Amorphous Fraction Research Articles

Investigation on tunable ferroelectricity of polymer-based composites via changing the surface state of ferroelectric nanosheets

Polymer-based materials play a crucial role in progressive electronic devices and electric power systems due to their excellent ferroelectric and dielectric performances. High ferroelectricity of the polymers could be achieved even at a low electric field with ferroelectric nanofillers introduced. However, the origin of the extra ferroelectricity enhancements of these composites besides the intrinsic ferroelectricity of fillers are still ambiguous. Herein, the Ca2Nb3O10 (CNO) nanosheets were selected as ferroelectric nanofillers, which were embedded into the polyvinylidene fluoride (PVDF)-based polymer matrix. Numerous characterization results manifested that the surface of ferroelectric nanofillers could induce the formation of polar-oriented amorphous fractions (OAFs) in the composites. Experimental results indicated these formed OAFs are the origination of the enhanced ferroelectricity of the composites. In contrast, the aluminum oxide layer was constructed onto the CNO surface to cut off the interaction between the CNO nanosheets and the polymer matrix. These effects could be eliminated when the direct contact between the nanofiller and polymer matrix is interrupted, which facilitates the energy storage performance enhancements of the composites. This work reveals the origin of the extra ferroelectricity enhancements of polymer-based composites containing ferroelectric nanofillers. It may blaze new trails in realizing multifunctional applications of polymer-based composites via surface engineering.

Drying of non-chemically prepared nanofibrillated cellulose from lime residue: Effects of drying methods on fiber morphology, physicochemical properties and redispersibility

The effects of drying methods, i.e., hot air oven and spray drying, defibrillation step and use of maltodextrin as a drying aid on microstructure as well as water redispersibility of dried nanofibrillated cellulose (NFC) were investigated; NFC-to-maltodextrin ratios were varied at 1:0, 1:1, 1:1.5 or 1:2. Spray-dried NFCs possessed lower crystallinity index (CI) values than oven-dried NFC. Higher added maltodextrin content resulted in lowered CI values due to an increase in the amorphous fraction within the samples. Freshly prepared NFC suspension exhibited gel-like behavior, while suspensions prepared by redispersing any dried NFC in water lost such a behavior. High pressure (100 MPa) applied during microfluidization defibrillation is postulated to cause the degradation of pectin, leading to the loss of its capability to serve as Van der Walls interactions and hydrogen bonding formation inhibitor. Although maltodextrin could somehow help form a dispersed fibrillar structure upon water reconstitution, the extents of such a structure in all cases were still not adequate to regain the gel-like behavior.

Pulse sequence induced variability combined with multivariate analysis as a potential tool for 13C solid-state NMR signals separation, quantification, and classification

Multivariate Curve Resolution (MCR) is a multivariate analysis procedure commonly used to analyze spectroscopic data providing the number of components coexisting in a chemical system, the pure spectra of the components as well as their concentration profiles. Usually, this procedure relies on the existence of distinct systematic variability among spectra of the different samples, which is provided by different sources of variation associated to differences in samples origin, composition, physical chemical treatment, etc. In solid-state NMR, MCR has been also used as a post-processing method for spectral denoising or editing based on a given NMR property. In this type of use, the variability is induced by the incrementation of a given parameter in the pulse-sequence, which encodes the separation property in the acquired spectra. In this article we further explore the idea of using a specific pulse sequence to induce a controlled variability in the 13C solid-state NMR spectra and then apply MCR to separate the pure spectra of the components according to the properties associated to the induced variability. We build upon a previous study of sugarcane bagasse where a series of 13C solid-state NMR spectra acquired with the Torchia-T1 CPMAS pulse sequence, with varying relaxation periods, was combined with different sample treatments, to estimate individual 13C solid-state NMR spectra of different molecular components (cellulose, xylan and lignin). Using the same pulse sequence, we show other application examples to demonstrate the potentiality, parameter optimization and/or establish the limitations of the procedure. As a first proof of principle, we apply the approach to commercial semicrystalline medium density polyethylene (MDPE) and polyether ether ketone (PEEK) providing the estimation of the individual 13C ssNMR spectra of the polymer chains in the amorphous (short T1) and crystalline (long T1) domains. The analysis also provided the relative intensities of each estimated pure spectra, which are related to the characteristic T1 decays of the amorphous and crystalline domain fractions. We also apply the analysis to isotactic poly (1-butene) (iPB-I) as an example in which the induced T1 variability occurs due to the mobility difference between the polymer backbone and side-chains. A jack-knifing procedure and a student t text allow us to stablish the minimum number of spectra and the range of relaxation periods that need to be used to achieve a precise estimation of the individual pure spectra and their relative intensities. A detail discussion about possible drawbacks, applications to more complex systems, and potential extensions to other type of induced variability are also presented.

THE EFFECT OF DISPERSION TIME ON THE STRUCTURE AND THERMOPHYSICAL PROPERTIES OF SYSTEMS BASED ON POLYETHYLENE GLYCOL AND MONTMORILLONITE

In this work, the effect of ultrasonic dispersion time on the structural and thermophysical properties of nanocomposites was studied. Model systems were made based on polyethylene glycol and montmorillonite. All samples had the same composition and filler content (5% by weight), the ultrasonic treatment time was from 5 to 12 minutes. The methods of wide-angle X-ray scattering and differential scanning calorimetry were used to establish the dependence of the properties of the systems on the dispersion time. Data analysis of the obtained results showed that the variation of ultrasonic dispersion time significantly affects the properties of polymer nanocomposites. As the mixing time increases, the interplanar distance of montmorillonite increases, which indicates an increase in the degree of intercalation of the polymer matrix. At the same time, the crystallinity of the nanocomposite decreases, which corresponds to the increase in the area of the polymer/filler boundary layer. The melting temperature of the nanocomposite increases with increasing dispersion time. This trend is a consequence of the complication of the thermal movement of polymer molecules due to the presence of a developed surface of the filler. It is shown that with an increase in the sonication time, the part of the immobilized amorphous fraction of the polymer increases. This is explained by the fact that the polymer intercalated in the interlayer space of montmorillonite loses its ability to cooperative movement, that is, to glass transition. It was established that the maximum improvement of system properties is observed at a dispersion time of 10 min. In this state, the montmorillonite particles are most stratified, which leads to the maximum increase in the area of the boundary layer. During further mixing, processes of aggregation of montmorillonite particles and destruction of polymer molecules occur, which leads to the loss of the desired properties of the nanocomposite. Finding the optimal mixing time of a polymer nanocomposite makes it possible to obtain the desired properties of systems with a defined composition.

Characterization of Natural Fiber Extracted from Corn (Zea mays L.) Stalk Waste for Sustainable Development

Corn stalk fibers were extracted from corn stalk using sodium hydroxide for textile application. The extraction conditions were optimized on the basis of the quality and quantity of extracted fibers. The optimum conditions were obtained by treating corn stalk with 5 g/L concentration of sodium hydroxide for 60 min at boiling temperature using a 1:50 material-to-liquor ratio. Extracted fibers were bleached and tested for different physical and chemical properties. Besides these properties, the characterization of extracted fibers was carried out by a scanning electron microscope (SEM), X-ray diffraction analysis (XRD) and Fourier–transform infrared (FTIR) analysis. SEM was used to study the morphological changes in the raw and bleached fibers. The crystallinity changes of the raw and bleached samples were measured with an X-ray diffractometer by peak height method. FTIR was used to examine the compositional changes in the bleaching process. It was found that raw fibers contained the cellular residues such as lignin and hemicelluloses, which cement the fibers together. The chemical treatments such as alkali and bleaching partially removed hemicelluloses, lignin, and amorphous fractions of cellulose. This led to the gradually increasing crystallinity of the treated fiber. Peak height values were obtained by measuring the transmittance of the spectra through FTIR analysis. Different physical and chemical properties of the extracted corn stalk fibers indicated that it can be used for making biodegradable composite materials.

Decomposition Behavior of Stereocomplex PLA Melt-Blown Fine Fiber Mats in Water and in Compost

This study introduces systematic and comparative investigations of various PLA fine fiber mats prepared by melt blowing. A series of PLLA and PDLA melt-blown fibers from various L and D enantiomers blends were produced. Their morphological, mechanical, and thermal properties were studied, and their decomposition in water and compost was investigated. It was found that the 1:1 ratio blend with stereocomplex crystals had an 80% lower average fiber diameter, 60% higher specific strength and better thermal stability than the PLLA and PDLA fiber mats. In the case of composting, the crystalline peak melting temperature, crystallinity, and thermogravimetric decomposition temperatures marginally decreased after 14 days. The high surface of the fine fiber mats played a crucial role in fast decomposition, as they entirely disintegrated in less than only 40 days. In the case of water, the homocrystalline domains were more susceptible to hydrolysis than the stereocomplex ones. All the PLA fiber mats underwent decomposition and extensive disintegration for 70 days in water. Hydrolysis reduced the amorphous and crystalline fraction of the fibers via surface and bulk erosion, while the decomposition of stereocomplex-crystalline-rich domains mainly exhibited surface erosion. Findings revealed that high porosity and the high surface area of PLA melt-blown fine fiber mats undergo fast decomposition in compost and in water.Graphical

Strong, ductile and durable Poly(glycolic acid)-based films by constructing crystalline orientation, entanglement network and rigid amorphous fraction

Poly(glycolic acid) as a promising substitution for traditional petroleum-based polymers suffers from its inherent brittleness and rapid hydrolysis. The improvement of toughness and hydrolysis resistance is usually accompanied with a sacrifice of strength and stiffness. In the present work, we propose a simple and effective strategy to address this obstacle by introducing poly (butylene adipate-co-terephthalate) (PBAT) and combining extensional stress and temperature field. The robust, ductile and durable PGA/PBAT films are fabricated via “high temperature melting-quenching-stretching-annealing” (QSA) process. Highly oriented crystallites constructed by stretching and annealing lead to the improvement of strength and stiffness. Stress-induced strengthened entanglement network contributes to stabilization of craze growth and slipping, thus leading to the ductile behavior. As the result, the QSA films exhibit 89 MPa and 131% in tensile strength and the elongation at break, which are increased by 195%, 126% respectively compared to the unstretched film. More impressively, stretching resulted in abundant rigid amorphous chains, which worked with oriented crystallites to impart enhanced heat and hydrolysis resistance for the films. This work paves an avenue toward a wider range of applications for PGA materials and might be guidance for other semi-crystalline polymer blends.

Quantification of amorphous Si, Al, and Fe in palagonitic Mars analogs by chemical extraction and X-ray spectroscopy

Data collected by orbiters and landers have consistently shown abundant amorphous materials on Mars. Developing analytical techniques that target amorphous materials in terrestrial analogs can help determine the environmental conditions under which the amorphous assemblages on Mars were formed. However, the inherent short-range order, chemical heterogeneity, and nanoscale phase interactions create challenges in characterizing these phases. This study is aimed to overcome these challenges by combining chemical dissolution rate analysis with spectroscopy-based mass balance calculations (MBC) to characterize different pools of amorphous materials in terrestrial analogs. The differences in dissolution rates between rapidly dissolving Si, Al, and Fe amorphous materials and the slowly dissolving crystalline minerals were modeled to predict amorphous composition in five palagonitic samples from Hawaii (Mauna Kea) and four hyaloclastite tuffs from the subglacial volcanoes in southwest Iceland. These samples were selected as potential analogs because of their spectral and compositional resemblance to Mars's surface materials.The amorphous Si, Al, and Fe compositions from both sites varied based on the degree of aqueous alteration, grain size, and location. The amorphous fractions of the unconsolidated palagonitic analogs from Hawaii are composed of alteration products such as opal CT and ferrihydrite with minor amounts of unaltered basaltic glass. In contrast, the amorphous fractions of the cemented Icelandic samples were composed mainly of unaltered glass and mixed Fe(II, III) iron phases. Amorphous compositions of the loose Hawaiian and the consolidated Icelandic palagonites are comparable with the Martian modern and ancient aeolian materials respectively. The amorphous compositions in the Hawaiian sample, HWMK101, closely resemble the amorphous materials in Rocknest, a modern aeolian material from the sand shadow, and the Icelandic samples are comparable to the ancient aeolian materials from the Stimson formation. Our analysis indicates the presence of hydrated secondary alteration products such as ferrihydrite and allophane in Rocknest, and a mixture of reduced iron phases and silicate glass outlined with finer altered opaline silicates in Greenhorn. The chemical extraction analysis combined with MBC can be used to characterize amorphous phases in terrestrial analogs to better constrain the formation and characterization of the abundant amorphous materials on Mars.

Centrifugal Atomization of Glass-Forming Alloy Al86Ni8Y4.5La1.5.

Centrifugal atomization is a rapid solidification technique for producing metal powders. However, its wide application has been limited to the production of common metal powders and their corresponding alloys. Therefore, there is a lack of research on the production of novel materials such as metallic glasses using this technology. In this paper, aluminum-based glassy powders (Al86Ni8Y4.5La1.5) were produced by centrifugal atomization. The effects of disk speed, atomization gas, and particle size on the cooling rate and the final microstructure of the resulting powder were investigated. The powders were characterized using SEM and XRD, and the amorphous fractions of the atomized powder samples were quantified through DSC analysis. A theoretical model was developed to evaluate the thermal evolution of the atomized droplets and to calculate their cooling rate. The average cooling rate experienced by the centrifugally atomized powder was calculated to be approximately 7 × 105 Ks-1 for particle sizes of 32.5 μm atomized at 40,000 rpm in a helium atmosphere. Amorphous fractions from 60% to 70% were obtained in particles with sizes of up to 125 μm in the most favorable atomization conditions.

Structure of an amorphous calcium carbonate phase involved in the formation of Pinctada margaritifera shells.

Some mollusc shells are formed from an amorphous calcium carbonate (ACC) compound, which further transforms into a crystalline material. The transformation mechanism is not fully understood but is however crucial to develop bioinspired synthetic biomineralization strategies or accurate marine biomineral proxies for geoscience. The difficulty arises from the simultaneous presence of crystalline and amorphous compounds in the shell, which complicates the selective experimental characterization of the amorphous fraction. Here, we use nanobeam X-ray total scattering together with an approach to separate crystalline and amorphous scattering contributions to obtain the spatially resolved atomic pair distribution function (PDF). We resolve three distinct amorphous calcium carbonate compounds, present in the shell of Pinctada margaritifera and attributed to: interprismatic periostracum, young mineralizing units, and mature mineralizing units. From this, we extract accurate bond parameters by reverse Monte Carlo (RMC) modeling of the PDF. This shows that the three amorphous compounds differ mostly in their Ca-O nearest-neighbor atom pair distance. Further characterization with conventional spectroscopic techniques unveils the presence of Mg in the shell and shows Mg-calcite in the final, crystallized shell. In line with recent literature, we propose that the amorphous-to-crystal transition is mediated by the presence of Mg. The transition occurs through the decomposition of the initial Mg-rich precursor into a second Mg-poor ACC compound before forming a crystal.

Pressure-dependent microstructure evolution of Fe-based amorphous alloy powders via high-pressure gas atomization

The pressure-dependent microstructure evolution of the gas-atomized Fe-based amorphous alloy powders at gas pressures in the range from 5 MPa to 8 MPa has been studied experimentally and theoretically. The results showed that the amorphous fraction and nucleation mode of particles were sensitive to the gas pressure. Specifically, at 7 MPa, the particles presented a fully amorphous structure with the maximal size class of 45–75 µm, and preferred to nucleate heterogeneously within the sphere rather than on the surface. These interesting phenomena were associated with the difference of cooling rate within droplets, reflected by the gas-liquid relative velocity at the cooling and solidification stage in the atomization process. It was also indicated that the amorphous phase underwent three stages of reactions upon devitrification with the precipitations of {Fe, Ni} and Fe 2 P, α-Fe and M 23 B 6 , respectively. Finally, glass-forming features of powders at different pressures have been elucidated based upon a kinetics analysis of the cooling rates of droplets and the phase selection competition. This work provides beneficial guidance for the devisal of metallic glass powder with different microstructures via gas atomization. • The amorphous fraction and nucleation mode of particles were sensitive to the gas pressure. • The cooling histories of droplets with typical diameters were simulated in the atomization process. • At 7 MPa, the largest gas-liquid relative velocity resulted in the largest cooling rate for droplets. • The microstructure evolution and transition depend on the calculation of R c and cooling histories of droplets.

Temperature-Dependent Rotational Dipole Mobility and Devitrification of the Rigid Amorphous Fraction in Unpoled and Poled Biaxially Oriented Poly(vinylidene fluoride)

Temperature-Dependent Rotational Dipole Mobility and Devitrification of the Rigid Amorphous Fraction in Unpoled and Poled Biaxially Oriented Poly(vinylidene fluoride)

Efficient co-production of xylooligosaccharides and glucose from lignocelluloses by acid/pentanol pretreatment: Synergetic role of lignin removal and inhibitors

A novel technology for co-production of xylooligosaccharides (XOS) and glucose from Monterey pine sawdust and wheat straw was introduced using dilute acid (DA)/pentanol pretreatment. Effects of pretreatment severity (PS), lignin removal, and inhibitors with byproduct concentrations on XOS production were investigated. Optimal identified conditions (PS: 3.71; 170 °C, 45 min) resulted in maximum XOS of 48.65 % (pine sawdust) and 46.85 % (wheat straw), due to appropriate lignin removal (pine sawdust, 88.5 %; wheat straw, 89.7 %) and formation of small amounts of inhibitors and byproducts. Enzymatic hydrolysis of optimal pretreated solid residues yielded 88.65 % and 93.34 % glucose in pine sawdust and wheat straw, respectively. Biomass characterization revealed that DA/pentanol pretreatment enhanced porosity and pore size along with removal of amorphous fractions in both samples, thereby increasing cellulose accessibility and glucose yield. This study demonstrated lignin removal and low formation of inhibitors and byproducts, effectively enhancing XOS and glucose production from lignocellulosic biomass.

Effect of multiphase content on temperature‐dependent electrical conductivity in silicone rubber composites

AbstractTo investigate how the multiphase structures affect the electrical conductivity in semi‐crystalline polymer composites, herein, an accurate multiphase content calculation method is proposed and verified, which combines amorphous phase information in broadband dielectric spectroscopy and crystalline phase information in differential scanning calorimetry. Taken aluminium hydroxide (ATH) filled silicone rubber as an example, it is found that the rigid amorphous fraction (RAF) corresponding to the chains constrained by crystals (RAFcry) is not linearly increased with crystalline fraction (CF). Compared to non‐isothermal crystallisation, RAF caused by ATH/silicone rubber interface (RAFint) after isothermal crystallisation at 213 K changes little, while mobile amorphous fraction and RAFcry is attenuated and CF is strengthened. Based on the calculated structures of ATH filled silicone rubber, activation energy of conductivities during cooling is dominated by the thermal transition for conductive ions and shortened distance among the conductive ions through shrunk volumes of the amorphous phase. Our findings deepen the understanding of multiphase content in semi‐crystalline polymer composites and its relationship with electrical conductivity, which can be applied in manipulating electrical performance of semi‐crystalline polymers by fillers.

Large carbon dioxide adsorption in ZTC at medium pressure: Effects of surface functionalization

Zeolite-Template-Carbon is a new class of porous ordered carbon-based materials synthetized using 3-D zeolites as a template, then for these features have been investigated CO2 capture. This work presents the synthesis and characterization of ZTC on beta-type zeolite with a tuning-surface properties procedure resulting from different post-synthesis strategies aimed to tune the surface O-containing functional groups. It shows how a suitable micropore size distribution, a high specific surface area and a pore wall functionalization could maximize the reversible CO2 adsorption. Structural, chemical and morphological characterization has been obtained by X-ray diffraction, Thermo-gravimetric analysis, Raman/FT-IR spectroscopy, Branauer-Emmett-Teller analysis and scanning electron microscopy, while adsorption properties were investigated with Sievert's-type apparatus. XRD patters showed good replica of 3-D zeolite frameworks without presence of graphene and FT-IR spectroscopy indicated the presence of different carbon-oxygen functional groups. Adsorption measurements, at room temperature and pressure range 0–15 bar, showed a reversible CO2 uptake of 76.5 wt%. Furthermore, using deconvolution approach, a deep Raman spectroscopy analysis allowed us to assess the change in the structural order and in oxygen atomic coordination induced by post-synthesis treatment in correlation with the adsorption capacity. Post-synthesis treatments induced structure modification elucidated by evidence of an increased order of the porous structure and variation of the amorphous carbon fraction.

Positron annihilation lifetime spectroscopic analysis of Nafion and graft‐type polymer electrolyte membranes for fuel cell application

AbstractThe subnano free‐volume hole features of Nafion‐212 and poly(styrene sulfonic acid) grafted poly(ethylene‐co‐tetrafluoroethylene) polymer electrolyte membranes are investigated by using the positron annihilation lifetime spectroscopic analyses with three‐ and four‐component models (i.e., one‐ and two‐ortho‐positronium [o‐Ps] components). The four‐component model provides a more adequate description of free‐volume hole features for both membranes, in which the longer o‐Ps lifetime is assigned to the larger free volume hole sizes in the mobile side chains, while the shorter o‐Ps lifetime is associated with the smaller free volume hole sizes within the backbones (the rigid amorphous fractions and the crystalline‐amorphous interfaces) and the interfaces between the main chains and the side chains. The o‐Ps annihilation is found to occur primarily in the side chains. The subnano volume hole features revealed by the positron annihilation lifetime (PAL) spectra suggest the primary water uptake and conductance in the side chains and the possible presence of water molecules in the rigid amorphous fractions and the interfaces. Note that the three‐component model as usually reported in the literature may underestimate the lifetime and intensity of ortho‐positronium annihilation.

New opportunities for sustainable bioplastic development: Tailorable polymorphic and three-phase crystallization of stereocomplex polylactide by layered double hydroxide

Stereocomplexation between enantiomeric poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) is a promising sustainable approach and gaining momentum to overcome the shortcomings of polylactide (PLA) for its use as a replacement for fossil-based plastics. Filler addition in tailoring the crystallization of stereocomplex PLA (SC-PLA) has attracted extensive attention; however, research has primarily focused on the heterogeneous nucleation effect of filler. The impact of filler on the chain behavior of SC-PLA during crystallization has not been exclusively discussed, and the rigid amorphous fraction (RAF) development remains unknown. In this study, the crystallization of PLLA/PDLA blends was modified by low loading of layered double hydroxide (LDH) (≤ 1 wt%) with the proposed local effect of such filler, and additional RAF development was incurred. In the early stage of crystallization, LDH facilitates the pairing of PLLA and PDLA and arrests the ordered SC pairs during the dynamic balance between the separation and pairing of racemic segments. This explains the severely suppressed homochiral (HC) crystallization, promoted SC crystallization, and additional RAF formation driven by the nucleation-induced chain ordering. This work, for the first time, highlights the role of LDH in creating SC-PLA with tailorable polymorphism and RAF, where the mechanism can be extended to other filler-type nucleator systems.

Resistance Drift Convergence and Inversion in Amorphous Phase Change Materials

AbstractPhase change materials (PCMs) are key to the development of artificial intelligence technologies such as high‐density memories and neuromorphic computing, thanks to their ability for multi‐level data storage through stepwise resistive encoding. Individual resistance levels are realized by adjusting the crystalline and amorphous volume fraction of the memory cell. However, the amorphous phase exhibits a drift in resistance over time that has so far hindered the commercial implementation of multi‐level storage schemes. In this study, the underlying physical process of resistance drift with the goal of modeling is elucidated that will help minimize and potentially overcome drift in PCM memory devices. Clear evidence is provided that the resistance drift is dominated by glass dynamics. Resistivity convergence and drift inversion for the amorphous chalcogenide Ge15Te85 and the PCM Ge3Sb6Te5 are experimentally demonstrated and these changes are successfully predicted with a glass dynamics model. This new insight into the resistance drift process provides tools for the development of advanced PCM devices.

Rapid solidification of simulated radioactive contaminated soil by continuous microwave sintering: Effects of temperature and doping level

In order to find a rapid, efficient, safe and reliable treatment technology for radioactive contaminated soil, the microwave sintering method was used to sinter the simulated radioactive contaminated soil with different CeO2 content at 1100 °C, 1200 °C and 1300 °C for 30 min to achieve vitrification. The phase, microstructure, morphology, mechanical properties, and chemical durability of the sintered samples were investigated. XRD and SEM-EDS results showed that Ce4+ did not participate in the formation of the glass network, but was fixed in the glass network structure. The amorphous fraction of the samples sintered at 1300 °C can reach up to 98%. EDS results showed that the element distribution was uniform. In addition, the density and hardness values of the sintered matrices were in the range of 1.875–2.543 g/cm3 and 6.667–7.112 GPa, respectively. Our results show that the density and hardness values are related to the sintering temperature and CeO2 content. The normalized leaching rate of Ce in samples reached 10−7 g/(m2·d) after 28 d.

Morpho-Structural Investigations and Carbon Nanoclustering Effects in Cr-Al-C Intermetallic Alloys.

Intermetallic Cr-Al-C thin films from the 211 class of MAX phases were fabricated via ion beam deposition and structural investigations were undertaken to obtain information about morpho-structural effects propelled by carbon excess in the stoichiometry of the films. In order to promote the occurrence of the Cr2AlC MAX phase, the stoichiometric thin films were subsequently annealed at two temperature values: 650 °C and 700 °C in UHV conditions for 30 min. The morpho-structural effects in both as-deposited and annealed films were monitored using scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. XRD analysis showed that the as-deposited sample was almost completely crystallized in the hexagonal Cr2AlC structure, with a remaining amorphous fraction of about 17%, most probably rich in carbon. Raman analysis allowed the identification of three spectral regions, two of them encompassing the Raman optical modes belonging to the Cr2AlC 211 MAX phase, while the third one gave strong evidence of highly intense and large D- and G-bands of carbon. Structural parameters such as the crystal lattice parameters as well as the volume of the crystal unit cell were found to decrease upon annealing; this decrease is attributed to the grain growth. The average crystallite dimension was proven to increase after annealing, while the lattice micro-strain lowered to approximately 63% in the annealed thin film compared to the as-deposited one. Well-formed and intense Raman peaks attributed to D- and G-bands of carbon were also observed and, corroborated with the structural data, seemed to indicate an overall increased level of crystal ordering as well as potential carbon nanoclustering after thermal treatments with thin Cr2AlC films. This observed phenomenon concords with previously documented reports on ab initio modelling of possible Cr2AlC structures with carbon excess.