Negative thermal expansion coefficient of graphene measured by Raman spectroscopy

The thermal expansion coefficient (TEC) of single-layer graphene is estimated with temperature-dependent Raman spectroscopy in the temperature range between 200 and 400 K. It is found to be strongly dependent on temperature but remains negative in the whole temperature range with a room temperature value of (-8.0 ± 0.7) × 10(-6) K(-1). The strain caused by the TEC mismatch between graphene and the substrate plays a crucial role in determining the physical properties of graphene, and hence its effect must be accounted for in the interpretation of experimental data taken at cryogenic or elevated temperatures.     

Micro/nanoscale spatial resolution temperature probing for the interfacial thermal characterization of epitaxial graphene on 4H-SiC

Limited internal phonon coupling and transfer within graphene in the out-of-plane direction significantly affects graphene-substrate interfacial phonon coupling and scattering, and leads to unique interfacial thermal transport phenomena. Through the simultaneous characterization of graphene and SiC Raman peaks, it is possible, for the first time, to distinguish the temperature of a graphene layer and its adjacent 4H-SiC substrate. The thermal probing resolution reaches the nanometer scale with the graphene (≈1.12 nm) and is on the micrometer scale (≈12 μm) within SiC next to the interface. A very high thermal resistance at the interface of 5.30 (-0.46) (+0.46) x 10(-5) Km2 W(-1) is observed by using a Raman frequency method under surface Joule heating. This value is much higher than those from molecular dynamics predictions of 7.01(-1.05) (+1.05) x 10(-1) and 8.47(-0.75) (+0.75) x 10(-10) Km2 w(-1) for surface heat fluxes of 3 × 10(9) and 1 × 10(9) and 1 x 10(10) W m(-2) , respectively. This analysis shows that the measured anomalous thermal contact resistance stems from the thermal expansion mismatch between graphene and SiC under Joule heating. This mismatch leads to interface delamination/separation and significantly enhances local phonon scattering. An independent laser-heating experiment conducted under the same conditions yielded a higher interfacial thermal resistance of 1.01(-0.59) (+1.23) x 10(-4) Km2 W(-1). Furthermore, the peak width method of Raman thermometry is also employed to evaluate the interfacial thermal resistance. The results are 3.52 × 10(-5) and 8.57 × 10(-5) K m2 W(-1) for Joule-heating and laser-heating experiments, respectively, confirming the anomalous thermal resistance between graphene and SiC. The difference in the results from the frequency and peak-width methods is caused by the thermal stress generated in the heating processes.     

MC尼龙和MC尼龙/纳米ZnO复合材料晶面间距的膨胀系数和收缩系数

采用原位聚合反应制备铸型(MC)尼龙6/纳米ZnO复合材料,并用动态高温X射线衍射(XRD)法对合成的MC尼龙/纳米ZnO复合材料进行研究,考察其在升温和降温过程中α<,1>和α<,2>晶面间距的变化、结晶结构的热稳定性.结果表明,随着温度的升高,MC尼龙的α<,1>(200)和α<,2>(002+202)晶面分别发生...     

Pressure-mediated doping in graphene

Exfoliated graphene and few layer graphene samples supported on SiO(2) have been studied by Raman spectroscopy at high pressure. For samples immersed on a alcohol mixture, an electron transfer of ?n/?P ～ 8 × 10(12) cm(-2) GPa(-1) is observed for monolayer and bilayer graphene, leading to giant doping values of n ～ 6 × 10(13) cm(-2) at the maximum pressure of 7 GPa. Three independent and consistent proofs of the doping process are obtained from (i) the evolution of the Raman G-band to 2D-band intensity ratio, (ii) the pressure coefficient of the G-band frequency, and (iii) the 2D band components splitting in the case of the bilayer sample. The charge transfer phenomena is absent for trilayer samples and for samples immersed in argon or nitrogen. We also show that a phase transition from a 2D biaxial strain response, resulting from the substrate drag upon volume reduction, to a 3D hydrostatic compression takes place when going from the bilayer to the trilayer sample. By model calculations we relate this transition to the unbinding of the graphene-SiO(2) system when increasing the number of graphene layers and as function of the surface roughness parameters. We propose that the formation of silanol groups on the SiO(2) substrate allows for a capacitance-induced substrate-mediated charge transfer.     

Ridge formation and removal via annealing in exfoliated graphene

It is well known that graphene is a very promising material due to its excellent physical, chemical, and thermal properties. Previously, ridges in graphene on a substrate were found in epitaxial graphene on a SiC substrate. It was found in this study that ridges can be made on a graphene layer via mechanical exfoliation on a sapphire substrate, and that ridges can be created or removed through heating and cooling. Due to the difference of the thermal-expansion coefficients of the substrate and graphene, it can be said that thermal cycling causes compressive strain, which is released by forming ridges. Annealing was carried out in a vacuum chamber within the pressure range of 10(-3)-10(-6) Torr and at 900-1100 degrees C. To analyze the shapes and mechanical properties of the ridges, Raman spectroscopy and AFM measurement were performed. It was found that the ridges can be extended by defect as a nucleation center, and the graphene layer can be folded along the preexisting ridge during heating and cooling.     

Thermal coefficients of the expansion and refractive index in YAG

The thermal expansion coefficient and dn/dT are measured by interferometry techniques in undoped YAG below 300 K. The thermal expansion coefficient at 125 K is measured to be 2.70 x 10(-6) K(-1) and dn/dT at 633 nm is 2.5 x 10(-6) K(-1), compared with 7 x 10(-6) K(-1) and 9 x 10(-6) K(-1) for these quantities at 300 K.     

Electrical and thermal conductivity of low temperature CVD graphene: the effect of disorder

In this paper we present a study of graphene produced by chemical vapor deposition (CVD) under different conditions with the main emphasis on correlating the thermal and electrical properties with the degree of disorder. Graphene grown by CVD on Cu and Ni catalysts demonstrates the increasing extent of disorder at low deposition temperatures as revealed by the Raman peak ratio, IG/ID. We relate this ratio to the characteristic domain size, La, and investigate the electrical and thermal conductivity of graphene as a function of La. The electrical resistivity, ρ, measured on graphene samples transferred onto SiO2/Si substrates shows linear correlation with La(-1). The thermal conductivity, K, measured on the same graphene samples suspended on silicon pillars, on the other hand, appears to have a much weaker dependence on La, close to K～La1/3. It results in an apparent ρ～K3 correlation between them. Despite the progressively increasing structural disorder in graphene grown at lower temperatures, it shows remarkably high thermal conductivity (10(2)-10(3) W K(-1) m(-1)) and low electrical (10(3)-3×10(5) Ω) resistivities suitable for various applications.     

Temperature stability of thin-film narrow-bandpass filters produced by ion-assisted deposition

Four types of single-cavity, thin-film, narrow-bandpass filter whose full width at half-maximum ranges from 0.5 to 1.1 nm are produced by ion-assisted deposition of alternating TiO(2)/SiO(2) or Ta(2)O(5)/SiO(2) layers upon eight substrates having differing coefficients of linear expansion, and the temperature stability of their center wavelengths is examined in the 1540-nm wavelength region. The temperature stability is shown to be greatly dependent on the coefficient of linear expansion of the substrate upon which the filter is deposited. For the eight substrates whose coefficients of linear expansion range from 0 to 142 × 10(-7)/°C, the temperature stability of the filters ranges from +0.018 to -0.005 nm/°C. Calculations based on a newly developed elastic strain model reveal that the main reason temperature stability of the center wavelengths exhibits substrate dependency is due to a reduction in film packing density brought about by volumetric distortion of the film, which is caused by stress induced from the substrate.     

Elastic stiffness and thermal expansion coefficients of various refractory silicides and silicon nitride films

The elestic stiffness parameter E_f/(1−ν_f) and the thermal expansion coefficient _f were obtained for four different silicides (TiSi₂, TaSi₂, MoSi₂ and WSi₂) and for two different nitrides (chemically vapor-deposited Nitrox Si₃N₄ and r.f. plasma SiN) from stress-temperature measurements on identical films deposited on two different substrate materials. The values determined for _f and E_f/(1−ν_f) were quite similar for all silicides and averaged 15 ppm °C⁻¹ and 1.1 × 10¹² dyncm⁻² respectively. The thermal mismatch of these silicides is such that, once safely formed, the silicide film should be able to withstand high temperature processing steps without cracking. For the nitrides the values were essentially the same (approximately 1.5 ppm°C^-1), although the larger value of E_f/(1−ν_f) chemically vapor-deposited Si₃N₄ film (3.7 × 10¹² as opposed to 1.1 × 10¹² dyn cm^-2) indicates that it is somewhat stiffer than the SiN film.     

微波ECR—CVD低温SiNx薄膜的氢含量分析

利用红外光谱技术研究了微波电子回旋共振（ＥＣＲ）等离子体化学气相沉积（ＣＶＤ）法在低温条件下制备的ＳｉＮ。膜的键的结构和氢含量，分析了微波 功率和后处理条件对膜含量的影响及其成因，提出适当提高微波功率是降低微波ＥＣＲ－ＣＶＤ低温ＳｉＮｘ膜中氢含量的可能途径。     

氧化石墨烯的制备及其对NH3的敏感特性研究

石墨烯独特的原子结构赋予其电学、热学、力学等方面的优异性能,在诸多领域具有广泛的应用。氧化石墨烯不仅具有石墨烯结构特点,而且具有大量的含氧官能团,增强了对气体的吸附能力,更适合应用于气敏传感器。通过改进的Hummer方法制备了片状多层氧化石墨烯,并对不同浓度的NH3进行敏感特性测试。结果表明氧化石墨烯对NH3具有良好的响应,在(1.5～3.5)×10-4范围内呈线性关系。     

Lattice expansion in seamless bilayer graphene constrictions at high bias

Our understanding of sp(2) carbon nanostructures is still emerging and is important for the development of high performance all carbon devices. For example, in terms of the structural behavior of graphene or bilayer graphene at high bias, little to nothing is known. To this end, we investigated bilayer graphene constrictions with closed edges (seamless) at high bias using in situ atomic resolution transmission electron microscopy. We directly observe a highly localized anomalously large lattice expansion inside the constriction. Both the current density and lattice expansion increase as the bilayer graphene constriction narrows. As the constriction width decreases below 10 nm, shortly before failure, the current density rises to 4 × 10(9) A cm(-2) and the constriction exhibits a lattice expansion with a uniaxial component showing an expansion approaching 5% and an isotropic component showing an expansion exceeding 1%. The origin of the lattice expansion is hard to fully ascribe to thermal expansion. Impact ionization is a process in which charge carriers transfer from bonding states to antibonding states, thus weakening bonds. The altered character of C-C bonds by impact ionization could explain the anomalously large lattice expansion we observe in seamless bilayer graphene constrictions. Moreover, impact ionization might also contribute to the observed anisotropy in the lattice expansion, although strain is probably the predominant factor.     

硅砖线膨胀率和平均线膨胀系数测定的不确定度评定

按照GB／T7320．1—2000的方法测试并计算了一批硅砖试样的线膨胀率和平均线膨胀系数,对影响其线膨胀率和平均线膨胀系数的不确定度分量进行了分析,并对不确定度的各个分量进行了计算和合成,给出了该硅砖的不确定度：ρ=1．06％,U=0．01％,k=2; α=10．8×10^-6℃-1,U=0．1×10^-6℃-1,k=2。     

Giant two-photon absorption in bilayer graphene

We present a quantum perturbation theory on two-photon absorption (2PA) in monolayer and bilayer graphene which is Bernal-stacked. The theory shows that 2PA is significantly greater in bilayer graphene than monolayer graphene in the visible and infrared spectrum (up to 3 μm) with a resonant 2PA coefficient of up to ～0.2 cm/W located at half of the bandgap energy, γ(1) = 0.4 eV. In the visible and terahertz region, 2PA exhibits a light frequency dependence of ω(-3) in bilayer graphene, while it is proportional to ω(-4) for monolayer graphene at all photon energies. Within the same order of magnitude, the 2PA theory is in agreement with our Z-scan measurements on high-quality epitaxial bilayer graphene deposited on SiC substrate at light wavelength of 780 and 1100 nm.     

Superior thermal conductivity of single-layer graphene

We report the measurement of the thermal conductivity of a suspended single-layer graphene. The room temperature values of the thermal conductivity in the range approximately (4.84+/-0.44)x10(3) to (5.30+/-0.48)x10(3) W/mK were extracted for a single-layer graphene from the dependence of the Raman G peak frequency on the excitation laser power and independently measured G peak temperature coefficient. The extremely high value of the thermal conductivity suggests that graphene can outperform carbon nanotubes in heat conduction. The superb thermal conduction property of graphene is beneficial for the proposed electronic applications and establishes graphene as an excellent material for thermal management.     

Ca1-xBaxZr4(PO4)6磷酸盐陶瓷材料的热膨胀特性

Ca1-xBaxZr4(PO4)6(0≤X≤1.0)属于NZP族磷酸盐陶瓷材料,它是新的一类低热膨胀陶瓷材料。研究发现CaZr(PO4)6(X=0)及BaZr4(PO4)6(X＝1)时的轴膨胀特性正好相反,因此当X为某一值时热膨胀系数将为零。本研究采用共沉淀法合成了Ca1-xBaxZr4(PO4)6磷酸盐陶瓷材料,当X＝0时,热膨胀系数为－0．86×10-61／℃;当X＝0．25时热膨胀系数为2．7×10-61／℃;当X＝0．75时其热膨胀异向性最小。为了得到热膨胀异向性和热膨胀系数均小的Ca1-xBaxZr4(PO4)6磷酸盐陶瓷材料,X应在0．6至0．9之间,并通过控制烧结条件抑制晶粒的生长。     

K2O对LZAS微晶玻璃微观结构及热膨胀性能的影响

采用DTA、XRD、SEM、TEC(热膨胀系数)等分析手段研究了加入K2O前后LZAS系统微晶玻璃的微观结构和热膨胀性能.结果表明,650℃晶化时,试样析出γⅡ-LZS和方石英晶体;725℃时,γⅡ-LZS逐渐转变为γ0-LZS晶体,并且出现β-石英固溶体;800℃时,β-石英固溶体转变为β-锂辉石固溶体,晶粒尺寸逐渐长大.制得微晶玻璃的热膨胀系数在(50～130)×10-7℃-1(20～500℃)之间,其大小取决于晶相的种类和含量.并且K2O的加入降低了玻璃的转变温度、粘度,抑制了玻璃的析晶倾向,增大了微晶玻璃的热膨胀系数.     

热电材料CoSi和CrSi_2的晶格热膨胀性

利用动态高温X射线衍射技术分别对立方CoSi和六方CrSi2化合物在298~973 K温度范围内的晶格热膨胀性进行了研究。结果表明:化合物CoSi的点阵参数随温度升高呈线性增长关系,其平均线热膨胀系数aα和平均体热膨胀系数αV分别为1.14×10-5K-1和3.42×10-5K-1,两者之间符合立方晶系关系式,即3αa=αV;化合物CrSi2的点阵参数随温度升高而显著增大,其中沿a轴和c轴的平均线热膨胀系数及平均体热膨胀系数分别为αa=0.96×10-5K-1,cα=0.73×10-6K-1和αV=2.45×10-5K-1,三者之间符合六方晶系关系式,即2αa+cα=αV;化合物CrSi2沿a轴方向的线热膨胀系数远大于沿c轴方向的,呈较强的各向异性。     

Transport in nanoribbon interconnects obtained from graphene grown by chemical vapor deposition

We study graphene nanoribbon (GNR) interconnects obtained from graphene grown by chemical vapor deposition (CVD). We report low- and high-field electrical measurements over a wide temperature range, from 1.7 to 900 K. Room temperature mobilities range from 100 to 500 cm(2)·V(-1)·s(-1), comparable to GNRs from exfoliated graphene, suggesting that bulk defects or grain boundaries play little role in devices smaller than the CVD graphene crystallite size. At high-field, peak current densities are limited by Joule heating, but a small amount of thermal engineering allows us to reach ～2 × 10(9) A/cm(2), the highest reported for nanoscale CVD graphene interconnects. At temperatures below ～5 K, short GNRs act as quantum dots with dimensions comparable to their lengths, highlighting the role of metal contacts in limiting transport. Our study illustrates opportunities for CVD-grown GNRs, while revealing variability and contacts as remaining future challenges.     

煅烧条件对共沉淀法合成ZnZr_4(PO_4)_6粉体影响的研究

采用共沉淀法制备了磷酸锆锌(α-ZnZr_4(PO_4))粉体,探讨了磷酸锆锌粉体合成机理,研究了煅烧温度、保温时间等煅烧条件对粉体合成的影响,并用TG-DTA、FT-IR、XRD、TEM等方法进行表征。实验表明前驱体在900～950℃煅烧2h可以获得平均粒径200nm、结晶完全的α-ZnZr4(PO4)6粉体,采用此粉体制备的陶瓷材料1200℃时热膨胀系数为-2.0×10-6℃-1。