCl under 3.4 GPa is between 25 and 30 V, approximately (27.5±2.5) V. At this pressure, the melting point of NaCl is
1582 K, suggesting that a voltage of 27.5 V induces an instantaneous temperature of approximately1582 K in the sample chamber.)图 3 3.4 GPa下KCl瞬间放电后的SEM图像(21 V时未发现晶粒细化现象,而在25 V时发现红圈处的晶粒细化现象,说明KCl在3.4 GPa压力下的熔化电压在21~25 V之间,约为(23±2) V,此压力下KCl的熔点为
1416 K,即 23 V的电压使样品腔的瞬间温度达到约1416 K。)Figure 3. SEM images of KCl samples after flash-heating under 3.4 GPa (No grain refinement was observed at 21 V, but appeared at 25 V within the red-circled areas. It indicates that the melting voltage of KCl under 3.4 GPa is between 21 and 25 V, approximately (23±2) V. At this pressure, the melting point of KCl is
1416 K, suggesting that a voltage of 23 V induces an instantaneous temperature of approximately1416 K in the sample chamber.)图 4 3.4 GPa下Al2O3瞬间放电后的SEM图像(46 V时未发现晶粒细化现象,而在51 V时发现红圈处的晶粒细化现象,说明Al2O3在3.4 GPa下的熔化电压在46~51 V之间,约为(48.5±2.5) V,此压力下Al2O3的熔点为
2701 K,即 48.5 V的电压使样品腔的瞬间温度达到约2701 K。)Figure 4. SEM images of Al2O3 samples after flash-heating under 3.4 GPa (No grain refinement was observed at 46 V, but appeared at 51 V within the red-circled areas. It indicates that the melting voltage of Al2O3 under 3.4 GPa is between 46 and 51 V, approximately (48.5±2.5) V. At this pressure, the melting point of Al2O3 is
2701 K, suggesting that a voltage of 48.5 V induces an instantaneous temperature of approximately2701 K in the sample chamber.)图 5 3.4 GPa下MgO瞬间放电后的SEM图像(65 V时未发现晶粒细化现象,而70 V时发现红圈处的晶粒细化现象,说明MgO在3.4 GPa下的熔化电压在65~70 V之间,约为(67.5±2.5) V,此压力下MgO的熔点为
3419 K,即 67.5 V的电压使样品腔的瞬间温度达到约3419 K。)Figure 5. SEM images of MgO samples after flash-heating under 3.4 GPa (No grain refinement was observed at 65 V, but appeared at 70 V within the red-circled areas. It indicates that the melting voltage of MgO under 3.4 GPa is between 65 and 70 V, approximately (67.5±2.5) V. At this pressure, the melting point of MgO is
3419 K, suggesting that a voltage of 67.5 V induces an instantaneous temperature of approximately3419 K in the sample chamber.)图 7 3.4 GPa下h-BN经过不同电压瞬间放电后样品的SEM图像(红圈部分为细晶出现的位置。300 V时,有非常明显的晶粒细化现象;96 V时,仍能观察到晶粒细化现象;而90 V时,未见晶粒细化区域,说明在该压力下h-BN熔化,对应的电压约为(93±3) V,对应的熔化温度为(
4251 ±150) K。)Figure 7. SEM images of h-BN samples after flash-heating at different voltages under 3.4 GPa (The red circled areas indicate the locations where fine grains are observed, with a distinct grain refining at 300 V. Grain refinement is still observable at 96 V, but cannot be observed at 90 V. It indicates that the melting voltage of h-BN under this pressure is approximately (93±3) V. The corresponding melting temperature is approximately (
4251 ±150) K.)图 8 4.3 GPa下h-BN经瞬间放电后样品的SEM图像(97 V时,未发现晶粒细化现象,而105 V时,红圈处出现晶粒细化现象,说明在该压力下,h-BN的熔化电压约为(101±4) V,对应的熔化温度为(
4531 ±200) K。)Figure 8. SEM images of h-BN samples after flash-heating at 4.3 GPa (No grain refinement is observed at 97 V, but appeared at 105 V within the red-circled areas. It suggests that the melting voltage of h-BN under this pressure is approximately (101±4) V. The corresponding melting temperature is approximately (
4531 ±200) K.)图 9 实验得到的c-BN的熔化数据与Zhang等[16]得到的数据的延伸和比较(蓝色区域为c-BN熔化曲线可能存在的区间,绿色区域为本实验得出的h-BN熔化曲线可能存在的区间,中间红色区域为三相点可能存在的范围。h-BN的熔化温度随压力的升高而升高,且斜率逐渐增大,但不为负值,与Solozhenko 等[14] 预测的结果不同。)
Figure 9. Extension and comparison of the results obtained in this study with data of Zhang et al.[16] on the melting curve of c-BN (The blue region indicates the possible range of the c-BN melting curve, the green region represents the possible range of the h-BN melting curve obtained from this study, and the middle red region indicates the possible range of the triple point. The melting point of h-BN is expected to increase with pressure, with the slope gradually steeper but not negative. This differs from the predictions made by Solozhenko et al.[14].)
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[1] SONG Z Y, WANG W, CAI G X, et al. Investigation of optical spectrum properties of hexagonal boron nitride from metal to dielectric transition [J]. Plasmonics, 2018, 13(2): 563–566. doi: 10.1007/s11468-017-0544-y [2] ZHOU W Y, ZUO J, ZHANG X Q, et al. Thermal, electrical, and mechanical properties of hexagonal boron nitride: reinforced epoxy composites [J]. Journal of Composite Materials, 2014, 48(20): 2517–2526. doi: 10.1177/0021998313499953 [3] PEREVISLOV S N. Structure, properties, and applications of graphite-like hexagonal boron nitride [J]. Refractories and Industrial Ceramics, 2019, 60(3): 291–295. doi: 10.1007/s11148-019-00355-5 [4] KIM T Y, SONG E H, KANG B H, et al. Hydrolyzed hexagonal boron nitride/polymer nanocomposites for transparent gas barrier film [J]. Nanotechnology, 2017, 28(12): 12LT01. doi: 10.1088/1361-6528/aa5f2e [5] TURKOGLU M, SAHIN I, SAN T. Evaluation of hexagonal boron nitride as a new tablet lubricant [J]. Pharmaceutical Development and Technology, 2005, 10(3): 381–388. doi: 10.1081/PDT-65684 [6] DECKER R, WANG Y, BRAR V W, et al. Local electronic properties of graphene on a BN substrate via scanning tunneling microscopy [J]. Nano Letters, 2011, 11(6): 2291–2295. doi: 10.1021/nl2005115 [7] REVABHAI P M, SINGHAL R K, BASU H, et al. Progress on boron nitride nanostructure materials: properties, synthesis and applications in hydrogen storage and analytical chemistry [J]. Journal of Nanostructure in Chemistry, 2023, 13(1): 1–41. doi: 10.1007/s40097-022-00490-5 [8] SHTANSKY D V, FIRESTEIN K L, GOLBERG D V. Fabrication and application of BN nanoparticles, nanosheets and their nanohybrids [J]. Nanoscale, 2018, 10(37): 17477–17493. doi: 10.1039/C8NR05027A [9] 王艳芝, 张旺玺, 孙长红, 等. 氮化硼系列材料的合成制备及应用研究进展 [J]. 陶瓷学报, 2018, 39(6): 661–671. doi: 10.13957/j.cnki.tcxb.2018.06.002WANG Y Z, ZHANG W X, SUN C H, et al. The development of the applications and synthesis of boron nitride materials [J]. Journal of Ceramics, 2018, 39(6): 661–671. doi: 10.13957/j.cnki.tcxb.2018.06.002 [10] WANG P, HE D W, WANG L P, et al. Diamond-cBN alloy: a universal cutting material [J]. Applied Physics Letters, 2015, 107(10): 101901. doi: 10.1063/1.4929728 [11] WATANABE T, SATAKA R, YAMAMOTO K. Effect of bias application on c-BN synthesis by induction thermal plasmas under atmospheric pressure [J]. Thin Solid Films, 2008, 516(13): 4462–4467. doi: 10.1016/j.tsf.2007.10.018 [12] WENTORF JR R H. Cubic form of boron nitride [J]. The Journal of Chemical Physics, 1957, 26(4): 956. doi: 10.1063/1.1745964 [13] BUNDY F P, WENTORF JR R H. Direct transformation of hexagonal boron nitride to denser forms [J]. The Journal of Chemical Physics, 1963, 38(5): 1144–1149. doi: 10.1063/1.1733815 [14] SOLOZHENKO V L, TURKEVICH V Z, HOLZAPFEL W B. Refined phase diagram of boron nitride [J]. The Journal of Physical Chemistry B, 1999, 103(15): 2903–2905. doi: 10.1021/jp984682c [15] LIANG A K, LIU Y J, SHI L T, et al. Melting temperature of diamond and cubic boron nitride at 15 gigapascals [J]. Physical Review Research, 2019, 1(3): 033090. doi: 10.1103/PhysRevResearch.1.033090 [16] ZHANG J W, LIU F M, LI S Q, et al. Recrystallization behaviour of cubic boron nitride under high pressure [J]. Journal of the European Ceramic Society, 2021, 41(16): 132–138. doi: 10.1016/j.jeurceramsoc.2021.09.036 [17] HRUBIAK R, MENG Y, SHEN G Y. Microstructures define melting of molybdenum at high pressures [J]. Nature Communications, 2017, 8(1): 14562. doi: 10.1038/ncomms14562 [18] WU H Y, ZHUANG X L, NIE Y, et al. Effect of heat treatment on mechanical property and microstructure of a powder metallurgy nickel-based superalloy [J]. Materials Science and Engineering: A, 2019, 754: 29–37. doi: 10.1016/j.msea.2019.03.064 [19] IKESUE A, AUNG Y L, YODA T, et al. Fabrication and laser performance of polycrystal and single crystal Nd: YAG by advanced ceramic processing [J]. Optical Materials, 2007, 29(10): 1289–1294. doi: 10.1016/j.optmat.2005.12.013 [20] ZHOU X F, MA D J, WANG L F, et al. Large-volume cubic press produces high temperatures above 4 000 Kelvin for study of the refractory materials at pressures [J]. Review of Scientific Instruments, 2020, 91(1): 015118. doi: 10.1063/1.5128190 [21] BOEHLER R, ROSS M, BOERCKER D B. High-pressure melting curves of alkali halides [J]. Physical Review B, 1996, 53(2): 556–563. doi: 10.1103/PhysRevB.53.556 [22] SHEN G Y, LAZOR P. Measurement of melting temperatures of some minerals under lower mantle pressures [J]. Journal of Geophysical Research: Solid Earth, 1995, 100(B9): 17699–17713. doi: 10.1029/95JB01864 [23] BOEHLER R, ROSS M, BOERCKER D B. Melting of LiF and NaCl to 1 Mbar: systematics of ionic solids at extreme conditions [J]. Physical Review Letters, 1997, 78(24): 4589–4592. doi: 10.1103/PhysRevLett.78.4589 [24] KIMURA T, OHFUJI H, NISHI M, et al. Melting temperatures of MgO under high pressure by micro-texture analysis [J]. Nature Communications, 2017, 8(1): 15735. doi: 10.1038/ncomms15735 -