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Using Flixweed Seed as a Pore-former to Prepare Porous Ceramics | OMICS International
ISSN: 2169-0022
Journal of Material Sciences & Engineering
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Using Flixweed Seed as a Pore-former to Prepare Porous Ceramics

Hedayat N and Du Y*

College of Applied Engineering, Sustainability and Technology (CAEST), Aeronautics & Technology Building (ATB), Kent State University, Lefton Esplanade, Kent, Ohio, USA

*Corresponding Author:
Du Y
College of Applied Engineering Sustainability and Technology (CAEST)
Aeronautics & Technology Building (ATB)
Kent State University,1400 Lefton Esplanade Kent
Ohio 44242, USA
Tel: 3306721910
Fax: 3306722894
E-mail: [email protected]

Received Date: April 25, 2016; Accepted Date: May 16, 2016; Published Date: May 26, 2016

Citation: Hedayat N, Du Y (2016) Using Flixweed Seed as a Pore-former to Prepare Porous Ceramics. J Material Sci Eng 5: 255. doi: 10.4172/2169-0022.1000255

Copyright: © 2016 Hedayat N, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

Flixweed (Descurainia Sophia L.) seeds, known as an herbal medicine, for the first time are used as a promising pore-forming agent (pore-former) in ceramic technology. Flixweed seeds were selected because of their unique constant shape (oblong, 1.2 mm long with the aspect ratio of about 2) and narrow size distribution as well as their low-cost. Porous zirconia ceramics have been fabricated using flixweed seeds by tape casting technique. The dried tape-cast cut into disk-shaped pieces and were fired at 1400°C for 2h, resulting in porous zirconia disks with a bulk density of 3.96 g/cm3, total porosity of 34.6 ± 0.9% (open porosity 25.5 ± 0.7%, closed porosity 9.1 ± 0.3%) and a linear shrinkage of 21.5 ± 0.3%. The pore shape and size were similar in shape and size to the original pore-former.

Keywords

Pore-former; Porous ceramics; Microstructure; Tape casting; Flixweed seeds

Introduction

Porous ceramics are used for an ever-expanding range of applications in bone tissue engineering, membrane separation, and catalytic reactors. Different applications of porous ceramics have been described in a book edited by Scheffler and Colombo [1]. A number of processing routes are used to prepare porous ceramics including partial sintering [2], gel casting [3], sol-gel technique [4], dry foaming method [5], and the incorporation of pyrolizable pore-forming agents (poreformers) that burn out during firing. Various types of pyrolizabale pore-formers have been examined as sacrificial templates or fugitive materials to obtain the different shape and size of pores [6-11]. The pyrolizable pore-formers have behaved as template in forming the pores that correspond closely to the shape and size of original pore-former [12]. The number and nature of the pores (open pores and closed pores) can be determined by the combination of characterization techniques including Archimedes method, scanning electron microscopy (SEM) imaging, optical microscopy, focused ion beam (FIB)-SEM studies, mercury porosimetry, and gas permeability tests [13].

The pore diameter size that is closely connected to the size of the pore-former is determinable by microscopic image analysis or three dimension (3D) tomography, but the pore size determined by mercury intrusion is the size of interconnections between open pores and refers to pore throat size, which is usually in the range of about 1–10 μm [14]. Pore-formers with the diameter of about 3 μm or smaller have significant impact on the sintering kinetics and shrinkage, and poreformers with the diameter of about 20 μm are used to tailor the porosity and to improve the gas diffusion in porous electrodes for solid oxide fuel cells [7]. The pore size can be controlled by varying the following variables: pore-former size [12]; thermal decomposition profile of the pore-former [15]; and volume ratio of pore-former/ceramic particles [12,16]. Hu et al. [17] reported that the porosity and also the number of large pores are increased with the pore-former loading. A composite pore-former containing two or more pore-formers in the different size range can be used to improve the porous structure and adjust the shrinkage [17,18]. Small pores (<100 µm) have been generated using common pore-formers such as different graphite types [9], different starch types [6], polymethyl methacrylate (PMMA) [7], carbon microspheres [8], cellulose [10], and paper-fibers [19]. Larger pore-formers that can generate larger pores are desirable for many purposes such as acoustic and thermal insulation materials, lightweight structured ceramics, and bone tissue ingrowth into bio-ceramic he pore diameter size that is closely connected to the size of the pore-former is determinable by microscopic image analysis or three dimension (3D) tomography, but the pore size determined by mercury intrusion is the size of interconnections between open pores and refers to pore throat size, which is usually in the range of about 1–10 μm [14]. Pore-formers with the diameter of about 3 μm or smaller have significant impact on the sintering kinetics and shrinkage, and poreformers with the diameter of about 20 μm are used to tailor the porosity and to improve the gas diffusion in porous electrodes for solid oxide fuel cells [7]. The pore size can be controlled by varying the following variables: pore-former size [12]; thermal decomposition profile of the pore-former [12]; and volume ratio of pore-former/ceramic particles [12,16]. Hu et al. [17] reported that the porosity and also the number of large pores are increased with the pore-former loading. A composite pore-former containing two or more pore-formers in the different size range can be used to improve the porous structure and adjust the shrinkage [17,18]. Small pores (<100 µm) have been generated using common pore-formers such as different graphite types [9], different starch types [6], polymethyl methacrylate (PMMA) [7], carbon microspheres [8], cellulose [10], and paper-fibers [19]. Larger pore-formers that can generate larger pores are desirable for many purposes such as acoustic and thermal insulation materials, lightweight structured ceramics, and bone tissue ingrowth into bio-cera

Flixweed (Descurainia sophia L.) seeds, a known medical herb commonly used in traditional medicine [21], is one of the most abundant weeds in North America and China [22]. Felixweed seed is very small, dark yellow or brown, possess an uneven surface in a stretched oval form, one end of which is cut and maintains a transparent yellowish ring [23]. Flixweed seeds might be a potentially interesting and unique pore-former for ceramics, and there is no report so far on the use of flixweed seeds as a pore-former in ceramic fabrication. In the present study, we report the first results on the use of flixweed seeds to fabricate porous ceramics. A composite pore-former containing microcrystalline cellulose and flixweed seeds together used to generate zirconia ceramic with a hierarchical porosity including small and large pores derived from the burnout of microcrystalline cellulose and flixweed seeds. Cellulose as a natural pore-former has a number of advantages including easy availability and processing (narrow decomposition temperature range of 300-350°C), and lowcost [24]. Alumina ceramics prepared using poppy seeds [14,25], and commercially milled coffee [25] are compared to the samples fabricated using flixweed seeds by tape casting.

Materials and

Reagents and apparatus

Flixweed seeds were purchased from Sadaf (Soofer Co., Inc., USA), and microcrystalline cellulose (PH-301) was provided by FMC BioPolymer, USA. TZ-3Y powder was obtained from Tosoh, Japan. Methyl ethyl Ketone (MEK, ≥99.0%) and polyethylene glycol (PEG 200) were purchased from Sigma-Aldrich. Hypermer KD-1, Butvar (polyvinyl butyral (PVB), B-98) and Ethanol (94-96%) were prepared form Tape Casting Warehouse, Inc., USA, Solutia, Inc., and Alfa Aesar, respectively. A lab roll ball mill (Tencan, Model No.: QM-5, China) using yttria stabilized zirconia (YSZ) cylinders as milling media (5×5 mm, Inframat Advanced Materials) was used for the ball milling of slurry. Tape casting was performed using a Lab scale tape caster (Richard E. Mistler, Inc.), in which slurry was cast onto a polyethylene carrier film (Mylar sheet). A muffle furnace (Across International, NJ) was used for the firing of samples. The S-2600N scanning electron microscope that was used to take the SEM images is from Hitachi, Japan.

Procedure

In the present study, tape casting (TC) or doctor blade process that is a low-cost and simple method to fabricate thin flat sheets of ceramics was used. Figure 1 depicts a schematic representation of fabricating porous zirconia ceramic using flixweed seeds by tape casting. Zirconia tape was produced by tape casting of the slip that was prepared by (i) weighing the TZ-3Y powder in the required amount to prepare a 60 wt% zirconia suspension, (ii) dispersing the TZ-3Y and dispersant powder (KD-1) in a binary solvent system of ethanol and methyl ethyl ketone with the ratio of EtOH/MEK : 34/66 wt% for 3h (dispersion step), (iii) introducing tape casting additives such as binder (B-98), plasticizer I (S-160), and plasticizer II (PEG 200), and (iv) ball milling the resulting mixture for 24h (thickening step). Finally, flixweed seeds were mixed, and dispersed using a mechanical stirrer to make the dispersion uniform for 15 min. The flixweed seeds were mixed and dispersed right after ball milling and before de-airing to prevent the breakage of the flixweed seeds during ball milling, and to take advantage of the presence of solvents that could help to dispersion of the seeds before de-airing. De-airing under a mechanical vacuum is necessary to remove the extra solvent and also to prevent the formation of air bubbles during the casting. A mechanical vacuum pump and a vacuum chamber were used to perform the de-airing during magnetic stirring. Upon de-airing, the mechanical stirring continued for further 5 min. to make the dispersion more uniform. The slurry cast at the thickness of 3000 μm, and the casting speed was 2 mm/s. The tape-cast dried for 48h, and the thickness of dried tape was 1000 μm. The tapecast was cut into 25 mm diameter disks, and fired at 1400°C for 2h, resulting in disks without visible cracks.

Results

Figure 2a shows a SEM image of flixweed seeds with typical oblong shape. Figure 2b shows a SEM image of flixweed seeds and the network texture on the surface. Table 1 presents the maximum Feret diameter (length), minimum Feret diameter (width) and aspect ratio of flixweed seeds obtained by image analysis software (Image J) that revealed the particle size distribution of flixweed seed is narrow.

Table 2lists the density of ingredients used. The density of flixweed seed is required to calculate the volume percentage of pore-former in the system and it was determined by floating flixweed seeds in a sugar solution (saccharose). The addition of sugar to the water in which flixweed seeds had settled down continued until the flixweed seeds exhibited buoyancy. The measured density of flixweed seeds in this study is 1.14 ± 0.04 g/cm3.

Figure 3 shows the SEM micrograph of the pores generated by burning out of flixweed seeds. As expected, the shape of the pores due to flixweed seed is oblong. The maximum Feret diameter, the minimum Feret diameter, and the aspect ratio of the pores derived from the burnout of flixweed seeds are 740 μm, 380 μm, and 1.9, respectively. Figure 4 shows the SEM micrograph of the pores generated by burning out of microcrystalline cellulose. The pore diameter of the pores derived from the burnout of microcrystalline cellulose is 0.3 μm.

Archimedes relations are used to calculate the bulk density (Db), and open porosity (Po) of composites. WD , WS and WI are the weight of dry, weight of saturation and weight of immersed in distilled water (Dwater = 1 g/cm,sup>3), respectively.

(1)

(2)

material-sciences-engineering-Schematic-representation-5-255-g001

Figure 1: Schematic representation of the porous zirconia fabrication using fixweed seeds by tape casting.

material-sciences-engineering-SEM-micrography

Figure 2: (a) SEM micrography of fixweed seeds with typical oblong shape;(b) SEM micrography of fixweed seeds demonstrating the network texture on the surface, which is the reticulum of wider-than-long pits,like corn on the cob.

Feature Arithmetic mean value and standard deviation
Maximum Feret diameter (µm) 1277 ± 23
Minimum Feret diameter (µm) 680 ± 21
Aspect ratio 1.88 ± 0.08

Table 1: Size and shape characteristics of flixweed seeds (Feret diameters and aspect ratios determined by image analysis).

Ingredient Density (g/cm3)
TZ-3Y 6.05
EtOH 0.79
MEK 0.81
PVB 1.08
KD-1 0.88
PH-301 0.37
Flixweed seeds 1.14
S-160 1.12
PEG 200 1.12

Table 2:Densities of the ingredient used.

material-sciences-engineering-SEM-micrograph-large

Figure 3: SEM micrograph of large pores derived from the burnout of flixweed seeds after firing at 1400°C.

The total porosity (PT) is obtained from the following equation:

(3)

Where,

Dth is the theoretical density of zirconia (TZ-3Y), i.e., 6.05 g/cm3.

Table 3 summarizes the microstructural characteristics of fired porous zirconia ceramics with pores derived from microcrystalline cellulose and flixweed seeds. Microcrystalline cellulose and flixweed seeds were added to 165 g of the as-prepared zirconia suspension in the amounts of 5.2 and 1.2 g that according to the densities presented in Table 2 corresponds to a total pore-former volume of 28.1% on dry green tape basis. The total porosity of fabricated porous zirconia is 34.6% that is 6.5% higher than the pore-former content of dry green tape. This extra porosity must be a result of dispersant burnout (4.2 vol% on dry green tape basis) and also from the binder burnout (18.8 vol. % on dry green tape basis). Živcová et al. [25] used traditional slip casting (TS) into plaster molds (cylindrical rods, diameter 5 mm) and starch consolidation casting (SC) using metal molds (cylindrical rods, diameter 7 mm) for the fabrication of porous alumina ceramics. In the starch consolidation casting (SC), starch can be used as pore-former and at the same time binder [14,25]. Commercially milled coffee, and poppy seeds are of potential interest due to their specific size. Table 4 compares the alumina ceramics prepared using commercially milled coffee, and poppy seeds, and the zirconia ceramics prepared using flixweed seeds as pore-former. The data for alumina ceramics prepared using potato starch is also presented because in alumina ceramics prepared using coffee and poppy seed by starch consolidation method potato starch performs as binder. Fabriation of the zirconia with hierarchical porosity using flixweed seed by tape casting has generated ceramics with the total porosity of about 35% that is in the same range compared to other methods and pore-formers. About 26% of the formed pores in the prepared porous zirconia are closed, but they are functional for uses such as acoustic and thermal insulation. The lamination of ceramic sheets prepared by tape casting and multi-layer tape casting provides the advantage of manufacturing ceramic sheets with gradual porosity. The controlled pore size and porosity enables the technique to fabricate ceramic multi-layers with different thermal conductivity in each layer.

material-sciences-engineering-SEM-micrography-small

Figure 4: SEM micrograph of small pores derived from the burnout of microcrystalline cellulose after firing at 1400°C.

Feature Arithmetic mean value and standard deviation
Bulk density (g/cm3) 3.96 ± 0.05
Apparent density (g/cm3) 5.31 ± 0.03
Open porosity (%) 25.5 ± 0.7
Total porosity (%) 34.6 ± 0.9
Closed porosity (%) 9.1 ± 0.3
Linear shrinkage (%) 21.5 ± 0.3
Volumetric shrinkage (%) 44.5 ± 0.5
Weight loss (%) 8.1 ± 0.2

Table 3: Microstructural characteristics of porous zirconia with large and small pores due to flixweed seeds and microcrystalline cellulose fired in 1400°C.

Pore-former Fabrication method Pore-former [vol.%] Open porosity [%] Total porosity [%] Characteristic diameter of pore-former [µm] Ref.
Potato starch SC 10 22.0 ± 0.3 25.8 ± 0.3 46.3 [25]
30 32.9 ± 1.3 37.0 ± 0.5
Coffee TS 19.7 25.7 ± 1.5 32.7 ± 0.9 405 [25]
SC 36.7 (1) 44.6 ± 0.7 46.6 ± 0.4
Poppy seeds SC 32.3 (2) 29.6 ± 1.6 38.7 ± 1.3 max Feret: 1038 [14]
min Feret: 1265
Flixweed seeds TC 28.1(3) 25.5 ± 0.7 34.6 ± 0.9 max Feret: 1277 This work
min Feret: 680

Table 4: Alumina ceramics prepared using different pore-formers compared to the samples fabricated in this study.

using flixweed seeds as pore-former. The data for alumina ceramics prepared using potato starch is also presented because in alumina ceramics prepared using coffee and poppy seed by starch consolidation method potato starch performs as binder. Fabriation of the zirconia with hierarchical porosity using flixweed seed by tape casting has generated ceramics with the total porosity of about 35% that is in the same range compared to other methods and pore-formers. About 26% of the formed pores in the prepared porous zirconia are closed, but they are functional for uses such as acoustic and thermal insulation. The lamination of ceramic sheets prepared by tape casting and multi-layer tape casting provides the advantage of manufacturing ceramic sheets with gradual porosity. The controlled pore size and porosity enables the technique to fabricate ceramic multi-layers with different thermal conductivity in each layer.

Conclusion

In this study, it is shown that flixweed seeds are a potential poreformer for ceramic preparation. The defect-free burnout of flixweed seeds was feasible using a conventional firing profile. The density measurement revealed that the density of flixweed seed is a slightly higher than that of water. Therefore, not only for organic slurry formulations, but also for aqueous slurry formulations flixweed seeds are appropriate pore-former compared to many synthetic polymeric pore-formers that are exhibiting strong buoyancy effects at the same size. Among the main advantages for the use of flixweed seeds as a pore-former in ceramic technology are easy handling, defect-free burnout, lowcost, ready availability in the constant oblong shape with narrow size distribution and well-defined aspect ratio (about 2). Considering that the thermal barrier coating is one of the most important applications of zirconia-based ceramics [26], the compatibility of flixweed seeds with zirconia and its large size highlights the use of this natural pore-former.

Acknowledgements

The SEM data were obtained at the SEM lab of the Characterization Facility of the Liquid Crystal Institute, Kent State University.The authors acknowledge the FMC BioPolymer, USA for providing microcrystalline cellulose (PH-301).

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