EP2539141A1

EP2539141A1 - Method of making a densified body by isostatically pressing in deep sea

Info

Publication number: EP2539141A1
Application number: EP11710353A
Authority: EP
Inventors: Randy L. Rhoads; Paul M. Schermerhorn
Original assignee: Corning Inc
Current assignee: Corning Inc
Priority date: 2010-02-24
Filing date: 2011-02-17
Publication date: 2013-01-02
Also published as: JP2013520339A; WO2011106221A1; KR20130004306A; TW201139331A; CN102762362B; CN102762362A; JP5795339B2

Abstract

A method of making a large block comprising a step of isostatically pressing a green-body in the deep sea. Very large blocks can be made by this method at a relatively low cost. The deep sea provides the necessary high pressure for the isopressing.

Description

METHOD OF MAKING A DENSIFIED BODY BY ISOSTATICALLY

PRESSING IN DEEP SEA

[0001] This application clais the benefit of priority to US Patent Application No. 61/307657 filed on February 24, 2010.

FIELD

[0002] The present invention relates to methods of forming large blocks of materials. In particular, the present invention relates to methods of forming large blocks of compressed particle green-bodies and sintered large blocks involving a step of isostatic pressing. The present invention is useful, for example, in making large zircon ceramic blocks suitable for an isopipe of a fusion down-draw process for making glass sheets. TECHNICAL BACKGROUND

[0003] Large ceramic blocks are useful in construction of many engineering structures due to their attractive mechanic properties and refractoriness. For example, large blocks of ceramics comprising AI2O3, Zr0₂, zircon (Zr0₂ Si0₂), Ti0₂, BeO, MgO, Si0₂, and mixtures and combinations thereof can be used in making components for melting, delivering and forming metal and/or glass articles. Specifically, zircon has been used in making the large forming ceramic block (called isopipe) in fusion down-draw processes for making glass sheets due to high mechanical strength and dimensional stability at high temperatures.

[0004] It is highly desirable that the structural ceramic blocks be present in the form of a unitary densified body. Ceramic blocks have been made by a process comprising the following steps: (a) packing particles of the ceramic material into a bag; (b) isostatically pressing the bag to form a green-body inside an isopress at a high pressure; and (c) sintering the green-body to obtain a densified ceramic block. The sintered ceramic block can be subsequently machined into desired dimensions and shapes.

[0005] The cost of fabrication, installation and maintenance of an isopress is very high and grows to a prohibitive level if a dimension of the ceramic block exceeds about 3 meters.

[0006] Therefore, there remains a need for a practical and cost-effective method of making large-size ceramic blocks.

[0007] The present invention satisfies this and other needs. SUMMARY

[0008] Several aspects of the present invention are disclosed herein. It is to be understood that these aspects may or may not overlap with one another. Thus, part of one aspect may fall within the scope of another aspect, and vice versa.

[0009] Each aspect is illustrated by a number of embodiments, which, in turn, can include one or more specific embodiments. It is to be understood that the embodiments may or may not overlap with each other. Thus, part of one embodiment, or specific embodiments thereof, may or may not fall within the ambit of another embodiment, or specific embodiments thereof, and vice versa.

[0010] A first aspect of the present invention is directed to a method for making a densified body having a dimension of at least 1 meter, comprising the following steps:

(I) packing a plurality of particles into a hermetically sealable bag;

(II) removing gas from inside the bag;

(III) hermetically sealing the bag; and

(IV) lowering the bag into a water column having a depth of at least 1000 meters to a pressing location at least 1000 meters below the surface of the water column, whereby an isostatically pressed green-body is formed.

[0011] In certain embodiments of the first aspect of the present invention, in step (IV), the green-body water column is part of the sea on the planet earth, and the pressing location is at least 5000 meters below the surface of the sea.

[0012] In certain embodiments of the first aspect of the present invention, in step (IV), the green-body water column is part of the sea on the planet earth, and the pressing location is at least 10000 meters below the surface of the sea.

[0013] In certain embodiments of the first aspect of the present invention, in step (IV), the water column is part of the sea on the planet earth, and the pressing location is at least 10500 meters below the surface of the sea.

[0014] In certain embodiments of the first aspect of the present invention, in step (I), the plurality of particles comprise a ceramic, and the method further comprises a step (V) as follows:

(V) sintering the green-body obtained in step (IV), desirably under normal atmospheric pressure at a temperature higher than 1000°C to obtain a densified ceramic block. [0015] In certain embodiments of the first aspect of the present invention, the ceramic comprises a material selected from BeO, MgO, Zr0₂, Zr0₂ Si0₂, A1₂0₃, Ti0₂, and mixtures and combinations thereof.

[0016] In certain embodiments of the first aspect of the present invention, step (IV) comprises:

(IV.1) placing the hermetically sealed bag into a cage; and

(IV.2) lowering the cage from the surface of the water column to the pressing location.

[0017] In certain embodiments of the first aspect of the present invention, in step (IV.2), the cage is attached to a cable, and the cable is extendably attached to a vessel on the surface of the water column.

[0018] In certain embodiments of the first aspect of the present invention, the cage travels at a vertical speed of at most 10 ms^-1.

[0019] In certain embodiments of the first aspect of the present invention, the cage travels at a vertical speed of at most 1 ms^"1.

[0020] In certain embodiments of the first aspect of the present invention, a pressure sensor is installed on or near the cage, which provides information of the pressure the bag is subjected to.

[0021] In certain embodiments of the first aspect of the present invention, a heating element is provided surrounding the bag during step (IV) to maintain a desired temperature thereof.

[0022] In certain embodiments of the first aspect of the present invention, the power of the heating element is controlled via a temperature sensor near or on the bag of the green-body.

[0023] In certain embodiments of the first aspect of the present invention, the method further comprises the following step (IV-A):

(IV -A) raising the bag to above the surface of the water column after step (IV).

[0024] In certain embodiments of the first aspect of the present invention, in step (IV-A), the bag travels at a vertical speed of at most 10 m-s-1 , in certain embodiments at most 5 m-s-1 , in certain other embodiments at most 1 m-s-1, in certain other embodiments at most 0.5 m-s-1, in certain other embodiments at most 0.1 m-s-1. [0025] In certain embodiments of the first aspect of the present invention, a bladder box capable of providing adjustable difference between weight and buoyancy thereof is attached to the cage.

[0026] In certain embodiments of the first aspect of the present invention, at the end of step (IV), the weight of the bladder is reduced to provide an overall upward force for the assembly comprising the cage and the bladder.

[0027] In certain embodiments of the first aspect of the present invention, the densified body has a dimension of at least 2 meters.

[0028] In certain embodiments of the first aspect of the present invention, the densified body has a dimension of at least 3 meters.

[0029] In certain embodiments of the first aspect of the present invention, the densified body has a dimension of at least 4 meters.

[0030] In certain embodiments of the first aspect of the present invention, the densified body has a dimension of at least 5 meters.

[0031] In certain embodiments of the first aspect of the present invention, the densified body has a dimension of at least 10 meters.

[0032] In certain embodiments of the first aspect of the present invention, the densified body has at least two dimensions perpendicular to each other each of at least 2 meters.

[0033] In certain embodiments of the first aspect of the present invention, the densified body has three dimensions perpendicular to each other each of at least 2 meters.

[0034] In certain embodiments of the first aspect of the present invention, the densified body has a porosity of at most 10% by volume, in certain embodiments at most 8%, in certain other embodiments at most 5%, in certain other embodiments at most 3%.

[0035] One or more embodiments of the preset invention have one or more of the following advantages. First, given the size of the ocean, the size of the green-body that can be isopressed in the deep sea is virtually unlimited. Second, since the vessel for transporting the green-body before and after the pressing can be a general purpose vessel not specifically dedicated to the isostatic pressing step, the cost of transportation for the green bodies can be shared with the transportation of other goods, and therefore reduced. Third, multiple green bodies can be pressed in a relatively short period of time or even simultaneously without the need of a supersized isopress. The final result would therefore be the capability to manufacture large-size ceramic bodies with a low cost. [0036] Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.

[0037] It is to be understood that the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed.

[0038] The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In the accompanying drawings:

[0040] FIG. 1 is a schematic illustration of an isopipe operating to make a glass sheet by a fusion down-draw process.

[0041] FIG. 2 is a diagram showing the temperature of sea water as a function of depth thereof in a tropical area that is suitable for carrying out the isostatic pressing step according to one embodiment of the present invention.

[0042] FIG. 3 is a flow chart showing the steps of making a Zr0₂ Si0₂ ceramic block according to one embodiment of the present invention.

DETAILED DESCRIPTION

[0043] As mentioned supra, the present invention may be utilized for making any large bodies of any material involving a step of isostatically pressing a green-body in a sealed bag, especially a green-body for a large ceramic block based on oxides, phosphates or other inorganic materials. The present invention is especially useful for making large ceramic blocks having at least one dimension, desirably at least two dimensions perpendicular to each other, of at least 2 meter in size, in certain embodiments at least 2.5 meters, in certain other embodiments at least 3 meters.

[0044] One particularly advantageous use of the present invention is for the fabrication of large zircon (Zr0₂ Si0₂) based ceramic blocks useful for making large-size unitary ceramic components in the melting, delivery and conditioning of glass melt and forming of glass articles. As mentioned supra, the isopipe of a fusion down-draw process for making glass sheets, especially high-precision, high surface quality glass sheets suitable as LCD substrates, is typically made of a unitary piece of ceramic material such as zircon, xenotime (YP0₄) or other refractory materials having high dimensional stability at the target forming temperature. The present invention will be further illustrated by the embodiment of making a zircon-based isopipe below. However, it should be understood that, upon reading the present disclosure and with the benefit of the teachings herein, the present invention can be used by one having ordinary skill in the art in making other articles from large blocks of materials, mutatis mutandis.

[0045] FIG. 1 schematically illustrates an isopipe 100 in operation to make a glass ribbon 111 via a fusion down-draw process. The isopipe 100 comprises a unitary piece of zircon comprising a trough-shaped upper part 103 over a wedge-shaped lower part 105. Molten glass enters into trough 103 through an inlet tube 101, overflows both the top surfaces of the trough 103, forms two ribbons over the external side surfaces of the trough 103 and the two converging side surfaces of the wedge-shaped lower part 105, and fuse at the root 109, which is the lower tip of the wedge-shaped part, to form a single glass ribbon 111 which is drawn downward in the direction 113. The glass ribbon 111 is further cooled down downstream (not shown) to form a rigid glass sheet, which is further severed from the glass ribbon, cut to size and used for making LCDs. Because both external surfaces of the glass ribbon 111 were exposed only to ambient air and never came into contact with the surfaces of the isopipe, they are of pristine surface quality essentially free of scratches without the need of further surface finishing such as grinding and polishing. The glass sheets made by this process tend to have very high mechanical strength due to the lack of surface flaws. To make precision glass sheets having desired properties, such as high thickness uniformity, low stress, high stress uniformity, and the like, the stability of the shape of the isopipe 100 during the production cycle is very important. Unfortunately, the isopipe can sag overtime due to the gravity of itself and the load of molten glass, leading to shape and dimension shift. The larger the span of the isopipe, the more pronounced the sagging is, assuming a substantially constant creep ratio of the material and a given operating temperature. The porosity of the zircon ceramic affects the sagging and other properties of the isopipe. Therefore, in general, the zircon ceramic for an isopipe is desirably densified to having a porosity of lower than about 10% by volume. Desirably, the pores in the zircon ceramic of an isopipe are mostly closed pores, i.e., they are not exposed to the ambient air surrounding the surface of the isopipe.

[0046] United States Patent No. 6,974,786 and United States Patent Application Publication Nos. US2008/0125307 Al and US2009/0111679 Al and WO09/054951 disclose zircon materials suitable for making isopipes in fusion down-draw processes and processes for making such materials, the contents of which are incorporated herein by reference in their entirety.

[0047] FIG. 3 shows the flow chart of the process of making an isopipe based on zircon ceramics according to one embodiment of the present invention. In step 301, a powder of zircon having desired chemical composition, particle size and particle size distribution is made. To achieve a high final density of the zircon ceramic, particle distribution of the powder should desirably enable a dense packing of the particles under pressure. An exemplary particle size range is from about 0.1 μπι to about 100 μπι, with a median particle size in the range from 3 μπι to 20 μνα. The particles can be made by ball- milling bulk zircon having the desired composition such as those disclosed in the above- mentioned patent references by using the synthesis methods disclosed therein, followed by selection at the proper sieve size.

[0048] In step 303, the zircon powder having the desired particle size distribution are mixed with optional binders and placed into a hermetically sealable, flexible bag. The powder inside the bag is compacted by, e.g., vibration, to improve particle packing. The bag can be made of, e.g., nylon or other water-proof flexible fabric. The bag should have the shape and volume to house a green-body having the desired shape and dimension. For example, in order to make a final ceramic block having a dimension of 3mx2mxlm, the bag should be able to contain such a green-body having a size significantly larger due to the shrinkage of the green-body during the firing and sintering step, described in greater detail infra. To help support the weight of the green-body and define the shape of the filled bag, a metal container such as steel cage or box may be used to house the bag.

[0049] In step 305, the bag is vacuumed and then hermetically sealed. The removal of gas from the bag allows the particles to be pressed intimately without trapping large air pockets inside in the subsequent pressing step. Steps 301, 303 and 305 are

advantageously carried out in a land-based factory, although they may be performed on a floating vessel or a rig on the sea. If the green-body is shipped over a long distance on the sea, it may be advantageous to perform step 305 on the sea immediately before step 307.

[0050] Next, in step 307, the bag, desirably housed inside a metal cage, is lowered into the sea at a location where the depth is at least 1000 meters (e.g., 5000 meters, 6000 meters, 7000 meters, 8000 meters, 9000 meters, 10000 meters, and the like) in a controllable manner. The lowering of the bag can be carried out by using a submarine or a extendable cable tethered to a floating vessel or a rig or the like. In one embodiment, the bag is placed inside a stainless steel cage having a plurality of holes on all six sides, which, in turn, is tethered to one end of a roll of steel cable. The cage is brought to above the Mariana Trench in the Pacific Ocean by a vessel, and then lowered into sea water from the vessel in a controllable manner. The sea at the Mariana Trench is known to have a maximal depth of more than 10500 meters. Advantageously, a pressure sensor is installed on or near the cage, so pressure information can be transmitted to the vessel along the cable or by other means such as wireless transmission. Alternatively, a sonar or other device may be used to monitor and locate the cage during the lowering process. It is known that the pressure P exerted by a water column can be calculated according to the following equation:

P = p - g - h

where p is the density of the water column, g is the gravitational accelerator, and h is the height of the water column. Thus the deeper the sea is at the perssing location, the higher the pressure it exerts on the green-body. While the green-body travels from the surface of the ocean down to the pressing location, the pressure it experiences grows. The higher the pressure the green-body is subjected to, the more compact it becomes, and the better the packing of the particles. To make highly densified sintered ceramic bodies, it is desired that the pressing location is at least 8000 meters below sea level, in certain embodiments at least 9000 meters, in certain other embodiments at least 10000 meters. At 10000 meters below sea level, the pressure is about 16 kPsi.

[0051] FIG. 3 shows the temperature of sea water in a tropical area. As can be seen from this figure, the temperature variation of sea water is not significant, from about 20°C from the surface to about 0°C on the sea floor. This is a temperature range that can be tolerated by most bag material suitable for isopressing. Because the cage has holes on all sides, once it is submerged by sea water, the whole bag is subjected to a substantially even pressure exerted by the water column above it. Hence the green-body inside the bag is pressed isostatically. The combination of the gravity of the cage, the bag and the green-body would allow the cage to descend to the sea floor if the cage is not restricted by the tethered cable. The controlled extension of the cable via, e.g., a pulley and/or a motor, can limit the descending to a substantially constant vertical speed, e.g., of at most 10 meters per second (m-s ¹), in certain embodiments at most 5 ms^"1, in certain other embodiments at most 1 ms^"1. A relatively slow and constant descending speed allows the particles in the green-body to shuffle, shift, rearrange and pack gradually, resulting in a substantially even overall density and porosity without large cracks and cavity trapped inside. Fast and abrupt change of pressure to the bag and thus the green body can result in unwanted pressure gradient locked inside the green-body, preferential pressing in the surface region than in the core, leading to different levels of packing and compaction in the green-body, which can be detrimental for the final density, porosity and porosity distribution in the sintered ceramic block. In addition, the slow and steady denscending speed of the green body allows the temperature of the green body to equilibrate with the environment surrounding it at a sufficiently slow speed to avoid any thermal shock which could be detrimental for a uniform packing of the particles.

[0052] It is desirable that the cage should not be allowed to reach the sea floor to avoid contact with sharp objects such as rock which could puncture the bag, causing contamination of the green-body by sea water. Desirably, once the cage reaches the target depth, it is held statationery at the depth for a given period of time so that the green-body is compressed to a stable stage before the cable is retracted to retrieve the cage. The holding period can range from several minutes to several months, desirably from several hours to several weeks.

[0053] The target depth is desirably at least 5000 meters below sea level, in certain embodiments at least 8000 meters, in certain other embodiments at least 9000 meters, in certain other embodiments at least 10000 meters. The maximal depth of the sea at a given location can be determined according to available public data, or by using a depth meter such as a sonar. In general, the target depth should be relatively free from highspeed current and relatively far from geothermal eruptions and volcanos on the sea floor to provide a steady and even compaction environment.

[0054] In step 309, the cage is raised to above the sea surface by, e.g., retracting the cable tethered to the cage. Again, the cable retraction speed should be carefully controlled. Unlike the cable extension step, the retracting cable has to counter the weight of the cage and green-body, the weight of the cable, and provide the upward acceleration needed for the whole assembly, minus the buoyancy provided by sea water. While the cage travels upward, the pressure exerted on the green-body and the bag decreases gradually. To maintain the structural integrity of the compressed green-body, sudden pressure change should be avoided. Thus, a gradual, slow cable retraction speed is desired. In certain embodiments, the cable is retracted at a speed of at most 10 ms^"1, in certain embodiments at most 1 ms^"1, in certain embodiments at most 0.5 ms^"1, in certain other embodiments at most 0.1 ms^"1. Similar to the descending step, speed control in the ascending step can have significant impact on the packing of the particles due to abrupt changes of pressures and temperature can impose on the green-body. A slow and constant ascending vertical speed allows the green-body to equilibrate with the surrounding environment both in terms of pressure and temperature, allows the internal pressure inside the green-body to release gradually, and avoids detrimental thermal shock.

[0055] In other embodiments, a sealable metal box equipped with a valve that allows controllable ingress and egress of water into the box may be used in lieu of the cage to contain the bag and the green-body. During the decending process, the valve can be kept open to allow sea water to flow into the box and maintain sutantially the same pressure in and outside of the box. The box can be cubic, oblong or spherical. During the ascending process, the valve can be kept open to maintain a pressure equilibrium in and outside of the box. On the other hand, it may be desirable to close the valve at a certain point during the descending process, at the pressing location, or during the ascending process, so that the bag is maintained in a safe and steady environment with a constant pressure. The use of a box is advantageous in that it prevents unwanted disturbance due to current, innocent or malicious wild life in the sea, and the like. Alternatively, it is possible that at a certain point while the box is in the sea, such as at the pressing location, the valve is closed, and then the box is raised to a differing location or even above sea level, where the green- body is subjected to the same pressure at the pressing location. It is also contemplated that the box with the green-body and high-pressure water can be shipped to a location on land, where it may be held for a sufficient period of time before the pressure is relieved by opening the valve, and then opened to retrieve the isopressed green-body. In this embodiment, it is highly desired that the box is designed to have a shape such as a spherical shape to maximize its ability to withstand such high pressure.

[0056] In certain embodiments, it is possible to attach a bladder to the container (cage or box) of the bag. The bladder is a metal box with function similar to that of a fish or a submarine, which can contain adjustable amount of water. Thus during the descending step, the bladder can be filled with water such that the whole assembly can be lowered down to the target location due to gravity. During the ascending step, at least part of the water inside the bladder can be removed and replaced with gas, so that an upward force is provided to the container of the bag and green-body.

[0057] Still in other embodiments, a heating element can be provided to the bag and the green-body, so that the temperature of the green-body can be controlled and adjusted or maintained at a higher level than the water in the sea. Such heating element can provide the termal energy by electricity supplied from the vessel, or from reactions of chemicals stocked around the bag. A temperature sensor can be provided to the bag of the green-body and used in a temperature control loop.

[0058] While it is desirable to tether the cage/box or other container of the green- body to a vessel, a rig or other surface object, it is also possible to allow the container of the bag and the green-body to drift into the deep sea, and then later retrieved by a fishing net, a submarine, or other means.

[0059] Next, in step 311, the green-body is then shipped to a sintering facility, where it is taken out of the bag, placed into a furnace, first fired to burn out the binder, if any, and then sintered to a temperature of at least 1000°C, in certain embodiments at least

1300°C, in certain other embodiments at least 1500°C, whereby a densified zircon block is formed. During firing and sintering, the adjacent particles join each other to form a strong, unitary body. Shrinkage of the block is typically observed and pores in the green- body were reduced. The final densified zircon desirably has a density of at least 90% of the theoretic limit of zircon under normal conditions. The sintering step can take from several hours to several months, desirably from several hours to several weeks. Upon completion of the sintering, the block is slowly cooled down to room temperature to allow annealing and prevent cracking due to thermal shock. For example, the sintering step can take from 1 hour to 150 days, in certain embodiments from 2 hours to 100 days, in certain embodiments from 10 hours to 90 days, in certain embodiments from 20 hours to 80 days, in certain embodiments from 20 hours to 60 days.

[0060] Finally, in step 313, the cooled large block of zircon ceramics is machined to the desired shape, i.e., having an upper trough and a lower wedge, having desired sizes.

[0061] It will be apparent to those skilled in the art that various modifications and alterations can be made to the present invention without departing from the scope and spirit of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. A method for making a densified body having a dimension of at least 1 meter, comprising the following steps:

(I) packing a plurality of particles into a hermetically sealable bag;

(II) removing gas from inside the bag;

(III) hermetically sealing the bag; and

2. A method according to claim 1, wherein in step (IV), the water column is part of the sea on the planet earth, and the pressing location is at least 5000 meters below the surface of the sea.

3. A method according to claim 1 or claim 2, wherein in step (IV), the water column is part of the sea on the planet earth, and the pressing location is at least 10000 meters below the surface of the sea.

4. A method according to any of the preceding claims, wherein in step (IV), the water column is part of the sea on the planet earth, and the pressing location is at least 10500 meters below the surface of the sea.

5. A method according to any of the preceding claims, wherein in step (I), the plurality of particles comprise a ceramic, and the method further comprises a step (V) as follows:

(V) sintering the green-body obtained in step (IV) at a temperature higher than 1000°C to obtain a densified ceramic block.

6. A method according to claim 5, wherein the ceramic comprises a material selected from BeO, MgO, Zr0₂, Zr0₂ Si0₂, A1₂0₃, Ti0₂, and mixtures and combinations thereof.

7. A method according to any of the preceding claims, wherein step (IV) comprises:

(IV.1) placing the hermetically sealed bag into a cage; and

8. A method according to claim 7, wherein in step (IV.2), the cage is attached to a cable, and the cable is extendably attached to a vessel on the surface of the water column.

9. A method according to claim 7 or claim 8, wherein in step (IV.2) the cage travels at a vertical speed of at most 10 m-s^"1.

10. A method according to claim 9, wherein in step (IV.2) the cage travels at a vertical speed of at most 1 m-s^"1.

1 1. A method according to any of claims 7 to 10, wherein a pressure sensor is installed on or near the cage, which provides information of the pressure the bag is subjected to.

12. A method according to any of the preceding claims, wherein a heating element is provided surrounding the bag during step (IV) to maintain a desired temperature thereof.

13. A method according to claim 12, wherein the power of the heating element is controlled via a temperature sensor near or on the bag of the green-body.

14. A metod according to any of the preceding claims, further comprise the following step (IV-A):

15. A method according to any of the preceding claims, wherein in step (IV-A), the bag travels at a vertical speed of at most 10 m-s^"1, in certain embodiments at most 5 m-s^"1, in certain other embodiments at most 1 m-s^"1, in certain other embodiments at most 0.5 m-s^"1, in certain other embodiments at most 0.1 m-s^"1.

16. A method according to any of the preceding claims 7 to 15, wherein a bladder box capable of providing adjustable difference between weight and bouyancy thereof is attached to the cage.

17. A method according to claim 16, wherein at the end of step (IV), the weight of the bladder is reduced to provide an overall upward force for the assembly comprising the cage and the bladder.

18. A method according to any of the preceding claims, wherein the densified body has a dimension of at least 2 meters.

19. A method according to claim 18, wherein the densified body has a dimension of at least 3 meters.

20. A method according to claim 18, wherein the densified body has a dimension of at least 4 meters.

21. A method according to claim 18, wherein the densified body has a dimension of at least 5 meters.

22. A method according to claim 18, wherein the densified body has a dimension of at least 10 meters.

23. A method according to any of the preceding claims, wherein the densified body has at least two dimensions perpendicular to each other each of at least 2 meters.

24. A method according to claim 23, wherein the densified body has three dimensions perpendicular to each other each of at least 2 meters.

25. A method according to any of the preceding claims, wherein the densified body has a porosity of at most 10%, in certain embodiments at most 8%, in certain other embodiments at most 5%, in certain other embodiments at most 3%.