Mesophase evolution in heat-treated solid petroleum pitches

Abstract

Uma combinação de técnicas analíticas foi utilizada para acompanhar a evolução da mesofase em pixes de petróleo em fase sólida. As amostras foram caracterizadas por dessorção a laser assistida por matriz, seguida de espectrometria de massas por tempo de vôo (MALDI-TOF-MS), ressonância magnética nuclear de 13 C ( 13 C NMR) de alto campo, 1 H NMR de baixo campo, ressonância eletrônica de spin (EPR), microscopia ótica polarizada e análise termogravimétrica (TGA). A análise por MALDI-TOF-MS revelou uma distribuição oligomérica de pesos moleculares no pixe: monomérica (m/z 200 a 400), dimérica (m/z 400 a 650), trimérica (m/z 650 a 950) e tetramérica (m/z 950 a 1600), fator essential para a produção de fibras de carbono, bem como outros compósitos de carbono. Estudos de tempo de relaxação (T 1 ) permitiram a observação de duas grandes áreas: a dos aromáticos e a de mesofase. Estes resultados também mostraram que o tempo de aquecimento aumenta a área de mesofase. Através de medidas não destrutivas de 13 C NMR, foram obtidos os conteúdos de carbonos aromáticos e alifáticos, e também o grau de protonação desses carbonos. A microscopia ótica com luz polarizada mostrou as interfaces entre as regiões anisotrópicas e isotrópicas, bem como o processo de coalescência da mesofase. Os resultados obtidos pelas outras técnicas corroboram com os resultados de MALDI-TOF-MS e NMR.A combination of analytical techniques was used to follow the mesophase evolution of petroleum pitches in the solid phase. The samples were characterized by matrix assisted laser desorption and time of flight analysis by mass spectrometry (MALDI-TOF-MS), high field 13 C nuclear magnetic resonance ( 13 C NMR), low field 1 H NMR, electron spin resonance (EPR), polarized optical microscopy and thermogravimetric analysis (TGA). MALDI-TOF-MS revealed an oligomeric distribution of molecular weights in the pitch: monomeric (m/z 200 to 400), dimeric (m/z 400 to 650), trimeric (m/z 650 to 950) and tetrameric (m/z 950 to 1600), an essential factor for the production of carbon fibers, among other advanced composite of carbon. Relaxation time (T 1 ) studies permitted to observe two large areas: the aromatics and the mesophases. These results also showed that the heating time increases the mesophasic area. Also through direct and non-destructive 13 C NMR measurements, we obtained the content of aromatic and aliphatic carbons, as well as the degree of protonation of these carbons. Optical microscopy with polarized light showed the interfaces between the anisotropic and isotropic regions as well as the process of coalescence of mesophase. The results obtained by the other techniques corroborate the MALDI-TOF-MS and NMR results.

Robust Pitch on Silicon Nanolayer–Embedded Graphite for Suppressing Undesirable Volume Expansion

“…Note that at the softening point (corresponding to 240 °C), pitch converts into a viscous liquid (inset of Figure b and Video S5, Supporting Information) . Beyond 400 °C, the proportion of hydrogen gradually decreases due to the volatilization of H 2 and CH 4 . Therefore, the proportion of carbon increases consistently up to 900 °C, resulting in a carbon‐dominant structure after carbonization (≈99.3 wt% at 900 °C) (Table S1, Supporting Information).…”

“…Using in situ transmission electron microscopy (TEM), we have found that pitch‐coated Si nanoparticles do not crack upon lithiation, whereas conventional acetylene‐coated Si nanoparticles show complete fracturing of the coating layers. Comparing the other carbon coating methods, pitch, which is specifically derived from petroleum, is well known as a cheap carbon coating source for conventional graphite anodes and as an ingredient of synthetic graphite and carbon fiber . However, the factors responsible for its superior mechanical strength have not been fully investigated .…”

“…Since the Si carbide (SiC), which can cause resistance at Si anodes, can be formed from Si and carbon above 900 °C, we carbonized the pitch below 900 °C. As Figure 2 a shows, the structural changes in the pitch were investigated using various analytical tools, including elemental analysis (EA), thermogravimetric analysis (TGA), 13 C‐nuclear magnetic resonance ( 13 C‐NMR), and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectroscopy (MALDI‐TOF MS) …”

“…According to the 13 C‐NMR spectra (Figure d), the pristine pitch has both aromatic rings and aliphatic chains evenly, while the carbonized pitch consists of aromatic rings with few aliphatic chains. This is accompanied by an increase in molecular weight, which was observed by MALDI‐TOF MS (Figure S4, Supporting Information). Thus, the aliphatic chains do not just disappear; but they undergo aromatic‐growth polymerization between pitch molecules occurs with the volatilization, thereby evolving into the graphene‐like sheets .…”

“…Thus, the aliphatic chains do not just disappear; but they undergo aromatic‐growth polymerization between pitch molecules occurs with the volatilization, thereby evolving into the graphene‐like sheets . Under viscous liquid conditions, i.e., after the softening point, we could expect the aromatic‐growth polymerization to facilitate π–π stacking between graphene‐like sheets (Figure a), thus developing into the highly oriented liquid crystalline domains called mesophase . This mesophase structuring plays a vital role in imparting the high Young's modulus and tensile strength to the carbonized pitch .…”