Modern Physics Letters B

We present a systematic study of the local distortions produced upon doping metal ions to lithium niobate (LiNbO₃, LN) single crystals. The impurity bond length can be predicted by a radial force constant model, when the dopant ions substitute for Li⁺ or Nb⁵⁺ ions in the LN crystallographic frame. From the viewpoint of constituent chemical bonds, the lattice energy can be described as the function of bond valence on the basis of Born–Haber cycle for the formation of an ionic oxide M_mO_n. The dopant occupancy in the LN matrix can be determined by comparing the deviation of its lattice energy in different locations at both Li⁺ and Nb⁵⁺ sites, on the basis of the bond length relaxation of impurity ions, which can agree well with the experiment results. The effect of impurity ions on the property modification of LN crystals is also discussed according to our calculated results.

The selective polymorph synthesis of aragonite, vaterite and calcite was done by changing the conformation of chitosan molecules. Aragonite crystals in hexagonal prism-like shape were obtained in chitosan precipitates, vaterite crystals in hexagonal symmetrical plate-like shape were prepared in chitosan gel, and calcite nanoparticles were obtained in chitosan sol. The phenomenon of polymorph selection of biominerals by the morphology of biomacromolecules indicates that the studies on the relation between biominerals and biomacromelecules have general interest not only in bionics, but also in biometrics and related fields.

Magnetic properties and crystal structure of melt spun SmCo_7-xM_x (M = Hf, V, Nb, and Ta; x = 0–0.3) ribbons have been investigated. The experimental results showed that not just Hf but also V, Nb and Ta prefer to occupy 2e site in the TbCu₇ structure and remarkably increase coercivity of the ribbons from 1.7 kOe for x = 0 to 10–12 kOe for x = 0.2, which would be due to the increase of magnetic anisotropy field by the element substitution. Furthermore, the effect of C addition on the magnetic properties and microstructure of melt spun Sm(Co, M)₇ ribbons were also studied. A small amount of C addition in the ribbons could effectively refine their microstructure, and leads to further improvement of the magnetic properties. The optimal magnetic properties of B_r = 6.9 kG, _iH_c = 11.8 kOe and (BH)_max = 10.6 MGOe were attained for SmCo_6.8Hf_0.2C_0.12 nanocomposites.

The surface of commercially pure titanium (grade 4) was modified by micro-arc oxidation treatment. A porous TiO₂ coating layer was formed on the Ti surface and its crystal structure was analyzed as rutile phase. Both the thickness of the TiO₂ layer and the pore size on the surface were strongly dependent on the applied current density as well as the micro-arc oxidizing time during the oxidation treatment. When a current density of 50 mA/cm² had been applied for 10 minutes, the thickness and the pore size of the oxide layer were 1.2 µm and 230 nm, respectively. The thickness and the pore size of the layer continued to increase with increasing the applied current density. The TiO₂ passive layer of the titanium surface can attribute to the excellent biocompatibility and the formation of nano-sized pores may maximize the interlocking between mineralized bone and the surface of the Ti implant.

The ferrite added with Bi-Zn-Al-B-Si-O nano-glass was sintered at 840-900°C, 2 h and the initial permeability, quality factor, density and saturation magnetization of the samples were measured. The initial permeability of the nano-glass added sample was about 62.6 at 0.1 wt%, and showed a sharp increase from 0.4 to 0.5 wt%. Then, it reached a maximum value (211.2) at 1.0 wt% addition. Above 1.0 wt% addition, the initial permeability decreased again with a further increase in the nano-glass content. The quality factor and saturation magnetization of 0.5 wt% nano-glass added NiZnCu ferrites sintered at 880 °C was about 143 and 386 emu/cc, respectively. It was shown that the Bi-Zn-Al-B-Si-O nano-glass systems were useful as a sintering aid in the fabrication of MLCIs.

The sintering behavior and high frequency electro-magnetic properties of Ba₃Co₂Fe₂₄O₄₁ ceramics were investigated for the small antenna application. All samples of the Ba₃Co₂Fe₂₄O₄₁ ceramics were prepared by the solid-state reaction method and sintered at 1150°C 1400°C. From the XRD patterns of calcined Ba₃Co₂Fe₂₄O₄₁ powders, the most suitable condition for calcining was 600°C–1000°C. Ba₃Co₂Fe₂₄O₄₁ phase was observed in sintered Ba₃Co₂Fe₂₄O₄₁ ceramics as main phase. Bulk densities increased with sintering temperature and decreased at 1400°C. Permittivity and permeability of the Ba₃Co₂Fe₂₄O₄₁ ceramics increased or decreased with sintering temperature, respectively. On the other hand, loss tangent of permittivity and of permeability showed contrary tendency with permittivity and permeability. The permittivity and loss tangent of permittivity of Ba₃Co₂Fe₂₄O₄₁ ceramics sintered at 1300°C were 19.896 and 0.171 at 210 MHz. and the measured value of permeability and loss tangent of permeability were 14.218 and 0.204, respectively.

In the present study, we prepared and characterized electrically conductive adhesives based on polyurethane filled with two kinds of multiwalled carbon nanotubes (MWCNTs), viz. pristine MWCNTs and acid treated MWCNTs, respectively. The influence of the type of filler on the dispersibility within the polyurethane matrix and the corresponding electrical conductivity is investigated. The electrical conductivity of the prepared specimens was measured using a four-point probe. The morphology and dispersibility of the fillers were observed by field emission scanning electron microscopy and transmission electron microscopy.

The nucleation and growth behaviors of undoped and phosphorus doped polycrystalline Si thin films were investigated by in-situ TEM observations. Polycrystalline Si thin films were partially changed to amorphous by ion implantations. A normal grain growth was observed in the undoped Si thin films during heating. On the other hand, the P-doped sample showed the recovery and growth at grain boundary as well as the nucleation of Si nanocrystals at amorphous regions. Although the amorphous hindered the grain growth and acted as the nucleation source of Si nanocrystals at lower temperature, the final grain size of polycrystalline Si at 650°C was larger in the P-doped sample. The carrier mobility of the P-doped Si thin films not only increased with heat treatments, but also was corresponding to the microstructural evolution.

The effect of OH^- ions on the branching growth of cuprous oxide microcrystals was systematically studied by a reduction route, where copper-citrate complexes were reduced by glucose under alkaline conditions. Different copper salts including Cu(NO₃)₂, CuCl₂, CuSO₄, and Cu(Ac)₂ were used in this work. The results indicate that the Cu2O branching growth habit is closely correlated to the concentration of OH^- ions, which plays an important role in directing the diffusion-limited branching growth of Cu₂O and influencing the reduction power of glucose. A variety of Cu₂O branching patterns including 6-pod, 8-pod and 24-pod branches, have been achieved without using template and surfactant. The current method can provide a good platform for studying the growth mechanism of microcrystal branching patterns.

Some efficient methods have been developed to chemically prepare some magnesium salt products with multiscale morphologies such as MgCO₃·3H₂O and 5Mg(OH)₂·MgSO₄·2H₂O whiskers, Mg₅(CO₃)₄(OH)₂·4H₂O microplates and aggregates, and Mg(OH)₂ plates and spheres. The crystallization processes of the as-prepared products were studied by designing different reaction routes and corresponding growth morphologies. In these solution chemical systems, Mg²⁺ cation was found to be easily coordinated by various ligands, and can be easily transformed into various compounds with variable compositions. The current crystallization observations of some magnesium salts can be readily simulated by the chemical bonding theory of single crystal growth. These results can guide the further studies of various magnesium salts with controllable morphologies.

A simple procedure for one-step growing Nb₂O₅ hollow nanospheres and nanotubes was proposed. The final products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The as-obtained hollow spheres present a polycrystal surface while the nanotubes possess single crystal walls. Moreover, the factors that could impact the product morphology and crystal structure were discussed, and the corresponding mechanism was proposed accordingly. The crystal structure and morphology of products can be easily tuned by adjusting the molar ratio of Nb/Ti used, which determines the amount of F^- ions existing in the reaction system. The as-obtained Nb₂O₅ hollow nanospheres and nanotubes may exhibit enhanced applications in a wide range of areas. Additionally, it is rational to expect that this simple method can be extended to systems of other inorganic materials with desired morphologies.

A template-free and surfactant-free hydrothermal method has been successfully developed to fabricate hierarchically dendritic metal sulfides (PbS and CdS). It has been found that the reaction temperature play important roles in the formation of well-defined sulfide dendritic nanostructures. A possible mechanism for the formation of present dendrites was proposed. The as-obtained transition-metal sulfide dendritic nanostructures may bring wide applications in optics, electricity, gas sensors, and other related fields. The synthetic route present in this work provides a new principle for the designing synthesis of dendritic metal sulfide nanomaterials and can be regarded as a general way to fabricate other metal chalcogenide materials.

The heterolayered BT/BNT thick films were fabricated by screen printing techniques on alumina substrates electrodes with Pt. Their structure and ferroelectric properties were investigated with the heterolayered tetragonal/rhombohedral structure composed of the BT and the BNT thick films. The structural and electrical properties of the heterolayered BT/BNT thick films were studied. The dielectric properties such as dielectric constant, loss and remanent polarization of the heterolayered BT/BNT thick films were superior to those of single composition BNT, and those values for the heterolayered BT/BNT thick films were 1455, 0.025 and 12.63 µC/cm².

In this work, fabrication and investigation of substituing high-valence W⁶⁺ for Ti⁴⁺ ion on B-site of La³⁺-doped Bi₄Ti₃O₁₂ or BLT structure to form Bi_3.25La_0.75(Ti_1-xW_x)₃O₁₂ or BLTW (when x = 0, 0.01, 0.03, 0.05 0.07, 0.09 and 0.10) ceramics were carried out. X-ray diffraction patterns of BLTW ceramics indicated orthorhombic structure with lattice distortion, especially with higher concentration of WO₃ dopant. An increase in WO₃ doping content reduced not only length and thickness of grain, but also density of the ceramics. Electrical conductivity was found to decrease, while dielectric constant increased with W⁶⁺ doping. Ferroelectric properties were found to be improved with increasing WO₃ content and were optimized at x = 0.05.

This study aims to fabricate and characterize a series of new complex-structured ceramics with formula (1 - x)Pb(Zr_0.52Ti_0.48)O_3-xBi_3.25Dy_0.75Ti₃O₁₂ or (1 - x)PZT-xBDT (when x = 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0) using a solid state mixed oxide and sintering method. Optimum sintering temperature for this system was found to be 1000°C for 4 h dwell time. Phase analysis using X-ray diffraction indicated existence of BDT- as well as PZT-based solid solutions with corresponding lattice distortion. Scanning electron micrographs of ceramic surfaces showed plate-like structure in BDT-rich phase while typical grain structure was observed for PZT-rich phase. PZT-rich ceramics also possessed high densities due to their more equiaxed grains with high packing capability compared to the ceramics with addition of plate-shaped BDT.

Crystallization represents a kind of phase transition, which can be regarded as a process related to bond formation and breaking. According to the chemical bonding characteristics of constituent atoms, the ideal morphology of LiNbO₃ crystals has been successfully predicted by using the chemical bonding theory, which consists well with our experimental results. Some observations concerning the morphology evolution of KH₂PO₄ (KDP), NH₄H₂PO₄ (ADP) and ZnO crystals can also be kinetically simulated by the present theory. The obtained results indicate that the microscopic constituent chemical bonds can help us to comprehensively understand the crystallization mechanism on the basis of quantitative calculations of growth rate in different crystal faces. The chemical bonding theory builds up a link between the crystallographic structure, growth morphology and microscopic chemical bonds of crystal materials, which provides us a useful microscopic tool to quantitatively understand single crystal growth behaviors.

A template-free solution method based on solid-liquid interface reaction has been developed for fabricating metal chalcogenide dendritic nanostructures. Well-defined PbSe dendrites with four branches can be large-scale synthesized with metal lead foil as Pb²⁺ source and Na₂SeO₃ as Se²⁺ source in solvothermal conditions. The nanorods in each branch are parallel to each other in the same plane and are perpendicular to the trunk. It was found that PbSe cubes first formed on the lead substrate through a direct surface reaction and that subsequent crystal growth preferentially took place on the vertex of these PbSe cubes, leading to the formation of final branched dendrites. The current solid-liquid interface reaction has been extended to the synthesis of CuS micro-pine dendrites with copper foil and sulfur powder as reaction reagents.

A benign room temperature synthesis process has been developed to fabricate peroxy niobates with controllable composition and morphology. X-ray diffraction and scanning electronic microscopy were used to monitor the change of the phase composition and corresponding microstructure of the obtained niobates. Furthermore, UV/Vis spectra were used to study the optical properties of these ammonium and potassium peroxy niobates. The results showed that these peroxy niobates with {O₂} units present larger bandgap than those with {O₂F} units. Photoluminescence results well demonstrated that the Nb group environment is sensitive to its optical properties, which offers a marvelous chance for its further applications, such as sensors.

Large scale MnO₂ nanorods were controllably synthesized from the inexpensive precursors (e.g., manganese acetate, ammonium persulfate) via a facile one-step low temperature hydrothermal strategy. The crystal phase and microscopic morphology of the as-prepared MnO2 nanorods were characterized by X-ray powder diffraction (XRD) and scanning electron microscope (SEM). Through investigating the morphology evolution of MnO₂ products in the whole synthesis process, a novel growth mechanism of these MnO₂ nanorods was proposed, which may be efficiently extended to other material systems as a general approach towards one-dimensional nanostructures. The obtained MnO2 nanorods may have potential applications in Li-ion batteries and supercapacitors.

We herein demonstrated the preparation of nickel sulfide hierarchical architectures via an emulsion-assisted route in hydrothermal system. Scanning electron microscope images of the products at different reaction stages indicated that the construction of the nanostructured hierarchical nickel sulfide was accomplished through oriented attachment accompanied by an encapsulation procedure, and the as-obtained products exhibited uniform architectures assembled by various primary units (e.g. nanorods, nanosheets). In addition, the effect of different surfactants on the shape of nickel sulfide nanostructures was further studied. The results indicated that the hierarchical architectures can only be achieved with the assistance of anionic surfactant (e.g. sodium dodecyl sulfate). These as-prepared novel hierarchical nanostructures may be used in various areas, including industrial catalysis, and energy storage etc.

Homogenous core-shell structured Cu₂O microspheres have been successfully fabricated by a reductive transformation of in situ formed intermediate templates in an aqueous solution. The morphological evolution for these unique hollow structures was monitored via SEM observations and a detailed formation process has been proposed: (i) formation of intermediate microspheres; (ii) reductive conversion of this template to Cu₂O; (iii) inside-out Ostwald ripening of Cu₂O microspheres. In this ethanol-water system, ethanol serves as reductant in the reductive conversion process and the reaction temperature has an important effect on the formation of unique core-shell architectures. This effective solution-based synthetic route may shed some light on the design and fabrication of homogenous core-shell micro/nanostructures of other promising functional materials.

In this work, silicon-steel magnesium oxide was fabricated from light-burned magnesia by several steps. Light-burned magnesia was first slaked and carbonated to afford magnesium bicarbonate liquor and then the impurities in the liquor were removed by activated carbon. The purified liquor was then pyrolyzed to obtain basic magnesium carbonate which was calcined to result in silicon-steel magnesium oxide. The effects of carbonization time and solid to liquid ratio on the leaching process were discussed, and the impacts of adsorption temperature and time on removing impurities were also investigated. An insight into pyrolytic temperature, time and temperature rise rate indicates that fast temperature rising and high temperature lead to magnesia powders with large particle size while pyrolytic time has little effect on the particle size of magnesium oxide. The results show that the mass fraction of MgO in silicon-steel magnesium oxide is more than 99.5%, the mass fractions of CaO and Fe are less than 0.09% and 0.05%, boron content between 30–1000 ppm and more than 72.3% of magnesia particles are less than 5 µ in size by means of this process.

Calcium fluoride additive was used to produce high thermal conductivity AlN ceramics which has no grain boundary phase. Thermal conductivity of AlN is determined by the point defect, represented as oxygen related defect, within the AlN grain. The defect density characterization of high thermal conductivity CaF₂ doped AlN ceramics after heat treatment was conducted by Raman spectroscopy. As measure Raman linewidth broadening, the point defect density variation after heat treatment and corresponding thermal conductivity change was investigated.

Silicon carbide (SiC) coatings were fabricated using a chemical vapor deposition (CVD) process onto a graphite substrate at different deposition temperatures. The mechanical properties such as hardness, modulus, and creep properties from room temperature to 500°C were investigated using nanoindentation techniques. The SiC coatings deposited at 1300°C exhibited a small grain size (0.2 ~ 0.4 µm) and [111] preferred orientation, while the coatings obtained at 1350°C had a large grain size (0.5 ~ 1.0 µm) and [220] preferred orientation. The hardness was decreased with testing temperature, but no significant change in the modulus was measured with testing temperature from high temperature nanoindentation test, and the apparent creep behavior was observed in the high temperature. The high temperature mechanical properties of CVD-SiC coatings were relate to their microstructure and crystal orientation, and CVD-SiC coatings deposited at 1300°C exhibited high stability and reliability at high temperature.

We have examined superconducting property of MgB₂, containing Type 1 elemental superconductor, Pb. The results showed double transition temperatures of superconductivity. Although the addition of Pb considerably improved grain connectivity of MgB₂, the presence of Type I superconducting phase (Pb) degraded the superconductivity of composites by flux creep.

Copper sputtering target should be developed which has low enough electric resistance for the element that performs high-speed operation for a computer or the like and which is capable of forming a thin film with excellent thickness uniformity. In order to acquire the deposited film thickness uniformity, crystalline orientation has to be controlled because intensified orientation in the (111) plane rather than wholly random orientation produces better film thickness uniformity. In the present study, the Cu sputtering target with average grain size of ~30 µm and un-uniformity of 17% could be archived through an annealing temperature of 0.4 Tm.

This paper presents a novel single-step method for preparation copper nanofluids by electrical explosion of wire in liquid. Three types of fluids were used as a medium of the wire explosion process: deionised water, cetyl trimethyl ammonium bromide (CTAB) solution 0.001 M and ethylene glycol. The X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), energy dispersive spectroscopy (EDS) and UV-vis spectroscopy were used to investigate the properties of the nanofluids. Pure copper phase was detected in the nanofluids using ethylene glycol and mixture of copper and oxide phase was observed in the nanofluids using water and CTAB solution. FE-SEM analysis showed that size of particles formed in ethylene glycol was about 90 nm, the smallest in three samples.

Zn powder has been used for many years as anticorrosive pigments in various paints. In this work, the nano-sized Zn powder was synthesized by wire explosion method in deionised water and ethanol. The morphology of the prepared powders was observed by field emission-scanning electron microscopy (FE-SEM) and the oxidation of the powders was determined using energy-dispersive X-ray spectroscopy (EDS). The size of particles was estimated from transmission electron microscopy (TEM) and FE-SEM. Dispersion properties of the as-prepared Zn nanoparticles in ethanol were evaluated by Turbiscan lab device. It is found that Zn nanoparticle prepared in ethanol had spherical shape with average particles size ~150 nm while the one prepared in water had a mixture of hexagonal and spherical shape. Ethanol can reduce the oxidation of Zn nanoparticles in comparison with water. Stability of Zn nanoparticles dispersed in ethanol exhibited the agglomeration in the dispersion.

Hydrothermal technique is demonstrated for achieving Zn₅(OH)₆(CO₃)₂ microstructure consisting of nanosheets on zinc foils in the presence of formamide. Formamide concentration and reaction temperature have significant influences on the size and shape of hierarchical Zn₅(OH)₆(CO₃)₂ structures. A self-assembly mechanism is proposed to explain the growth of Zn₅(OH)₆(CO₃)₂ microstructure, which can be transformed into sheet-like dense ZnO architectures without loss of the shape features by calcining in air.

Sm-16.7wt%Co alloy powders were prepared by a high energy ball milling under various milling times, and their micro-structure and magnetic properties were investigated. After 5 h milling, the irregular shaped powders were changed to spherical shape and the size decreased to approximately 5 µm. Based on TEM results, the obtained powders consisted of nano-sized particles with approximately 13 nm, which showed a slight decrease with increasing the milling time. The XRD peaks of powders including the SmCo₅ phase disappeared after milling and became broader with increasing the milling time, and the peaks increased again after 30 h milling. The reduction in coercivity and remanence values appeared with increasing the milling time, whereas the saturated magnetization increased.

In this study, we investigated the effects of nanoclay additives on the electrical and mechanical properties of diglycidyl ether of bisphenol A (DGEBA) epoxy resin. Epoxy-clay nanocomposites were synthesized using organically modified two montmorillonite clays (MMT) with different interlamellar spacing (31.5 Å and 18.5 Å). The electrical and mechanical properties of epoxy-clay nanocopomosites were measured with variation of the amount and type of clay. The nanocomposites were found to be homogenous materials although the nanocomposites still have clay aggregates with increasing nanoclay contents. The dielectric constant showed between 3.2 ~ 3.5 and the dielectric loss showed between 3.2 ~ 5.7% in all nanocoposites. The dielectric strength and tensile strength of the 5 wt% Cloisite 15A added epoxy-oclay nanocomposite were 23.9 kV/mm and 86.7 MPa, respectively.

Nano-B₄C powder whose surface was coated with polyvinylalcohol (PVA) was prepared by using a ball milling mechanical activation process. As-prepared powders exhibit the structure of PVA layer on the surface of nano-sized B₄C and the size of the produced core/shell structured nano-B₄C/PVA particles was in the range of 50 to 200 nm. The sheets of the HDPE composite reinforced by the nano- and micro-B₄C fillers were fabricated by hot pressing following the melt mixing process respectively. The structures of prepared nano-B₄C/PVA powders and the degree of particles dispersion in HDPE were observed by means of X-ray diffraction as well as the SEM/TEM images. It was found that the dispersion of the core/shell structured nano-B₄C/PVA fillers in HDPE was more homogeneous than the surface-untreated micro-B₄C fillers in HDPE.

A novel optical fiber co-doped with Ag nanoparticles and Tm-ions was developed for S-band emission. Enhanced emission around 1470 nm was obtained upon pumping at 633 nm by the He-Ne laser.

The original length d₀ of N—H and H⋯O bonds in various inorganic systems was comprehensively studied from a chemical bond viewpoint. Two linear relationships between d₀ and the average bond lengths of each system, d_{0, N-H}, versus and d_{0, H⋯O} versus were respectively established. It is indicated that d₀ is affected by the crystalline environment evidently, therefore, the valence electron distribution of hydrogen atom which depends on the lengthening degree of the original bond length is strongly affected by the chemical environment of hydrogen atoms. The obtained valence electron distributions of hydrogen are in a good agreement with the bond valence sum rule, and their overall applicability to ammonium ion interactions was discussed.

Structural characteristics of O—H⋯O hydrogen bonds in inorganic crystals were comprehensively investigated on the basis of a database study. It is shown that the multi-furcated hydrogen bonds are very common, therefore, the structures of hydrogen bonds in inorganic crystals are extremely flexible. The direction and distance preferences of hydrogen bonds were systematically analyzed through frequency distribution histograms and normalized spatial frequency distribution scatter plot. In addition, new bond valence parameters were proposed for O—H⋯O bonds in inorganic crystals fully taking into account the multi-furcated hydrogen bonds, which can be used to properly evaluate hydrogen bond strengths in inorganic crystals. The current work sheds some light on the usage of hydrogen bonds in inorganic crystal design.

From the chemical bond viewpoint, the microscopic characterstatics of hydrogen bonds in M_i—OH₂⋯O (M is the metal cation coordinated to water molecule and i is the number of M) systems were comprehensively studied. It is shown that the original O—OH and H⋯O bond lengths of each hydrogen bonding system are evidently influenced by the crystalline environment and strongly dependent on the corresponding average bond lengths of each system, and . Furthermore, the hydrogen bonding capability of water molecules coordinated to various metal cations was properly estimated and found to be related to the ionic electronegativities of these metal cations. The current work provides a useful route to calculating hydrogen bond valences within reasonable accuracy and sheds light on the rational utilization of hydrogen bonds in crystal design.

Morphologies of hydroxyapatite (HAp) powders have influence on the mechanical properties of HAp/polymer composites. In this paper we reported a synthetic route for nanospherical and ultralong fibrous HAp powders and compared the influence of HAp morphologies on composite mechanical properties. HAp fibers with the length of ~250 µm along c-axis direction and nanospheres with the diameter of ~80 nm were produced, respectively, in the acidic solution containing glutamic acid and in the alkaline solution containing polyacrylic acid. The ultralong HAp fibers synthesized were used to reinforce biodegradable chitosan biomaterials with the significant improvement of bending strength because of the pull-out effect of long fibers.