Latini Alessandro, Tomellini Massimo, Lazzarini Laura, Bertoni Giovanni, Gazzoli Delia, Bossa Luigi, Gozzi Daniele
Dipartimento di Chimica, Università di Roma La Sapienza, Roma, Italy.
Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma Tor Vergata, Roma, Italy.
PLoS One. 2014 Aug 25;9(8):e105788. doi: 10.1371/journal.pone.0105788. eCollection 2014.
The thermodynamic stability of onion-like carbon (OLC) nanostructures with respect to highly oriented pyrolytic graphite (HOPG) was determined in the interval 765-1030 K by the electromotive force (emf) measurements of solid electrolyte galvanic cell: (Low) Pt|Cr3C2,CrF2,OLC|CaF2s.c.|Cr3C2,CrF2,HOPG|Pt (High). The free energy change of transformation HOPG = OLC was found positive below 920.6 K crossing the zero value at this temperature. Its trend with temperature was well described by a 3rd degree polynomial. The unexpected too high values of [Formula: see text] jointly to the HR-TEM, STEM and EELS evidences that showed OLC completely embedded in rigid cages made of a Cr3C2/CrF2 matrix, suggested that carbon in the electrodes experienced different internal pressures. This was confirmed by the evaluation under constant volume of [dP/dT by the α/κ ratio for OLC (0.5 MPa K(-1)) and HOPG (8 Pa K(-1)) where α and κ are the isobaric thermal expansion and isothermal compressibility coefficients, respectively. The temperature dependency of the pressure was derived and utilized to calculate the enthalpy and entropy changes as function of temperature and pressure. The highest value of the internal pressure experienced by OLC was calculated to be about 7 GPa at the highest temperature. At 920.6 K, ΔrH and ΔrS values are 95.8 kJ mol(-1) and 104.1 JK(-1) mol(-1), respectively. The surface contributions to the energetic of the system were evaluated and they were found negligible compared with the bulk terms. As a consequence of the high internal pressure, the values of the enthalpy and entropy changes were mainly attributed to the formation of carbon defects in OLC considered as multishell fullerenes. The change of the carbon defect fraction is reported as a function of temperature.
通过固体电解质原电池的电动势（emf）测量，在765 - 1030 K温度区间内确定了洋葱状碳（OLC）纳米结构相对于高度取向热解石墨（HOPG）的热力学稳定性：（低温）Pt|Cr₃C₂，CrF₂，OLC|CaF₂（固溶体）|Cr₃C₂，CrF₂，HOPG|Pt（高温）。发现HOPG = OLC转变的自由能变化在920.6 K以下为正值，并在此温度下穿过零值。其随温度的变化趋势可用三次多项式很好地描述。与高分辨透射电子显微镜（HR - TEM）、扫描透射电子显微镜（STEM）和电子能量损失谱（EELS）证据相结合的[公式：见原文]意外过高值表明OLC完全嵌入由Cr₃C₂/CrF₂基质制成的刚性笼中，这表明电极中的碳经历了不同的内压。通过在恒定体积下根据α/κ比值评估OLC（0.5 MPa K⁻¹）和HOPG（8 Pa K⁻¹）的dP/dT得到了证实，其中α和κ分别是等压热膨胀系数和等温压缩系数。推导了压力的温度依赖性，并用于计算焓变和熵变随温度和压力的函数关系。计算得出OLC在最高温度下所经历的内压最高值约为7 GPa。在920.6 K时，ΔrH和ΔrS值分别为95.8 kJ mol⁻¹和104.1 J K⁻¹ mol⁻¹。评估了系统能量的表面贡献，发现与体相项相比可忽略不计。由于高内压，焓变和熵变的值主要归因于被视为多壳富勒烯的OLC中碳缺陷的形成。报道了碳缺陷分数随温度的变化。
