Essay: Cost-Effective Insulation: Silica Aerogels and Xerogels for Thermal Efficiency

Abstract

Silica aerogels and xerogels are effective thermal insulation. Unfortunately the preparation of these materials is both time consuming as well as costly. The preparation of wet silica gels which can be effectively dried either super critically or under atmospheric pressure has both time consuming and expensive steps. For example, drawing the necessary quantity of water out of the gel to prepare for drying makes the procedure both times consuming and expensive. As a result of such problem, silica aerogels and xerogels have not yet reached full commercial effectiveness.

Chapter 1 – Introduction

An aerogel is a former gel from which the liquid phase has dried out. To be an aerogel, as opposed to a xerogel, the solid structure in the gel needs to be preserved throughout the drying (Iler, 1979). The concept of aerogels was first investigated by Kistler (1931a) in the early thirties. Kistler’s hypothesis was that the shrinkage was created by capillary forces during the process. To test the supposition Kistler heated his gel samples to the critical temperature of the liquid while the pressure was kept above the vapor pressure of the liquid. Above the critical temperature, the phase change form liquid to gas will not subject the gel structure to tension, which leaves a dry gel with the structure secure. The tests were successfully tested for a wide variety of gels (Kistler, 1931b).

Main objective of dissertation

The main objective of this dissertation is to use xerogel as an insulator in different applications to reduce heat transfer and to check its effectiveness compared to other insulators. These applications can be engineering applications or other. Xerogel can also be used to reduce power consumption that is also one of its applications and it is already being used in some applications.

Dissertation outline

The main goal of this dissertation to make xerogel in a laboratory by one of the convenient making processes and to use it as an aid or in a mechanism to reduce heat transfer in any engineering elements or   other mechanisms or structure and to substantiate its effectiveness in corresponding applications.

Chapter 2 – Literature review

Micro-porous silica aerogel and xerogel structures are described which are suitable as insulation. These xerogels and aerogels are made by the process described herein which carefully controls the ratios and proportions of ingredients, catalyst, and solvent. Thermal insulation is an important and valuable product. Although many insulative compositions are already being applied, there is a continuing desire for energy conservation pushing a drive to achieve insulation with lower thermal conductivity. Micro-porous silica aerogel structures have excellent characteristics for insulation. M. Antonieta Maeip-Boulis and  Aheed G.Boulis has worked on microporous aerogel.

Ravindra Deshpande worked on manufacturing of wet silica gels.A wet silica gel can be prepared by a process comprising the steps of contacting a stable, aqueous, fluid, silica system with a pH greater than 7.5 with an ion exchange resin which removes metal ions and substitutes them with H+ ions and thereby reducing the pH to less than 5.0, adding an organic liquid to the silica system, however, the organic liquid added stays in one phase with the aqueous, silica, system, and stops the silica precipitation, and adding a base to the silica system so that the pH of the silica system is in the range of from 5.0 to 7.5, and allowing the wet silica gel to form. The wet silica gel produced by this process is characterized by its intense organic solvent content and the low concentration of basic metal ions.

Xerogel is a dried gel which is formed by drying the gel at atmospheric pressure, typically using elevated temperatures; shrinkage of the gel’s network occurs during drying. But, in fact, the resulting xerogel is often reduced in volume 5 to 10 times compared to the original wet gel. Dierk Frank from Germany worked on dried xerogels. The undesired loss of volume has been noted in “The Concise Chemical and Technical Dictionary” (Chemical Publishing Co. 1974) which tells that the removal of volatile fluid from the gel during the preparation of xerogel results in a harder and stronger mass. That shrinkage is not advantageous for properties such as porosity, density and thermal conductivity, which are very important for insulation. Less-dense xerogels are discussed in D. M. Smith, Better Ceramics through Chemistry V, Mat. Res. Soc. Symp. Proc. Volume 271. According to this reference, aging and surface derivatization can be used to manipulate the capillary pressure and gel structure strength of base-catalyzed alkoxide derived silica gels and subsequent formulation of xerogels under normal pressure conditions. Generally, the removal of pore fluids during drying in a non-super-critical drying process causes the gel network to break due to the high capillary pressure and further chemical reactions between surface sites.

Fig.1.1 Difference between densities of gels

Chapter 3 – Formulation

Preparation of Silica xerogels with different drying conditions

1) Preparation of a Hydrophobic Silica Alcogel

A silica sol obtained in the same conditions as example 1 by hydrolysis of alkoxysilane in the presence of hydrochloric acid is gelled in the presence of ammonia. After an ageing phase of 4 h under ethanol refluxing, hydrochloric acid and hexamethyldisiloxane (3:97) (hydrophobizing agent) were added to the reactor so as to fully cover the silica alcogel. The reaction medium was heated and held under reflux for 4 h. The reaction medium was then separated from the hydrophobic silica alcogel by percolation.

The hydrophobic silica alcogel (250 g) thus obtained was then divided in pieces having a size comprised between 1 and 20 mm and introduced in a crystallizing dish.

2) Preparation of a “Condensed” Hydrophobic Silica Alcogel

The crystallizing dish containing the hydrophobic silica alcogel (250 g) divided in pieces was placed in a ventilated oven and the sample was dried at 80° C. until it has lost about 50% of its initial weight.

3a) Obtaining a Hydrophobic Silica Xerogel by Convective Drying in a Ventilated Oven

The “condensed” hydrophobic silica alcogel previously obtained was dried in a ventilated oven at 160° C. for 60 min. The bed of hydrophobic silica xerogel granules obtained exhibited a bulk density of 0.06 g•cm−3, and the xerogel granules obtained had dimensions comprised between about 0.1 and 10 mm. The thermal conductivity value measured on the granules having a size comprised between 1 mm and 1.2 mm, using the guarded hot plate method of standard NF EN 12667 at 20° C. and at atmospheric pressure, was 19.8 mW/m•K.

3b) Obtaining a hydrophobic silica xerogel by dielectric drying under lowered pressure

The “condensed” hydrophobic silica alcogel previously obtained was dried in a microwave drier under vacuum of 40 to 60 mbar, under gentle stirring, and by applying an incident power of 0.5 kW/kg of alcogel. After 20 min of drying, the power reflected by the system was above 160 W•h/kg of alcogel initially introduced. At this stage, the incident power was adjusted to 0.3 kW/kg of alcogel initially introduced and samples were collected at various drying times (33 min, 39 min, 46 min, 51 min and 55 min) corresponding to a total power absorbed by the system of 0.225 Wh, 0.233 Wh, 0.234 Wh, 0.236 Wh and 0.238 Wh per kg of alcogel initially introduced in the drier. The surface temperatures recorded during the drying of the various samples were between room temperature (recorded during the first 20 min of drying) to 78° C. (recorded at the end of the drying). The contents in volatile components of these samples were respectively 16%, 2.5%, 1.6%, 1.0% and 0.9%. The bulky densities of these samples were respectively 0.249 g/cm3, 0.086 g/cm3, 0.078 g/cm3, 0.078 g/cm3 and 0.078 g/cm3. The thermal conductivity values measured on granules having a size of between 1 mm and 1.2 mm, using the guarded hot plate method of standard NF EN 12667 at 20° C. and at atmospheric pressure, were respectively 45.8 mW/mK, 19.7 mW/mK, 17.6 mW/mK, 18.0 mW/mK and 18.1 mW/mK.

3c) Obtaining a hydrophobic silica xerogel by dielectric drying at ambient pressure

The “condensed” hydrophobic silica alcogel previously obtained was dried in a microwave drier at atmospheric pressure, in a stream of nitrogen having a flow of 1.0 L/min, under gentle stirring, and by applying an incident power of 6.7 kW/kg of alcogel. After 3.5 min of drying, the power reflected by the system was 2.7 kW/kg of alcogel initially introduced. At this stage, the incident power was adjusted to 1.85 kW/kg of alcogel initially introduced for 45 min, until the reflected power of the system was 2 kW/kg of alcogel. During the drying, the surface temperature of the sample was between room temperature and 50° C. for the first 3.5 min of the drying and between 50 and 78° C. during the last 45 min of this drying. The silica xerogels obtained were in the form of translucent granules having dimensions comprised between about 0.1 and 10.0 mm. The bulky density of the bed of granules thus obtained was 0.076 g/cm3. The thermal conductivity value measured on granules having a size of between 1 mm and 1.2 mm, using the guarded hot plate method of standard NF EN 12667 at 20° C. and at atmospheric pressure, was 18.0 mW/mK.

Fig 1.2 chemical structure of hydrophobic xerogel

Our selected procedure

Xerogel is an ultra light material with a low density and lower thermal conductivity. It is fairly translucent, but has a rough texture when felt. Traditionally, xerogel is made through the process of super critical drying, so it is needed to build private super-critical dryer at workstation if it is required to be manufactured at there. There are versions that can be made without a super-critical dryer, but they are more sensitive and dense.

Part 1 Building the Super-critical Dryer

Fig. 1.3 Super-critical dryer

1.  Obtain a 9 kg cylinder of carbon dioxide.

Make sure that siphon container is available, with a tube extending from the bottom to the outside of the cylinder. This type of cylinder allows getting liquid carbon dioxide out of the container rather just a gas.

2.

Fig 1.4 heating chamber

Take a look at the schematics.

Instructions on how to assemble a supercritical dryer will be included here, but it is highly recommended to look at the actual visual schematics provided by the aerogel website.

3. Attach stainless steel 316 pipe fitting and valves to a non-welding stainless steel pipe tee.

This pipe tee body should be 1.9 cm. To the two sides of the tee body, attach the pipe plugs or door. Or install a sight window on one side instead of a second pipe plug, if required . At the bottom of the tee body, connect a ball valve using a 6.35 mm reduction bushing. To the top of the body, screw on, in order: 1.2 cm reduction bushing, 1.2 cm nipple fitting, and 1.2 cm cross.

4.

Fig 1.5 schematic diagram of heating chamber

Finish to assemble the top of the machine.

The remainder of valves and gauges will be connected to this second cross pipe.

Mount a bimetal thermometer to the top of the cross pipe.

To the left side, attach by 6.35 mm nipple fitting. To date, attach the ball valve.

To the right side, attach another 6.35 mm nipple fitting. To date, attach a 6.35 mm cross pipe with a pressure gauge fixed to the top and a pop safety valve fixed to the base.

On the open side arm of the small cross pipe, attach another 6.35 mm nipple fitting and a needle valve.

5 Know what to use and what not to use.

Stainless steel is preferable since it is clean and strong.

Use gauges having brass threads and valves made of carbon steel.

Don’t use pipe fittings made of brass or carbon steels, and don’t use any part which is rated for lower than 2000 psi pressure.

6. Connect the carbon dioxide tank to the supercritical dryer.

Make a secure connection of  the tank to the dryer so that the liquid carbon dioxide can easily flow into the dryer.

It is recommended to look at the official schematics, found here:

Connect to the carbon dioxide tank, in this order- 1> CGA320 male thread, 2> Teflon gasket, 3>CGA inlet nipple, 4> CGA inlet nut, 5> 6.35 mm NPT female quick disconnect socket, 6> 6.35 mm NPT male quickdisconnect plug, and 7> a braided high-pressure hose, rigid 6.35 mm NPT, with the female fittings.

To the opposite side of the hose, attach in this order: 6.35 mm NPT male quick disconnect plug and 6.35 mm NPT male quickdisconnect socket.

Attach the final socket to the intake ball valve of the dryer.

Part 2 Preparing Silica Aerogel

1 Dilute concentrated ammonium hydroxide.

Combine 4.85 g or 5.40 ml concentrated ammonium hydroxide with 1000 ml of water in a glass or plastic bottle.

This bottle is labeled by “ammonium hydroxide stock solution.” It can be stored and sealed at room temperature to be reused in future.

2 Mix the tetramethyl orthosilicate and methanol.

Combine 10.2 g or 10 ml tetramethyl orthosilicate (TMOS) with 7.80 g or about 10 ml methanol in a glass beaker. Stir till it doesn’t get mixed.

This should be labeled as “alkoxide solution” or “solution A.”

Note that TMOS is very difficult to purchase since many chemical suppliers are hesitant to sell chemicals to individuals. Even so, make sure to obtain the TMOS from a reliable source so that there are no dangerous impurities in it.

3. Mix ammonium hydroxide solution with methanol.

Combine 5 g or 5 ml of the stock solution previously prepared with 7.9 g or about 10 ml of methanol in another glass beaker. Stir until well mixed.

Name this beaker as “catalyst solution” or “solution b.”

4. Pour the catalytic solution into the alkoxide solution.

Carefully pour the catalyst solution into the alkoxide, that is, solution A and stir with a glass stirring rod until completely combined.

The resulting solution is called the “sol.”

5 Transfer the sol into molds.

Line molds with silicone based baking paper before pouring the liquid sol gel into the bolds. Let the sol sit until gel forms.

This can take from 15 minutes to 1 hour.

Also pour the sol into syringes with no needles or tops. That way, removal of finished gel by pushing it out with the plunger is possible.

TMOS is the main source of the silica in this method. The water allows the TMOS to polymerize. The methanol lets both the water and TMOS enter into the same phase and they can react. The ammonium hydroxide makes the reaction go faster.

6. Let the gel age slowly.

After the gel sets, put it under methanol and let it age for a full 24 hours, minimum.

7 Drain the water out.

Change the methanol for fresh methanol, 200 proof ethanol, or acetone at least four to five times over a week.

This process drains the water out and leaves a pure methanol gel.

8. Dry the gel in supercritical dryer.

Put the gel in the supercritical dryer’s compartment and heat the CO2 above its critical point, 31.10 °C and 73 bars, to about 45 degrees at a pressure of about 100 bars. Super critical drying process causes the methanol to get drawn out of the gel.

Depressurize the machine at a rate of about 7 bar h1.

When done, you should be left with a finished silica aerogel.

Part 3 Alternate Drying Instructions – Subcritical Drying

1 Prepare the aerogel as usual.

Prepare the silica aerogel from the previous process, with the same chemical ingredients to produce the “sol,” the liquid state of the gel.

2 Purify the gel through solvent exchange as normal practice.

Surround it in ethanol or acetone, changing the solution out four times over a week.

3. Make a hexane and ethanol solution.

Mix one part hexane with three parts ethanol until you get a volume at least five times the volume of gel currently being processed.

Note that acetone can be used instead of ethanol, if desired.

As an example, if 20 ml of aerogel is being processed, prepare a solution of 25 ml hexane and 75 ml acetone or ethanol.

Label this as your 2575 mixture.

4 Prepare two additional solutions of hexane and ethanol.

For the second, use equal amounts of both and for the third, use three parts hexane to on part ethanol.

Acetone can be used instead of ethanol as before

Label the second solution as your 5050 mixture and the third as your 7525 mixture.

5 Soak the gel in all three mixtures.

Let the gel soak in your 2575 mixture for 12 to 48 hours.

Afterward, let it soak in your 5050 mixture for the same amount of time, finishing up by letting it soak in the 7525 mixture for another 12 to 48 hours.

6 Let the gel soak in hexane.

Change out the pure hexane three times over the lap of 24 to 72 hours.

7 Prepare a trimethyl chlorosilane(TMCS) solution.

Add TMCS to hexane so that 6 percent of the solution’s weight is because of TMCS.

Make enough so as to have roughly 15 times as much solution as that of gel.

Label this as your TMCS solution.

8 Soak the aerogel in the trimethyl chlorosilane(TMCS) solution.

Place the gel in a sealed, chemical resistant container with a wider mouth. Add enough TMCS solution so as to have 5 to 10 times the volume of the gel. Seal the container.

Lubricate the lid with silicone vacuum grease if using glass. Otherwise, the lid might get stuck.

9. Heat and cool the container.

Heat the container for 12 to 24 hours at a steady 60 °C temperature using a hot plate or oven. Let it cool to room temperature before changing the TMCS solution with fresh solution.

Repeat this procedure twice more.

10. Soak the cooled gel in pure hexane.

Swap out the TMCS solution for pure hexane and allow the aerogel to soak in it for another 12 to 24 hours.

Change out the hexane in the container for fresh hexane at least twice over the course of 24 to 72 hours.

11. Dry the gel subcritically.

Place the gel in a jar with a little hexane and secure the lid only halfway. Let the gel dry out for 24 to 48 hours in this manner.

At the conclusion of this step, your aerogel should be complete.

Fig 1.6  Alcogel and xerogel

Chapter 4 – Conclusion and future scope

The xerogel is a highly low density material but slightly dense compared to aerogel. It is a thermal insulator and is now available commercially and also can be prepared at home by the above mentioned processes. Hence it can be used at different domains to reduce heat transfer.

Xerogel is gel prior to aerogel.It is made before aerogel can be made. Our selected procedure from various processes is one of the easiest methods to manufacture xerogel and is also less time consuming.  Carbon dioxide is used as super-critical dryer to make aerogel.

In the upcoming future it will be used as a thermal insulator in buildings to reduce power consumption and also in other engineering applications to reduce heat transfer. Its effectiveness will be compared with other insulators.

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