A single particle Hamiltonian for electro-magnetic properties of graphene nanoribbons

Graphene zigzag edges are known to show the spin polarized ferromagnetic states, which are well described by the mean field treatment of Hubbard model. The parameter of onsite Coulomb interaction U is estimated to be comparable to the kinetic hopping parameter t so as to fit the electronic band structures obtained by the spin–polarized density functional theory (DFT). In this paper, we propose a simple way to transfer the electronic band structures obtained by DFT onto the mean-field Hubbard Hamiltonian by adopting site-dependent U parameter, which is taken as the decaying function from the edge. This approach is applicable to both anti-ferromagnetic and ferromagnetic states between two edges of graphene nanoribbons and will serve to perform the further large-scale simulation of electro-magnetic transport properties of graphene-based nanodevices.     

A theoretical investigation and synthesis of layered ternary carbide system U-Al-C

This article presents the theoretical predictions and experimental synthesis of the uranium-containing layered ternary carbides UAl₃C₃ and U₂Al₃C₄. The electronic structures and mechanical properties of UAl₃C₃ and U₂Al₃C₄ have been investigated by first-principles computations. The chemical bonding characteristics of UAl₃C₃ and U₂Al₃C₄ have been compared to those of nanolaminated ternary carbides and strong interactions have been found between uranium and carbon atoms. Furthermore, UAl₃C₃ and U₂Al₃C₄ powders have been fabricated by the solid state reaction method and the crystal structures of UAl₃C₃ and U₂Al₃C₄ have been determined by X-ray diffraction (XRD). Based on the present results, the layered ternary carbides UAl₃C₃ and U₂Al₃C₄ may provide an expansion of the derivative MAX phase (layered compounds) family as well as a new option for nuclear fuels.     

Understanding the adsorption of calcium sulfate on tetracalcium aluminoferrite and tricalcium aluminate surfaces

Calcium sulfate (CaSO₄), an essential retarder in cement, retards the hydration of tricalcium aluminate (C₃A) and tetracalcium aluminoferrite (C₄AF) phases. However, its retarding mechanism remains unclear. This paper focused on the adsorption of CaSO_4f on C₄AF and C₃A surfaces based on isothermal calorimetry, the measurement of the ionic concentrations in a diluted system, and density functional theory to enhance the understanding of the retardation mechanism. The results showed that the retarding effect of CaSO₄ on C₄AF was stronger than that on C₃A due to the slower CaSO₄ consumption rate, lower driving force for CaSO₄ adsorption, and surface coverage of Fe(OH)₃ gel. The adsorption of CaSO₄ hindered Ca dissolution more markedly on C₄AF than C₃A, which was pronounced on Fe-free C₄AF surfaces. The adsorption of CaSO₄ weakened the affinity of water on C₄AF and C₃A surfaces, lowering the driving force for H₂O adsorption. The adsorption of H₂O and CaSO₄ promoted the dissolution of Al on the [AlO₆] octahedral surface of C₄AF, which may be responsible for the maintenance of a higher Al concentration in the solution. Based on the above results, the adsorption of CaSO₄ on initial C₄AF and C₃A hydration was explained.     

Early performances of cement paste in the presence of triethanolamine: Rheology,setting and microstructural development

The effect of triethanolamine (TEA) at various dosages on the early performance of cement paste was systematically evaluated through the techniques of rheological measurements, penetration tests, and ultrasonic pulse velocity. The correlation of early performance to the chemical hydration process was analyzed by calorimetry, zeta potential, in situ XRD, and pore solution analysis. It is found that the effect of TEA on the early performance of cement paste is strongly dependent on its dosage. With the TEA dosage below 0.1 wt%, the setting and microstructural development of cement paste are retarded. Meanwhile, the yield stress of fresh paste is decreased due to the increasing zeta potential of cement grains. The promoted formation of ettringite (AFt) and monosulfate (AFm) caused by TEA decreases the rheological retention ability. At dosages ≥0.2 wt%, the reaction of aluminate-containing phases is greatly accelerated and a flash setting is observed. Besides, the importance of ferric phase on the reaction of cement with TEA is highlighted. At a low dosage, TEA prefers to accelerate the dissolution of tetracalcium aluminoferrite (C₄AF) first and increases the [Fe] in the pore solution of cement paste. In cement without C₄AF, the retardation of TEA on silicate phase hydration is significantly alleviated.     

First-principles calculation of mechanical properties and electronic structures of W4Co4−nXnB4 (X = V,Mn, Fe,Ni; n = 0,1)

The first-principles calculation is performed to explore the mechanical properties and electronic structures of transition elements X (X = V, Mn, Fe, Ni) doped WCoB (tungsten cobalt boron), which has shown high oxidation resistance and melting point under high pressure. The energy analysis indicates that the high pressure leads to the lower lattice constants and less stable structures. The deviation of cohesive energy and formation enthalpy between doped and undoped structures indicates that W₄Co₃FeB₄ and W₄Co₃NiB₄ have similar stability. The high pressure contributes to the increasing of elastic, shear, and bulk moduli, which indicates the increase of covalence. The increase of Poisson's ratio, B/G ratio, and anisotropy index A^U indicates the higher ductility and higher anisotropy under high pressure. Based on bulk modulus and shear modulus, the hardness of W₄Co₄B₄, W₄Co₃FeB₄, and W₄Co₃NiB₄ increases under high pressure, which consists of the variation of electronic structures. The density of states (DOS) and partial DOS analysis indicate that the high pressure leads to lower density around Fermi level and higher hybridization. W₄Co₄B₄, W₄Co₃FeB₄, and W₄Co₃NiB₄ show similar variation of mechanical properties, which is determined by the similar atom properties of Co, Fe, and Ni. Similarly, W₄Co₃VB₄ and W₄Co₃MnB₄ also imply similar variation of mechanical properties and electronic structures.     

Theoretical Study of Cubic and Orthorhombic Nd1–xSrxMnO3 as a Potential Solid Oxide Fuel Cell Cathode

In this paper, the atomic and electronic structures of cubic and orthorhombic Nd_1–xSr_xMnO₃ are investigated using the projector augmented‐wave (PAW) methods within the spin‐polarization generalized gradient approximation (GGA+U), where U is on‐site Coulomb interaction correction. The optimized structure parameters of both cubic and orthorhombic bulk phases are obtained. The difference between the AFM and FM structures for NdMnO₃ is very small, indicating a small magneto‐elastic effect. In Nd_1–xSr_xMnO₃, the pseudo‐cubic lattice constant decreases on increasing Sr doping due to the oxidation of Mn³⁺ cations to the smaller Mn⁴⁺ cations. The result of the total density of states shows the majority spin without gap and the minority spin with 4.8 × 10⁻¹⁹ J gap, indicating a half‐metallic ground state for NdMnO₃ in GGA+U treatment. The Bader effective charges in both cubic and orthorhombic phases are analyzed. The oxygen vacancy formation energy of Nd_1–xSr_xMnO₃ becomes smaller as the Sr doping concentration increases. The oxygen vacancies can be formed more easily on Nd_0.5Sr_0.5MnO₃ than other systems. These results suggest that Nd_0.5Sr_0.5MnO₃ could be a promising candidate for application in SOFC.     

The hydration of synthetic brownmillerite in presence of low Ca-sulfate content and calcite monitored by quantitative in-situ-XRD and heat flow calorimetry

We present here experimental data of the hydration of C₄AF in a Portland cement environment with low sulfate content. Calorimetric data indicate a distinct two-step main reaction of C₄AF, which was successfully deconvoluted by laboratory scale in-situ-XRD in combination with the G-factor method and a quantitative approach by mass balance estimation with normalized peak areas of hydrate phases. The presented data here suggest a rapid unhindered dissolution of C₄AF after depletion of sulfate in the pore solution, which is then rapidly leading to an oversaturation with respect to sulfate-AFm-16 (C₄A$H₁₆). Since the sulfate content of the mixture is low, a second acceleration of C₄AF dissolution, due to precipitation of hydroxy-AFm-19 (C₄AH₁₉) was observable after the decline in ettringite (C₆A$₃H₃₂) content and the subsequent conversion to sulfate-AFm-16 (C₄A$H₁₆).     

Influence of ordered carbon‐vacancy networks on the electronic structures and elastic properties of Nb4AlC3−x

Carbon‐vacancy‐bearing Nb₄AlC_3?x has the best high‐temperature mechanical robustness among MAX phases. The existing form of the vacancies has been long overlooked. Recently, the vacancies in Nb₄AlC_3?x have been identified to be ordered, establishing an ordered compound Nb₁₂Al₃C₈. Here, the spatial distribution of the ordered vacancies and their influences on bonding characteristics and elastic properties are unraveled by thoroughly comparing Nb₁₂Al₃C₈ and vacancy‐free Nb₄AlC₃. In Nb₁₂Al₃C₈, the carbon vacancies break only relatively weak Nb–C bonds and form ordered equilateral triangular carbon‐vacancy networks (OETCVNs) to maximize the bond strengthening effect. The networks slightly shift partial and total density of states toward the Fermi energy level, and bring about a feature of “de‐metallization”. Meanwhile, the presence of OETCVNs results in the softening of elastic modulus, decreasing of the anisotropy of Young's modulus, yet increasing that of shear modulus. These results shed lights on the carbon‐vacancy ordering behavior of MAX phases, and provide an opportunity to tailor their electronic structures and elastic properties through defect engineering.     

A sorption balance study of water vapour sorption on anhydrous cement minerals and cement constituents

The phenomenon of water vapour sorption by powdered cement constituents exposed to different relative humidities and temperatures was studied. The individual clinker phases C₃S, C₂S, C₃A, C₄AF, calcium sulfates and CaO were tested. Using a water sorption balance, the amount of chemically and physically sorbed water per unit of surface area of the powders and the relative humidity at which water sorption starts to occur on the phases were determined. Various cement clinker phases prehydrate very differently. CaO and C₃A were found to be most reactive towards water vapour whereas the silicates react less. CaO starts to sorb water at very low RHs and binds it chemically. Beginning at 55% RH, orthorhombic C₃A also sorbs significant amounts of water and binds it chemically and physically. Water sorption of C₃S and C₂S only begins at 74% RH, and the amount of water sorbed is minor. Calcium sulfates sorb water predominantly physically.     

Synthesis and physical properties of (Zr1−x,Tix)3AlC2 MAX phases

MAX phase solid solutions physical and mechanical properties may be tuned via changes in composition, giving them a range of possible technical applications. In the present study, we extend the MAX phase family by synthesizing (Zr_1?xTi_x)₃AlC₂ quaternary MAX phases and investigating their mechanical properties using density functional theory (DFT). The experimentally determined lattice parameters are in good agreement with the lattice parameters derived by DFT and deviate <0.5% from Vegard's law. Ti₃AlC₂ has a higher Vickers hardness as compared to Zr₃AlC₂, in agreement with the available experimental data.     

Mössbauer and X-ray investigations of some portland cements

The Mössbauer spectroscopic and x-ray diffraction investigations have been carried out on a variety of ordinary portland, portland pozzolanic, portland slag and sulphate resisting portland cements, using dry as well as hydrated samples. The discussion of the Mössbauer parameters shows that Fe atoms occupying distorted octahedral and tetrahedral sites in the dry cements are hydrated to form ferrite monosulphate without producing Fe(OH)₃ and its gel; hydration of the slag cement proceeds much faster than other cements; and that the composition of the iron-bearing phase in the sulphate resisting portland cement, studied in detail, is close to C₄AF.     

Characterization by solid-state NMR and selective dissolution techniques of anhydrous and hydrated CEM V cement pastes

The long term behaviour of cement based materials is strongly dependent on the paste microstructure and also on the internal chemistry. A CEM V blended cement containing pulverised fly ash (PFA) and blastfurnace slag (BFS) has been studied in order to understand hydration processes which influence the paste microstructure. Solid-state NMR spectroscopy with complementary X-ray diffraction analysis and selective dissolution techniques have been used for the characterization of the various phases (C₃S, C₂S, C₃A and C₄AF) of the clinker and additives and then for estimation of the degree of hydration of these same phases. Their quantification after simulation of experimental ²⁹Si and ²⁷Al MAS NMR spectra has allowed us to follow the hydration of recent (28 days) and old (10 years) samples that constitutes a basis of experimental data for the prediction of hydration model.     

Realization of diversity in physical properties of Zr2Se(B1-xSex) MAX phases through DFT approach

The discovery of a series of MAX phases, Zr₂Se(B_1-xSe_x), with Se at both A- and X-sites, drives a new chemical diversity to the MAX family. Here, we employed the density functional theory (DFT) approach to realize the diversity in physical properties of Zr₂Se(B_1-xSe_x). All compositions of Zr₂Se(B_1-xSe_x) are mechanically stable and the dynamical stability of the end member Zr₂SeSe is confirmed. The elastic constant C₃₃ and bulk moduli B show a decrease almost monotonically with Se-content x while other constants and moduli change irregularly. All elastic constants and moduli except C₁₂ and C₁₃ are highest for the end member Zr₂SeB. Additionally, the Vickers hardness, Debye temperature, minimum thermal conductivity, and lattice thermal conductivity are highest for Zr₂SeB. The increase of Se-content x at X-site reduces most of the properties of Zr₂Se(B_1-xSe_x). The electronic band structures change drastically with increasing Se-content x. This diversity in electronic band structures is mainly the reason for the diversity in physical properties of Zr₂Se(B_1-xSe_x). All compositions of Zr₂Se(B_1-xSe_x) have the potential to be thermal barrier coating materials, and Zr₂SeB has the potential to be etched into 2D MXene, Zr₂B.     

Incorporation of trace elements in Portland cement clinker: Thresholds limits for Cu, Ni, Sn or Zn

This paper aims at defining precisely, the threshold limits for several trace elements (Cu, Ni, Sn or Zn) which correspond to the maximum amount that could be incorporated into a standard clinker whilst reaching the limit of solid solution of its four major phases (C₃S, C₂S, C₃A and C₄AF). These threshold limits were investigated through laboratory synthesised clinkers that were mainly studied by X-ray Diffraction and Scanning Electron Microscopy. The reference clinker was close to a typical Portland clinker (65% C₃S, 18% C₂S, 8% C₃A and 8% C₄AF). The threshold limits for Cu, Ni, Zn and Sn are quite high with respect to the current contents in clinker and were respectively equal to 0.35, 0.5, 0.7 and 1 wt.%. It appeared that beyond the defined threshold limits, trace elements had different behaviours. Ni was associated with Mg as a magnesium nickel oxide (MgNiO₂) and Sn reacted with lime to form a calcium stannate (Ca₂SnO₄). Cu changed the crystallisation process and affected therefore the formation of C₃S. Indeed a high content of Cu in clinker led to the decomposition of C₃S into C₂S and of free lime. Zn, in turn, affected the formation of C₃A. Ca₆Zn₃Al₄O₁₅ was formed whilst a tremendous reduction of C₃A content was identified. The reactivity of cements made with the clinkers at the threshold limits was followed by calorimetry and compressive strength measurements on cement paste. The results revealed that the doped cements were at least as reactive as the reference cement.     

Impact of triethanolamine on the sulfate balance of Portland cements with mixed sulfate carriers

The differences between the hydration of Portland cements with single and with mixed sulfate carriers in the presence of triethanolamine (TEA) were investigated, and possible mechanisms were proposed. Without TEA, cements with different types of sulfate carriers (gypsum, hemihydrate, anhydrite, and mixture of these) have a comparable hydration process at the same molar amount of calcium sulfate. At a TEA dosage of 0.5 wt.%, the sample with a mixture of three sulfate carriers shows substantially stronger retardation of the C₃S (This publication uses the cement chemist notation: C₃S = Ca₃SiO₅, C₂S = Ca₂SiO₄, C₃A = Ca₃Al₂O₆, C₄AF = Ca₂(Al, Fe)₂O₅.) hydration than the cements with only one of these sulfate carriers, which is likely caused by the rapid formation of ettringite and the fast depletion of all sulfate carriers. These effects indicate that TEA influences the balance of sulfate carriers with aluminate-containing clinker phases. On the one hand, TEA can disturb the original sulfate balance due to the accelerated dissolution of aluminate-containing clinker phases, especially C₄AF. On the other hand, these effects are closely related to the types and amounts of the sulfate carriers in the cement. A higher amount of sulfate carriers can minimize the TEA-related retardation of the C₃S hydration, and hemihydrate shows the strongest impact at the same calcium sulfate quantity.     

First-principles investigation of elastic and electronic properties of double transition metal carbide MXenes

We use first-principles-based density functional theory (DFT) calculations to investigate the structural, elastic, and electronic properties of various pristine and oxygen (O)-functionalized double transition metal (DTM) MXenes with general formulas of M₂′M′′C₂ and M₂′M′′C₂O₂, where M′ = Mo, Cr and M′′ = Ti, V, Nb, Ta. The dynamic stability of the DTM MXenes are assessed and elastic stiffness constants (C_ij) are used to investigate the mechanical stability and properties of the compositions. The calculated elastic properties of the pristine Mo-based MXenes are found to be superior compared to Cr-based compounds. Furthermore, the O-functionalized MXenes exhibit improved in-plane elastic constants, Young's moduli, and shear moduli compared to their pristine counterpart. We observe that the hybridization of the energy states results in stronger covalent interactions as such increased elastic properties for the M₂′M′′C₂O₂ MXenes. Ashby plot clearly demonstrates superior materials properties of O-functionalized Mo-based DTM MXenes compared to other commonly known two-dimensional materials. All the MXenes exhibit metallic character evident from the density of states (DOS) calculations. Additionally, the work functions are studied and the calculated values are higher in the case of O-functionalized MXenes. Overall, this work will be a guide for future investigations on the mechanical properties of DTM MXenes for their targeted applications in structural nanocomposites.     

The work of Powers and Brownyard revisited: Part 2

Powers and Brownyard [T.C. Powers, T.L. Brownyard, Studies of the physical properties of hardened Portland cement paste, Bull. 22, Res. Lab. of Portland Cement Association, Skokie, IL, U.S., 1948 reprinted from J. Am. Concr. Inst. (Proc.), 43, 1947, pp. 101-132, pp. 249-336, pp. 469-505, pp. 549-602, pp. 669-712, pp. 845-880, pp. 933-992. [1]] were the first to systematically investigate the reaction of cement and water and the composition of cement paste. In Part I to this paper, their work was recapitulated (Brouwers [H.J.H. Brouwers, The work of Powers and Brownyard revisited: Part 1, accepted for publication in Cem. Concr. Res. 34 (2004) 1697-1716 [2]]). Here, it will be demonstrated that their water retention data also enables the study of the molar reactions of the aluminate (C₃A and C₄AF) and sulphate phases. It follows that the C₄AF most likely reacts with the C₃S and/or C₂S to form a Si containing hydrogarnet and portlandite. The remaining calcium silicates react to C-S-H (C_1.7SH_3.2 when saturated) and CH, as proposed in Part I [H.J.H. Brouwers, The work of Powers and Brownyard revisited: Part 1, accepted for publication in Cem. Concr. Res. 34 (2004) 1697-1716 [2]].The CS¯ seems to react exclusively with the C₃A. In case of carbonation, both phases react to hemi-carbonate, mono-sulphate, ettringite and tetra calcium aluminate hydrate. The concept “degree of carbonation” is introduced to quantify the fraction of mono-sulphate that is carbonated. This enables the quantification of all four hydration products, which represents a principal innovation. Subsequently, using the molar reactions and known specific volumes of the crystalline hydration products, the specific volumes of non-evaporable water (ν_n) and gel water (ν_g) are determined. These values are in line with the values suggested by Powers and Brownyard [T.C. Powers, T.L. Brownyard, Studies of the physical properties of hardened Portland cement paste, Bull. 22, Res. Lab. of Portland Cement Association, Skokie, IL, U.S., reprinted from J. Am. Concrete Inst. (Proc.), 43, 1947, pp. 101-132, pp. 249-336, pp. 469-505, pp. 549-602, pp. 669-712, pp. 845-880, pp. 933-992. [1]], which were based on their shrinkage data, implying a successful coupling of the molar reactions and their original paste model.     

Early hydration of cement constituents with organic admixtures

The hydration of C₂S, C₃S, C₃A, C₄AF and type 1 portlant cement in the presence of calcium lignosulfonate and salicylic acid was studied at a high(20/1) water-cement ratio. The effect of these admixtures on the development, microstructure and surface area of the hydration products was investigated.     

Raman Spectroscopy of Anhydrous and Hydrated Calcium Aluminates and Sulfoaluminates

Recent investigations have revealed the great potential of Raman spectroscopy for the characterization of clinker minerals and commercial Portland cements. The usefulness of this technique for the identification of anhydrous, hydrated, and carbonated phases in cement‐based materials has been demonstrated. In the present work, the application of micro‐Raman spectroscopy for the characterization of the main clinker phases of calcium aluminate cements and calcium sulfoaluminate cement is explored. The main stable hydrated phases as well as several important carbonated phases are investigated. Raman measurements on the following phases are reported: (i) pure, unhydrated phases: CA, C₁₂A₇, CA₂, C₂AS, cubic‐C₃A, C₄AF, and C₄A₃; (ii) hydrated phases: ettringite, monosulfoaluminate, and hydrogarnet (C₃AH₆); (iii) carboaluminate phases: hemicarboaluminate and monocarboaluminate. The present results, which are discussed in terms of the internal vibrational modes of the aluminate, carbonate, and sulfate molecular groups as well as stretching O–H vibrations, show the ability of Raman spectroscopy to identify the main hydrated and unhydrated phases in the aluminate and sulfoaluminate cements. The Raman spectra obtained in this work provide an extended database to the existing data published in the literature.     

Adsorption characteristics of superplasticizers on cement component minerals

Adsorption characteristics of various superplasticizers on portland cement component minerals were investigated. Adsorption isotherms of various types of superplasticizers and ζ-potentials of cement component minerals at the maximum adsorption of the superplasticizers were measured. The value of the adsorption isotherm was calculated from the amount of the superplasticizer adsorbed on a cement component mineral in an equilibrated solution. The maximum amounts of adsorption and the adsorption isotherms varied with types of component mineral and superplasticizer. For all types of superplasticizers, a larger amount of superplasticizer was adsorbed on C₃A and C₄AF than C₃S and C₂S. However, the equilibrated concentration of each superplasticizer at the maximum adsorption was not influenced by types of superplasticizer. Without superplasticizer, C₃S and C₂S had negative ζ-potential. On the contrary, C₃A and C₄AF had positive ζ-potential. Therefore, accelerated coagulation of cement particles might occur due to their electrostatic potentials that are opposite each other. However, all component minerals of cement had negative ζ-potential when they were mixed with any superplasticizer. Fluidity of fresh cement paste is improved due to electrostatic repulsion acting between particles.