Ice Growth Direction Research Articles

Magnetic freeze casting inspired by nature

Magnetic field aligned freeze casting is a novel method to fabricate porous, anisotropic ceramic scaffolds with a hierarchy of architectural alignment in multiple directions. A weak rotating magnetic field applied normal to the ice growth direction in a uniaxial freezing apparatus allowed the manipulation of magnetic nanoparticles to create different pore structures and channels with long-range order in directions parallel and perpendicular to the freezing direction. Porous scaffolds consisting of different host ceramics (hydroxyapatite (HA), ZrO2, Al2O3, or TiO2) mixed with varying concentrations (0–9wt%) of Fe3O4 nanoparticles were fabricated by freeze casting under three different conditions: (1) no magnetic field, (2) a static magnetic field of 0.12T, or (3) a rotating magnetic field of 0.12T at 0.05rpm. The HA, ZrO2, and Al2O3 scaffolds showed biphasic material properties with separate Fe3O4-rich and Fe3O4-poor regions. The TiO2 scaffolds showed homogeneous distributions of Fe3O4 throughout the macrostructures, which resulted in aligned pore channels parallel to the magnetic field, normal to the ice growth direction. In the magnetic field direction, the compressive strength and stiffness of the TiO2 scaffolds containing Fe3O4 was doubled. The enhanced mechanical performance of the field aligned TiO2 scaffolds are the result of the long-range microstructural order in multiple directions—(1) the magnetic field direction and (2) the ice growth direction.

Particle redistribution and structural defect development during ice templating

The freezing of colloidal suspensions is encountered in many natural and engineering processes. It can be harnessed, through a process known as ice templating, to produce porous materials and composites exhibiting unique functional properties. The phenomenon by itself appears simple: a solidification interface propagates through a colloidal suspension. We are nevertheless still far from a complete understanding and control of the phenomenon. Such lack of control is reflected in the very large scattering of mechanical properties reported for ice-templated ceramics, largely due to the formation of structural defects. Through systematic in situ investigations, we demonstrate here the role of suspension composition and the role of particle–particle electrostatic interactions on defect formation during ice templating. Flocculation can occur in the intercrystal space, leading to a destabilization of the solid–liquid interface and triggering the growth of crystals perpendicular to the main ice growth direction. This mechanism contributes significantly to the formation of structural defects and largely explains the scattering of compressive strength values reported in the literature.

Temperature distribution and local heat flux in the unidirectional freezing of antifreeze-protein solution

Experiments have been conducted on the unidirectional freezing of dilute aqueous solutions of winter flounder antifreeze protein, which are 0.02mm thick, between two cover glasses on the stage of a microscope. The instantaneous temperature field has been obtained by measuring the intensity of near-infrared light with a near-infrared camera. In addition, the local protein concentration has been measured separately using the brightness of fluorescence emitted from molecules tagged to the protein. It is found that the temperature distribution in the ice region near the ice/water interface is similar to that predicted from the modified Neumann solution. Furthermore, the temperature measurement made using the near-infrared light with a specific wavelength is verified. In addition to this, in the case of antifreeze protein solutions, serrated interfaces are observed. The sum of the conduction heat flux of a protein solution near the front edge of the serrated interface and the heat flux for solidification is lower than the conduction heat flux of ice. On the other hand, the sum of the conduction heat flux of protein solution near the bottom edge of the serrated interface and the heat flux for solidification is higher than the conduction heat flux of ice. The balance of heat flux is obtained by taking account of heat convection due to high-concentration regions of protein. These regions move to the deepest parts of the interface and form narrow liquid regions inside the ice. The convection is maintained by the heat conduction in a direction perpendicular to the direction of ice growth. Not only protein adsorption to the interface but also the heat conduction/convection contributes to the modification of ice growth in the non-equilibrium state.

Microstructure and mechanical properties of calcium phosphate cement/gelatine composite scaffold with oriented pore structure for bone tissue engineering

The macroporous calcium phosphate(CPC) cement with oriented pore structure was prepared by freeze casting. SEM observation showed that the macropores in the porous calcium phosphate cement were interconnected aligned along the ice growth direction. The porosity of the as-prepared porous CPC was measured to be 87.6% by Archimede’s principle. XRD patterns of specimens showed that poorly crystallized hydroxyapatite was the main phase present in the hydrated porous calcium phosphate cement. To improve the mechanical properties of the CPC scaffold, the 15% gelatine solution was infiltrated into the pores under vacuum and then the samples were freeze dried to form the CPC/gelatine composite scaffolds. After reinforced with gelatine, the compressive strength of CPC/gelatine composite increased to 5.12 MPa, around fifty times greater than that of the unreinforced macroporous CPC scaffold, which was only 0.1 MPa. And the toughness of the scaffold has been greatly improved via the gelatine reinforcement with a much greater fracture strain. SEM examination of the specimens indicated good bonding between the cement and gelatine. Participating the external load by the deformable gelatine, patching the defects of the CPC pores wall, and crack deflection were supposed to be the reinforcement mechanisms. In conclusion, the calcium phosphate cement/gelatine composite with oriented pore structure prepared in this work might be a potential scaffold for bone tissue engineering.

석탄회의 재활용을 통한 다공질 뮬라이트 복합체의 제조

Porous mullite/alumina composites have been fabricated by a freeze casting technique using TBA-based coal fly ash/alumina slurry. After sintering, unidirectional macropore channels aligned regularly along the TBA ice growth direction were developed; simultaneously, small sized micropores fromed in the outer walls of the pore channels. The physical and mechanical properties (e.g. porosity and compressive strength) of the sintered porous composites were roughly dependant of processing conditions, due to the complexity of the factors affecting them. However, with increasing solid loading and sintering temperature, the compressive strength generally increased and the porosity decreased. After sintering 1500℃ for 2 h, the porous specimen (porosity: 52.1%) showed a maximum compressive strength of 70.0㎫.

Macroporous silicate ceramics prepared by freeze casting combined with polymer sponge method

A combined processing route using freeze casting and a polymer sponge has been introduced for preparing macroporous silicate materials with controlled “designer” pore structures. Water/ceramic slurry systems were used in this work. The typical processing characteristics resulting from freeze casting and the polymer sponge were carried through to the sintered microstructure. A reticulated structure consisted of the struts with highly interconnected, large open pores together the inner walls with fine dendritic pores were developed and both types of characteristic pores were aligned along the ice growth direction. The pore structure developed was relatively independent of the concentration of the starting slurry and the sintering temperature.

Fabrication of ice-templated macroporous TiO2–chitosan scaffolds for photocatalytic applications

Higher-ordered 3D macroporous TiO2-functionalized chitosan scaffolds were prepared by ice segregation induced self-assembly (ISISA), in which homogenous dispersions of 6, 27 or 200 nm sized TiO2nanoparticles and chitosan cross-linked with glycidoxypropyltrimethoxysilane were unidirectionally frozen at −196 °C. Scanning electron microscopy showed that under established optimum conditions the monoliths comprised well-aligned 30–50 µm sized micro-channels running parallel to the direction of ice growth. The surface morphology of the internal wall structure was influenced by the size of the titania nanoparticles. Scaffolds prepared in the presence of 6 nm sized anatase particles showed featureless walls with concealed/embedded nanoparticles, whereas larger TiO2 particles remained exposed at the surface of the channel walls. Mechanical properties of the composite scaffolds were studied under compressive loads and stress–strain measurements obtained, and the effect of particle size on the elastic properties of the macroporous constructs determined. The hybrid scaffolds were shown to be reusable substrates for the photocatalytic degradation of methylene blue and Orange II dye molecules.

Morphological analysis of pores in directionally freeze-cast titanium foams

Synchrotron x-ray tomography was performed on titanium foams with aligned, elongated pores, initially created by sintering directionally freeze-cast preforms using two different powder sizes. Three-dimensional reconstructions of the pore structures were analyzed morphologically using interface shape and interface normal distributions. A smaller powder size leads to more completely sintered titanium walls separating the dendritic pores, which in turn created a more compact distribution of pore shapes as well as stronger pore directionality parallel to the ice growth direction. The distribution of pore shapes is comparable to trabecular bone reported in the literature, indicating the foam's potential as a bone replacement material.

Evaluation of Pore Architecture in Hydroxyapatite/Collagen Scaffold Using Micro Computed Tomography

Hydroxyapatite/collagen (HAp/Col) composite scaffold with unidirectionally elongated pores (scaffold-U) was fabricated by the unidirectional solidification with ice growth and subsequent freeze-dry process. The pore architecture in the composite was evaluated by using scanning electron microscopy (SEM) and micro computed tomography (micro-CT) with a high resolution. The SEM observation showed that the scaffold-U had unidirectional pores elongated along the vertical direction (i.e. ice growth direction), however the horizontal cross-section showed quite different pore morphology: spindle-shaped pores with random direction. The 3-D micro-CT image of the scaffold-U simultaneously showed the microstructure of the unidirectionally elongated pore and the cross-sectional pore, indicating that the interconnected micropores were successfully fabricated along the ice growth direction. The micro-CT is a powerful tool for the visualization of 3-D pore structure.

Control of pore structure and mechanical property in hydroxyapatite/collagen composite using unidirectional ice growth

The porous hydroxyapatite/collagen (HAp/Col = 80/20 in weight ratio) composite with unique three-dimensional pore structures was fabricated by the control of ice crystal growth using the liquid N 2 as a cooling medium and subsequent freeze-dry process, and the pore structure and mechanical properties were evaluated by SEM and compression tests. The porous composite had the unidirectionally interconnected micropores along the ice growth direction with a spindle-shaped cross-section (distributed from 40.1 × 11.0 to 110 × 21.8 μm in size). When the container was set on a cooling stage at − 30 °C, the length of the long-axes (distributed from 75.2 to 134.3 μm) reached the range that is necessary to bone tissue penetration. The defined pore structure improved its mechanical resistance against the compression and gave anisotropic compression properties against the pore directions. The porous HAp/Col composite with flexibility and shape recover properties will be a promised biomaterial for bone regeneration and other scaffold for tissue engineering.

Modeling of multidimensional freezing problem during cryosurgery by the dual reciprocity boundary element method

The main difficulties encountered in cryosurgery are the unknowns on the extent of the irregular shape of the frozen region, the direction of ice growth, and the temperature distribution within the ice balls during the freezing process. In this study, a numerical algorithm based on the dual reciprocity boundary element method (DRBEM) is developed to solve multidimensional phase change problem of biological tissues subject to cryosurgery. The tissues are treated as nonideal materials, freezing over a temperature range, and the effects of blood perfusion and metabolic heat generation in the unfrozen tissues are also taken into account in the heat transfer model. The accuracy of this DRBEM algorithm is validated through comparisons of the calculation results with the one-dimensional numerical solutions by the finite difference method. Comprehensive analyses are performed respectively, on the freezing behaviors of normal biological tissue and diseased tissue with tumors. For treatment of large tumors where large cooling power is required, a single probe will not be able to address a sufficiently large volume. On this account, the freezing problems by two- and three-cryoprobe system are solved for illustration purpose. The present algorithm can serve as a useful tool for post-treatment planning in cryosurgery.

Preparation of NiO–YSZ tubular support with radially aligned pore channels

A porous NiO–YSZ tubular support with radially aligned pore channels was prepared by freeze-drying of a water-based slurry. The slurry was frozen, controlling the growth direction of the ice, and it was sublimated at a reduced pressure. After sintering, the support tube showed radially aligned large pore channels parallel to the ice growth direction, and fine pores are formed around the outer surface of the tube. Bilayer consisting of dense thin electrolyte film of YSZ onto the support has been successfully fabricated using a slurry-coating process followed by co-firing. It is regarded that such a unique bilayer structure is suitable for constructing electrode-supported type electrochemical cells.

Special Issue Ceramics Integration. Preparation of Dense Thin-Film Solid Electrolyte on Novel Porous Structure with Parallel Pore Channel.

A porous La0.6Sr0.4Co0.2Fe0.8O3-Ce0.9Gd0.1O1.95 (LSCF-CGO) green body was prepared from a water-based slurry by a freeze-drying process. The LSCF-CGO slurry was frozen, controlling the growth direction of ice, and it was sublimated at a reduced pressure. After sintering the green body at 1350°C, the obtained porous substrate has macroscopically aligned open pores parallel to the ice growth direction, and micro pores on their surfaces. It was found that a porous body with such a unique pore structure is the most suitable for integrating a thin film with a porous substrate. A bi-layer consisting of a dense thin-film of CGO on the porous LSCF-CGO substrate has been then fabricated using a slurry-coating process.

Pore structure of porous ceramics synthesized from water-based slurry by freeze-dry process

A unique porous ceramic with complex pore structure was synthesized by the freeze-dry process. A water-based ceramic slurry was frozen while controlling the growth direction of ice, and sublimation of the ice were generated by drying it at a reduced pressure. By sintering this green body, a porous ceramic with complex pore structure was obtained, where macroscopically aligned open pores exceeding 10 μm in size contained minute pores of about 0.1 μm in their internal walls. Wide control of the porosity was possible by changing the concentration of the starting slurry. The pore size distribution as well as the microstructure were substantially affected by the freezing and sintering temperatures. Optimization of the synthesis conditions was investigated in order to obtain the desired pore structure.

Factors Influencing The Coefficient Of FrictionBetween Sea Ice And Various Materials

A coefficient of friction test between sea ice and various construction materials was conducted to clarify ice-material interaction. The properties of the ice and material are necessary to calculate the ice forces on sloping structures in cold coastal and offshore regions. From experimental studies through six years, the following conclusions were made. The coefficient of friction was found to be affected by: i) the relative velocity: ii) the sea ice temperature: and iii) the surface roughness of the construction materials. It was found to be relatively unaffected by: iv) the contact area: v) normal stress: vi) the growth direction of sea ice: and vii) the water at the sea ice-material interface.

Factors Influencing the Coeffecient of Friction Between Sea Ice and Various Materials

Coefficient of friction test between sea ice and various construction materials were conducted to clarify the ice-material interaction. This ice-material properties are necessary to calculate the ice forces on sloping structures in cold coastal and offshore regions. Through the six years experimental studies, the following conclusions have been drawn (Saeki et al. 1986) The coefficient of friction have been found to be affected by the following: i) relative velocity; ii) sea ice temperature; and iii) surface roughness of construction materials. They have been found to be relatively unaffected by the following; iv) contact area; v) normal stress; vi) growth direction of sea ice; and vii) water at the sea ice-material interface.

Triaxial Testing of Freshwater Ice at Low Confining Pressures

Tests have been conducted on freshwater columnar ice samples to determine the uniaxial and triaxial compressive strength of the ice. Four parameters were varied. The confining pressure was increased up to 2.85 MPa (400 psi) in steps of 0.7 MPa (100 psi). The strain rate was varied in order to obtain the ductile to brittle transition peak. Temperature was kept mainly at −2°, with two series at −10°C and −20°C, and samples were machined with axes parallel, perpendicular or at a 45-deg angle to the direction of ice growth. The results are presented within their original context, but analyzed with reference to recent studies conducted on the multiaxial behavior of ice.

The Coefficient of Friction Between Sea Ice and Various Materials Used in Offshore Structures

When structures having inclined surfaces, such as cone-type and inclined-pile structures, are constructed in coastal and offshore cold regions, sea ice forces must be considered in their design. In order to estimate these forces, the relationships of the coefficients of static and kinetic friction between sea ice and construction materials must be evaluated. The authors have been conducting, for four years, experiments on the coefficients of friction between sea ice and various commonly used offshore construction materials such as concrete and steel. This paper summarizes the results of this study. The coefficients of friction have been found to be affected by the following: (i) relative velocity (i.e., velocity of construction material relative to sea ice); (ii) sea ice temperature; and (iii) surface roughness of construction material. They have been found to be relatively unaffected by the following: (i) contact area, (ii) normal stress, (iii) growth direction of sea ice, and (iv) water at the sea ice-material interface.

Experimental Study on Direct Shear Strength of Sea Ice

When structures such as oil drilling rigs are constructed on or through the ice plate in coastal and offshore regions, the shear strength of sea ice must be estimated to determine the loading on these structures. Testing methods for shear strength must be established so that shear strength of sea ice in various conditions can be determined. The authors have been conducting, for five years, direct shear strength experiments using sea ice samples from the Okhotsk Sea coast. Physical characteristics of sea ice, including shear strength, depend to a great extent on such properties as salinity, porosity, grain size, etc; thus, there is variation in the test results of five years experimentation since the samples obtained varied from year to year. The following conclusions were drawn from this experiment: i) Under certain conditions the relation between shear strength and vertical stress can be represented by Coulomb’s equation of soil; ie, where Ts: shear strength, C* : apparent cohesion (shear strength at σv σv = 0), φ*: angle of internal friction, σv : vertical stress ii) The shear strength of sea ice increases, approaching a constant, with decreasing ice temperature. iii) The shear strength decreases with increasing ice shear area; an analogous relation exists in concrete. iv) The shear strength is not greatly dependent on either the shear velocity or stress rate. v) The shear strength is greater and generally increases more rapidly with decreasing ice temperatures in planes perpendicular to the ice growth direction than in planes parallel to it. vi) Two types of failure occurred in the sea ice samples. In the case of (sample diameter)/(sample length) less than 2 the failure was induced by shear only. With d/l>2, the failure was not solely caused by shear since the existence of a small gap between the ice sample and shear box introduced a bending moment.

An experimental study of uniaxial compressive strength of segregated ice in soil

Uniaxial compression tests of columnar-grained ice which was segregated in soil specimens were conducted. Specimens (5 cm in diameter and 9 cm in height) were prepared by using frost susceptible clay which was sampled from Tokyo. The tests were carried out systematically with respect to the dependence of compressive strength on temperature, direction of loading, strain rate and specimen height.When the load is applied ta the direction of ice growth, the longitudinal uniaxial compressive strength increases with the lowering of temperature, giving values of 190 kgf/cm2 at -10°C and 250 kgf/cm2 at -20°C. These values are larger than those of various types of ice, Fujinomori frozen clay and Toyoura frozen sand. When the loading is perpeiadicular to the growth directian, the lateral uniaxial compressive strength is approximately equal to the commercial ice strength, being about 50 kgf/cm2 at -20°C. The compressive strength, when the loading is applied to the ice growth direction, has a maximum at a strain rate of between 0.5%/min and 2%/min for -3°C. When the initial specimen temperature was about -0.4°C. the specimen temperature dropped with partial fusion during compression at a faster strain rate than 0.4%/min, and consequently the relation between the longitudinal strength and the specimen temperature agrees with the relation between pressure and freezing point calculated from the Clausius-Clapeyron equation. For a constant strain rate, the longitudinal strength decreases as the specimen height decreases from 15 cm to 1 cm For a height less than 1 cm, on the other hand, it decreases. When the load is applied to the direction of ice growth, the transition from ductile failure to brittle failure occurred at the strain rates of 0.6, 0.2 and 0.04%/min for constant temperatures of -1.7, -3 and -18°C, respectively. In the range of ductile failure, grain boundary cracks and grain inside cracks are distributed regularly in the growth direction and/or perpendicular to it.