Anomalous low-temperature saturation effects and negative thermal expansion in the c-axis of highly oriented pyrolytic graphite at the magic angle

We report a novel T-XRD and Rietveld-refinement investigation of pyrolytic-graphite samples with high degree of graphene-layer-orientation and misfit-rotational-angle of ~ 0.8o in the T-range from 12K to 298K. An anomalous variation of the graphitic c-axis which involves firstly negative-thermal-expansion (from 12K to ~50K), a saturation-effect (from 50K to ~160K) and then a positive expansion (from ~180K to 298K) is evidenced. The reported trend is significantly different with respect to that expected by considering the standard-thermal-expansion α-parameter where no saturation-effect is present. SQUID-magnetometry revealed further presence of superconducting-like hysteresis which resemble those observed by Scheike et al

EFFET DE LA TENEUR EN FIBRES DE PALMIER DATTIER ET DE LA CONTRAINTE DE COMPACTAGE SUR LES PROPRIÉTÉS DES BLOCS DE TERRE COMPRIMÉE

Afin de valoriser les matériaux locaux, de contribuer à la réduction des coûts de construction et de l’énergie consommée pour le chauffage ou la climatisation, des fibres de surface du palmier dattier mâle (FSPDM) ont été utilisées pour stabiliser les blocs de terre comprimée (BTC). L'objectif principal de ce travail est l'étude de l'effet de la teneur en fibres de palmier dattier et de la contrainte de compactage sur les propriétés mécaniques du BTC renforcée par ces fibres. L'étude s’est focalisée aussi sur sa sensibilité à l'eau, en étudiant son absorption totale. Dans ce cadre, on a utilisé trois teneurs en ciments (5, 6.5 et 8 %), quatre teneurs en fibres (0.05, 0.1, 0.15 et 0.2 %) du poids du mélange sec et trois contraintes de compactage (1.5, 5 et 10 MPa). Les résultats indiquent qu’il y a une amélioration de la résistance à la compression sèche des blocs avec 0,05% en fibres, 8 % en ciment et une contrainte de compactage de 10 MPa. Mais pour le reste des cas étudiés, l'ajout de fibres a un effet négatif sur les propriétés du BTC. Cette étude a montré aussi que l’augmentation de la contrainte de compactage conduit à l’amélioration de la résistance à la compression sèche In order to valorise locals materials, to contribute to the reduction of construction costs and energy consumed for heating or conditioning, the male date palm surface fibers (MDPSF) have been used to stabilize the compressed earth blocks (CEB). The main objective of this work is the study of the effect of date palm fibers content and compaction pressure on the mechanical properties of CEB reinforced with this fibers. The study also focused on its sensitivity to water, by studying its total absorption. In this framework, three cement contents (5%, 6.5% and 8%), four fibres contents (0.05%, 0.10%, 0.15% and 0.20%) of the weight of the dry mixture are used, and compacted the soil with a static loading by applying three compacting stresses (1.50 MPa,5 MPa and 10 MPa). The results indicate that, an improvement in dry compressive strength of the blocks with 0.05% fibres content and 8% cement content and compacted at 10MPa pressure, is recorded. But for the remaining cases studied, the addition of fibers has an adverse effect on the properties of CEB. This study also showed that the increase in compaction stress leads to the improvement of dry compressive strengt

Magnetic vortex and unsaturated magnetization components in highly oriented pyrolytic graphite

Observation of ferromagnetic and granular superconductive features in highly-oriented-pyrolytic-graphite (HOPG) has recently attracted an important attention. We report a novel temperature dependent XRD and SQUID investigation of HOPG in the temperature range from 300.15 to 77.15 K. Unusual hysteresis features indicate the possible presence of vortex states in conditions of magnetic field approximately perpendicular to the HOPG layers. This interpretation is further supported by additional measurements performed on intermediate lamellae extracted by exfoliation. Evidence of a possible structural-transition in the c-axis of HOPG in the temperature range between 77 K and 100K is also provided by using the Rietveld refinement method. ZFC and FC measurements performed at high field values of 5000-10000 Oe, together with mFC-mZFC subtraction, highlight absence of a sharp depletion of the difference between magnetization signals towards zero. These observations may indicate the possible presence of additional unsaturated weak features, which are ascribed to superconductive signals as previously predicted by Scheike et al. [8]

Scanning tunneling microscopy identification of van Hove singularities and negative thermal expansion in highly oriented pyrolytic graphite with hexagonal moiré superlattices

We report a novel investigation on the structural, electronic and magnetic properties of highly-oriented-pyrolytic-graphite (HOPG) by employing STM/S, TEM and SQUID. Van Hove singularities (vHs) were identified by STS-acquisition of local-density-of-states spectra (LDOS) acquired from moiré-superlattice-periodicities D ∼0.5 nm (θ rot ∼28.6°), ∼2 nm (θ rot of ∼7.08°) and ∼ 4 nm (θ rot ∼3.6°). A possible interlayer-coupling- or moiré-thickness-induced variation of the vHs-separation-parameter was identified. Investigation of the spatial-distribution of the moiré-superlattices by exfoliation revealed also D ∼13.87 nm (θ rot ∼1.02°), ∼13.0 nm (θ rot ∼1.09°), ∼12.65 nm (θ rot ∼1.12°) and ∼2.03 nm (θ rot ∼7.0°). These observations were further supported by additional LDOS spectra acquired on HOPG from moiré-superlattice-periodicities D ∼8 nm (θ rot ∼1.8°). In the latter, the unusual presence of four vHs peaks evidenced the existence of multiple rotational effects between internal sublattices. Extended analyses were further performed by T-XRD from 12K to 298K. Unknown peak-features exhibiting unusual T-dependent shifts were analysed at 2θ ∼ 21° (Fig. 6A-C) and ∼ 43 ° (Fig. 6D). ZFC and FC-signals from the exfoliated lamellae further evidenced an anisotropic ferromagnetic behaviour, possibly involving the coexistence of 1) disorder induced percolative ferromagnetism and 2) additional magnetic components arising from the locally twisted sublattices, which were found to exhibit the moiré superlattices

Unusual butterfly-shaped magnetization signals and spin-glass-like behaviour in highly oriented pyrolytic graphite

We report a novel investigation on the relationship between magnetic-ordering and graphitic-structure in highly-oriented-pyrolytic-graphite (HOPG). By employing orientation-dependent-X-ray-diffraction, Raman-spectroscopy and temperature-dependent-superconductive-quantum-interference-device (T-SQUID) we examined the presence of ferromagnetic- and superconductive-ordering in HOPG systems with 1) disordered (HOPG1, containing carbon-vacancy-rich weak-Bernal-stacking and Moiré-superlattices with θmisfit ∼ 0.5°) and 2) ordered (HOPG2, containing higher-degree of Bernal-stacking and Moiré-superlattices with θmisfit ∼ 0.5°, 0.8°, 11°) graphitic-layer-arrangement. A perfect-HOPG is expected to exhibit a diamagnetic-response to an applied-magnetic-field. Instead, additional 1) ferromagnetic-signals presenting a characteristic width-enhancement with the field increasing in HOPG1 and 2) complex butterfly-shaped ferromagnetic signals in HOPG2, are demonstrated. Temperature-dependent-magnetometry evidenced further the presence of randomly oriented ferromagnetic clusters originating from topological disorder in both HOPG1 and HOPG2. These magnetic signals were explained on the basis of the percolative-type model

Rhombohedral stacking-faults in exfoliated highly oriented pyrolytic graphite

The recent observation of possible granular superconductivity in highly oriented pyrolytic graphite (HOPG) has attracted significant research interest. Here we report a novel investigation on the structural-properties of exfoliated-HOPG. We investigated two types of exfoliation methods, involving either a full (method-1) or partial (method-2) contact between adhesive tape and the main HOPG. Structural characterization was obtained by employing X-ray diffraction (XRD), Raman spectroscopy and electron microscopy (SEM). In particular, Raman point and mapping spectroscopy revealed significant structural-transitions from ABA (Bernal) to ABC (rhombohedral) stacking (stacking-faults), in those samples obtained with the method-2. Interestingly, strained regions exhibiting structural-deformations with a ridge-like morphology were reproducibly identified. The acquired Raman-spectra revealed a local enhancement of the D and D’ bands-intensity together with contributions arising from Electronic Raman Scattering (ERS) across the band-gap of rhombohedral-graphite, at middle (∼1870 cm−1) and high (∼ 2680 cm−1) frequency. HRTEM of the samples produced with the method-2 allowed also for the identification of local-coexistence of ripplocation-like defects with moiré superlattices, an indicator of non-uniform c-axis configuration

Anomalous stepped-hysteresis and T-induced unit-cell-volume reduction in carbon nanotubes continuously filled with faceted Fe3C nanowires

Ferromagnetically-filled carbon nanotubes have been recently considered important candidates for application into data recording quantum disk devices. Achievement of high filling rates of the ferromagnetic materials is particularly desirable for applications. Here we report the novel observation of carbon nanotubes continuously filled along the capillary with unusual μm-long faceted Fe3C nanowires. Anomalous magnetic features possibly due to strain effects of the crystal facets are reported. Magnetization measurements revealed unusual stepped magnetic hysteresis-loops at 300 K and at 2 K together with an anomalous decrease in the coercivity at low temperature. The observed unusual shape of the hysteresis is ascribed to the existence of an antiferromagnetic transition within or at the boundary of the ferromagnetic facets. The collapse in the coercivity value as the temperature decreases and the characteristic width-enhancement of the hysteresis with the field increasing appear to indicate the existence of layered antiferromagnetic phases, possibly in the strain-rich regions of the nanowire facets. Zero field cooled (ZFC) and field cooled (FC) magnetic curves evidenced presence of magnetic irreversibilities, an indicator of a possible spin-glass-like behavior induced by competing antiferromagnetic and ferromagnetic interactions. Characterization performed with low temperature XRD measurements, further revealed a slight variation in the average Fe3C unit cell parameters, suggesting the absence of additional unit-cell volume induced ferromagnetic transitions at low temperature

ETUDE DU COMPORTEMENT PHYSICO-MECANIQUE DU BLOC DE TERRE COMPRIMÉE AVEC FIBRES

Afin de valoriser les matériaux locaux et de contribuer à la réduction des coûts de construction et de l’énergie consommée pour le chauffage ou la climatisation, des fibres de surface de palmier dattier ont été utilisées pour stabiliser les blocs de terre comprimée (BTC). L'objectif principal de ce travail est d’étudier l'effet de l’utilisation des fibres végétales, notamment les fibres de palmier dattier sur les propriétés mécaniques et sur le comportement mécanique du BTC. L'étude s’est focalisée aussi sur sa sensibilité à l'eau, à travers son absorption totale et son gonflement. L’utilisation de la paille d’orge hachée et des fibres de polypropylène dans cette étude est seulement à titre de comparaison. Dans ce cadre, on a utilisé trois teneurs en ciments (5, 6.5 et 8 %), trois teneurs en chaux vive (8, 10 et 12 %), quatre teneurs en fibres (0.05, 0.1, 0.15 et 0.2 %) du poids du mélange sec et trois contraintes de compactage (1.5, 5 et 10 MPa). Les résultats indiquent qu’il y a une légère amélioration de la résistance à la compression sèche des blocs avec 0,05% en fibres, 8 % en ciment et une contrainte de compactage de 10 MPa. Alors que pour le cas de la chaux vive, l’amélioration a touché la résistance à la traction sèche des blocs avec 0,05% en fibres et pour 8 % et 12 % en chaux. Mais pour le reste des cas étudiés, l'ajout des fibres brutes ou traitées à la solution de soude a un effet négatif sur les propriétés du BTC. En revanche, la présence des fibres a un effet positif sur le comportement mécanique du composite, en augmentant sa ductilité par rapport au comportement fragile de la matrice seule. Cette étude a montré aussi que l’augmentation de la contrainte de compactage a un effet bénéfique sur l’ensemble des propriétés étudiées. Les résultats montrent aussi une baisse générale de l’absorption totale d’eau et du gonflement des blocs avec l’augmentation de la teneur en liant et avec la diminution de la teneur en fibres de palmier dattier. La résistance à la compression sèche des BTC à base de chaux augmente avec le temps de cure à l’étuve. Les blocs curés à l’étuve ont donné une résistance meilleure que celle des BTC curés au laboratoire ou avec de la vapeur naturelle, tout en signalant que ce dernier type de cure présente l’avantage d’absence de dépense énergétique. Mots Clés : bloc de terre comprimée; fibres de palmier dattier ; résistance ; absorption totale d'eau; gonflement, comportement mécanique

Characterization of a sustainable mortar based on mineral additions and prepared sand

The present study is to investigate the physical and mechanical properties of the prepared sand mortars, mainly based on the local Oued Souf sand region south of Algeria, this in accordance with standard sand grading. The principal aim of the research is to replace the restricted fine part between (0.08and 0.16 mm) which is of the order 15% and to substitute it with additions active slag (S) and inert glass powder(GP) with dosages of 5, 10 and 15 % and maintain the rest of the granular fractions of the sand studied fixed. Thus, to see and analyze the influence of these additions on the physical properties and in regards the mechanical performances of confected prepared sand mortars. The results showed the advantageous effect of replacing the said prepared sand with two additions (slag and glass powder) on physical, mechanical properties mortars based on this local sand by improving the performance of these cement products, especially at an optimal percentage of 10%