氮化矽(silicon nitride, S3N4)是常用於引擎與切銷工具的結構陶瓷；在奈米氮化矽中添加導電碳化鈦(titanium carbide, TiC)或氮化鈦(titanium nitride, TiN)奈米顆粒可使燒結體導電有助於放電加工，具有成為未來切削工具材料的潛力。SPS則是一種可將氮化矽在短時間內燒結致密的新技術。由於spark plasma sintering (SPS)是利用脈衝電流加熱試片達到燒結致密之效果，其電流在試片中的分布狀況深深影響氮化矽-碳化鈦和氮化矽-氮化鈦奈米複合陶瓷的微結構跟性質；我們因此可以從這些複合陶瓷的導電特性得知他們具有的微結構。
本實驗對添加20%重量百分比(wt%)以內碳化鈦和30%重量百分比以內氮化鈦的氮化矽基複合陶瓷試片進行電化學阻抗(electrochemical impedance spectrum, EIS)分析，以了解不同試片內的電流分佈狀況與微結構；然後再使用掃描電化學阻抗顯微鏡(scanning impedance microscope, SIM)針對一個添加20%碳化鈦的氮化矽基試片分析，以分辨存在同一個試片內的不同相態。電化學阻抗結果顯示在未添加碳化鈦的試片中，氮化矽晶粒會像一個純電容一樣阻礙所有電流，而在添加10%和20%的試片中，電流分別流過非晶質晶界和相連的碳化鈦晶粒網絡。此外，我們也藉由固定試片組成並調整SPS製程來研究氮化矽基奈米複合物中不同微結構對電化學阻抗行為的影響，發現當SPS溫度上升時添加10%碳化鈦試片在1 mHz的阻抗先由15GΩ下降至5.25MΩ再回升至300MΩ。此阻抗上升與下降的原因分別是試片致密化和晶粒成長；對於已致密的10%碳化鈦試片，提升SPS溫度也會使碳化鈦結晶造成的電阻由3.6Ω上升至32Ω。相反的，即使我們添加了30%的氮化鈦，氮化矽基氮化鈦複合物卻保持相當絕緣的狀態。用掃描式電子顯微鏡(scanning electron microscope, SEM)觀察後發現氮化矽基氮化鈦複合物中氮化鈦晶粒之間的距離遠大於氮化矽基碳化鈦複合物中碳化鈦的晶粒間距。以上電化學阻抗行為對試片微結構的關係可以穿隧電流來解釋。試片中的孔洞或晶粒成長現象會增加導電相晶粒之間的距離，而使得導電相晶粒之間穿隧電流減少、電化學阻抗增加。
這證明了我們可以透過電化學阻抗分析與建立等效電路模型(equivalent circuit model)來了解氮化矽基複合陶瓷內的電流分部與微結構。
藉著掃描式電化學阻抗顯微鏡，可從同一個添加20%碳化鈦的氮化矽基試片觀察到三種阻抗圖譜。其一由碳化鈦晶粒網絡造成，其二由含有較高碳化鈦比例的晶界所導致，其三則是由碳化鈦含量較少的晶界產生。這證明了三種導機制同時存在於此添加20%碳化鈦的試片中，但只有碳化鈦晶粒網絡會主導此試片的巨觀電化學阻抗行為，並且在SPS過程中主導微結構的變化。
掃描探針使用時會被磨耗而影響掃描探針顯微術的影像解析度與可靠度，因此我們將掃描電化學阻抗顯微鏡的探針鍍上氮化鈦薄膜，並和商用的白金(platinum, Pt)鍍膜探針在相同測量條件下比較其抗磨耗性質；在進行掃描電化學阻抗顯微術的每一步驟之後，使用掃描式電子顯微鏡 觀察每支探針的磨損情況。結果發現不論是氮化鈦鍍膜探針還是白金鍍膜探針，用來測量過掃描電化學阻抗之後針尖寬度都會變成500nm左右，遠大於氮化矽基複合陶瓷內約100nm大小的導電微結構。雖然氮化鈦鍍膜保護探針的效果不如預期，我們發現掃描電化學阻抗顯微鏡的探針磨耗現象主要是由電流所引發。
未來我們須要對氮化矽基複合陶瓷的微結構進行更詳細的分析，來加強甚至定量微結構與電化學阻抗性質之間的關係。同時我們也在尋找更耐磨或可承受的更大電流的探針改質材料與方法，讓掃描式電化學阻抗顯微鏡應用至固態奈米材料，例如電子元件中奈米線和奈米複合物的時候，能有效減輕探針磨耗現象、延長探針壽命。

Nanocomposites are known to exhibit multifunctional and attractive properties and are identified as potential candidates for structural, mechanical, electrical and other applications. The enhanced properties are closely associated to the grain size, grain shape, distribution, and the grain boundaries; all of which depend on the material composition and processing. The electrical properties also change with these factors; therefore, it is important to understand the connection between the microstructures of such nanocomposites and their electrical properties. In this study, the investigated nanocomposites are silicon nitride-titanium carbide (Si3N4-TiC) and silicon nitride-titanium nitride (Si3N4-TiN) samples sintered using spark plasma sintering (SPS), which densified its samples with current pulses.
The current distribution and microstructure of SPS Si3N4-based nanocomposites in different TiC and TiN weight percentages (indicated as xTiC/Si3N4 and xTiN/Si3N4 in which x= 0, 5, 10, 20, 30 wt%) were measured by electrochemical impedance spectrum (EIS), while the different phases exist simultaneously in the same 20TiC/Si3N4 sample were distinguished by scanning impedance microscope (SIM). The EIS analysis suggests that the pulsed current is blocked by Si3N4 grains in samples without TiC addition, but it can flow through grain boundary in 10TiC/Si3N4 and through networked TiC grains in 20TiC/Si3N4 nanocomposites. The microstructures of Si3N4 based nanocomposites can be tailored by varying SPS parameters. Through EIS study, we found that when either SPS temperature or time increases, the impedance of 10TiC/Si3N4 samples at 1 mHz first decreases severely from 15GΩ to 5.25MΩ and then increases to 500MΩ. The increase is caused by sample densification and the decrease is a result of grain growth. The densified 20TiC/Si3N4 samples, which have networked TiC grains, exhibit pure resistor behaviors. Raising the SPS temperature can increase the resistance of TiC grain network as well. In contrast to Si3N4-TiC composites, Si3N4-TiN nanocomposites, which have much larger distribution distance between TiN grains than that in Si3N4-TiC nanocomposites, show capacitive behavior. Such behavior persists for more than 30 weight percent TiN in the Si3N4 matrix. The tunneling-percolation electrical transport model is used to explain the aforementioned results with high consistency. The electrochemical impedance analysis through the establishment of equivalent circuit model has proven to be useful in verifying the microstructure and conducting mechanism in Si3N4–TiC and Si3N4–TiN nanocomposites. Impedance increases with either increasing distribution distance between conductive phase as a result of grain growth or porosity in loose structure.
SIM were conducted on the 20TiC/Si3N4 sample, from which three distinct impedance spectra were identified. One represents the behavior of a networked conductive TiC grain (networked TiC), while another was resulted from the grain boundary composed of percolating TiC (percolation mode) and the third one was associated with the grain boundary of few TiC (capacitive mode). Different SIM data proved that three conducting mechanisms take place simultaneously in the composites, but networked TiC dominates the EIS behavior of the 20TiC/Si3N4 sample and contributes to the microstructure development in SPS.
Since tip wear can severely degrade the image resolution and reliability of scanning probe microscopy (SPM), our SIM probes were modified with TiN coatings. Both TiN coated probes and commercial platinum (Pt) coated probes were used to take SIM measurements with the same configuration and parameters. These probes were characterized by scanning electron microscope (SEM) after each step to observe the degree of tip wear. The SEM images showed that the tip width of both Pt coated and TiN coated probes became much larger than the 100nm grains of the Si3N4-based nanocomposites. Although the performance of the protecting TiN coating didn’t meet our goal, we found that the tip wear in SIM is mainly induced by current.
In the future, more effort should be put to more detailed characterizations of the nanocomposites to establish clear relationships between the microstructure and the impedance spectra. We also need to find better materials and more appropriate processes for probe modification. With a conductive probe coating that is robust mechanically and electrically, SIM can be applied to nano-sized solid samples, such as conducting nano-wires and nanocomposites in electronic devices.

[1] C.-H. Lee, et al., "Influence of Conductive Nano-TiC on Microstructural Evolution of Si3N4-Based Nanocomposites in Spark Plasma Sintering," Journal of the American Ceramic Society, vol. 94, 2011.
[2] M. E. Orazem and B. Tribollet, Electrochemical Impedance Spectroscopy, xxv ed.: John Wiley & Sons., Ink., 2008.
[3] R. O'Hayre, et al., Scanning Probe Microscopy: Electrical and Electromechanical Phenomena at the Nanoscale vol. 1: Springer Verlag, 2007.
[4] K. L. Westra, et al., "Tip artifacts in atomic force microscope imaging of thin film surfaces," Journal of Applied Physics, vol. 74, 1993.
[5] (2011). AppNano products. Available: http://www.appnano.com/products/
[6] Basics of Electrochemical Impedance Spectroscopy, Rev. 5 ed.: Gamry Instruments, 2007.
[7] D. B. Rodgers. (2011). Electrochemical Impedance Spectroscopy (EIS). Available: http://www.consultrsr.com/resources/eis/index.htm
[8] J.-B. Jorcin, et al., "CPE analysis by local electrochemical impedance spectroscopy," Electrochimica Acta, vol. 51, 2006.
[9] 汪建民, 陶瓷技術手冊(下): 經濟部技術處, 1994.
[10] AniZhecheva, et al., "Enhancing the microstructure and properties of titanium alloys through nitriding and other surface engineering methods," Surface & Coatings Technology, vol. 200, p. 2192, 2005.
[11] (2004). FactSage SGTE Solution Database. Available: http://www.factsage.cn/fact/documentation/SGTE/SGTE_list.htm
[12] (2010). NIST Standard Reference Databases: Materials. Available: http://www.nist.gov/srd/materials.cfm
[13] J. C. Avelar-Batista, et al., "Plasma Nitriding and PAPVD Hard Coating: a Critical Overview of Duplex Coating Processing," JORNADAS SAM/ CONAMET/ SIMPOSIO MATERIA, vol. 06-50, p. 600, 2003.
[14] M. Miyayama and H. Yanagida, "Dependence of Grain-Boundary Resistivity on Grain-Boundary Density in Yttria-Stabilized Zirconia," Communications of the American Ceramic Society, p. 194, 1984.
[15] M. J. Jørgensen and M. Mogensen, "Impedance of Solid Oxide Fuel Cell LSMÕYSZ Composite Cathodes," Journal of The Electrochemical Society, vol. 148, p. 433, 2001.
[16] S. Luo, et al., "Gas-sensing properties and complex impedance analysis of Ce-added WO3 nanoparticles to VOC gases," Solid-State Electronics, vol. 51, 2007.
[17] T. B. Adams, et al., "Characterization of grain boundary impedances in fine- and coarse-grained CaCu3Ti4O12 ceramics," PHYSICAL REVIEW B, vol. 73, 2006.
[18] P. Dhak, et al., "Impedance spectroscopy study of LaMnO3 modified BaTiO3 ceramics," Materials Science and Engineering B, vol. 164, 2009.
[19] D. A. Neamen, Semiconductor Physics and Devices, II ed. Homewood, IL: Irwin, 2002.
[20] A. P. d. Kroon, et al., "Ionic conductivity of dense K-b-alumina ceramics: microstructural dependence and the influence of phase transformations," Solid State Ionics, vol. 133, 2000.
[21] F. Favre, et al., "Influence of relative humidity on electrical properties of α-Al2O3 powders: Resistivity and electrochemical impedance spectroscopy," Journal of Colloid and Interface Science, vol. 286, 2005.
[22] C.-A. Wang, et al., "Complex Impedance Analysis on the Orientation Effect of Whiskers in Oriented Silicon Carbide Whisker/Silicon Nitride Composites," Journal of Amercan Ceramic Society, vol. 83, 2000.
[23] S. A. Siddiqi, et al., "Impedance spectroscopy (IS), XRD and SEM of oxide-added Si3N4," Modern Physics Letters B, vol. 16, p. 525, 2002.
[24] L. F. Senna, et al., "Comparative Study Between the Electrochemical Behavior of TiN, TiCxNy and CrN Hard Coatings by Using Microscopy and Electrochemical Techniques," Materials Research, vol. 4, 2001.
[25] A. Arutunow, et al., "Atomic force microscopy based approach to local impedance measurements of grain interiors and grain boundaries of sensitized AISI 304 stainless steel," Electrochimica Acta, vol. 56, 2011.
[26] Aji A. Anappara, et al., "Impedance spectral studies of sol-gel alumina-silver nanocomposites," Acta Materialia, vol. 51, 2003.
[27] G. Binnig and C. F. Quate, "Atomic Force Microscope," Physical Review Letters, vol. 56, 1986.
[28] G. Binnig, et al., "Surface Studies by Scanning Tunneling Microscope," Rhysical Review Letters, vol. 49, 1982.
[29] R. O'Hayre, et al., "Quantitative impedance measurement using atomic force microscopy," Journal of Applied Physics, vol. 96, 2004.
[30] "NT-MDT product catalog," ed. Moscow, Russia: NT-MDT, 2009.
[31] I. W. division. (2007). BudgetSensors product. Available: http://www.budgetsensors.com/tapping_mode_afm_probes_electri.html
[32] Z. Tao and B. Bhushan, "Surface modification of AFM silicon probes for adhesion and wear reduction," Tribology Letters, vol. 21, 2006.
[33] 陳政諴, "自組裝單分子層應用在原子力顯微鏡探針的磨耗行為研究," 2010.
[34] B. Bhushan, Introduction to Tribology. New York: John Wiley & Sons., Ink., 2002.
[35] C.-H. Lee, et al., "Microstructure and mechanical properties of TiN/Si3N4 nanocomposites by spark plasma sintering (SPS)," Journal of Alloys and Compounds, vol. 508, 2010.
[36] 李昇頤, 離子束濺鍍鈦酸鍶鋇/氮化鋁鈦薄膜之研究. 台南, 台灣, 中華民國: 國立成功大學材料科學及工程學系博士論文, 2009.
[37] E. Barsoukov and J. R. Macdonald, Eds., Impedance spectroscopy :theory, experiment, and applications. Hoboken, N.J.: Wiley-Interscience, 2005, p.^pp. Pages.
[38] O. Schneegans, et al., "Capacitance measurements on small parallel plate capacitors using nanoscale impedance microscopy," APPLIED PHYSICS LETTERS, vol. 90, 2007.
[39] G. Ambrosetti, et al., "Solution of the tunneling-percolation problem in the nanocomposite regime," PHYSICAL REVIEW B, vol. 81, 2010.
[40] A. Layson, et al., "Resistance measurements at the nanoscale: scanning probe ac impedance spectroscopy," Electrochimica Acta, vol. 48, 2003.
[41] D. Manova, et al., "Investigation of d.c.-reactive magnetron-sputtered AlN thin films by electron microprobe analysis, X-ray photoelectron spectroscopy and polarised infra-red reflection," Surface and Coatings Technology, vol. 106, 1998.
[42] A. Brudnik, et al., "Microstructure and optical properties of photoactive TiO2:N thin films," Vacuum, vol. 82, 2008.
[43] C. D. Wagner, et al. (2007). The NIST X-ray Photoelectron Spectroscopy (XPS) Database. Available: http://srdata.nist.gov/xps/DataDefinition.aspx
[44] M. Bielawski, "Residual stress control in TiN/Si coatings deposited by unbalanced magnetron sputtering," Surface & Coatings Technology, vol. 200, 2006.