|Title of Invention||
HIGHLY POROUS CERAMIC OR METALLIC MATERIAL AND SIMPLE ENVIRONMENT FRIENDLY PROCESS FOR FABRICATION OF THE SAME
|Abstract||A highly porous ceramic or metallic material with porosity ranging from 20% to 95% and comprising large number of ceils and area of interconnections between cells or pores. The porous material is fabricated by a simple environment friendly process comprising of preparation of aqueous solution of soap nut extract, forming of slurry, foaming and drying of the slurries at defined temperature.|
|Full Text||FIELD OF INVENTION
The present invention relates to highly porous ceramic or metallic materials and a simple environment friendly process for fabrication of the same. More particularly, the present invention involves naturally available organic materials, namely pericarp of the fruits belonging to the Sapindus family or more specifically the Sapindus Mukorossi or Sapindus trifoliatus (known as soap nut or Reetha) for preparation of the highly porous ceramic or metallic materials
BACKGROUND AND PRIOR ART
Cellular ceramic materials with high pore volume fraction are referred to as ceramic foams Ceramic foams are distinct in terms of their chemical inertness, high temperature resistance, low thermal conductivity, thermal shock resistance, and high specific strength Owing to these exceptionaVpr6pefties"ceramic foams are used in a large number of applications such as high temperature gas purification, molten metal filtration, thermal insulation, high efficiency combusticn burner and catalyst carriers for chemical plants and automobiles and large surface areas for the exchange of ions and catalysis. Ceramic foams are also being used for biomedical implant applications, impact absorbing structures tailor able dielectric constant material, sensors, gas detectors Ceramic foams are now being considered as a better alternative to fibrous ceramic insulation since they are much more (environmentally friendly and non-hazardous as compared to the fibrous materials Similarly metallic foams have a large number of commercial applications that include their use in making impact absorbing structures light weight high specific strength structures, structures for rapid heat dissipation, in sound absorption applications etc.
There are a number of processes that are being used commercially or have been proposed for fabrication of foams from use of particulate slurries. In US 5082607, P. Sepulveda et al.(Gel casting Foams for Porous Ceramics," Am, Ceram. Soc. Bull, 76  61-65 (1997); "Properties of Highly Porcus Hydroxyapatite obtained by the Gel casting of Foams." J Am. Ceran Soc 83 3021-24 (2000); "Processing of
Cellular Ceramics by Foaming and Insitu Polymerization of Organic Monomers," J. Eur, Ceram Soc, 19  2059-66 (1999)). J G P Bmner (Production and Properties of Low Density Engineering Ceramic Foams/ Bri Ceram Trans 96  247-49 (1997)), ceramic foams with similar microstructures have been produced by gel casting. In this process, aqueous slurries containing water-soluble monomer and cross linker are foamed in presence of surfactants v;a mechanical agitation. Although the process has a lot of flexibility m terms of varying the ceramic foam microstructure, it has a number of disadvantages that makes it unfavorable for commercial use Since gelation is achieved through polymerization of the water-soluble monomer and cross linker, the process requires mixing of a small volume of initiator (ammonium persulphate - APS) and catalyst (tetra methyl ethylene diamine -TEMED) homogeneously into a mucn larger volume of the foamed suspension. Homogeneous mixing of initiator and catalyst into the foamed slurries is usually difficult./Also as air (oxygen) inhibits the free radical polymerization, all "steps beginning from foaming of the slurries to gelation have to be carried out under inert conditions. The requirement of use of inert environment for celation Doses difficulties for the overall processing.
Ceramic foams fabricated by infiltration of reticulated poiyurethane preforms usually with aqueous particulate ceramic slurries have been reported m US 3090094, F. F. Lange ("Open-Cell. Low Density Ceramics Fabricated from Reticulated Polymer Substrate," Adv. Ceram. Mater , 2  827-31 1967) S J. Powell (The Structure of Ceramic Foams Prepared from Pofyurethane-Ceiamic Suspensions.' Mater. Manu fact Process, 10  757-71 1995). US 4.866.011. US 5.066 432, US Pat. 5,441,919. in the above process the poiyurethane samples saturated with the slurries following infiltration are squeezed to force out part of the slurry such that the remaining slurry coats the struts of the poiyurethane foam The samples are then dried and the poiyurethane preform is burned cut leaving the ceramic structure, which is then sintered to obtain the ceramic foam The structure of the ceramic foam produced by this process of infiltration is limited by the structure of the poiyurethane preform and the process results in foams witn pool mechanical properties due to the
hollow struts. Not only the preparation of polyurethane preforms, but also its burn out especially in high amounts as in the case here, is not environmental friendly as it produces toxic NOx emissions.
In US 6,015,517, PT 101910, porous ceramics are fabricated based on use of organic fugitive materials that are insoluble in aqueous ceramic slurries. Organic fugitives burn out leaving pores in the material. The percentage porosity produced through the fugitive based processes depends on the amount of fugitive used. The size of pores is limited by the size of the insoluble fugitive particles in the slurries.
Thus the prior art methods of gel casting, infiltration, and use of organic fugitive materials suffer from certain drawbacks. For gel casting method separate foaming agent is to be used and the steps from foaming to gelation need to be carried out in inert atmosphere, which is costly as well as cumbersome. The infiltration process requires a preform and burn out process poses environmental hazards and the hollow struts leads to poor strength of the ceramic foams. In the method using organic fugitives the size of pores is limited by the size of fugitive particles and involves large amount of organic material that has to be burnt out.
It has been now found that highly porous ceramic or metallic materials and fabrication of the same by direct foaming of soap nut based aqueous ceramic slurries overcomes these and other drawbacks of the prior art. The aqueous soap nut solution containing carbohydrates, water-soluble gums (which serve as binder) and saponin (which serves as a foamer in water) serves to overcome the problem for processing of ceramic foams. The porous materials thus produced can be tailored to have different total porosity, pore morphology and permeability for different applications. At one extreme the pores or cells can be made nearly isolated from each other and at the other extreme the pores can be made completely interconnected. An example application of the former type of structure could be in thermal insulation while for the latter it could be in filters, combustion burners etc.
OBJECTS OF THE INVENTION
Thus one of the basic objects of the present invention is to make highly porous sintered ceramic and metallic structures.
Another object of the present invention is to make bodies with percent porosity similar to that produced through reticulated polymeric precursor route but with strength higher than the latter.
Another object of the present invention is to make porous bodies with controlled pore morphology without using fugitives or polymeric preforms.
Another object of the present invention is to make porous bodies with graded porosity directly from the slurries or suspensions.
Another object of the present invention is to make porous ceramic or metallic materials with porosity ranging from 20 % to 95%.
Another object of the present invention is to make ceramic or metallic foams with highly interconnected pores or cells.
Another object of the present invention is to make ceramic or metallic foams with completely isolated pores or cells.
Another object of the present invention is to exercise control over cell size through manipulation of viscosity of the suspensions.
Another object of the present invention is to control the percentage porosity through the time of foaming.
Another object of the present invention is to control the cell morphology by varying the different additives.
Another object is to provide a simple environment friendly process for fabrication of highly porous materials, which obviates the drawbacks as detailed above
A further object of the present invention is to develop a process for producing ceramic or metallic foams by direct foaming and casting without the use of a preform-Yet further object of the present invention is to develop a process for stabilizing the foam by using guar gum.
Another object of the present invention is to develop a process that eliminates the requirement of use of chemicals to initiate gelation for development of strength in green ceramic foams.
Another object of the present invention is to develop a process for forming near net shaped porous structures based on casting of foamed ceramic or metallic suspensions.
'Another object of the present invention is to develop a process for fabrication of porous ceramic or metallic structures starting from powders using least amount of organic processing additives that have to be burnt out later,
A further object of the present invention is to develop a process for producing ceramic foams directly from slurries or suspensions without the need for inert environment for development of strength of the green body
SUMMARY OF INVENTION
According to one aspect of the present invention there is provided a highly porous ceramic or metallic material (foam) with porosity ranging from 20% to 95% with large number of cells and area of interconnections between cells or pores
According to another aspect of the present invention there is provided a process for preparation of a highly porous ceramic or metaiiic materials (foam) comprising.
(i) preparation of aqueous solution of soap nut extract;
(ii) preparation of slurry;
(iii) foaming of the slurries;
(iv) drying of the foamed slurries at defined temperature;
(v) slow binder burn out, followed by heating
(vi) sintering the foams depending on the powder particle size, desired densification of the struts.
DETAILED DESCRIPTION OF THE INVENTION
The highly porous ceramic or metallic material (foam) of the present invention according to one preferred aspect is produced using 35 Vol % alumina slurry mixed with 5 wt % soap nut extract and drying the sample at 60°C. According to another preferred aspect the foam is produced mixing 45 Vol % alumina slurry with 5 wt % soap nut extract and drying the sample at 40°C.
The interconnections between cells or the macro pores are formed during drying. Increase in drying temperature resulted in greater number and area of interconnections between cells or pores thus leading to a higher permeability. The average sizes of the foam cells achieved are typically in the range of 100 to 600 um. The numbers of interconnections per cell in sintered ceramic foams are in the range of 0.5 to 4
The process of the present invention involves the use of naturally available organic materials -pericarp of the fruits, which belong to the Sapindus family, more specifically Sapindus Mukorossi or Sapindus trifoliatus also popularly known as soap nut (or Reetha). The aqueous solution of the soap nut pericarp contains mixture of both saponin (17.2 %), carbohydrates and gums which are responsible for inducing
foam formation m suspensions, stabilizing the foam as wen as for providing strength to the green dried ceramic or metallic foams respectively Sopmclus Mukorossi used in the present invention is a relatively low cost, naturally available material, which serves multiple roles in fabrication of the ceramic or metallic foam that can make it a success in commercial manufacturing
The present process utilizes aqueous solution of soap not extract by making different wt % solutions of the water-soluble components of soap nut pericarp
The ceramic slurry is prepared using the soap nut extract solution from the beginning Alternatively, first the aqueous ceramic slurry without the soap nut extract is made and then the soap nut extract is added after the slurry preparation is completed This is preferred as this avoids foam formation clunng the step of slurry preparation Also this minimizes mechanical degradation of the soap nut extract
The prerrnx of different concentrations of the soap nut is used to make slurries with different volume % ceramic or metallic loading (Table 1) The slurries are prepared by ball milling the components
Slurry Wt % of Solids ioad:ng
soap nut in slurry
A1 5 Al2O3 (35 Voi%)
B1 2 Alp03(45Voi%)
C1 2 Al2O3(55Vol%)
C3 45 Aluminium (45
The as-prepared ceramic slurries are foamed, cast in molds and dried in a preheated oven. Drying is a critical step in processing of foams by the present process. Drying temperature plays an important part in influencing the ceramic foam microstructure.
Drying could be carried out under ambient ambitions or under controlled conditions where the temperature and humidity is controlled as in conventional ovens or microwave ovens etc. Drying at room temperature (25 - 35 °C) prolongs the drying time opening up the possibility of collapse of the foamed slurry. Collapse of foam is evident from a depression on the surface of the cast sample resulting in reduced porosity. Foam samples when dried in the range of 40 - 50 °C there are no foam collapse and the associated depression on the sample surface. Drying at higher temperature of 60 °C results in slightly higher porosity and cell size than the sample dried at 50 °C. Drying temperature is limited to around 70 °C as temperature above 70 °C results in bursting of foams forming cracks on the sample surface. The dried foams were subjected to binder burn out and the samples are sintered to get the foams.
Pericarp of dried soap nut fruits is ground using a food processor. The ground powder is mixed in water using a magnetic stirrer fixed rpm. The soap nut powder-water mix is kept in a water bath at 80 oC for 3 hours for extracting the water-soluble components of the soap nut. Then the insoluble components of the soap nut are removed by filtration and weighed to determine the percent of the soluble components. The solution containing water-soluble components is used to prepare slurries with different ceramic loading. Preparation of ceramic foams requires use of the water-soluble soap nut component, which has to be extracted by boiling the ground soap nut powder in water and then filtering away the insoluble components.
For lower Vol % of ceramic loading in the slurries use of higher wt % binder is desirable. This is because for example 35vol % alumina loading in slurries and 2 wt % soap nut the foam produced was unstable.
Another/Dossible approach involves preparation of the ceramic slurry and foamed solution of the soap nut separately and then mixing them at a later stage to get the foamed ceramic slurry.
The process of present invention can be used to make ceramic or metallic foams from aqueous slurries of powders with size range from a few nanometers to 100's of microns.
Foaming of the slurries could be carried out through various means such as stirring, ball milling, tumbling, mixing with air, or any other standard foam producing method. Depending on the method used for foaming of the slurries, the foaming time could vary from few minutes to several hours or 10's of hours.
Partial replacement of soap nut by guar gum is a preferred aspect. In such preferred embodiment the soap nut continues to serve the purpose of generating the foams. The guar gum serves to stabilize the foam by associating with water molecules as well as by enhancing the viscosity. Mostly foaming depends on the viscosity of the slurries, which can be controlled by varying the additives. There is a certain limit of viscosity above and below which the slurries did not go for foam formation. Slurries over a much wider range of viscosities can be foamed by using different types of foaming methods such as high speed agitator, turbulent mixing with air, injection of fine air bubbles, and any other mechanical methods of turbulent mixing that involve incorporation of air in the slurries. For high viscosity slurries methods that involve higher shear are required while for slurries with lower viscosities method that apply lesser shear will be applicable.
The aqueous slurries used for making ceramic or metallic foams may contain powder loading of 15 - 55 Vol % and 2 - 50 wt % soap nut extract besides other additives such as the dispersant. Through the use of the above process the volume percent porosity in foams could be varied from 20 - 95 %.
Binder burn out is carried out slowly preferably up to 400 °C.
Sintering is preferably carried out at temperature depending on the size of the
powdered particle and desired densification of strut.
The components used for the composition and processes are non-toxic and the materials left after burnt out are eco friendly. Thus no environmental hazards are associated with the present composition and its process of preparation.
The present invention is now described by way of some non-limiting illustrative examples-
In this example Al203 foams were produced with 92 % porosity. Al203 powder of particle size 0.7mm was dispersed with the help of dispersant Darvan 821A in 5 wt% premix solution (aqueous extract of the soap nut pericarp). Initially, slurry was prepared with 55 Vol% alumina loading by ball milling method for better dispersion and then diluted with the corresponding premix to 35 Vol%. Foaming was carried out by tumbling the jars containing the slurry for 12 hours. Simply drying soon after casting of the foamed suspension gave sufficient green strength to handle or machine the green foam samples. Binder burn out was carried out in steps up to 400°C and then directly to 900°C. Sintering was done at 1600°C for two hours.
The present example demonstrates that the porosity in ceramic foams could be varied by varying the solids loading (Vol%), the concentration of the pericarp extract. Alumina slurry was prepared with 55vol% alumina loading and 5 wt% aqueous soap nut extract. The as-prepared slurry was foamed by tumbling the jars containing the slurry for 12 hours. Foamed suspension was dried soon after casting and followed by binder burn out and sintering steps. The obtained porous body had 80 % porosity.
The example shows that porosity in ceramic foams could also be controlled, by changing the slurry amount and the container capacity. In examples 1 and 2, 45 ml of alumina slurry was foamed in 300 ml. In the present example, 60 ml batch of alumina slurry (55 Vol% alumina loading, 5 wt % aqueous extract) was foamed in a container of 600 ml capacity. The foamed suspension were cast and processed to achieve final sintered body. The obtained sintered body had nearly completely open pores with 88 % porosity.
The example describes preparation of metallic foams. Aluminium metal particulate slurries were prepared with 45vol % aluminium metal powder and 45wt% aqueous soap nut extract The foamed slurry was cast into a mold and dried in a preheated oven to obtain the green aluminium metal foam.
An aqueous soap nut solution was prepared and foamed separately using a mechanical stirrer. The alumina slurry with 55 volume % loading was mixed with a 15 wt % soap nut solution aqueous solution foamed separately. The mixing was achieved using a mechanical stirrer for 10 minutes. The resultant particle loading in the foam was 28 Vol % alumina and 10 wt % soap nut. The foam produced was stable and could be cast into a mold The cast foam was dried at 40°C followed by unmolding, burn out of the organics including the soap nut upto 900°C and then sintered at 1600°C.
Partial replacement of soap nut in the ceramic slurries by guar gum provided another means to control slurry rheology This was useful for processing of ceramic foams as foaming of aqueous ceramic slurries depends on the rheology of the slurries. Partial replacement of guar gum and resulting enhancement in slurry viscosity also stabilized special foam compositions that were initially unstable due to extremely low
slurry viscosity. By replacing 0.5 wt% soap nut with guar gum in 2 and 5 wt % soap nut extract, viscosity of ceramic slurries (55vol%) increased from 2.8 to7.9 Pa.s measured at a shear rate of 10s"1, while for 35 Vol% slurries viscosity increased from 0.13 to 1.08 Pa.s. The modified slurries containing guar gum were foamed, cast into molds and dried at 40°C. Binder burn out was carried out up to 900°C and the samples were sintered at 1600°C for two hours. The change in viscosity as a result of partial replacement of soap nut by guar gum also provided a means of controlling the foam microstructure including the total porosity, cell size, number and area of interconnections.
Addition of a coagulating agent (NH4NO3) was used as an alternative means of modifying the rheology of the soap nut based ceramic slurries. Just before foaming 2 cc of NH4NO3 (10wt%) solution was added to 35vol% alumina slurries prepared with 5 wt% soap nut extract. The above slurries were foamed, cast and the foamed body was dried at 40°C. Binder burn out and sintering was carried out at 900°C and 1600°C respectively. From the microstructure analysis, the sample obtained by the above process showed lesser number of interconnection and area of interconnection per cell was also less in comparison to the sample where coagulating agent had not been used.
Brief Description of Accompanying Figures
Figure 1 illustrates the foam microstructure with negligible interconnection area between cells in foams dried at 40°C.
Figure 2 illustrates the foam microstructure with very high area of interconnections between cells in foams dried at 60°C.
In figure 1 the microstructure is representative of foams prepared as described in example 7. It is found that the average cell size is 120 urn (micrometer) and the number of interconnections between cells (1) is 0.5 at a drying temperature of 40°C
In figure 2 the foam is prepared by method as described in example 1. It is found that the average cell size is 480 pm (micrometer) and the number of interconnections (2) between cells is 3.8 at a drying temperature of 60°C.
The main advantages of the present invention are:
1. Ceramic foam samples could be machined easily in the dried state. Different features such as corners, edges, grooves or holes could be produced to form components with desired features. Machining in the green state as mentioned here saves energy and does not produce defects in the body.
2. Foams with a very wide range of pore connectivity or permeability
3. The natural materials, belonging to the family Sapindus, that serve both as foaming agent and binder are available commonly in large quantities.
4. The material used for foaming and binding action is bio-degradable and thus environment friendly
5. The process involves use of absolutely non-toxic materials.
6. The overall process has low relative cost as compared to other processes used for making ceramic foams.
7. Processes like gel casting requires use of separate foaming agent while here the same material works as the binder and foaming agent.
8. The process involves fewer steps and is much simpler to use than other processes for making ceramic foams
9. Unlike the process of gel casting, the present process does not require use of inert environment - the whole process can be carried out in air.
10.The present process does not need any chemical for gelation, simply drying at 40°C provides enough green strength necessary for handling or machining.
11. With use of the natural foaming materials the process of foaming is very easy.
12. Unlike the limitation with use of ionic foaming agent, soap nut pericarp (Reetha) can be used without causing coagulation in the ceramic suspensions.
13. The amount of organic material (binder cum surfactant) in the dried green foam is very small thus reducing the material that has to be burnt Pmt later.
1. A highly porous ceramic or metallic material (foam) with porosity ranging from 20% to 95% with large number of cells and area of interconnections between cells or pores.
2. A highly porous ceramic or metallic material (foam) as claimed in claim 1 wherein the size of the foam cells vary in the range of 100 to 600 micrometers.
3. A highly porous ceramic or metallic material (foam) as claimed in any one of claims 1 to 2 wherein number of interconnections per cell in the range of 0.5
4. A process for preparation of a highly porous ceramic or metallic material (foams) comprising the following steps:
i. preparation of aqueous solution of soap nut extract such as
herein described; ii. preparation of slurry such as herein described; iii. foaming of the slurries such as herein described; iv. drying of the foamed slurries at defined temperature such as
herein described; v. slow binder burn out such as herein described; vi. sintering the foams such as herein described.
5. A process as claimed in claim 4 wherein aqueous solution of soap nut extract is prepared by drying soap nut fruits; grounding in a food processor; mixing of the so formed ground powder in water using a magnetic stirrer having a fixed rpm; extraction of water soluble components of soap nut.
6. A process as claimed in claim 5 wherein extraction of water soluble components of soap nut is carried out by keeping it in a water bath at 80 °C for 3 hours.
7. A process for preparation of a highly porous ceramic or metallic material (foam) as claimed in claims 4 to 6 wherein the slurry is prepared by ball milling of water soluble components of soap nut with ceramic loading.
8. A process as claimed in claim 4 to 6 wherein the ceramic slurry containing powder loading of 15 - 55 Vol % is prepared separately and then 2 - 50 wt % soap nut extract is added and the components are ball milled to form the slurry.
9. A process as claimed in claims 4 to 6 wherein ceramic slurry is prepared without soap nut extract and foamed soap nut extract added at later stage after slurry preparation.
10. A process as claimed in claims 4 to 9 wherein foaming of the slurry is selectively carried out by stirring, ball milling, tumbling, mixing with air, or any other standard foam producing method with slurry comprising soap nut extract with ceramic loading.
11. A process as claimed in claims 4 to 10 wherein drying of the foamed slurries is carried on at around 65 °C but below 70 °C.
12. A process as claimed in claims 4 to 11 wherein guar gum is preferably added for stabilizing the foam on association with water molecules and enhancing the viscosity.
13. A process as claimed in claims 4 to 12 wherein binder burn out is carried out slowly preferably upto 400°C and then directly to 900°C.
14. A process as claimed in claims 4 to 13 wherein sintering is preferably carried out at temperature depending on the powder particle size, desired densification of the struts.
15. A highly porous ceramic or metallic material as substantially described and illustrated with reference to examples and figures.
16. A process for preparation of a highly porous ceramic or metallic material (foam) as substantially described and illustrated with reference to examples and figures.
A highly porous ceramic or metallic material with porosity ranging from 20% to 95% and comprising large number of ceils and area of interconnections between cells or pores. The porous material is fabricated by a simple environment friendly process comprising of preparation of aqueous solution of soap nut extract, forming of slurry, foaming and drying of the slurries at defined temperature.
|Indian Patent Application Number||473/KOL/2003|
|PG Journal Number||20/2007|
|Date of Filing||08-Sep-2003|
|Name of Patentee||INDIAN INSTITUTE OF TECHNOLOGY & COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH|
|Applicant Address||KHARAGPUR ,PIN-721302,|
|PCT International Classification Number||B 22 P 3/10|
|PCT International Application Number||N/A|
|PCT International Filing date|