使用Ni中间层SPS扩散连接金刚石/铜复合材料界面的导热性能
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Microstructure and Thermal Conductivity of Interface within Composite of Diamond Particulates/Cu Fabricated via Spark Plasma Sintering Technique with Thin Ni-foil as Interlayer
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通讯作者: 胡道春,教授,hudc@niit.edu.cn,研究方向为微通道高效换热结构制造技术
责任编辑: 黄青
收稿日期: 2025-02-06 修回日期: 2025-06-25
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Corresponding authors: HU Daochun, Tel:
Received: 2025-02-06 Revised: 2025-06-25
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作者简介 About authors
胡道春,男,1977年生,博士
使用Ni箔中间层SPS扩散连接金刚石/铜复合材料并分析了界面的元素,根据第一性原理密度泛函理论研究了扩散过程中的界面行为。结果表明,Ni中间层金刚石/铜复合材料扩散连接的界面是良好的冶金结合,Ni元素向界面两侧扩散与铜生成了CuNi合金的α-单相固溶体。铜/镍界面的声子、电子态密度图表明,这种二元界面的导热性能优于金刚石/铜、金刚石/镍界面,界面处高比例的铜/镍界面连接是其高导热性能的重要原因。添加Ni箔中间层的SPS扩散连接接头界面,其热导率最高可达703.83 W/(m·K),镍与铜的无限固溶使接头的导热性能提高。
关键词:
Composite of diamond particulates/copper was fabricated through diffusion bonding with a thin Ni-foil as interlayer via a spark plasma sintering technique. Then the formed bonding interface was characterized by means of SEM+EDS, XRD, universal testing machine and thermal conductivity meter, in terms of its microstructure, fracture morphology, shear strength, and thermal conductivity. Meanwhile the effect of process parameters on the microstructure and thermal conductivity of the bonding interface was investigated, and the variation of the interface behavior during diffusion bonding process was also studied by the first principle calculation. The results showed that the formed interface presents excellent metallurgical bonding between the diamond particulates and the Cu. The presence of the thin Ni foil interlayer significantly reduced the formation of voids and holes at the diffusion-bonded interface. The diffusion of Ni led to the formation of an α single-phase Cu-Ni solid solution and the production of the Ni3C phase. The phonon and electron density of states maps of the Cu/Ni-interface showed that the thermal conductivity of the binary interface was better than that of the diamond/nickel interface, and the high proportion of Cu/Ni-interface connection at the interface was an important factor in achieving high thermal conductivity. The spark plasma sintering induced diffusion joint interface presents thermal conductivity up to 703.83 W/(m·K), while the infinite solid solution of Ni-Cu, as well as the generation of carbide is conducive to the thermal conductivity of the joint.
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本文引用格式
胡道春, 王蕾, 见小琪, 陈明和, 周颖.
HU Daochun, WANG Lei, JIAN Xiaoqi, CHEN Minghe, ZHOU Ying.
电子信息技术中使用的芯片从最初的大体积逐步转型为小体积和高集成[1],使电子元器件局部的热流密度从200 W/cm2升高到1000 W/cm2 [2],极大的影响其性能和使用寿命。因此,亟须研制高导热电子封装材料[3,4]。为了避免高温对高集成芯片的影响,传统的散热材料已经不能满足要求[5]。即使铜的热导率高达400 W/(m·K)[6],也不能满足高热流密度电子元器件的热传导。金刚石的热导率为1200~2500 W/(m·K),热膨胀系数为1 × 10-6/K,可用于封装高热流密度电子芯片[7]。但是,金刚石的硬度极高且与铜之间不湿润,难以加工成型[8]。为此,可将铜基体合金化然后将金刚石颗粒添入制备出金刚石/铜复合材料[9,10]。金刚石/铜复合材料的热导率高达900 W/(m·K)[11],可用于芯片微通道热沉和高热流密度芯片封装。
芯片热沉的连接封装,有传统的钎焊[12]、激光焊[13]、电子束焊[14]等方法。但是,这些方法不能满足微通道热沉的封装要求;Constantin等[15]用激光粉末熔融技术打印金刚石和铜复合粉,制备出高热导率(330 W/(m·K))、高相对密度(96%)且金刚石不石墨化的复合材料。在激光粉末床熔化技术的基础上重涂和重熔,即可制备出致密(97%)和高热导率(349 W/(m·K))的金刚石/铜复合材料[16]。Wang等[17]对金刚石增强相颗粒进行表面金属化处理和空间位置约束,并用超声波低温固结技术制备出镀铬金刚石/铜复合材料,其热导率高达428.07 W/(m·K)。Su等[18]提出一种基于快速原位固化工艺调控粘结剂喷射增材制造中胚体的精度-饱和度,实现了金刚石/铜复合材料高精度/饱和度胚体的成形。Ma等[19]用高能激光束和高温熔池影响增材制造过程中金刚石石墨化行为,用粉末床熔融技术和CuSn10粉末金属结合剂制备出CuSn10-金刚石复合材料。Wang等[20]将电子束熔化技术用于3d结构NiCu/金刚石复合材料的粉末床熔合(PBF),制备出相对密度较高的(> 95%)、没有石墨化和机械性能优异的金刚石铜/复合材料。
1 实验方法
实验用金刚石/铜复合材料的热导率为712.75 W/(m·K),热膨胀系数为6 × 10-6 /K。将金刚石表面金属化后在其上镀一层钨金属以使金刚石与金属铜湿润,再用无压浸渗法制备金刚石体积分数为60%、尺寸为20 mm × 10 mm × 2 mm的金刚石/铜复合材料。这种复合材料中金刚石的平均粒径为150 μm。
图1
图2
用激光切割SPS连接后的镍中间层金刚石/铜复合材料以制样;用平面磨床将表面精磨以使制样表面的粗糙度低于1 μm;在金相抛光仪器上抛光。用扫描电镜(SEM)观察连接界面的微观组织和断口的形貌。用能谱仪(EDS)分析连接界面元素的含量。用X射线衍射仪(XRD)测试连接界面的XRD谱。用万能试验拉伸机测试界面的剪切强度。测试制样连接前后厚度方向的变形率以评价接头的变形[26]。用激光闪光分析仪测量制样的导热系数。在室温(25 ℃)下,用激光照射块状金刚石/铜复合材料的表面,一段时间后测量另一面的温度,则制样的热导率λ = αρc[27],其中α为制样的扩散系数(m2/s);ρ为制样的密度(kg/cm3);c为制样的比热容(J/(kg·K))。基于密度泛函理论的第一性原理研究连接界面的导热特性。
2 结果和讨论
2.1 SPS连接温度的影响
为了研究温度对Ni中间层金属SPS扩散连接质量的影响,在压力和保压时间相同的条件下控制连接温度。在压力为10 MPa、保压时间为60 min、连接温度为700 ℃、750 ℃、800 ℃条件下进行SPS连接。由图3a可以看出,连接温度为700 ℃连接界面处有明显的连接分割线,金刚石、铜、镍金属没有完全扩散,表明连接质量不高。在图3b中可见连接界面处有明显的孔洞,表明在700 ℃不能形成稳定可靠的连接界面;连接温度提高到750 ℃,连接界面处的焊缝消失,但是仍然有孔洞,表明金刚石、铜、镍元素已经能很好的连接(图3c);连接温度提高到800 ℃连接界面的质量进一步提高,孔洞全部闭合,连接界面处于稳定的连接状态。
图3
图3
镍中间层SPS扩散连接在不同温度下的微观形貌
Fig.3
Microscopic morphology of SPS diffusion bonding with Ni interlayer at different temperatures (a, b) 700 oC, (c, d) 750 oC, (e, f) 800 oC
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图4
在750℃下SPS连接界面的微观形貌和EDS图
Fig.4
Microscopic morphology and EDS image of SPS connection interface at 750 oC (a) microscopic morphology of the interface, (b) distribution of interface elements, (c) distribution of C element, (d) distribution of Cu element, (e) distribution of nickel element (f) distribution of W element
加入镍中间层的金刚石/铜复合材料的变形率和剪切强度,如图5所示。可以看出,连接温度为700 ℃时,其剪切强度仅为33.97 MPa,变形率仅为3.95%;连接温度提高到750 ℃,剪切强度显著提高到49.24 MPa,变形率提高到4.75%;连接温度为800 ℃时,剪切强度的提高不明显(为52.38 MPa),变形率提高到6.9%。以上结果表明,SPS连接的最佳温度为750 ℃。
图5
图5
在不同温度下镍中间层的剪切强度和变形率
Fig.5
Shear strength and deformation rate with Ni interlayer at different temperatures (a) shear strength, (b) deformation rate
2.2 SPS保温保压时间的影响
图6
图6
不同保压时间镍中间层的微观形貌和EDS元素
Fig.6
Microscopic morphology and EDS elemental analysis of nickel interlayer at different test times (a, b) 30 min, (c, d) 90 min
图7给出了不同实验时间镍中间层金刚石/铜复合材料连接接头的剪切强度和变形率。可以看出,随着试验时间的延长连接接头的剪切强度和变形率随之提高。保温保压时间为60 min时,连接接头的剪切强度达到49.24 MPa,而变形率为4.75%。但是保温保压时间延长到90min,连接接头剪切强度的提高不明显,而变形率却大大提高。由此可见,温度为750 ℃、压力为10 MPa、保压时间为60 min时,连接接头的质量最好。
图7
图7
镍中间层不同试验时间的剪切强度和变形率
Fig.7
Shear strength and deformation rate of nickel interlayer at different test times (a) shear strength, (b) deformation rate
2.3 连接界面断口的形貌、热导率以及第一性原理计算结果
图8
不加中间层金属的SPS扩散连接金刚石/铜复合材料的热导率最高只有347.73 W/(m·K)[22],热导率下降的原因是金刚石与铜界面不湿润而使界面结合能力降低[30,31]。在连接界面添加0.01 mm厚的镍箔进行SPS扩散连接,在室温25 ℃条件下制样的热扩散系数最高为3.0819 × 10-4 m2/s,热导率最高达到703.83 W/(m·K),与金刚石/铜母材的热导率相差不大,SPS扩散连接后其导热性能没有降低。其原因是,加入镍中间层的连接界面中的镍金属与铜金属生成了铜镍合金,使连接界面的结合强度较高。镍元素消除了界面中的空隙和孔洞等缺陷,使界面结合强度和导热性能提高[32]。另一方面,界面中的缺陷使金刚石中的声子和铜、镍金属中的电子在孔洞处散射,使导热性能大大降低[33]。中间层金属消除了这些缺陷,使导热性能提高。
根据基于密度泛函理论的第一性原理计算了界面的导热性能以揭示其微观机理。使用CASTEP(学术版)软件建立了不同元素的界面模型,计算中使用交换关联泛函广义梯度近似(GGA)中的Perdew-Burke-Ernerhof(PBE),选用OTFG Ultrasoft超软赝势,布里渊区k点采用Monkhorst-Pack网格。如图9所示,根据不同实验时间连接界面的XRD谱和EDS元素分析,计算出连接界面中有金刚石、铜和镍元素。
图9
图9
不同时间连接界面的XRD谱
Fig.9
XRD patterns of the connection interface at different test times
图10
图10
界面的模型结构示意图
Fig.10
Interface model structure diagram (a) diamond/Cu, (b) diamond/Ni, (c) Cu/Ni
图11
图11
金刚石、铜和镍的声子态密度图
Fig.11
Phonon density of states of diamond/Cu (a), diamond/Ni (b) and Cu/Ni (c)
图11a、b表明,金刚石与铜、镍元素之间的声子耦合性能较低。其原因是,金刚石的声子振动频率分布在10~40 THz,而铜的声子振动频率分布在0~9 THz,镍的声子振动频率分布在1~9 THz,这导致金刚石和铜、镍金属直接接触的导热性能不高。图11c表明,铜和镍元素的声子振动频率重合较好,在1~9 THz出现大量的共振峰,使连接界面中的铜元素和镍元接触时导热性能较高。根据EDS元素含量分析和XRD谱,连接界面中铜(含量为42.49%)和镍(含量为41.93%)的总含量超过80%,极大的提高了界面的导热性能。声子态密度进一步验证了,加入中间层金属Ni的金刚石/铜复合材料SPS扩散连接其界面高导热性能较高。这个结果,与制样热导率的实验值703.83 W/(m·K)一致。
图12
图12
金刚石/铜、金刚石/镍、铜/镍界面电子态密度
Fig.12
Interface electronic density of states diagramof diamond/Cu (a), diamond/Ni (b) and Cu/Ni (c)
如图12所示,金刚石/铜、金刚石/镍、铜/镍界面在Fermi能级附近的电子态密度均不为零。根据金刚石/铜界面的电子态密度,金刚石的2p轨道和铜的3p、4s轨道在-7 eV~13 eV内存在大量的共振峰;而金刚石/镍界面中,金刚石的2s、2p轨道与镍的3p、4s轨道在-9 eV~13 eV内也存在大量的共振峰;在铜/镍界面,镍元素的3p、4s轨道与铜元素的3p、4s轨道在-9 eV~22 eV内存在大量的共振峰。这表明,金刚石/铜、金刚石/镍界面电子的导热性能没有铜/镍界面电子的导热性能高,与根据声子态密度得出的结论一致。在SPS连接界面,铜、镍金属中的电子在电场的作用下迁移,使连接界面的导热性能与母材相比没有明显降低。因此,基于Ni中间层金刚石/铜的SPS扩散连接能满足高热流密度微通道热沉的要求。
3 结论
加入Ni中间层SPS扩散连接金刚石/铜复合材料,Ni元素向界面两侧扩散,与铜生成CuNi合金的α-单相固溶体,与碳形成Ni3C相。在接头界面,镍与铜的无限固溶提高了连接界面的冶金结合质量和导热性能。随着温度的提高和保压时间的延长SPS扩散连接界面剪切强度随之提高,界面内的孔洞和缝隙等缺陷减少,界面冶金结合的质量提高。Ni中间层的加入提高了连接界面处的声子振动、电子杂化峰耦合,极大地提高了金刚石/铜复合材料的热导率。
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