基体对应力诱导的纳米晶W膜开裂行为的影响

采用磁控溅射方法同时在Si(100)和聚酰亚胺(PI)基体上沉积W膜,对比研究不同基体约束对纳米晶W膜微观结构及应力诱导的开裂行为的影响.结果发现,在两种基体上W膜的裂纹形态明显不同.在Si基体上W膜的裂纹呈楔形,而在PI基体上W膜的裂纹呈半圆柱形凸起于薄膜表面.这种裂纹形态的差异源于两种基体上W膜的变形机理不同.在刚性Si基体上,W膜的裂纹扩展是通过晶粒平面内的转动实现的,而在柔性PI基体上W膜裂纹扩展是通过排列晶粒在平面内、外的转动协调完成的.分析表明,两种截然不同的开裂行为与不同基体上薄膜内应力的变 关键词： W膜 残余应力 裂纹 晶粒     

Desiccation of a clay film: Cracking versus peeling

We report a simulation study on competition between cracking and peeling, in a layer of clay on desiccation and how this is affected by the rate of drying, as well as the roughness of the substrate. The system is based on a simple 2-dimensional spring model. A vertical section through the layer with finite thickness is represented by a rectangular array of nodes connected by linear springs on a square lattice. The effect of reduction of the natural length of the springs, which mimics the drying is studied. Varying the strength of adhesion between sample and substrate and the rate of penetration of the drying front produces an interesting phase diagram, showing cross-over from peeling to cracking behavior. Changes in the number and width of cracks on varying the layer thickness is observed to reproduce experimental reports.     

Experimental study of cracking induced by desiccation in 1-dimensional systems

We designed a simple experiment to study both the dynamical and statistical properties of cracking that occurs in a one-dimensional system composed of wet clay (or similar material) exposed to shrinkage induced by desiccation. We study both the dynamical formation of cracks and the statistical characteristics of the final cracks pattern. We observe that the drying rate has a strong influence on the way cracks appear and grow. We find that the final crack width is related to the order of apparition of the cracks. We discuss the statistical distributions of cracks width and separation between two adjacent cracks. We also study the correlations between these two quantities. Our results are compared to the predictions of existing models. Finally, a comparison with another kind of clay is made. Received 6 May 2002 and Received in final form 5 July 2002     

Designing micro-patterned Ti films that survive up to 10% applied tensile strain

Reducing the strain in brittle device layers is critical in the fabrication of robust flexible electronic devices. In this study, the cracking behavior of micro-patterned 500-nm-thick Ti films was investigated via uniaxial tensile testing by in situ SEM and 4-point probe measurements. Both visual observations by SEM and 4-pt resistance measurements showed that strategically patterned oval holes, off-set and rotated by 45°, had a significant effect on limiting the extent of cracking, specifically, in preventing cracks from converging. Failure with regard to electrical conduction was delayed from less than 2% to more than 10% strain.     

Cracking in drying colloidal films

It has long been known that thick films of colloidal dispersions such as wet clays, paints, and coatings crack under drying. Although capillary stresses generated during drying have been recently identified as the cause for cracking, the existence of a maximum crack-free film thickness that depends on particle size, rigidity, and packing has not been understood. Here, we identify two distinct regimes for crack-free films based on the magnitude of compressive strain at the maximum attainable capillary pressure and show remarkable agreement of measurements with our theory. We anticipate our results to not only form the basis for design of coating formulations for the paints, coatings, and ceramics industry but also assist in the production of crack-free photonic band gap crystals.     

Microstructure and properties of manganese dioxide films prepared by electrodeposition

Nanostructured manganese dioxide films were obtained by galvanostatic, pulse and reverse pulse electrodeposition from 0.01 to 0.1 M KMnO₄ solutions. The deposition yield was investigated by in situ monitoring the deposit mass using a quartz crystal microbalance (QCM). Obtained films were studied by electron microscopy, X-ray diffraction analysis, energy dispersive spectroscopy, thermogravimetric and differential thermal analysis. The QCM and electron microscopy data were utilized for the investigation of deposition kinetics and film formation mechanism. It was shown that the deposition rate and film microstructure could be changed by variation of deposition conditions. The method allowed the fabrication of dense or porous films. The thickness of dense films was limited to ∼0.1 μm due to the insulating properties of manganese dioxide and film cracking, attributed to drying shrinkage. Porous and crack-free 1-2 μm films were obtained using galvanostatic or reverse pulse deposition from 0.02 M KMnO₄ solutions. It was shown that film porosity is beneficial for the charge transfer during deposition and crack prevention in thick films. Moreover, porous nanostructured films showed good capacitive behavior for applications in electrochemical supercapacitors. The porous nanostructured films prepared in the reverse pulse regime showed higher specific capacitance (SC) compared to the SC of the galvanostatic films. The highest SC of 279 F/g in a voltage window of 1 V was obtained in 0.1 M Na₂SO₄ solutions at a scan rate of 2 mV/s.     

Flow and fracture in drying nanoparticle suspensions

Drying aqueous suspensions of monodisperse silica nanoparticles can fracture in remarkable patterns. As the material solidifies, evenly spaced cracks invade from the drying surface, with individual cracks undergoing intermittent motion. We show that the growth of cracks is limited by the advancement of the compaction front, which is governed by a balance of evaporation and flow of fluid at the drying surface. Surprisingly, the macroscopic dynamics of drying show signatures of molecular-scale fluid effects.     

Morphological evolutions of iron films on PDMS substrates under uniaxial compression/tension

We report on the morphological evolutions of iron films sputtering deposited on elastic polydimethylsiloxane (PDMS) substrates under uniaxial compression/tension. The experiment shows that the as-prepared film (no external strain) spontaneously forms cracks and wrinkles due to the residual thermal stresses stored up during/after the film deposition. The external uniaxial compression can generate delaminated buckles perpendicular to the loading direction (transverse direction) and new cracks in the loading direction (longitudinal direction). Subsequent reloading and further straining result in the formation of transverse cracks and longitudinal buckles. It is found that there exists a significant coupling effect between the cracking and buckling patterns during the compression/tension process. The morphological evolution behaviours and underlying physical mechanisms are discussed and analysed in depth in this paper.     

Growth mechanism and stress relief patterns of Ni films deposited on silicone oil surfaces

The growth mechanism and stress relief patterns of nickel (Ni) films, deposited on silicone oil surfaces by a thermal evaporation method, have been studied systematically. Our experiment shows that the growth mechanism of the Ni films approximately obeys a two-stage growth model. Characteristic cracks with sinusoidal appearance resulted from the internal stress can be frequently observed in the continuous Ni films after the samples are removed from the vacuum chamber. Several crack modes including the regularly sinusoidal cracks, zigzag cracks, attenuation cracks and self-similar cracks are described and analyzed by using the general theory of buckling of plates in detail. The internal stress and propagating velocity of the sinusoidal cracks are also discussed in this paper.     

Pattern formation and selection in quasistatic fracture

Fracture in quasistatically driven systems is studied by means of a discrete spring-block model. Developed from close comparison with desiccation experiments, it describes crack formation induced by friction on a substrate. The model produces cellular, hierarchical patterns of cracks, characterized by a mean fragment size linear in the layer thickness, in agreement with experiments. The selection of a stationary fragment size is explained by exploiting the correlations prior to cracking. A scaling behavior associated with the thickness and substrate coupling, derived and confirmed by simulations, suggests why patterns have similar morphology despite their disparity in scales.     

Indentation fracture and indentation delamination in ZnO film/Si substrate systems

ZnO films with thicknesses ranging from 0.202 to 1.535?µm were deposited by using the magnetron sputtering technique on Si (100) substrates 525?µm thick. Then, Vickers indentation tests were carried out on the ZnO/Si systems at room temperature, in which the applied load varied from 10?mN to 2.0?N. The experimental results show that only indentation-induced radial cracking occurred in the systems with film thicknesses equal to and thinner than 0.554?µm, from which the residual stress in the films was extracted to be 387?MPa in compression. For the systems with film thicknesses equal to and thicker than 0.832?µm, only indentation-induced delamination occurred when indentation loads were low. Under high indentation loads, radial cracking concurrently occurred with delamination. The radial cracks were invisible at the film surfaces because the crack length was smaller than the delamination size. The critical film thickness for indentation-induced delamination was found to be around 0.7?µm for the ZnO/Si systems. Combining the composite hardness models with the indentation-induced delamination model, we developed a method to determine the interfacial fracture energy between a film and its substrate. The novel method is particularly useful for indentation equipment without any displacement measurement devices. Using the new method, we extracted the interfacial fracture energy to be about 12.2?J?m^?2 and from 9.2 to 11.7?J?m^?2 for the cases without and with buckling respectively of delaminated films. Consequently, the pure mode I interfacial fracture energy was calculated to be 10.4?J?m^?2 for the ZnO/Si systems.     

The influence of surface geometry and topography on the fatigue cracking behaviour of laser hybrid welded eccentric fillet joints

Laser hybrid welding of an eccentric fillet joint causes a complex geometry for fatigue load by 4-point bending. The weld surface geometry and topography were measured and studied in order to understand the crack initiation mechanisms. The crack initiation location and the crack propagation path were studied and compared to Finite Element stress analysis, taking into account the surface macro- and micro-geometry. It can be explained why the root and the upper weld toe are uncritical for cracking. The cracks that initiate from the weld bead show higher fatigue strength than the samples failing at the lower weld toe, as can be explained by a critical radius for the toe below which surface ripples instead determine the main stress raiser location for cracking. The location of maximum surface stress is related to a combination of throat depth, toe radius and sharp surface ripples along which the cracks preferably propagate.     

Influence of the substrate temperature on BCN films deposited by sequential pulsed laser deposition

The influence of substrate temperature on the composition and crystallinity of boron carbonitride (BCN) thin films deposited on (100) Si substrates by sequential pulsed laser deposition (PLD) has been investigated. A correlation between the target composition, the nitrogen pressure involved in the process, and the amount of B, C, and N elements (at % ) in the deposited films is established from energy dispersive spectroscopy (EDS) analysis. Electron microscopy studies show that the films deposited on heated substrates are mainly amorphous. Fourier-transform infrared spectroscopy (FTIR) analysis confirms the BCN-compound formation: the peak of C-BN and the peaks from B-N-B bending vibrations and C-N C-bond vibrations are present in the spectra. SEM studies show that the deposited films have a smooth surface, with no cracks and few droplets. Results were compared with those obtained on films deposited at room temperature under similar experimental conditions.     

In situ observation and analysis of multiple cracking phenomena in thin glass layers deposited on polymer films

The multiple cracking phenomena in thin SiO_x films deposited on 12 μm-thick polyethylene terephthalate (PET) substrates during the tensile test are investigated. Thicknesses of SiO_x films ranged from 43 to 320 nm. The multiple cracking progress is observed in situ by optical microscopy and from which the crack density in SiO_x films is measured. The predicted crack density by the shear lag analysis including residual strains, explains reasonably well the experimental results. The critical energy release rate, G_c, for the first film cracking is also evaluated from simple energy balance arguments. Although it depends on the analytical model, G_c is estimated to be a constant value of about 1.0 J/m² regardless of the thickness.     

Drying and Impregnation of Wood with Propiconazole Using Supercritical Carbon Dioxide

The process of wood drying is studied in supercritical (SC) CO₂ and SC-CO₂ containing 5 vol % ethanol at temperatures of 323, 343, and 353 K and pressures of 10, 20, and 30 MPa. It is established that 40–87% of moisture is removed from wood in the first cycle of drying. An increase in the duration of the decompression stage of the drying process decreases the number of cracks in the wood samples. The solubility of propiconazole is studied in SC-CO₂ at 323, 343, and 353 K in the pressure range of 10–30 MPa using a dynamic method. Rather high saturation concentrations of (3–5) × 10^–3 mol/mol CO₂ are obtained, which indicates the potential benefits of using SC-CO₂ as a solvent in wood impregnation with propiconazole. Continuous impregnation is achieved when impregnating wood with propiconazole from SC-CO₂. The impregnation efficiency increases with increasing pressure and duration of the process.     

Investigation on cracking behavior of Ni-based coating by laser-induction hybrid cladding

Based on the experiments of laser-induction hybrid cladding by powder feeding, the cracking behavior of Ni-based coating and solidification characteristic in molten pool were investigated. The results indicate that the hybrid cladding is effective to prevent from cracking in Ni-based coating. With the increase of induction energy density, the tensile stress and crack rate decrease obviously. When the induction energy density arrives at 36 J/mm², the free-cracks coating can be achieved. In laser-induction hybrid cladding, the martensite can be eliminated in the heat affected zone and the phase transformation stress is little. Moreover, the molten pool is solidified through two directions such as the coating surface and coating/substrate interface, i.e., firstly the top and bottom in molten pool are solidified, and then the middle in molten pool is solidified. Therefore, in hybrid cladding, the peak value of tensile stress is located in the middle of coating, which is different from that in laser cladding. This distribution status of residual stress is greatly helpful to restrict the cracks of Ni-based coating in laser-induction hybrid cladding.     

粒状物料常压吸附流化床冷冻干燥的传热研究

以粒状马铃薯为研究对象,在常压吸附流化床冷冻干燥的实验基础上,建立了吸附流化床内对流边界条件下的球坐标冻干模型,并采用变时间步长的有限差分法,求解了物料内部温度分布及升华界面的移动速率,与实验结果吻合较好,分析了有关因素对于冻干过程中传热及干燥过程的影响。     

Effects of the rate of evaporation and film thickness on nonuniform drying of film-forming concentrated colloidal suspensions

In this paper, we report on nonuniform distribution of film-forming waterborne colloidal suspensions above the critical concentration _c of the colloidal glass transition during drying. We found that colloidal suspension films dry nonuniformly when the initial rate of evaporation E and/or the initial thickness l₀ are high. We found that a Peclet number Pe, defined as Pe = El₀/D, where D is the diffusion coefficient of the colloids in the diluted suspensions, does not predict uniformity of drying of the concentrated suspensions, contrary to the reported work on drying of diluted suspensions. Since the colloidal particles are crowded and their diffusive motion is restricted in concentrated suspensions, we assumed that above _c water is transported to the drying surface by hydrodynamic flow along the osmotic pressure gradient. The permeability of water through channels between deforming particles is estimated by adapting the theory of foam drainage. We defined a new Peclet number Pe by substituting the transport coefficient of flow (defined as the permeability divided by the viscosity, multiplied by the osmotic pressure gradient) for the diffusion coefficient. This extended Peclet number predicted the nonuniform drying with a criterion of Pe > 1. These results indicate that the mechanism of water transport to the drying surface in concentrated suspensions is water permeation by osmotic pressure, which is faster than mutual diffusion between water and particles --that has been observed in diluted suspensions and discussed by Routh and Russel. The theory fits well the experimental drying curves for various thicknesses and rates of evaporation. The particle distribution in the drying films is also estimated and it is indicated that the latex distribution is nonuniform when Pe > 1.     

Large-scale ZnO inverse opal films fabricated by a sol–gel technique

We demonstrate that high-quality large-scale ZnO inverse opals can be fabricated by a simple sol–gel technique, comprising infiltration of polystyrene colloidal crystal films with zinc nitrate solution, drying and annealing at 300 C. This simple method yields continuous films, which consist of inverse opal domains (up to several hundreds of μm² in size), separated by small cracks filled with zinc oxide. Microradian X-ray diffraction was employed to verify the crystalline quality of ZnO inverse opals on the macroscale, revealing that the samples have a predominant face-centered cubic structure, and that the majority of domains have the same crystallographic orientation. The samples exhibit bright iridescence and possess photonic stop-bands in the visible to near-infrared spectrum.     

Growth imperfections in three-dimensional colloidal self-assembly

Self-assembled colloidal films produced from vertical deposition often show alternating bands of arrayed spheres and empty regions on the substrate as the solvent evaporates. We have identified three distinct growth zones, their relative extent being a function of colloidal concentration. We attribute the growth pattern to a ‘stick–slip’ motion of the meniscus growth front, a direct consequence of the dynamic balance of surface-tension forces and the convective transport of nanoparticles. Within these zones, the crystalline quality of the colloidal crystals is dependent on the wetting characteristics of the meniscus. These colloidal crystals also exhibit cracks of different morphologies within the different growth zones as a result of the drying process. We deduce that these macroscopic imperfections may be reduced by improving substrate wettability and reducing evaporation rate. PACS 42.70.Qs; 81.16.Dn; 81.40.Tv; 82.70.Dd