植布法制备镍基碳纳米管场发射阴极及其性能研究

提出了一种新型的制备碳纳米管场发射阴极的方法———植布法,详细描述了植布法工艺,并用扫描电镜观察植布法得到的样品的表面形态,讨论了球磨工艺、碳管和聚合物分散剂不同质量配比,及聚合物介质刻蚀时间的长短对最终场发射阴极性能的影响。实验表明植布法制备的场发射阴极具有良好的场发射性能,如低的开启电压（约为1.7V/μm）,高的电流密度（在3.6V/μm下,电流密度可以达到26mA/cm2）。该方法结合了传统直接生长和丝网印刷法制备碳纳米管阴极的优点,实现了碳纳米管与金属基底可靠结合的结构,广泛适用于以该结构作为核心功能单元的器件。     

衬底电极对丝网印刷CNT阴极场发射性能的影响

通过丝网印刷技术,将碳纳米管(carbon nanotube,CNT)浆料直接转移到CrCuCr薄膜衬底电极、掺Sn的In_2O_3(indium tin oxides,ITO)透明导电薄膜衬底电极和Ag浆导电厚膜衬底电极上,高温烧结后得到CNT阴极,并对CNT阴极进行表面形貌和场发射性能的研究.结果表明,不同衬底电极对CNT阴极场发射性能的影响不一样,CrCuCr薄膜衬底电极CNT阴极、ITO透明导电薄膜衬底电极CNT阴极及Ag浆厚膜导电衬底电极CNT阴极场发射的开启电场分别为0.99、2.05和2.46V/μm;当电场为3.0V/μm时,它们的亮度分别为2472、1889、587cd/m~2.CrCuCr薄膜衬底电极CNT阴极的场发射性能最优,ITO透明导电薄膜衬底电极CNT阴极次之,Ag浆厚膜导电衬底电极CNT阴极最差,并根据金属-半导体理论模型分析了原因.     

碳纳米管场发射电子源

从实验和理论上研究单根碳纳米管(CNT)场发射电子源的稳定问题.利用透射电镜/扫描探针显微镜(TEM/SPM)和场发射显微镜/场离子显微镜(FEM/FIM)对CNTs的场发射特性进行了实验研究.同时从密度泛函理论出发,利用相关程序模拟计算了吸附对单壁碳纳米管(SWNTs)场发射的影响.发现SWNTs荷电体系的总能量与荷电电荷数量关系具有抛物线形式,先减小,达到最小值,之后增加.通常荷4个电子时达到最小值,即体系处于最稳定状态.表明SWNTs有很大的电负性,是容易发生凝聚和吸附分子的根源.进而计算了对氢、氧和水的吸附特性,讨论了吸附对场发射的影响.这些结果对CNTs的场发射特性和作为新型电子源的应用都是有重要意义的.     

Ageing effect on the field emission reproducibility of multiwalled carbon nanotubes

The ageing effect on the field emission (FE) reproducibility of multiwalleded carbon nanotubes (MWCNTs) synthesised by plasma enhanced chemical vapour deposition was investigated by scanning electron microscopy. To investigate the ageing effect on the physical state of the MWCNTs, a scanning electron microscope (SEM), a Raman spectrometer, an X-ray photoelectron spectroscope and an X-ray diffractometer were used to characterise the MWCNTs comprehensively before and after the ageing. Through ageing, the FE reproducibility became better because the larger number and more evenly distributed shorter MWCNTs – (resulted from the local vacuum breakdown of higher MWCNTs due to Joule heating and oxidisation) – became the dominating emitters.     

管径相关的多壁碳纳米管膜的压阻效应

利用三点弯曲法研究了不同管径的碳纳米管膜的压阻效应,实验结果表明,管径较小的碳纳米管膜与管径大的碳纳米管膜相比具有更显著的压阻效应,并对电阻随应力变化的机制作了详细的讨论.     

碳纳米管膜/电解液界面的整流效应

对碳纳米管(CNT)膜/电解液(电解质溶液)界面的整流效应进行了研究。实验所用的碳纳米管用热灯丝化学气相沉积法(CVD)合成,整流效应通过分析其伏安特性来研究。结果发现,在室温下与一定浓度的电解液中,碳纳米管膜/电解液界面呈现出较强的整流效应,且该效应随电解液温度和浓度的增大而增强,重点讨论了碳纳米管膜/电解液界面整流效应的产生机制。     

Magnetic field enhances the graphitized structure and field emission effect of carbon nanotubes

This study uses a low temperature thermal chemical vapor deposition with an applied external magnetic field to grow carbon nanotubes (CNTs) on Ni/Ag-printed glass substrates. A mixture of C₂H₂ and H₂ gas was used for the growth of the CNTs. A Ni catalyst layer was deposited on the Ag-printed glass substrate by pulse electroplating. Scanning electron micrographs as well as the presence of two sharp peaks at 1320 cm⁻¹ (D band) and 1590 cm⁻¹ (G band) in the Raman spectra indicate that the graphitized structure of CNTs synthesized under a magnetic field has higher quality (i.e., a D-band to G-band intensity ratio of 0.303) than CNTs synthesized without a magnetic field. Transmission electron micrographs show a fine Ni catalyst at the tip of the tube for CNTs synthesized under a magnetic field, exhibiting a CNT “tip-growth” model. The synthesis of CNTs in the presence of a magnetic field also generates better field emission properties and better lighting morphology than without a magnetic field.     

用玻璃粉作粘结剂制备碳纳米管厚膜及其场发射特性

研究了用涂敷法制备的碳纳米管（CNT）厚膜及其场发射特性,裂解法获得的碳纳米管与玻璃粉等混合、研磨,直接涂敷在Si基底上,经烧结后制成碳纳米管厚膜,二极结构测量的结果表明,碳纳米管厚膜有较低的开启电场（1.0～1.25V/μm）,场强为5V/μm时,电流密度达到了50μA/cm^2.该工艺的烧结过程应控制好,加热时间稍长会使CNT厚膜的场发射性能很快下降,时间过长会使CNT处在厚膜表面之下,无法有效发射电子.浆料中的玻璃粉比例增大时,碳纳米管阴极的场发射性能会有所降低.     

表面覆铪改善碳纳米管膜发射性能

研究了表面沉积铪膜并进行后处理对碳纳米管膜场电子发射性能的影响.研究结果表明在适当的退火温度下碳纳米管表面形成了碳化铪,并显著提高了碳纳米管的发射电流密度、发射均匀性和发射稳定性.我们认为碳纳米管表面发射性能的提高归功于表面碳化铪膜良好的导电性、化学惰性和低逸出功.     

Flexible field emission of nitrogen-doped carbon nanotubes/reduced graphene hybrid films

The outstanding flexible field emission properties of carbon hybrid films made of vertically aligned N-doped carbon nanotubes grown on mechanically compliant reduced graphene films are demonstrated. The bottom-reduced graphene film substrate enables the conformal coating of the hybrid film on flexible device geometry and ensures robust mechanical and electrical contact even in a highly deformed state. The field emission properties are precisely examined in terms of the control of the bending radius, the N-doping level, and the length or wall-number of the carbon nanotubes and analyzed with electric field simulations. This high-performance flexible carbon field emitter is potentially useful for diverse, flexible field emission devices.     

双掺杂碳纳米管电子场发射性能的密度泛函研究

采用第一性原理的密度泛函理论(DFT)研究了(5,5)碳纳米管(CNT)顶端硼(B)、氮(N)、硅(Si)等元素双掺杂体系的电子场发射性能.结果表明,在外电场下,各种双掺杂CNT帽端态密度(DOS)向价带移动.电子轨道分布变化显著,电荷分布明显局城化.根据电子态密度、差分电荷密度、最高分子占据轨道(HOMO)/最低分子非占据轨道(LUMO)分布等计算结果可预期Si双掺杂后更有利于场致电子发射.     

电泳沉积碳纳米管场发射阴极研究进展

电泳法是一种工艺简洁、低能耗、低成本的薄膜制备工艺.基于电泳技术的碳纳米管薄膜具有对基底类型和形状要求低、常温操作等优势,尤其适宜于在复杂不规则基底和低熔点材料上的应用.在阐述了电泳法的工艺特点的基础上,本文总结了应用电泳技术制备碳纳米管薄膜的方法,讨论了丰富多样的碳纳米管电泳液制备工艺,介绍了碳纳米管薄膜作为场发射阴极在真空电子领域的应用开发新进展.     

Numerical analysis in focusing characteristics of the three-dimensional trapezoidal electrodes for carbon nanotube field emitters

In this study, the focusing characteristics of three-dimensional trapezoidal focusing electrodes for carbon nanotube field emitters are analyzed by the combination of the finite-difference time-domain and particle-in-cell methods. To investigate the divergences of the electrons, the three-dimensional trajectories of the electron beams are simulated by evaluating the electrons’ positions and momentums. The divergence angles of electrons in the emitters with three-dimensional focusing electrodes are proved to be smaller than those with the conventional triode structure or with the planar focusing electrodes. Characteristics of the 4 μm-thick three-dimensional geometries (trapezoidal, rectangular and inversed trapezoidal) are compared. The 1 V biased three-dimensional trapezoidal focusing electrode is optimal in regulating the divergence of the beams and forming uniform electron spots on the anode. The average divergence angle of the electrons is limited to 0.1309 rad on the anode plane while the maximum divergence angle is 0.3236 rad.     

Active vacuum brazing of CNT films to metal substrates for superior electron field emission performance

The joining of macroscopic films of vertically aligned multiwalled carbon nanotubes (CNTs) to titanium substrates is demonstrated by active vacuum brazing at 820 °C with a Ag–Cu–Ti alloy and at 880 °C with a Cu–Sn–Ti–Zr alloy. The brazing methodology was elaborated in order to enable the production of highly electrically and thermally conductive CNT/metal substrate contacts. The interfacial electrical resistances of the joints were measured to be as low as 0.35 Ω. The improved interfacial transport properties in the brazed films lead to superior electron field-emission properties when compared to the as-grown films. An emission current of 150 μA was drawn from the brazed nanotubes at an applied electric field of 0.6 V μm⁻¹. The improvement in electron field-emission is mainly attributed to the reduction of the contact resistance between the nanotubes and the substrate. The joints have high re-melting temperatures up to the solidus temperatures of the alloys; far greater than what is achievable with standard solders, thus expanding the application potential of CNT films to high-current and high-power applications where substantial frictional or resistive heating is expected.     

Active vacuum brazing of CNT films to metal substrates for superior electron field emission performance

Abstract

The joining of macroscopic films of vertically aligned multiwalled carbon nanotubes (CNTs) to titanium substrates is demonstrated by active vacuum brazing at 820 °C with a Ag–Cu–Ti alloy and at 880 °C with a Cu–Sn–Ti–Zr alloy. The brazing methodology was elaborated in order to enable the production of highly electrically and thermally conductive CNT/metal substrate contacts. The interfacial electrical resistances of the joints were measured to be as low as 0.35 Ω. The improved interfacial transport properties in the brazed films lead to superior electron field-emission properties when compared to the as-grown films. An emission current of 150 μA was drawn from the brazed nanotubes at an applied electric field of 0.6 V μm^?1. The improvement in electron field-emission is mainly attributed to the reduction of the contact resistance between the nanotubes and the substrate. The joints have high re-melting temperatures up to the solidus temperatures of the alloys; far greater than what is achievable with standard solders, thus expanding the application potential of CNT films to high-current and high-power applications where substantial frictional or resistive heating is expected.     

碳纳米管场发射阴极的制备及其场发射特性

采用电泳法将碳纳米管组装到电化学淀积的银台阵列上作为场发射阴极并研究了它的场发射特性.场发射特性测试结果表明:该阴极具有优异的场发射特性,开启电场为2.8V/μm,在应用电场为5.5V/μm时,发射电流密度达到1.7mA/cm2.具有优异的发射性能的原因可以归结到银台的边缘和银台类山状的表面增强了碳纳米管的场致电子发射.该阴极制备工艺简单、发射特性优异,且容易实现大面积制备,可以应用到大面积场发射显示器件中.     

Elimination of current non-uniformity in carbon nanotube field emitters

Activation of abnormal emitting sites in Carbon Nano Tube (CNT) field emitters and their elimination is reported. CVD grown, patterned CNT was used as cathode for field emission studies. We encountered the problem of current non-uniformity in CNT cathode. This non-uniformity was attributed to abnormally active emitting sites during voltage ramp-up. The sudden increase in current resulted in region of positive slope in F–N curve, which can’t be explained by conventional F–N theory. Also the grown CNTs can be a mixture of metallic and semi conducting nature, which may cause deviation from the conventional F–N theory. We could eliminate abnormally active sites by electric field treatment, thereby increasing current uniformity and stability. The work is underway to understand the deviation in FN curve at high fields.     

Study on effect of oxygen adsorption on characteristics of field electron emission from aligned carbon nanotubes grown by plasma-enhanced hot filament chemical vapor deposition

The characteristics of field electron emission (FE) from aligned carbon nanotubes (ACNTs) in a form of film, which were catalytically grown by plasma-enhanced hot filament chemical vapor deposition, were measured for many cycles. The results indicate that the FE characteristic for every measurement cycle is different and it follows the Fowler–Nordheim model in the low field region, but it deviates from the model in the high field region. The ACNTs were investigated by energy dispersive X-ray (EDX) and the EDX spectrum shows that oxygen is adsorbed on the ACNTs. The theory related to adsorption and space-charge effect was applied to analyze the experimental results.