Development of Carbon Foam and Silica Foam by Template route

Name : Nikunj Patel Parth M.Ka.patelSem. : 4th Roll no.: 11,12Department of Materials scienceDate : 16/4/2015

Guides : Dr. (Miss)R.H.Patel Mr. Milan Vyas

Outline What is Foam ?Two types of Foam: 1.Closed-cell Foam 2.Open-cell FoamBasic terminologies of foams: strut, cell and poreTypes of Foam: -Polymeric foam -Metallic Foam -Ceramic FoamVarious routes to fabricate the porous ceramics (A)Replica method (B)Direct foaming methods and (C)Sacrificial templateDevelopment of carbon foamDevelopment of silica foamCharacterization of Foam and ResultsReferences

What is Foam?- A Foam is a substance that is formed by

trapping pockets of gas in a liquid or solid. Now carbon foam means a carbonaceous

porous solid having well connected network of struts.

Many modern day natural occurring porous materials are being used for a long time, e.g. wood, cork, bone, etc. in many applications in day to day life by human beings.

Having derived inspiration from nature, several synthetic cellular materials have been developed.

e.g. polymeric foams for packaging, metallic in crush protection and ceramic for water purification. [1]

Two types of Foam The foams can be classified into

two types based on their pore structure:

1.Open cell structured foams (also known as reticulated foams i.e. like a net) and 2.Closed cell foams shown in figure.

1.Open cell foam

2.Closed cell foam

1.Open cell structured FoamOpen cell structured foams are

relatively soft as they contain pores that are connected to each other and form an interconnected network.

Open cell foams can be filled with whatever those are surrounded with.

i.e. If filled with air this could be a relatively good insulator, but if the open cells are filled with water, insulation properties would be reduced. The solid component of reticulated foam may be an organic polymer like polyurethane, a ceramic or a metal.

These foams are used in wide range of applications where high porosity and large surface area are needed, including filters, catalyst supports, fuel tank inserts, and loudspeaker covers.

Close cell structured FoamClosed cell foams have isolated pores. Normally

closed cell foams have higher compressive strength due to their structures.

However, closed cell foams are also generally denser, require more material, and consequently are more expensive to produce.

The closed cells can be filled with a specialized gas to provide improved insulation.

The closed cell structure foams have higher dimensional stability, low moisture absorption coefficients and higher strength compared to open cell structured foams.

All types of foam are widely used as core material in sandwich structured composite materials.

The disadvantage of the closed-cell foam is that it is denser, requiring more material, and therefore, more expensive.

Basic terminologies of foams

Strut, cell and pore :The microstructure of foam in below

figure shows cells, pores and struts. The basic structure of foam is made up of

interconnected cells. The cells are made of pores and

connected through struts. Struts are also called ligaments. In given foam, there are pores joined through ligaments.

Pore

cell

strut

Types of Foam

With specific application of foam, foam of any material or composition are being developed.

e.g. Polymeric foam for room to low temperature applications,

Metallic foam for structural applications and

Ceramic foam for high temperature insulation applications etc.

Polymeric foamPolymer foams are made up of a solid and

gas phase mixed together to form a foam. Polymer foams can be divided into either

thermoplastics or thermosets, which are further divided into rigid or flexible foams.

Examples of different polymeric foams are given in below figure.

Polymer foams are used in a wide variety of applications such as disposable packaging of fast-food, the cushioning of furniture and insulation material.

(i) PVC foam , (ii) polystyrene foam , (iii) polyethylene foam and (iv) polyurethane foam

Metallic foamsMetal foams are a new class of material.They are light and stiff, they have good energy-

absorbing characteristics (making them good for crash-protection and packaging) and they have attractive heat-transfer properties (used to cool electronic equipment and as heat exchangers in engines).

Below Figure shows examples of metallic foams such as 1.copper, 2. silicon, 3. gold foam.

1.Copper 2. silicon 3. gold foams

Ceramic Foams Ceramic foam is usually manufactured by

impregnating open-cell polymer foams internally with ceramic slurry and then firing in a kiln, leaving only ceramic material.

The foams may consist of several ceramic materials such as aluminium oxide, silica etc .

The foam is often used for thermal insulation, acoustic insulation, adsorption of environmental pollutants, filtration of molten metal alloys and as substrate for catalysts requiring large internal surface area.

Ceramic foams like zirconia (ZrO2), silicon carbide (SiC), alumina foam (Al2O3) and carbon foam are shown in below figure .

Various routes to fabricate the porous ceramics

The processing routes described have been classified into

(A)Replica method (B)Direct foaming methods and (C)Sacrificial template,

Schematically illustrated in figure (A, B & C).

Replica method

In this method ceramic suspension or precursor solution that contain ceramic is impregnated in cellular structure and after sintering it turns into a macroporous ceramic exhibiting replica of original porous material. Figure(A) shows schematic of sacrificial replica foam method.

Figure(A)

Direct foaming of liquid slurry

Direct foaming consists in generation of bubbles inside liquid slurry containing ceramic powders or inside a ceramic precursor solution to create foam which then needs to be set without collapsing obtained porous network before heating to high temperature. (figure (B))

Figure(B)

Sacrificial template method

The sacrificial template technique usually consists of the preparation of a biphasic composite comprising a continuous matrix of ceramic particles or ceramic precursors and dispersed pore former phase that was initially homogeneously distributed throughout matrix and was ultimately extracted to generate pores within the material.

The schematic diagram of burn-out pore former fugitive method is shown in figure(C).

Figure (C)

Conti….A wide variety of materials had

been used as pore formers, including compound of natural and synthetic organics, liquids, ceramics, metals, polymers, proteins, etc. which decompose or degrade or dissolve leaving behind the porous structure depending upon pore former used and removal method.

All method discussed above viz. replica method or direct foaming method or burn-out of fugitive pore former offers possibility to develop ceramic foams with wide range of morphology and properties.

Selection of processing technique basically depends on type of application aimed and thus microstructural characteristics.

Porous ceramics obtained with the sponge replica method can gives open porosity within the range 40%–95% and was characterized by a reticulated structure of highly interconnected pores with sizes between 200 mm and 3 mm.

Replication technique is a well established method to prepare ceramic foams in the range of 100 micron to 3 mm or more.

Direct foaming method yield porous ceramic foam with pore size in range of 10 micron to 1.2 mm. Incorporation of pore former fugitive method produce ceramic foams with pore size in the range of 1 to 100 micron. All the methods differ greatly in terms of processing parameters and final porous ceramic structure obtained.

Development of Carbon foamCarbon foams were developed by using

template route. Commercially available open cell polyurethane (PU) foam was used as template and phenolic resin was used as carbon precursor.

PU foams were impregnated with phenolic resin solution and cured.

Carbonization of cured foam was carried out up to 700 °C in an inert atmosphere to make carbon foam.

Materials used: Commercially available open cell polyurethane

foams Phenolic resin, Gas: Nitrogen.

Characteristics of Polyurethane foam: Open cell polyurethane (PU) foams,

commercially available were collected having different densities shown in below figure. There were of different colours.

Open cell PU foams used were characterized for their density. The densities of different foams were calculated and table-1 gives density of different types of foam. The density varies from 0.02 to 0.04 g/cc.

Figure: PU Foam having different densities

Table I: Density of different PU foams No. Colour PU foam density, (gm/cc) i White 0.020 ii Dark grey 0.026 iii Yellow 0.030 v Pink 0.040

Preparation of resin solution: =122 ml Phenol=122 ml Formaldehyde(HCHO)=7.300 ml NaOH solution

Resole Color is Reddish

Brown

Experimental work: Cleaned PU foam was impregnated with phenolic resin. The open cell PU foams having density of 0.04 g/cc were selected

for development of foam. The foams were washed in distilled water and dried in oven at

100 °C. The colour of PU foam was pink. Cleaned PU foams were cut into

small rectangular and square pieces. Dimensions of PU foam pieces were taken using vernier callipers

and mass by using electronic balance For that PU foam was dipped into beaker filled with phenolic

resin. The excess resin was removed from time to time to achieve

uniform impregnation. Drying and curing of foams: Drying of resin impregnated foam was carried out in oven at

between 60-70 °C over night and then after one day at 140 °C. To enhance the degree of cross linking between phenolic chains,

impregnated PU foams were cured at 150 °C overnight. The change in color of foam appeared to be dark brown as shown

in figure

Carbonization:The cured foams were carbonized at 700 °C in nitrogen atmosphere. The carbonization assembly used for making carbon foam is shown in figure (A). Carbonization was carried out in carbonization reactor having gas outlet and inlet on same side. The reactor was made up of stainless steel material. The S.S. container was placed in a muffle furnace which was heated electrically. Heating and cooling rate of the furnace was controlled by temperature programmer. The temperature of furnace was measured using thermocouple and temperature programmer/controller was used to maintain the temperature of furnace. High purity Nitrogen gas was used during carbonization as an inert gas.

Figure (A): Set up for carbonization

Figure (B): Schematic diagram for development of carbon foam

Phenolic resin

Development of silica foam: Cleaned PU foam was impregnated with silica sol The open cell PU foams having density of 0.03g/cc and 0.04 g/cc

were selected for development of silica foam. The foams were washed in distilled water and dried in oven at

100 °C. The colour of PU foam was yellow and pink. Cleaned PU foams

were cut into small rectangular and square pieces. Dimensions of PU foam pieces were taken using vernier callipers

and mass by using electronic balance For that PU foam was dipped into beaker filled with silica sol. The excess silica-sol was removed from time to time to achieve

uniform impregnation. Drying and curing of foams: Drying of resin impregnated foam was carried out in oven at

between 60-70 °C over night and then after one day at 100 °C. To enhance the degree of cross linking between silica-sol,

impregnated PU foams were cured at 100 °C. Then placing this bulk foam in the furnace in air for sintering.

Continue…..work..

Characterization of foams:Physical properties:Volume shrinkageThe reduction in length, breadth and thickness during

carbonization and sintering was measured by measuring dimensions both before and after heat treatment using vernier callipers. The shrinkage in all direction was calculated by using formula:

where, L.before – Length before heat treatment, cm L.after – Length after heat treatment, cm Likewise, shrinkage in breadth and thickness was

determined by above formula.

Carbon yield, (%): Carbon yield was calculated from the initial weight

of materials (W1) before carbonization and final weight of carbon (W2) obtained after carbonization using following relation.

Density Measurement: Bulk density of the samples was theoretically

calculated. Geometric volume of sample was calculated by measuring length, breadth and thickness of foam with the help of vernier callipers.

Volume of sample (V) = l * b * t Where, l = length of the sample b = breadth of the sample t = thickness of the sample Density of sample was determined using following

formula:

Density = mass of sample(W) Volume of sample(V) Kerosene porosity: The kerosene pick-up method was used for determination

of open porosity of developed carbon and silica foams. Figure(1) shows set-up used for determination of

kerosene porosity of foams. It consists of a glass flask connected to a kerosene

reservoir and vacuum system with stopcocks. One stopcock was joined in between flask and kerosene

reservoir and the second stop cork was joined in between flask and vacuum system.

A small piece of foam of known measured dimensions was taken and weighed. It was placed in the round bottom flask.

Flask was connected with vacuum system by opening stopcock 2.

The sample was evacuated at 10-3 torr for one hour with the help of vacuum pump to clean the sample.

The stopcock -2 was closed after evacuation for one hour and kerosene was allowed to flow by opening stopcock- 1. Kerosene was allowed to flow into open pores of sample. After equilibrium was attained, sample was taken out and excess of kerosene was wiped out. Sample was weighed using electronic balance. The density of kerosene was determined using specific gravity bottle. The kerosene porosity of sample was calculated using following reaction.

% porosity=[Wb-Wa] Vs * DkWhere,Wa = weight of sample before adding kerosene/water Wb = weight of sample after adding kerosene/water Vs = volume of sample Dk = Density of Kerosene/water

Figure (1): Set up for measurement of kerosene porosity

ResultsVolume shrinkage:% shrinkage in length= 4.66%%shrinkage in breath=12.4%%shrinkage in thickness=9.5%Carbon yield= 74.93%Density of bulk Carbon Foam

=0.682gm/cm³Kerosene Porosity =41.59%

Other tests: SEM

I. SEM image of dried PU Foam impregnated with Phenolic resin

SEM micrographs showed that pores get fractured during sample preparation or machining. At higher temperatures it is also possible that small pore walls collapse and create new bigger pores. The formation of these types of pores may be due to trapping of air between the surface of PU struts and phenolic resin. The presence of these types of pore may reduce mechanical properties of carbon foam.

Compressive test TGA

References:[1]Kalpesh patel,S.m.manocha& L.m. manocha,(oct.2010)

“Development of carbon foam from phenolic resin via templet route” pp.338-342

[2]Ashida kanetoshi(2006), “polyurethane and related foams chemistry and technology” CRP press, pp 333-342

[3]louis-philippe lefebvre,john Banhart,(2008) “porous metals and metallic foams: current status and recent developments”pp-775-787

[4]sulfizar ahmad,Marziana abdoll latif ,(2013), “ceramic foam fabrication technique for water treatment

application”pp-5-8[5]H.Schmidt, D.Koch, g.Grathwohl, P.Colombo, “Micro and

macroporous ceramics from preceramic precursors”, J.Am.Ceram.Soc.84(2001)pp-2252-2255

[6]W.p.Minnear, Processing of foamed ceramics, in:M.J.Cima (Ed.), Ceramic transactions, forming science and technology for ceramics,Am.Cerami.Soc.,(1992)pp.149-156

[7]P.Sepulveda, Gelcasting of foams for porous ceramics, J.Am.Ceram. Soc. 76(1997)pp.61-65

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