Lamellar Pores Research Articles

Freeze-casted TiO2 photoelectrodes with hierarchical porous structures for efficient light harvesting ability in dye-sensitized solar cells

Hierarchical porous TiO2 photoelectrodes fabricated using freeze-casting method were introduced for efficient light scattering effect in dye-sensitized solar cells (DSSCs). Unique porous microstructures (lamellar or interconnected pore structures) were obtained by controlling the concentrations of slurry and binder. The freeze-casted photoelectrodes exhibited stronger light scattering effects than conventionally screen-printed mesoporous photoelectrode. This is attributed to a great number of scattering sites (interconnected pores with the size range of 50–500nm), which are uniformly distributed throughout the entire freeze-casted photoelectrodes, resulting in an enhancement of the light pathway. As a result, despite a lower dye adsorption and less kinetically efficient charge transport, DSSCs based on freeze-casted photoelectrodes showed significant conversion efficiencies (3.64%) compared with those of DSSCs based on conventionally screen-printed photoelectrodes (2.57%). This study suggests that the hierarchical porous photoelectrodes fabricated using the freeze-casting are promising candidates for highly efficient DSSCs.

Fabrication of lamellar porous alumina with regular structure via directional solidification with multiple cold sources

Porous ceramics obtained with aqueous slurries by directional solidification have randomly distributed lamellar pore channels and thus have unstable mechanical properties. According to the anisotropic principle of ice crystal growth, the growth of lamellar ice crystals is regular when multiple cold sources are used, and porous ceramics with regular pore channels are then obtained after drying. Multiple cold sources are formed with a bottom cold plate and copper sides in rectangular molds. The copper sides are in contact with the bottom cold plate, thus forming the side cold source with temperature gradient distribution by heat transfer. The interaction between the side cold source and the bottom cold plate facilitates the regular distribution and continuous growth in parallel of ice crystals. The use of parallel copper sides of the mold results in porous ceramics with an axisymmetric pore structure and high aspect ratios of pore channel in porous alumina. The positive compressive strength of fabricated porous ceramics with an axisymmetric structure is similar with those of conventional directional solidification, but the lateral side direction compressive strength of fabricated porous ceramics with an axisymmetric structure is significantly increased.

Control of the structure and mechanical property of porous WS2 scaffold during freeze casting

Porous WS2 scaffolds with aligned lamellar pore were fabricated by freeze casting, and the pore morphology and size can be well controlled by adjusting the processing parameters during freeze casting process. The results indicated that the porosity and the compressive strength of porous WS2 were greatly affected by the concentration of gelatin. As the gelatin addition increased from 1 to 5 wt%, the porosity of porous WS2 scaffolds with initial solid content of 3 vol% decreased from 95.12 to 90.23%, and their compressive strength increased from 0.22 to 1.16 MPa. Moreover, the lamellar spacing and wall thickness could be tailored from 90 to 320 and 5 to 30 μm respectively by changing the cooling temperature. And the compressive strength of scaffolds has a slight increase with the decreases of cooling temperature. The porous WS2 scaffold with fine aligned lamellar structure and proper compressive strength are expected to be used for scaffold materials.

Water‐based freeze casting: Adjusting hydrophobic polymethylsiloxane for obtaining hierarchically ordered porous SiOC

AbstractThe hydrophobic properties of methyl poly siloxane (MK) were pushed into the “hydrophilic” range by cross‐linking it with (3‐aminopropyl)triethoxysilane (APTES) and subsequent pyrolysis to enable water‐based freeze casting. Filler properties are investigated by varying the ratios of MK to APTES (1:1, 1:2, 1:3, 1:4, 1:5), and pyrolysis temperatures (400°C, 500°C, 600°C) for the purpose of determining an optimal set of characteristics for freeze casting. Additionally, filler selection for this purpose is facilitated by analysis of zeta potential values and vapor adsorption. It was found that water‐based freeze casting with hybrid fillers, followed by a pyrolysis step (600°C‐700°C), leads to a SiOC ceramic monolith with a lamellar pore morphology and a hierarchically ordered micro/meso/macropore structure. Samples pyrolyzed at 1000°C contain mesopores, having a SSA as high as 51.6 m2/g. The hierarchically porous structure is very promising for applications involving gas or liquid transportation.

Aligned macroporous TiO2/chitosan/reduced graphene oxide (rGO) composites for photocatalytic applications

Abstract In this article ice templating is used to fabricate novel TiO2/chitosan/reduced graphene oxide (rGO) composites with a highly aligned macroporous structure for photocatalytic applications. The structure of the composites was readily tailored using the composite composition, for example the lamellar pore width decreased from 50–45 to 5–10 μm, while the lamellar thickness increased from 2–3 to 20–25 μm, with an increase of the TiO2 content from 45 to 77 vol%. Lamellar pore channels between the layers exhibited a more uniform distribution when the rGO content was 1.0 wt%. The increase in viscosity of the composites with high TiO2 contents led to the formation of smaller ice crystals and smaller lamellar pore sizes to enable the production of composite structures with improved mechanical strength. The TiO2/chitosan/rGO composites exhibited excellent photocatalytic degradation of methyl orange and the photocatalytic efficiency was optimized by control of the active material content and microstructure. The hybrid composites with 1.0 wt% rGO showed a degradation percentage of 97%, which makes these novel TiO2/chitosan/rGO freeze cast structures attractive materials as high performance and high strength substrates for photocatalytic degradation applications.

Formation of Lamellar Pores for Splats via Interfacial or Sub-interfacial Delamination at Chemically Bonded Region

To comprehensively understand the formation mechanism of lamellar pores in splats, the delamination morphologies and crack patterns of yttria-stabilized zirconia (YSZ) and lanthanum zirconia splats were examined. Results showed that both types of splats grew epitaxially on well-polished YSZ substrates, evidently confirming the formation of chemical bonding between splats and substrate. However, the interfacial or sub-interfacial delamination was observed in all kinds of splats in this study. Residual vertical cracks passing through delaminated domains (on bare substrate) were also observed, which clearly indicated that transverse delamination followed vertical cracking. Mechanical analysis about delamination was addressed, and the results were consistent with the experimental data.

Wicking into porous polymer-derived ceramic monoliths fabricated by freeze-casting

Silicon oxycarbide monoliths of different pore size distribution were fabricated by freeze-casting. The samples revealed a lamellar pore structure with an axial anisotropy. To evaluate the capillary transport abilities we performed wicking experiments. The sample weight measurement method was applied during the imbibition. The samples show deviations in permeability from 10% to 49% at different sample orientations that quantifies the impact of the anisotropy in the axial direction. The deviations were larger for the samples with smaller pore size. For these samples we also observed larger differences in the wicking behaviour. The samples with bigger pore size demonstrated higher permeability and faster wicking. Imbibition results at both sample orientations showed a good agreement with a prediction via the Lucas–Washburn equation with gravity effects. We demonstrate hereby, that our approach of macroscopic modelling predicts wicking behaviour in anisotropic structures reasonably well, providing a simple tool for further porous material investigations.

Influence of anisotropic grains (platelets) on the microstructure and uniaxial compressive response of ice-templated sintered alumina scaffolds

Ice-templated ceramics have unique lamellar pore morphology that offers better compressive mechanical properties in comparison to the typical ceramic foams with isotropic pore morphology. However, for very high-level porosity (>65 vol%) strength difference diminishes. This investigation reveals that by inducing anisotropic grains within the matrix of a fine-grained ceramic, uniaxial compressive response of the ice-templated sintered scaffolds can be markedly enhanced without causing any considerable modification of the total porosity. To address this innovative materials design strategy, we synthesized a series of microstructures by systematically varying the anisotropic grains content in an aqueous suspension and the freezing kinetics to investigate the process-microstructure correlations and understand the structure-property relationships. Microstructural investigations revealed the unique arrangements of the platelets within and out of the lamella walls, where the upward moving ice fronts aligned the platelets' in-plane direction to the ice-growth direction. In the low freezing front velocity regime, platelets were observed to be mainly within the lamella walls, whereas platelets started to develop lamellar bridges with the increasing velocity. As a result, a transition of the pore morphology occurred with the increasing platelets content and the freezing front velocity. A novel method based on the rigorous microstructural analysis is developed to estimate the distribution of the platelets within and out of the walls and the variation of the platelets distribution with the composition and freezing front velocity. A drastic improvement of the compressive mechanical properties (stiffness, strength, energy absorption capacity) was measured due to the platelets' addition, which has been related to the platelets' distribution within and out of the walls and the pore morphology modifications. Results are rationalized based on the role of the platelets during the compressive deformation.

Directional solidification of aqueous TiO2 suspensions under reduced gravity

Porous materials exhibiting aligned, elongated pore structures can be created by directional solidification of aqueous suspensions—where particles are rejected from a propagating ice front and form interdendritic, particle-packed walls—followed by sublimation of the ice, and sintering of the particle walls. Theoretical models that predict dendritic lamellae spacing—and thus wall and pore width in the final materials—are currently limited due to an inability to account for gravity-driven convective effects during solidification. Here, aqueous suspensions of 10–30 nm TiO2 nanoparticles are solidified on parabolic flights under micro-, lunar (∼0.17 g; gl = 1.62 m/s2), and Martian (∼0.38 g; gm = 3.71 m/s2) gravity and compared to terrestrially-solidified samples. After ice sublimation and sintering, all resulting TiO2 materials exhibit elongated lamellar pores replicating the ice dendrites. Increasing the TiO2 fraction in the suspensions leads to decreased lamellar spacing in all samples, regardless of gravitational acceleration. Consistent with previous studies of microgravity solidification of binary metallic alloys, lamellar spacing decreases with increasing gravitational acceleration. Mean lamellar spacing for 20 wt% TiO2 nanoparticles suspensions under micro-, lunar, Martian, and terrestrial gravity are, respectively: 50 ± 8, 34 ± 11, 30 ± 6, and 23 ± 9 μm, indicating that gravity-driven convection strongly affects lamellae spacing under terrestrial gravity conditions. Gravitational effects on lamellar spacing are highest at low TiO2 fractions in the suspension; for 5 wt% TiO2 suspensions, the microgravity lamellar spacing is more than twice that under terrestrial gravity (182 ± 21 vs. 81 ± 23 μm). Results of this study are in good agreement with previous studies of binary metallic alloy solidification where primary dendrite spacing increases under microgravity. Literature data from ice-templating systems are used to discuss a dependence on lamellae spacing of the density ratio of particles and fluid.

The phase behaviors of cationic and anionic aqueous mixtures and the usage as templates for synthesis of mesoporous materials

Liquid crystals formed in aqueous mixtures of cationic and anionic surfactants were used as structure- directing templates for preparation of mesoporous silicas. For this purpose, micellar particle sizes and liquid crystal regions in the mixed system containing cetyl trimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were investigated by Melvin laser particle size analyzer and polarizers, respectively. The formation of large organized assemblies was closely related to both the total surfactant concentrations and mixing ratios. Micelles formed at low total concentration while liquid crystals formed at high total concentration. The mixtures of CTAB and SDS favored the formations of large organized assemblies when the molar fraction of CTAB was 0.30 or 0.70. Using tetraethoxysilane as precursor and liquid crystals as templates, mesoporous silicas were prepared at different total concentrations of surfactants when molar fraction of CTAB was fixed at 0.70. Transmission electron microscopy results revealed that ordered mesoporous silicas with lamellar pore channels were prepared. Nitrogen adsorption-desorption determinations showed that these materials exhibited a pore size about 4.5 nm. In addition, the surface areas increased significantly with the increase of total surfactant concentrations. This work provided a simple and effective approach to synthesize lamellar mesoporous silicas.

Microstructure and compressive behavior of ice-templated copper foams with directional, lamellar pores

Copper foams are fabricated by directional freezing of aqueous suspensions of nanometric CuO powders followed by ice sublimation, reduction to Cu in Ar–5% H2 gas and sintering. During slurry solidification, parallel, lamellar, centimeter-long ice dendrites grow, pushing the CuO powders into lamellar interdendritic spaces. Upon subsequent ice sublimation, the ice dendrites create lamellar pores surrounded by CuO walls that are subsequently reduced to copper and sintered; these ice-templated walls display surface micropores and, depending on the reduction/sintering parameters, internal micropores. Varying the main processing parameters – powder fraction in the slurry (from 13 to 19vol%) and casting temperature (from −10 to −30°C) – has a strong effect on the foam microstructure: (i) porosity (varying from 45% to 73%) is inversely related to slurry powder fraction, (ii) oriented lamellar macropores width increases from 15 to 64µm with decreasing slurry fraction and increasing freezing temperature and (iii) oriented lamellar Cu wall width increases from 19 to 63µm with increasing slurry fraction and freezing temperature. The resulting Cu foams show oriented, lamellar macropores (beneficial to permeability) and walls micropores (which increase the surface area) and are promising for use in electrochemical cells given the simplicity, scalability, low cost, and microstructure tunability associated with the ice-templating process. The ice-templated Cu foams, with pore directions parallel and perpendicular to the direction of compressive loading, show ductile compressive behavior with high yield stress, ductility and energy absorption; they are compared to model predictions and literature data of Cu lotus foams with elongated, cylindrical pores.

Fabrication of alumina/copper heat dissipation substrates by freeze tape casting and melt infiltration for high-power LED

With the wide application of high-power LED, heat dissipation substrate not only demonstrates a high thermal performance but also requires a low thermal expansion coefficient that matches that of the chip. Metal/ceramic composite materials can combine the excellent heat dissipation performance of metals and low thermal expansion property of ceramics, satisfying the requirement for high-power LED. In this study, alumina/copper composite substrates were fabricated through freeze-tape casting and melt infiltration. Morphologies, infiltration rates, thermal properties, and heat dissipation properties of the fabricated composite substrates were investigated. This study found that copper was distributed in the lamellar pore channels of alumina substrates, and the infiltration rate of copper in alumina/copper composite substrates increased under increasing Ti content, infiltration temperature, and time. Furthermore, thermal conductivity and thermal expansion coefficient decreased as alumina content in the composite substrates decreases. The thermal resistance, junction temperature, and increase in junction temperature of the alumina/copper composite substrate when 57.33 vol% copper was used as heat dissipation substrates for 2W LED lamps are 22.5 K·W−1, 80.5 °C, and 40.5 °C, respectively. The heat dissipation performance of the LED module with the copper/alumina composite substrate with copper infiltration rate of 57.33 vol% was significantly better than that of the commercial alumina substrate. Fabricated composite substrate (57.33 vol% infiltration rate of copper) has a junction temperature of 80.5 °C and thermal resistance of 24.6 K·W−1, which are lower than the junction temperature (93.2 °C) and thermal resistance (32.1 K·W−1) of the commercial alumina substrate tested at same conditions. The reduction of junction temperature is important to improve the service life of the LED lamp.

Fabrication and properties of freeze-cast mullite foams derived from coal-series kaolin

Hierarchically porous mullite foams were fabricated from coal-series kaolin and Al2O3 slurries by freeze-casting using tert-butyl alcohol (TBA) and water as templates. TBA developed unidirectionally aligned pore channels along the freezing direction and water developed lamellar pores. The total porosity (60.2–83.4%), density (0.49–1.23g/cm3) and macro-pore size (10–50µm) of mullite foams were tailored by controlling the solid content of the slurries. The mullitization reaction was completed at the sintering temperature of 1500°C. The compressive strength of mullite foams along the freezing direction was 3.8–49.4MPa. Lower porosity and higher compressive strength of sintered mullite foams were obtained using TBA as solvent compared to water. The thermal conductivity of low density and mechanically stable TBA templated porous mullite with 80.2% porosity was 0.18W/mK, indicting that the freeze-cast porous mullite was a promising refractory material for applications in thermal insulation.

Submicron lamellar porous structure formed by selective dissolution of Ti-Al alloy

We created high-aspect-ratio sub-micron-sized lamellar porous structure by simple anodic selective dissolution of lamellar Ti-Al alloy in NaCl aqueous solution. The geometries of lamellar structures, which determined those of lamellar porous structure, were controlled by hot forging and subsequent ageing treatment before dissolution process. Although the changes in microstructure affected the selective dissolution behaviors, γ-TiAl phase could be dissolved selectively. As a result, the average sizes of unidirectional units of lamellar pores could be controlled in the range from to 25μm to 500μm. The pitches of the lamellar pores were controlled in the wide range from below 100nm to over 1μm.

Hierarchical Porous Zeolite Structures for Pressure Swing Adsorption Applications.

Porous adsorbents with hierarchical structured macropores ranging from 1 to 100 μm are prepared using a combination of freeze casting and additional sacrificial templating of polyurethane foams, with a zeolite 13X powder serving as adsorbent. The pore system of the prepared monoliths features micropores assigned to the zeolite 13X particle framework, interparticular pores of ∼1-2 μm, lamellar pores derived from freeze casting of ∼10 μm, and an interconnected pore network obtained from the sacrificial templates ranging from around 100 to 200 μm with a total porosity of 71%. Gas permeation measurements show an increase in intrinsic permeability by a factor of 14 for monoliths prepared with an additional sacrificial templated foam compared to monoliths solely providing freeze casting pores. Cyclic CO2 adsorption and desorption tests where pressure swings between 8 and 140 kPa reveal constant working capacities over multiple cycles. Furthermore, the monoliths feature a high volumetric working capacity of ∼1.34 mmol/cm(3) which is competitive to packed beds made of commercially available zeolite 13X beads (∼1.28 mmol/cm(3)). Combined with the faster CO2 uptake showing an adsorption of 50% within 5-8 s (beads ∼10 s), the monoliths show great potential for pressure swing adsorption applications, where high volumetric working capacities, fast uptakes, and low pressure drops are needed for a high system performance.

Macro- and micro-designed chitosan-alginate scaffold architecture by three-dimensional printing and directional freezing

While many tissue-engineered constructs aim to treat cartilage defects, most involve chondrocyte or stem cell seeding on scaffolds. The clinical application of cell-based techniques is limited due to the cost of maintaining cellular constructs on the shelf, potential immune response to allogeneic cell lines, and autologous chondrocyte sources requiring biopsy from already diseased or injured, scarce tissue. An acellular scaffold that can induce endogenous influx and homogeneous distribution of native stem cells from bone marrow holds great promise for cartilage regeneration. This study aims to develop such an acellular scaffold using designed, channeled architecture that simultaneously models the native zones of articular cartilage and subchondral bone. Highly porous, hydrophilic chitosan-alginate (Ch-Al) scaffolds were fabricated in three-dimensionally printed (3DP) molds designed to create millimeter scale macro-channels. Different polymer preform casting techniques were employed to produce scaffolds from both negative and positive 3DP molds. Macro-channeled scaffolds improved cell suspension distribution and uptake overly randomly porous scaffolds, with a wicking volumetric flow rate of 445.6 ± 30.3 mm3 s−1 for aqueous solutions and 177 ± 16 mm3 s−1 for blood. Additionally, directional freezing was applied to Ch-Al scaffolds, resulting in lamellar pores measuring 300 μm and 50 μm on the long and short axes, thus creating micrometer scale micro-channels. After directionally freezing Ch-Al solution cast in 3DP molds, the combined macro- and micro-channeled scaffold architecture enhanced cell suspension uptake beyond either macro- or micro-channels alone, reaching a volumetric flow rate of 1782.1 ± 48 mm3 s−1 for aqueous solutions and 440.9 ± 0.5 mm3 s−1 for blood. By combining 3DP and directional freezing, we can control the micro- and macro-architecture of Ch-Al to drastically improve cell influx into and distribution within the scaffold, while achieving porous zones that mimic articular cartilage zonal architecture. In future applications, precisely controlled micro- and macro-channels have the potential to assist immediate endogenous bone marrow uptake, stimulate chondrogenesis, and encourage vascularization of bone in an osteochondral scaffold.

New multifunctional porous Yb2SiO5 ceramics prepared by freeze casting

New porous Yb2SiO5 ceramics were prepared by a water-based freeze casting technique using synthesized Yb2SiO5 powders. The prepared porous Yb2SiO5 ceramics exhibit multiple pore structures, including lamellar channel pores and small pores, in its skeleton. The effects of the solid content and sintering temperature on the pore structure, porosity, dielectric and mechanical properties of the porous Yb2SiO5 ceramics were investigated. The sample with 20vol% solids content prepared at 1550°C exhibited an ultra-low linear shrinkage (i.e. 4.5%), a high porosity (i.e. 79.1%), a high compressive strength (i.e. 4.9MPa), a low dielectric constant (i.e. 2.38) and low thermal conductivity (i.e. 0.168W/(mK)). These results indicate that porous Yb2SiO5 ceramics are good candidates for ultra-high temperature broadband radome structures and thermal insulator materials.

Freeze cast fabrication of porous ceramics using tert-butyl alcohol–water crystals as template

Tert-butyl alcohol (TBA)–water (H2O) crystals are used as template to fabricate aligned porous alumina with special pore structures. Alumina slurries were prepared using TBA–H2O mixtures, which could undergo continuous crystallization by unidirectional freezing. The pore shape of porous alumina constantly changed from lamellar to snowflake, dendritic, needle-like, and hexagonal with the increase in the TBA content in alumina slurries. The hexagonal porous structure obtained by pure TBA slurry exhibited the best mechanical properties, whereas the mixture structure with irregular pore walls showed the worst, exhibiting inferior properties to lamellar pore structures.

Hierarchically ordered micro/meso/macroporous polymer-derived ceramic monoliths fabricated by freeze-casting

A hierarchically-ordered macro/meso/microporous SiOC monolith was obtained via freeze-casting using commercial polysiloxane as a raw material and silica sol as a binder and template source. The pre-ceramic polymer polysiloxane was pyrolyzed at 600°C to produce a hydrophilic surface; higher temperatures would fully decompose the organic groups. When silica sol and polysiloxane precursor were combined in freeze-casting method, after pyrolysis a polymer-derived SiOC ceramic monolith with a lamellar pore morphology and a hierarchically-ordered pore structure was obtained. Decomposition of the polysiloxane precursors results in the development of micropores, and particle packing is believed to be responsible for the mesopore formation. Macro/mesoporous hierarchically-ordered ceramics with a specific surface area of 74 m2/g are preserved at pyrolysis temperatures as high as 1000°C. The influence of H44-derived filler amount (10wt–40wt%), freezing temperature (−20°C, −80°C, −150°C), and pyrolysis temperature (600°C, 700°C, 1000°C) on open porosity, pore size distribution, and surface characteristics were investigated.

Mechanical performance and CO2 uptake of ion-exchanged zeolite A structured by freeze-casting

Zeolite 4A has been freeze-cast into highly porous monoliths with a cylindrical shape. The brittle monoliths, with lamellar or columnar pores and wall thicknesses between 8 and 35μm, show a compressive mechanical response along the main pore axis that could be modeled by a buckling behavior. The failure strength is proportional to the density and the amount of transverse bridging across lamella, which was shown to be related to the pore cross-sectional aspect ratio. Monoliths with highly anisotropic pores with a cross-sectional aspect ratio higher than 3 yielded sequentially from the top surface, whereas monoliths with a pore aspect ratio lower than 3 were found to delaminate into longitudinal splinters. The freeze-cast monoliths show a sharp gas breakthrough front with a 1:9 mixture of CO2 and N2, indicating rapid uptake kinetics of the lamellar structures.