Misfit strain–film thickness phase diagrams and related electromechanical properties of epitaxial ultra-thin lead zirconate titanate films

The phase stability of ultra-thin (0 0 1) oriented ferroelectric PbZr_1–xTi_xO₃ (PZT) epitaxial thin films as a function of the film composition, film thickness, and the misfit strain is analyzed using a non-linear Landau–Ginzburg–Devonshire thermodynamic model taking into account the electrical and mechanical boundary conditions. The theoretical formalism incorporates the role of the depolarization field as well as the possibility of the relaxation of in-plane strains via the formation of microstructural features such as misfit dislocations at the growth temperature and ferroelastic polydomain patterns below the paraelectric–ferroelectric phase transformation temperature. Film thickness–misfit strain phase diagrams are developed for PZT films with four different compositions (x = 1, 0.9, 0.8 and 0.7) as a function of the film thickness. The results show that the so-called rotational r-phase appears in a very narrow range of misfit strain and thickness of the film. Furthermore, the in-plane and out-of-plane dielectric permittivities ε₁₁ and ε₃₃, as well as the out-of-plane piezoelectric coefficients d₃₃ for the PZT thin films, are computed as a function of misfit strain, taking into account substrate-induced clamping. The model reveals that previously predicted ultrahigh piezoelectric coefficients due to misfit-strain-induced phase transitions are practically achievable only in an extremely narrow range of film thickness, composition and misfit strain parameter space. We also show that the dielectric and piezoelectric properties of epitaxial ferroelectric films can be tailored through strain engineering and microstructural optimization.     

High-temperature thermoelectric properties of Sr2RuYO6 and Sr2RuErO6 double perovskites influenced by structure and microstructure

The high-temperature thermoelectric properties of Sr₂RuYO₆ and Sr₂RuErO₆ double perovskites were evaluated and reported for the first time. These compounds show high Seebeck coefficients not only at room temperature, but also at high temperature (for Sr₂RuYO₆, S_RT ≈ ?475 μV K^?1 and S_1200K ≈ ?250 μV K^?1; Sr₂RuErO₆, S_RT ≈ ?400 μV K^?1 and S_1200K ≈ ?250 μV K^?1). The n-type semiconducting behaviour dominates the resistivity values. Both compounds crystallize in a monoclinic unit cell (space group P2₁/n). The lattice parameters are a = 5.7761(2), b = 5.7804(1), c = 8.1689(1), α = γ = 90° and β = 90.2087(8)° for the Sr₂RuYO₆, and a = 5.7760(1), b = 5.7722(0), c = 8.1544(4), α = γ = 90° and β = 90.2099(7)° for Sr₂RuErO₆. The unit cell can be described approximately as √2a_p × √2a_p × 2a_p, where a_p is the unit cell parameter of the ideal cubic perovskite structure. High-resolution transmission electron microscopy shows an interesting three-dimensional micro-twin-domain texture where the c axis is placed in the three space directions. Structural transitions at high temperatures (T_t(Sr₂RuYO₆) ≈920 K and T_t(Sr₂RuErO₆) ≈890 K) are observed by specific heat measurement in both compounds, which are found to have a strong influence on the Seebeck coefficient and electrical conductivity.     

Roles of interface roughness and internal stress in magnetic anisotropy of CoPt/AlN multilayer films

Magnetic anisotropy of CoPt/AlN multilayer films has been studied by systematically varying the nominal thickness of CoPt layers, t_CoPt (1–10 nm), and the annealing temperature, T_a (300–500 °C). The as-deposited films show in-plane magnetic anisotropy in the full range of t_CoPt, whereas the annealed films show perpendicular magnetic anisotropy (PMA) within small t_CoPt but change to in-plane magnetic anisotropy when t_CoPt is over a certain thickness. The critical thickness for such anisotropic transformations increases as the T_a increases. The maximum PMA obtained in this work is 1.13 × 10⁷ erg cm^?3. The interface roughness was analyzed by cross-sectional high-resolution electron microscopy and X-ray reflectivity using an abrupt interface model with a Debye exponent shape. The internal stress was analyzed by X-ray diffraction using an equal biaxial stress model. The results show that the CoPt/AlN interface roughness decreases from 0.385 nm to 0.158 nm and the internal stress increases from ?2.36 GPa (compressive) to 1.73 GPa (tensile), as the T_a increases to 500 °C. The roles of the interface roughness and the internal stress in the magnetic anisotropy of CoPt/AlN multilayer films are studied.     

Nonpolar ZnO film growth and mechanism for anisotropic in-plane strain relaxation

Using high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction, we investigated the strain relaxation mechanisms for nonpolar (1 1 ?2 0) a-plane ZnO epitaxy on (1 ?1 0 2) r-plane sapphire, where the in-plane misfit ranges from ?1.5% for the [0 0 0 1]ZnO6[1 ?1 0 ?1]sapphire to ?18.3% for the [?1 1 0 0]ZnO6[?1 ?1 2 0]sapphire direction. For the large misfit [?1 1 0 0]ZnO direction the misfit strains are fully relaxed at the growth temperature, and only thermal misfit and defect strains, which cannot be relaxed fully by slip dislocations, remain on cooling. For the small misfit direction, lattice misfit is not fully relaxed at the growth temperature. As a result, additive unrelaxed lattice and thermal misfit and defect strains contribute to the measured strain. Our X-ray diffraction measurements of lattice parameters show that the anisotropic in-plane biaxial strain leads to a distortion of the hexagonal symmetry of the ZnO basal plane. Based on the anisotropic strain relaxation observed along the orthogonal in-plane [?1 1 0 0] and [0 0 0 1]ZnO stress directions and our HRTEM investigations of the interface, we show that the plastic relaxation occurring in the small misfit direction [0 0 0 1]ZnO by dislocation nucleation is incomplete. These results are consistent with the domain-matching paradigm of a complete strain relaxation for large misfits and a difficulty in relaxing the film strain for small misfits.     

Thermal expansion of the V5Si3 and T2 phases of the V–Si–B system investigated by high-temperature X-ray diffraction

The thermal expansion anisotropy of the V₅Si₃ and T₂-phase of the V–Si–B system were determined by high-temperature X-ray diffraction from 298 to 1273 K. Alloys with nominal compositions V_62.5Si_37.5 (V₅Si₃ phase) and V₆₃Si₁₂B₂₅ (T₂-phase) were prepared from high-purity materials through arc-melting followed by heat-treatment at 1873 K by 24 h, under argon atmosphere. The V₅Si₃ phase exhibits thermal expansion anisotropy equals to 1.3, with thermal expansion coefficients along the a and c-axis equal to 9.3 × 10^?6 K^?1 and 11.7 × 10^?6 K^?1, respectively. Similarly, the thermal expansion anisotropy value of the T₂-phase is 0.9 with thermal expansion coefficients equal to 8.8 × 10^?6 K^?1 and 8.3 × 10^?6 K^?1, along the a and c-axis respectively. Compared to other isostructural silicides of the 5:3 type and the Ti₅Si₃ phase, the V₅Si₃ phase presents lower thermal expansion anisotropy. The T₂-phase present in the V–Si–B system exhibits low thermal expansion anisotropy, as the T₂-phase of the Mo–Si–B, Nb–Si–B and W–Si–B systems.     

Misfit dislocations of anisotropic magnetoresistant Nd0.45Sr0.55MnO3 thin films grown on SrTiO3 (1 1 0) substrates

Nd_0.45Sr_0.55MnO₃ is an A-type antiferromagnetic manganite showing obvious angular-dependent magnetoresistance, which can be tuned by misfit strain. The misfit strain relaxation of Nd_0.45Sr_0.55MnO₃ thin films is of both fundamental and technical importance. In this paper, microstructures of epitaxial Nd_0.45Sr_0.55MnO₃ thin films grown on SrTiO₃ (1 1 0) substrates by pulsed laser deposition were investigated by means of (scanning) transmission electron microscopy. The Nd_0.45Sr_0.55MnO₃ thin films exhibit a two-layered structure: a continuous perovskite layer epitaxial grown on the substrate followed by epitaxially grown columnar nanostructures. An approximately periodic array of misfit dislocations is found along the interface with line directions of both 〈1 1 1〉 and [0 0 1]. High-resolution (scanning) transmission electron microscopy reveals that all the misfit dislocations possess a〈1 1 0〉-type Burgers vectors. A formation mechanism based on gliding or climbing of the dislocations is proposed to elucidate this novel misfit dislocation configuration. These misfit dislocations have complex effects on the strain relaxation and microstructure of the films, and thus their influence needs further consideration for heteroepitaxial perovskite thin film systems, especially for films grown on substrates with low-symmetry surfaces such as SrTiO₃ (1 1 0) and (1 1 1), which are attracting attention for their potentially new functions.     

Effects of quenching speeds on microstructure and magnetic properties of novel SmCo6.9Hf0.1(CNTs)0.05 melt-spun ribbons

By adding carbon nanotubes (CNTs) to SmCo_6.9Hf_0.1, novel SmCo_6.9Hf_0.1(CNTs)_0.05 as-cast alloy has been prepared, which consists of Sm(Co,Hf)₇ as the main phase, a small amount of SmCo₅ and a particle-like grain boundary phase Hf(CNTs). SmCo_6.9Hf_0.1(CNTs)_0.05 ribbons melt-spun at speeds of 10–50 m s^?1 have a single TbCu₇-type structure. Increasing the quenching speed can result in a decrease in ribbon thickness and grain boundary width. Meanwhile, the grain size tends to be smaller and the grain boundary phase tends to be more dispersed. A new Sm(Co,Hf)₇(CNTs)_x boundary phase may be formed in SmCo_6.9Hf_0.1(CNTs)_0.05 ribbons. Increasing the quenching speed can also enhance coercivity, remanence and remanence ratio. The ribbons melt-spun at a speed of 50 m s^?1 display the best magnetic properties: H_ci = 18.781 kOe, M_s2T = 76.87 emu g^?1, M_r = 66.79 emu g^?1 and M_r/M_s2T  = 0.87.     

Crystal structure and properties of U2Co3Al9

A new ternary compound with stoichiometry U₂Co₃Al₉ has been synthesized. It adopts the orthorhombic Y₂Co₃Ga₉-type structure (space group Cmcm, Z = 4, a = 12.824(2) Å, b = 7.515(1) Å, c = 9.249(2) Å). Measurements of dc- and ac-magnetic susceptibility, electrical resistivity, and magnetoresistivity on polycrystalline samples have been performed. The Curie–Weiss law is strictly followed, with θ_CW = −48 K and μ_eff = 3.2 μ_B. A small kink observed in the temperature dependence of the resistivity is attributed to a phase transition at T_t = 8 K. The magnetoresistivity was found to be negative at all temperatures examined below 45 K, with a sharp minimum at T_t = 8 K.     

Evolution of microstructural parameters and flow stresses toward limits in nickel deformed to ultra-high strains

A quantitative analysis of microstructure and strength as a function of strain is presented for polycrystalline nickel (99.5%) deformed by high-pressure torsion in the strain range 1–300 (ε_VM, von Mises strain). Typical lamellar structures consisting of extended boundaries and short interconnecting boundaries have been found, with additional features at large strains which are equiaxed regions, small equiaxed subgrains and deformation twins. The evolution of microstructure and microstructural parameters falls in stages: (i) the first stage at ε_VM = 1–12; (ii) a transition stage at ε_VM = 12–34; and (iii) a saturation stage at ε_VM ⩾ 34. A scaling analysis of spacing between boundaries shows a universal behavior up to ε_VM = 300, indicating that the predominant deformation mechanism is dislocation glide whereas twin formation is of minor importance. A clear link is observed between the evolution in structure and flow stress, which can guide the development of strong metals with a structural scale extending below 50–100 nm.     

A new phase in stoichiometric Cu6Sn5

The intermetallic compound Cu₆Sn₅ is a significant microstructural feature of many electronic devices where it is present at the solder–substrate interfaces. The time- and temperature-dependent thermomechanical properties of Cu₆Sn₅ are dependent on the nature and stability of its crystal structure, which has been shown to exist in at least four variants (η, η′, η⁶ and η⁸). This research details an additional newly identified monoclinic-based structure in directly alloyed stoichiometric Cu₆Sn₅ using variable-temperature synchrotron X-ray diffraction (XRD) and transmission electron microscopy. The phase is associated with a departure from the equilibrium temperature of the polymorphic monoclinic–hexagonal transformation temperature. The new monoclinic phase can be treated as a modulation of four η⁸-Cu₅Sn₄ unit cells plus one η′-Cu₆Sn₅ unit cell. It has been labeled as η⁴⁺¹ and has cell parameters of a = 92.241 Å, b = 7.311 Å, c = 9.880 Å and β = 118.95° determined from electron diffraction patterns. The XRD results could be fitted well to a Rietveld refinement using the new crystal parameters.     

Electric-field-induced phase-change behavior in (Bi0.5Na0.5)TiO3–BaTiO3–(K0.5Na0.5)NbO3: A combinatorial investigation

The electric-field-induced strain behavior in (1 ? x ? y)(Bi_0.5Na_0.5)TiO₃–xBaTiO₃–y(K_0.5Na_0.5)NbO₃ electroceramics has been studied using a combinatorial technique. A stoichiometrically graded sample was produced to contain compositions across the ternary phase diagram between the two end-member components of 0.93(Bi_0.5Na_0.5)TiO₃–0.07BaTiO₃ and 0.86(Bi_0.5Na_0.5)TiO₃–0.14(K_0.5Na_0.5)NbO₃. Both composition and structural information were measured simultaneously during the application of electric fields using secondary X-ray fluorescence and high-energy X-ray microdiffraction, respectively. An initial electric-field-induced distortion from the pseudo-cubic structure is seen across all compositions, while those with a greater concentration of BaTiO₃ also undergo an electric-field-induced phase transformation. The microstructural contribution to the macroscopic strain within the 0.93(Bi_0.5Na_0.5)TiO₃–0.07BaTiO₃ end member is quantified at a field strength of 5.5 kV mm^?1; 0.08% and 0.11% of the measured macroscopic strain of 0.4% is contributed by the induced ferroelastic domain texture and the volumetric strain associated with the electric-field-induced phase transformation, respectively.     

Phase transition behavior and electrical properties of [(K0.50Na0.50)1−xAgx](Nb1−xTax)O3 lead-free ceramics

The effect of AgTaO₃ on the electrical properties of (K_0.5Na_0.5)NbO₃ lead-free ceramics was systematically investigated, and the phase transition behavior of the ceramics was also studied in terms of high temperature X-ray diffraction. The experimental results show that Ag⁺ and Ta⁵⁺ ions diffuse into the (K_0.5Na_0.5)NbO₃ lattices to form a stable solid solution with orthorhombic structure, and also lead to the decrease in the orthorhombic to the tetragonal phase transition temperature and the Curie temperature. The 0.92(K_0.5Na_0.5)NbO₃–0.08AgTaO₃ ceramics exhibit optimum electrical properties (d₃₃ = 183 pC/N, k_p = 41%, T_c = 356 °C, T_o–t = 158 °C, ?_r ～ 683, and tan δ ～ 3.3%) and good thermal-depoling behavior. The piezoelectric properties of (1 ? x)(K_0.5Na_0.5)NbO₃–xAgTaO₃ ceramics are much superior to pure (K_0.5Na_0.5)NbO₃ ceramics. X-ray diffraction patterns for the 0.92(K_0.5Na_0.5)NbO₃–0.08AgTaO₃ ceramic at different temperatures indicated a pure perovskite phase with an orthorhombic structure at below 160 °C, a tetragonal structure at 160–350 °C, and a cubic structure at above 360 °C. As a result, the (1 ? x)(K_0.5Na_0.5)NbO₃–xAgTaO₃ ceramic is one of the promising candidate materials for lead-free piezoelectric ceramics.     

In situ TEM study of microplasticity and Bauschinger effect in nanocrystalline metals

In situ transmission electron microscopy straining experiments with concurrent macroscopic stress–strain measurements were performed to study the effect of microstructural heterogeneity on the deformation behavior of nanocrystalline metal films. In microstructurally heterogeneous gold films (mean grain size d_m = 70 nm) comprising randomly oriented grains, dislocation activity is confined to relatively larger grains, with smaller grains deforming elastically, even at applied strains approaching 1.2%. This extended microplasticity leads to build-up of internal stresses, inducing a large Bauschinger effect during unloading. Microstructurally heterogeneous aluminum films (d_m = 140 nm) also show similar behavior. In contrast, microstructurally homogeneous aluminum films comprising mainly two grain families, both favorably oriented for dislocation glide, show limited microplastic deformation and minimal Bauschinger effect despite having a comparable mean grain size (d_m = 120 nm). A simple model is proposed to describe these observations. Overall, our results emphasize the need to consider both microstructural size and heterogeneity in modeling the mechanical behavior of nanocrystalline metals.     

Growth kinetics of oxide films at the polyaniline/mild steel interface

The growth rate of oxide film formed at the polyaniline/mild steel interface was investigated by open circuit potential (OCP) measurements. The OCP of polyaniline base (EB) coated electrode displayed three feature changes during immersion: t₁ ? t₂, t₂ ? t₃ and >t₃, and the oxide film grew mainly in t₂ ? t₃ time range. The oxide film growth followed a direct logarithm law, the growth rate decreased with increasing NaCl concentration and temperature. With increasing pH, the growth rate decreased first and then increased. The apparent activation energy of oxide film growth was calculated as ?39.8 kJ/mol, indicates that oxide film growth was under diffusion control.     

Experimental and computational study on the effect of yttrium on the phase stability of sputtered Cr–Al–Y–N hard coatings

The effect of Y incorporation into cubic Cr–Al–N (B1) was studied using ab initio calculations, X-ray diffraction and energy-dispersive X-ray analysis of sputtered quaternary nitride films. The data obtained indicate that the Y incorporation shifts the critical Al content, where the hexagonal (B4) structure is stable, to lower values. The calculated critical Al contents of x ≈ 0.75 for Cr_1?xAl_xN and x ≈ 0.625 for Cr_1?x?yAl_xY_yN with y = 0.125 are consistent with experimentally obtained values of x = 0.69 for Cr_1?xAl_xN and x = 0.68 and 0.61 for Cr_1?x?yAl_xY_yN with y = 0.02 and 0.06, respectively. This may be understood based on the electronic structure. Both Cr and Al can randomly be substituted by Y. The substitution of Cr by Y increases the phase stability due to depletion of non-bonding (anti-bonding) states, while the substitution of Al by Y decreases the phase stability mainly due to lattice strain.     

A method for measuring single-crystal elastic moduli using high-energy X-ray diffraction and a crystal-based finite element model

This paper presents a method – based on high-energy synchrotron X-ray diffraction data and a crystal-based finite element simulation formulation – for understanding grain scale deformation behavior within a polycrystalline aggregate. We illustrate this method by using it to determine the single-crystal elastic moduli of β21s, a body-centered cubic titanium alloy. We employed a polycrystalline sample. Using in situ loading and high-energy X-rays at the Advanced Photon Source beamline 1-ID-C, we measured components of the lattice strain tensor from four individual grains embedded within a polycrystalline specimen. We implemented an optimization routine that minimized the difference between the experiment and simulation lattice strains. Sensitivity coefficients needed in the optimization routine are generated numerically using the finite element model. The elastic moduli that we computed for the β21s are C₁₁ = 110 GPa, C₁₂ = 74 GPa and C₄₄ = 89 GPa. The resulting Zener anisotropic ratio is A = 5.     

Correlation between crystallization and strain hardening during homogeneous deformation of Cu54Ni6Zr22Ti18 bulk metallic glass

Homogeneous deformation behaviors of the Cu₅₄Ni₆Zr₂₂Ti₁₈ BMGs with fully amorphous structure (FA) and with high density of ordered domains (OD) in amorphous matrix were examined and compared. Strain rate vs. stress curves showed that the OD exhibited m = 0.5 at low strain rates and m < 0.5 at high strain rates whereas the FA exhibited m ∼ 1 at low strain rates and m < 0.5 at high strain rates. The degree of strain hardening measured from the stress–strain curves of the FA could be correlated with the volume fraction of crystalline phase measured by differential scanning calorimetry. This result allows prediction of the volume fraction of crystallized phase in amorphous matrix and the strength ratio between nanocrystalline phase and amorphous matrix at a given volume fraction.     

Solidification behavior of γ′-Ni3Al-containing alloys in the Ni–Al–O system

The chemical activities of Al and Ni in γ′-Ni₃Al-containing alloys were measured using the multi-cell Knudsen effusion-cell mass spectrometry technique, over the composition range 8–32 at.% Al and temperature range T = 1400 to 1750 K. From these measurements a better understanding of the equilibrium solidification behavior of γ′-Ni₃Al-containing alloys in the Ni–Al–O system was established. Specifically, these measurements revealed that (i) γ′-Ni₃Al forms via the peritectiod reaction, γ + β (+Al₂O₃) = γ′ (+Al₂O₃), at 1633 ± 1 K; (ii) the {γ + β + Al₂O₃} phase field is stable over the temperature range 1633–1640 K; and (iii) equilibrium solidification occurs by the eutectic reaction, L (+Al₂O₃) = γ + β (+Al₂O₃), at 1640 ± 1 K and a liquid composition of 24.8 ± 0.2 at.% Al (at an unknown oxygen content). When projected onto the Ni–Al binary, this behavior is inconsistent with the current Ni–Al phase diagram and a new diagram is proposed. This new Ni–Al phase diagram explains a number of unusual steady-state solidification structures reported previously and provides a much simpler reaction scheme in the vicinity of the γ′-Ni₃Al phase field.     

Electrosynthesis of polyaniline films on titanium by pulse potentiostatic method

Polyaniline (PANI) was synthesized on titanium electrode from aqueous solution containing 0.3 mol L⁻¹ aniline and 1 mol L⁻¹ HNO₃ by pulse potentiostatic method. The chronoamperogram during polymerization process of aniline was recorded. The effects of the synthesis parameters, such as anodic pulse duration (t_a), cathodic pulse duration (t_c), lower limit potential (E_c) and upper limit potential (E_a), on the morphology and electroactivity of the PANI films were investigated by scanning electron microscopy (SEM) and cyclic voltammetry (CV). SEM results present that flake, mica-like, quasi-fibrous and nano-fibrous PANI film could be synthesized with various polymerization parameters. Under the following conditions, t_a = 0.8 s, t_c = 0.1 s, E_c = 0 V and E_a = 1.0 V, high quality nano-fibrous PANI film with the best electroactivity was obtained. The CV results show that the PANI films with different morphologies, which were prepared under the same anodic polymerization charge, have obvious different characteristics. This means that the PANI films with different morphologies have different electrochemical activity.     

The characterization of structure,thermal stability and magnetic properties of Fe–Co–B–Si–Nb bulk amorphous and nanocrystalline alloys

Recently bulk amorphous alloys have attracted great attention due to their excellent magnetic properties. The glass-forming ability of bulk amorphous alloys depends on the temperature difference (ΔT_x) between glass transition temperature (T_g) and crystallization temperature (T_x). The increase of ΔT_x causes a decrease of the critical cooling rate (V_c) and growth of the maximum casting thickness of bulk amorphous alloys. The aim of the present paper is to characterize the structure, the thermal stability and magnetic properties of Fe₃₆Co₃₆B₁₉Si₅Nb₄ bulk amorphous alloys using XRD, Mössbauer spectroscopy, DSC and VSM methods. Additionally the magnetic permeability μ_i (at force H ≈ 0.5 A/m and frequency f ≈ 1 kHz) and the intensity of disaccommodation of magnetic permeability Δμ/μ(t₁) (Δμ = μ(t₁ = 30 s) ? μ(t₂ = 1800 s)), have been measured, where μ is the initial magnetic permeability measured at time t after demagnetisation, the Curie temperature T_C and coercive force H_c of rods are also determined with the use of a magnetic balance and coercivemeter, respectively.Fe–Co–B–Si–Nb bulk amorphous alloys were produced by pressure die casting with the maximum diameters of 1 mm, 2 mm and 3 mm.The glass transition temperature (T_g) of studied amorphous alloys increases from 807 K for a rod with a diameter of 1 mm to 811 K concerning a sample with a diameter of 3 mm. The crystallization temperature (T_x) has the value of 838 K and 839 K for rods with the diameters of 1 mm and 3 mm, respectively. The supercooled liquid region (ΔT_x = T_x ? T_g) has the value of about 30 K. These values are presumed to be the origin for the achievement of a good glass-forming ability of the Fe–Co–B–Si–Nb bulk amorphous alloy. The investigated amorphous alloys in the form of rods have good soft magnetic properties (e.g. M_s = 1.18–1.24 T). The changes of crystallization temperatures and magnetic properties as a function of the diameter of the rods (time of solidification) have been stated.