Jair C. C. Freitas

Investigation of biomass- and polymer-based carbon materials using 13C high-resolution solid-state NMR

Carbon materials produced through the thermal decomposition of biomass (rice hulls) and a polymer (polyvinyl chloride) were investigated by 13C high-resolution solid-state NMR under two different magnetic fields (2.0 and 9.4 T). The details revealed by the high-field NMR spectra provide important information about the chemical changes in the initial stages of pyrolysis: These are shown to be directly related to the original structure of the precursors and the results complement well some conclusions existent in the literature. From a heat treatment temperature of about 600°C upwards, the general shape of the 13C NMR spectra, attained with low applied magnetic field, is very similar for both chars, with a strong resonance line near 125 ppm from TMS (carbon nuclei in aromatic planes). The analysis of the evolution of the main parameters associated with this resonance line shows a behavior typical of heat-treated carbon materials, which is interpreted on the basis of the structural evolution of both chars. We show that the results are well understood when a comparison is made with the features of the 13C NMR spectrum of polycrystalline graphite.

Studies on crude oil‐water biphasic mixtures by low‐field NMR

Low‐field 1H NMR was used in this work for the analysis of mixtures involving crude oils and water. CPMG experiments were performed to determine the transverse relaxation time (T2) distribution curves, which were computed by the inverse Laplace transform of the echo decay data. The instruments ability of quantifying water and petroleum in biphasic mixtures following different methodologies was tested. For mixtures between deionized water and petroleum, one achieved excellent results, with root mean squared error of cross‐validation (RMSECV) of 0.8% for a regression between the water content (wt %) and the relative area of the water peak in the T2 distribution curve, or a standard deviation of 0.9% for the relationship between the water content and the relative water peak area, corrected by the relative hydrogen index of the crude. In the case of biphasic mixtures of Mn2+‐doped water and crude oils, the best result of RMSECV = 1.6% was achieved by using the raw magnetization decay data for a partial least squares regression. Copyright

Recent Advances in Solid-State 25Mg NMR Spectroscopy

Abstract In spite of the relatively unfavourable characteristics of the 25 Mg nuclide for NMR experiments—such as low natural abundance, small magnetogyric ratio and sizeable quadrupolar broadening—there is increasing evidence that solid-state 25 Mg NMR is a powerful tool for studies involving many different materials. Much of the recent boost in this field has been driven by instrumental improvements, such as the availability of high magnetic fields and fast-spinning speeds, as well as by the use of signal-enhancement methods developed in the past decade for half-integer spin quadrupolar nuclei. The most relevant advances in solid-state 25 Mg NMR spectroscopy are reviewed here, including a detailed account of the use of signal-enhancement methods and of the more recent applications of first-principles calculations of 25 Mg NMR parameters. Examples of the application of 25 Mg solid-state NMR are given for different classes of materials, including organic compounds, oxide-based materials, glasses, alloys and intermetallic compounds.

On the connection between structural distortion and magnetism in graphene with a single vacancy

Abstract The correlation between structural distortion and emergence of magnetism in graphene containing a single vacancy was investigated using first-principles calculations based on density functional theory (DFT). Our results have shown that a local distortion is formed around the vacancy, with reconstruction of two atomic bonds and with a dangling bond remaining at the third atom adjacent to the vacancy. A systematic investigation of the possible out-of-plane displacement of this third atom was then carried out, in order to ascertain its effects on the magnetic features of the system. The ground state was definitely found to be magnetic and planar, with spin-resolved σ and π bands contributing to the total magnetic moment. However, we have also found that metastable solutions can be achieved if an initial shift of the third atom above a minimum threshold from the graphene plane is provided, which leads to a nonplanar geometry and a nonmagnetic state.

Evidences for Tsallis non-extensivity on CMR manganites

We found, from the analysis of M vs. T curves of some manganese oxides (manganites), that these systems do not follow the traditional Maxwell-Boltzmann statistics, but the Tsallis statistics, within the normalized formalism. Curves were calculated within the mean-field approximation, for various ferromagnetic samples and the results were compared to measurements of our own and to various other authorss published data, chosen at random from the literature. The agreement between the experimental data and calculated Mq vs. T* curve, where T* is an effective temperature, is excellent for all the compounds. The entropic parameter, q, correlates in a simple way with the experimental value of Tc, irrespective of the chemical composition of the compounds, heat treatment or other details on sample preparation. Examples include q 1 (subextensivity) cases.

Preparação e caracterização de carvão ativado quimicamente a partir da casca de arroz

This work consists in a study about the chemical activation of charred rice hulls using NaOH as the activation agent. The influence of the naturally-occurring silica was particularly evidenced. X-ray diffraction patterns showed the formation of sodium carbonate and silicates in the activated samples, whereas thermogravimetric curves revealed a strong reduction in the ash content of these samples after washing with water. Nitrogen adsorption data indicated a microporosity development only in the washed samples, with BET surface area values of 450 and 1380 m2/g achieved for the samples activated at 800 °C starting from the precursor with or without silica, respectively.

Comparison of practical techniques to develop latent fingermarks on fired and unfired cartridge cases

We have tested some widely used and practical fingermark enhancement techniques such as powdering (regular powder dusting and magnetic powder application), cyanoacrylate fuming, fluorescent dying (basic yellow 40), gun blueing solutions and acidified hydrogen peroxide solutions. The results were evaluated and compared in order to establish best procedures on processing cartridge cases. The tests were performed on brass discs subjected to three different temperatures (room temperature, 63 and 200°C), and on fired and unfired cartridge cases. All the samples were processed after three different periods of time (24h, 7 days and 14 days) after deposition. The best results for both fired and unfired cartridge cases were obtained by the sequential application of cyanoacrylate, gun blueing solution and basic yellow 40. Some stages of the firing process were isolated in order to identify their effects over the final amount and quality of the remaining latent fingermarks on cartridge cases. Good state fingermarks were developed on unfired cartridge cases cycled through the gun, showing that friction inside the gun without firing does not cause significant damage to the fingermarks. On the other hand, fired cartridge cases are significantly affected by the firing effects, exhibiting low quality ridge details which are mainly located next to base. An unexpected phenomenon was observed on most of the brass discs heated to 200°C and developed with gun blueing solutions; they presented a reverse development compared to the expected one, with darkening of the ridges instead of the background.

Quantum information processing through nuclear magnetic resonance

We discuss the applications of Nuclear Magnetic Resonance (NMR) to quantum information processing, focusing on the use of quadrupole nuclei for quantum computing. Various examples of experimental implementation of logic gates are given and compared to calculated NMR spectra and their respective density matrices. The technique of Quantum State Tomography for quadrupole nuclei is briefly described, and examples of measured density matrices in a two-qubit I = 3/2 spin system are shown. Experimental results of density matrices representing pseudo-Bell states are given, and an analysis of the entropy of theses states is made. Considering an NMR experiment as a depolarization quantum channel we calculate the entanglement fidelity and discuss the criteria for entanglement in liquid state NMR quantum information. A brief discussion on the perspectives for NMR quantum computing is presented at the end.

Determination of the hyperfine magnetic field in magnetic carbon-based materials: DFT calculations and NMR experiments.

The prospect of carbon-based magnetic materials is of immense fundamental and practical importance, and information on atomic-scale features is required for a better understanding of the mechanisms leading to carbon magnetism. Here we report the first direct detection of the microscopic magnetic field produced at 13C nuclei in a ferromagnetic carbon material by zero-field nuclear magnetic resonance (NMR). Electronic structure calculations carried out in nanosized model systems with different classes of structural defects show a similar range of magnetic field values (18–21 T) for all investigated systems, in agreement with the NMR experiments. Our results are strong evidence of the intrinsic nature of defect-induced magnetism in magnetic carbons and establish the magnitude of the hyperfine magnetic field created in the neighbourhood of the defects that lead to magnetic order in these materials.Information on atomic-scale features is required for a better understanding of the mechanisms leading to magnetism in non-metallic, carbon-based materials. This work reports a direct evaluation of the hyperfine magnetic field produced at 13C nuclei in ferromagnetic graphite by nuclear magnetic resonance (NMR). The experimental investigation was made possible by the results of first-principles calculations carried out in model systems, including graphene sheets with atomic vacancies and graphite nanoribbons with edge sites partially passivated by oxygen. A similar range of maximum hyperfine magnetic field values (18-21T) was found for all systems, setting the frequency span to be investigated in the NMR experiments; accordingly, a significant 13C NMR signal was detected close to this range without any external applied magnetic field in ferromagnetic graphite.

Numerical simulation of NQR/NMR: Applications in quantum computing.

A numerical simulation program able to simulate nuclear quadrupole resonance (NQR) as well as nuclear magnetic resonance (NMR) experiments is presented, written using the Mathematica package, aiming especially applications in quantum computing. The program makes use of the interaction picture to compute the effect of the relevant nuclear spin interactions, without any assumption about the relative size of each interaction. This makes the program flexible and versatile, being useful in a wide range of experimental situations, going from NQR (at zero or under small applied magnetic field) to high-field NMR experiments. Some conditions specifically required for quantum computing applications are implemented in the program, such as the possibility of use of elliptically polarized radiofrequency and the inclusion of first- and second-order terms in the average Hamiltonian expansion. A number of examples dealing with simple NQR and quadrupole-perturbed NMR experiments are presented, along with the proposal of experiments to create quantum pseudopure states and logic gates using NQR. The program and the various application examples are freely available through the link http://www.profanderson.net/files/nmr_nqr.php.