BN粉体的结晶度对Cu-Si合金润湿行为的影响

图1 氮化温度为1000 ℃和1200 ℃，用不同摩尔比的硼酸-尿素制备的h-BN的XRD谱

Fig.1 XRD patterns of h-BN products from urea and boric acid with different molar ratios at 1000 ^oC(a) and 1200 ^oC (b)

2.2 氮化温度对 h-BN结晶度的影响

图2给出了硼酸与尿素摩尔比为1∶2，在不同温度氮化制备出的h-BN样品的XRD谱。可以看出，在氮化温度为1400 ℃样品的谱中2θ ≈ 26.5°处出现一个尖锐的衍射峰，对应h-BN的(002)晶面；在2θ ≈ 42°、55°和76°也出现了清晰的衍射峰，表明h-BN的结晶度较高。相比之下，氮化温度为1200 ℃和1000 ℃的谱中(002)晶面衍射峰的位置分别向低角度偏移至2θ ≈ 26.1°和2θ ≈ 25.7°，且衍射峰强度减弱、半高宽增大，表明合成的h-BN结晶度较低。

图2

图2 在不同氮化温度制备的h-BN样品的XRD谱

Fig.2 XRD patterns of h-BN samples prepared at different nitridation temperatures

图3给出了硼酸与尿素摩尔比为1∶2、氮化温度不同的h-BN样品的拉曼光谱。可以看出，谱中h-BN的拉曼特征峰位于约1370 cm^-1处，归属于面内B-N键振动引起的E₂g模式。随着氮化温度从1400 ℃降至1000 ℃，特征峰逐渐向高波数方向移动且峰形有所宽化，表明h-BN的结晶度降低。

图3

图3 在不同氮化温度制备的h-BN样品的拉曼光谱

Fig.3 Raman spectra of h-BN samples prepared at different nitridation temperatures

图4给出了在1400 ℃、1200 ℃和1000 ℃氮化制备的样品BN1400、BN1200和BN1000的XPS全谱。从图4可见，除了B和N元素，三种样品中还有C和O元素。C元素的峰可归因于样品表面吸附的污染碳，O元素峰则可归因于样品表面的吸附氧和B₂O₃等氧化物。图5给出了BN1400、BN1200和BN1000的B 1s、N 1s及O 1s精细谱的分峰拟合结果。对BN1400的B 1s谱分峰拟合得到两个特征峰(图5a)，结合能分别为190.75 eV和191.71 eV，分别对应B-N键和B-O键；N 1s精细谱在398.44 eV的单一主峰(图5b)归属于N-B键；O 1s谱在533.00 eV处的单一主峰(图5c)归属于对应O-B键。BN1200的B 1s精细谱(图5d)可分峰拟合为190.63 eV和192.28 eV两个峰，分别归属于B-N和B-O键；N 1s谱(图5e)在398.28 eV处出现单一N-B峰；O 1s谱(图5f)则在533.00 eV处出现单一O-B峰。BN1000的B 1s精细谱(图5g)可分峰拟合为190.72 eV和192.69 eV两个峰，分别对应B-N和B-O键；N 1s谱(图5h)在398.31 eV处出现单一N-B峰；O 1s谱(图5i)在532.94 eV处出现单一O-B峰。上述分峰拟合结果表明，BN1400，BN1200和BN1000样品中除含有BN，还有少量的B₂O₃。由表1可知，BN1400中O元素的含量为7.61%，BN1200中O元素的含量为11.30%，而BN1000中O元素的含量高达17.98%，表明BN1000中B₂O₃的含量最高。B₂O₃以无定型态存在，与XRD谱(图2)和拉曼谱(图3)给出的结论一致，进一步表明提高氮化温度有助于提高h-BN的纯度。

图4

图4 三种h-BN样品的XPS全谱

Fig.4 XPS full spectra of h-BN samples (a) BN1400, (b) BN1200, (c) BN1000

图5

图5 BN1400, BN1200和BN1000样品的XPS精细谱

Fig.5 XPS fine spectra of BN1400 (a-c), BN1200 (d-f), and BN1000 (g-i) samples (a, d, g) B 1s, (b, e, h) N 1s, (c, f, i) O 1s

表1 对XPS全谱分析得到的元素组成

Table 1 Elemental compositions obtained from XPS full spectra (atomic fraction, %)

Sample	B	C	N	O
BN1400	46.24	10.41	35.74	7.61
BN1200	48.46	8.26	31.98	11.30
BN1000	45.03	11.95	25.04	17.98

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图6给出了样品BN1400和BN1000的高分辨透射电子显微镜(HRTEM)照片和选区电子衍射(SAED)花样。根据沿a轴方向的平面尺寸(L_a)和沿c轴方向的堆叠厚度(L_c)，可评估h-BN的结晶度。如图6a所示，在1400 ℃氮化制备的h-BN具有高度有序的晶体结构，其晶格条纹平直、排列规整，尺寸达60 nm，c轴方向的堆叠厚度超过25 nm。测得BN1400的(002)晶面间距d₀₀₂ = 0.333 nm，与完全结晶h-BN的理论值相同。在BN1400的SAED花样中可清晰观察到(002)、(100)、(004)及(110)晶面的衍射环。这些衍射环连续且锐利，其中(002)和(004)晶面衍射环上斑点清晰可辨，进一步表明BN1400的结晶度较高。

图6

图6 BN1400和BN1000样品的HRTEM图像和SAED花样

Fig.6 HRTEM images and SAED patterns of BN1400 (a) and BN1000 (b) samples

如图6b所示，BN1000样品的原子排列有序度极低，微观结构呈准各向同性特征：其晶格条纹明显扭曲且排列无序，长度普遍小于8 nm，堆叠厚度低于5 nm。测得(002)晶面间距d₀₀₂ = 0.337 nm，略大于完全结晶h-BN的理论值(0.333 nm)，表明BN1000晶体由发育不完全的纳米级h-BN结构单元组成。从BN1000的SAED花样中可见中心区域存在衍射晕，(002)晶面衍射环与衍射晕相互融合，(100)晶面衍射环较为清晰，而(110)晶面衍射环较为模糊，表明其结晶度较低。上述TEM分析结果与XRD图谱(图2)和拉曼图谱(图3)的结论，都证实BN1000的结晶性较差。

2.3 h-BN的合成机理

图7给出了在氮气气氛中硼酸-尿素混合物(摩尔比为1∶2)的TG-DTA曲线。可以看出，温度为70~130  ℃的TG曲线缓慢下降，对应硼酸的分步脱水反应^[21]

图7

图7 摩尔比为1∶2的硼酸-尿素混合物的TG-DTA曲线

Fig.7 TG-DTA curves of the boric acid and urea mixture (molar ratio 1:2)

H_{3} B O_{3} \to H B O_{2} + H_{2} O

(1)

4 H B O_{2} \to H_{2} B_{4} O_{7} + H_{2} O

(2)

并在DTA曲线的85  ℃附近出现吸热峰。随着温度升至130~220  ℃ TG曲线急剧下降，表明发生了尿素的缩合和水解反应^[22,23]

2NH₂-CO-NH₂→NH₂-CO-NH-CO-NH₂ + NH₃(3)

NH₂-CO-NH₂ + H₂O→CO₂ + 2NH₃(4)

在DTA曲线的220 ℃附近出现吸热峰。在220~380 ℃区间TG曲线继续下降，表明硼酸完全脱水生成氧化硼且部分聚脲发生水解释放出氨气^[21,22]

H_{2} B_{4} O_{7} \to 2 B_{2} O_{3} + H_{2} O

(5)

NH₂-CO-NH-CO-NH₂ + 2H₂O→3NH₃ + 2CO₂(6)

温度进一步升高氧化硼与氨气反应生成无定型氮化硼放出热量

B_{2} O_{3} + 2 N H_{3} \to 2 B N + 3 H_{2} O

(7)

并在DTA曲线的600 ℃附近出现放热峰。温度高于800 ℃后TG曲线趋于平缓，随着温度的升高和时间的延长无定形氮化硼逐渐转变为六方氮化硼。

2.4 h-BN结晶度对BN与Cu-Si合金润湿性的影响

图8给出了测试润湿过程中Cu-Si合金形貌的动态变化。可以看出，随着温度的升高Cu-Si合金依次经历固态、开始熔化、部分熔化和完全熔化三个阶段：温度低于1300 ℃时合金为初始固态形貌；温度高于1300 ℃后合金开始熔化，至1400 ℃以上完全熔化。在熔化过程中，合金的形状由圆柱形逐渐转变为球形，即使在1450 ℃的高温Cu-Si合金也未铺展在基板上，表明其与两种h-BN样品(BN1400和BN1000)都不润湿。

图8

图8 润湿测试过程中Cu-Si合金形貌的变化

Fig.8 Macro-images of Cu-Si alloy during wetting test

图9给出了Cu-Si合金与h-BN之间的接触角(θ)与温度(T)关系。可以看出，BN1400和BN1000对应的θ-T曲线基本重合。温度升至~1380 ℃时Cu-Si合金与两种h-BN样品之间的接触角突然增大到140~150°，表明合金开始熔化(图8)。随着温度的继续升高接触角趋于稳定，在1450 ℃接触角稳定在~135°。上述结果表明，Cu-Si合金与h-BN之间不润湿，且h-BN的结晶度对其润湿性没有显著的影响。

图9

图9 Cu-Si合金与BN1400和BN1000之间的润湿曲线

Fig.9 Wetting curves of Cu-Si alloy on BN1400 (a) and BN1000 (b)

上述现象，可归因于h-BN表面没有悬空键和其表面能极低。h-BN具有与石墨类似的层状结构，每一层内的硼原子和氮原子通过sp²杂化形成键角为120°的强共价键，构成蜂窝状六元环结构，而层与层之间通过较弱的van der Waals力结合。h-BN最常见的解理面(001)面没有悬空键，解理时只需克服层间van der Waals力，因此能实现几乎“无损”的完全解理^[24,25]。

固-液-气三相界面张力与接触角之间的关系符合Young方程

c o s θ = (γ_{S V} - γ_{S L}) / γ_{L V}

(8)

其中γ_SV为固-气界面张力(即固体表面能)，γ_SL为固-液界面张力，γ_LV为液-气界面张力(即液体表面张力)，θ为固-液接触角。γ_SV远小于γ_LV时接触角θ大于90°，表现为不润湿行为。

h-BN具有极低的表面能，而Cu-Si合金熔体具有较高的表面张力^[26]。低表面能的h-BN不能为Cu-Si合金熔体在其表面铺展提供足够的驱动力，导致熔体收缩成球状而形成较大的接触角。虽然BN1000的结晶度比BN1400的低，但其仍由纳米尺度的h-BN结构单元构成(图6)。这些单元保留的h-BN本征表面特性使BN1000与BN1400的表面属性高度接近。因此，Cu-Si合金与两者之间的润湿行为没有显著的不同。

3 结论

(1) 以硼酸和尿素为原料可制备h-BN粉体。在硼酸-尿素摩尔比为1∶2、氮化温度为1400 ℃条件下制备的h-BN的纯度和结晶度最高、晶体结构高度有序，晶格条纹高度平直、排列规整；而在1000 ℃合成的h-BN纯度和结晶度都较低，原子排列的有序度也较低，微观结构呈准各向同性特征且其晶格条纹扭曲和无序。

(2) 硼酸-尿素合成h-BN的反应分为四个阶段：在70~130 ℃区间硼酸分步脱水，在130~220 ℃尿素发生缩合与水解并释放氨气；温度高于220~380 ℃硼酸完全脱水生成氧化硼且聚脲进一步水解；在约600 ℃氧化硼与氨气反应生成无定形氮化硼；温度高于800 ℃无定形氮化硼转变为结晶完整的六方氮化硼。

(3) h-BN与Cu-Si合金的润湿性对其结晶度变化不敏感。h-BN的表面没有悬空键、表面能极低且在不同氮化温度合成的h-BN的本征表面属性相似，Cu-Si合金与两者都不润湿，在1450 ℃接触角稳定在~135°。

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