various advantageous features and embodiments of the process of the invention will now be described in greater detail . step ( a ) of the process of the invention comprises the production of an aqueous dispersion of nanotubes . within the scope of the present description , “ nanotube ” is understood as being a tubular carbon - based structure , which is generally based substantially on carbon in the form of graphene sheets , and which has a diameter of from 1 to 200 nm ( dimensions can be measured especially from transmission electron microscopy photographs ). these compounds belong to the family of the so - called “ nanostructured materials ”, which have at least one characteristic dimension of the order of a nanometer . for further details regarding these materials and their methods of synthesis , reference may be made especially to the articles “ nanotubes from carbon ” by p . m . ajayan ( chem . rev ., vol . 99 , p . 1787 , 1999 ) and “ carbon nanofibers : catalytic synthesis and applications ” by k . de jong and j . w . geus ( catal . rev . sci . eng ., vol . 42 , p . 481 , 2000 ). the nanotubes used in the process of the invention are typically nanotubes of the multiwalled type , namely hollow cylindrical structures based on graphene sheets rolled up on themselves , comprising a plurality of concentric graphene - based cylinders . advantageously , there will be used in step ( a ) nanotubes of the multiwalled type having an average diameter of from 10 to 100 nm , the diameter in most cases being at least 30 nm , for example from 50 to 80 nm . in addition , the average length of the nanotubes used in step ( a ) is generally from 1 to 20 microns , typically from 5 to 15 microns ( for example of the order of 10 microns ). in step ( a ), the dispersion of the nanotubes in water is carried out in the presence of a suitable surfactant , namely a surfactant suitable for stabilising the nanotube / water interface . dispersion can be carried out by any means known per se . it is typically effected by subjecting a water / nanotube / surfactant mixture to sufficient shear , advantageously by subjecting it to ultrasound ( sonication ). the surfactant used to produce the dispersion is preferably an ionic surfactant . anionic surfactants of the sulfate , sulfonate or carboxylate type especially , for example alkyl sulfates or alkylsulfonates , or carboxymethylcellulose salts ( especially carboxymethylcellulose sodium salt ), have been found to be especially suitable for carrying out the invention . in general , the surfactant employed in step ( a ) is used in the nanotube dispersion in a concentration below its critical micelle concentration ( cmc ), above which threshold the nanotubes would tend to aggregate by depletion interaction . the concentration of nanotubes in the dispersion of step ( a ) can vary fairly widely as a function of the surfactant used . nevertheless , in the most general case , it is preferred for the dispersion prepared in step ( a ) to comprise at least 0 . 5 g of nanotubes per liter , more advantageously at least 1 g of nanotubes per liter , more advantageously at least 5 g of nanotubes per liter or even at least 10 g of nanotubes per liter , the concentration in most cases being from 1 to 15 g of nanotubes per liter . it should be noted that the concentration of nanotubes is limited when certain surfactants are used . for example , maximum concentrations of the order of 1 g of nanotubes per liter are achieved with surfactants such as sodium dodecyl sulfate sds . on the other hand , other surfactants allow higher concentrations to be achieved . according to an advantageous embodiment , therefore , the surfactant used in step ( a ) is a carboxymethylcellulose salt , especially the sodium salt , which allows a nanotube concentration of up to 15 g of nanotubes per liter of dispersion to be obtained , while a homogeneous dispersion is retained . in step ( b ) of the process of the invention , a foam is produced from the dispersion produced in step ( a ), by swelling under the action of a gas . swelling is in most cases carried out by bubbling a gas through the dispersion produced in step ( a ), the gas advantageously being delivered through sintered glass . the foam produced within the scope of step ( b ) is a foam of the liquid / gas type , the structure of which determines the morphology of the final aerogel . the foam is formed in the presence of a foaming agent , namely an agent capable of stabilising the water / gas interface in the foam that is formed . to that end , a foaming agent is generally added to the dispersion as obtained at the end of step ( a ), for example a surfactant suitable for that purpose , for example a non - ionic ethoxylated surfactant such as tergitol ( especially tergitol np9 ( having the formula c 15 h 24 o ( c 2 h 4 o ) 9 ). according to an interesting embodiment of the process of the invention , there can be used as the foaming agent in step ( b ) starch or , alternatively , a sugar , for example a polysaccharide . the use of starch , or of sugar , especially of the polysaccharide type , is found to be advantageous especially where step ( d ) is carried out . the starch introduced in step ( b ) is in fact found in the material obtained at the end of step ( c ). when step ( d ) is carried out , at least some of the starch is carbonised , which generally allows the nanotubes constituting the material to be bonded together , which is reflected in an increase in the cohesion of the material and therefore an improvement in its mechanical properties , which is measurable especially by young &# 39 ; s modulus of elasticity . in order to obtain such an effect of improving the mechanical properties when using starch , it is preferred for step ( d ) to be conducted at a temperature of at least 600 ° c ., preferably at least 800 ° c ., more preferably at least 1000 ° c . or even at least 1200 ° c . in some cases , the surfactant used in step ( a ) is capable of providing the effect of stabilising the foam of step ( b ). this is the case especially with sds , which is found to be useful both as a surfactant for stabilising the water / nanotube interface in step ( a ) and for stabilising the interface ( water / gas ) in step ( b ). in this particular case , no foaming agent is added to the dispersion produced in step ( a ), but it can prove advantageous to use the surfactant of step ( a ) in excess relative to the required amount in order to ensure that the nanotubes are stabilised in the dispersion . accordingly , for example , in the case of sds , the surfactant will advantageously be used in the dispersion of step ( a ) in an amount greater than 1 g / l or even greater than 2 g / l . nevertheless , sds is generally used in a concentration below its critical micelle concentration ( cmc ), above which threshold the nanotubes would aggregate by depletion interaction . whatever the nature of the foaming agent used in step ( b ), the formation of the foam in that step is advantageously carried out by injecting bubbles of gas into the dispersion obtained at the end of step ( a ), if necessary ( which is most often the case ) with the addition of a foaming agent , the injection of gas bubbles advantageously being carried out through a porous membrane such as sintered glass , above which the dispersion is placed . in that case , the solution is generally placed in the bottom of a receptacle of sufficient size and of a shape suitable for permitting swelling of the foam . there is typically used a tubular cylindrical receptacle equipped with a porous membrane of the sintered glass type in its base ( bottom ), the volume of the receptacle being at least 20 times the initial volume of the dispersion before foaming . in step ( b ), the gas used to carry out the foaming can vary to a fairly large degree . it is advantageously air or nitrogen , to which a hydrophobic organic compound , for example perfluorohexane , can optionally be added . this additional organic compound serves especially to stabilise the air / water interfaces of the foam , thus minimising the ostwald ripening and coalescence phenomena which destabilise newly formed foams . the morphology of the foam obtained in step ( b ) can be controlled by acting on several parameters , especially the following : as it rises under the action of the gas that effects foaming , the foam that forms tends to lose its water by gravity , the water tending to flow downwards . this phenomenon tends to form a relatively “ dry ” foam with non - spherical cavities which are polygonal in shape and relatively unstable . in order to obtain more spherical cavities and improve the stability , the degree of moisture in the foam can be increased , especially by offsetting the loss of water due to gravity by “ feeding ” the developing foam with water , in its upper portion . to that end , in order that the composition of the foam is not modified at the same time , the top of the developing foam in step ( b ) can be sprinkled with a dispersion having the same composition as the dispersion that is being foamed by swelling , sprinkling preferably being carried out at a constant flow rate , which allows a homogeneous composition of the foam to be obtained while additionally avoiding any coalescence phenomena . accordingly , in step ( b ), controlling the volume fraction of water in the foam is a good way of controlling the shape of the cells that are produced . a foam having a low volume fraction of water ( dry foam ) will have cells of polygonal shapes , while a foam having a high volume fraction of water ( moist foam ) will have cells which are much more spherical in shape . in addition , controlling the volume fraction of water in the starting foams also enables the width of the cell walls ( called “ plateau borders ”) to be controlled . this size can be controlled by acting on the size of the gas bubbles initially introduced to carry out the swelling ( especially by acting on the size of the pores of the porous membrane used ), as well as the flow rate of the gas used . at a constant gas flow rate , the size of the cavities of the resulting foam is generally larger , the larger the gas bubbles that are injected . with constant flow rates of the order of , for example , from 0 . 1 to 1 ml / s and pore sizes of the porous membrane of the order of from 10 to 250 microns , the average size of the cells of the resulting foam can typically be varied from 50 to 600 microns . there is generally obtained in step ( b ) a foam in which the volume ratio liquid / gas is less than or equal to 0 . 1 , that ratio typically being from 0 . 01 to 0 . 1 . broadly , the aim of step ( c ) of the process is to fix the structure of the foam obtained in step ( b ). the foam obtained at the end of step ( b ) is relatively stable , but not sufficiently so that drying thereof by evaporation can be considered . accordingly , in step ( c ), the water is removed by freezing and then freeze - drying . the freezing of step ( c ) is advantageously carried out so as to fix the structure of the foam as quickly as possible , so as to avoid any coalescence phenomenon . to that end , freezing is advantageously carried out by placing the foam obtained in step ( b ) at a temperature below − 50 ° c ., more advantageously below − 80 ° c . the sublimation step is in most cases a low - temperature freeze - drying carried out by any means known per se ( especially according to the conventional technique described , for example , on the site www . lyo - san . ca / lyophilisation / htm ). the heat treatment step ( d ), which is optional , can be conducted especially in a hot cabinet or in an oven , generally in an inert , non - oxidising atmosphere , preferably with a gradual rise in temperature and gradual cooling , typically with temperature rises and drops of the order of several degrees per minute . in order to optimise this step it can be advantageous for the material subjected to the heat treatment to comprise carbon - containing adjuvants used as carbon precursor . during the heat treatment , these carbon - containing adjuvants decompose into carbon , whereby they are capable of bonding the nanotubes together in the structure that is formed , thus resulting in an improvement in the cohesion and the mechanical properties of the resulting material . such carbon - containing adjuvants can be introduced , for example , during steps ( a ) and / or ( b ). to that end , as emphasised hereinbefore , starch or a sugar ( for example polysaccharide ) is advantageously used as the foaming agent in step ( b ). it is also possible to introduce other carbon - containing adjuvants during steps ( a ) and / or ( b ), provided that they do not stand in the way of the formation of the desired foam , for example saccharose , melamine or a phenolic resin . alternatively , such carbon - containing adjuvants can be introduced after step ( c ), before step ( d ), for example by impregnating the structure obtained at the end of step ( c ), prior to the heat treatment of step ( d ). at the end of these various steps there is generally obtained an aerogel in the form of a generally monolithic , macroscopic material typically having dimensions of the order of a centimeter or of about ten centimeters or even more . in these aerogels , the nanotubes are advantageously bonded together , the materials preferably having a young &# 39 ; s modulus of elasticity greater than or equal to 5 mpa , for example from 5 to 10 mpa . for some applications , the resulting aerogels can be post - treated , for example in order to be impregnated with catalytic species . fig1 is a scanning electron microscopy photograph showing the structure of an aerogel according to the invention . fig2 is a scanning electron microscopy photograph showing the structure of an aerogel according to the invention . the invention will be made further explicit in view of the exemplary embodiment below and with reference to the accompanying fig1 and 2 , which are scanning electron microscopy photographs of a material as prepared in the example , showing the very specific porosity that is obtained . an aerogel according to the invention is produced under the following conditions . within the scope of this specific example , multiwalled carbon nanotubes are used ( commercial pyrograph iii , of type pr - 24 - ps ). an aqueous dispersion of these nanotubes was prepared using carboxymethylcellulose sodium salt as dispersing surfactant . to that end , 1 g of carboxymethylcellulose sodium salt in 100 ml of water was introduced into a 150 ml erlenmeyer flask , and then 10 g of nanotubes were introduced into the resulting mixture . the resulting medium was placed in a bath at 0 ° c . ( liquid water + ice ) and was then subjected to sonication for 30 minutes with the aid of a branson 250 sonicator equipped with a type 12 probe , the end of which measures 3 mm in diameter ( 80 % pulse duration — 20 w power ). the water and ice bath is used to prevent heating during the sonication . a dispersion ( d ) comprising 10 % by mass nanotubes and 1 % by mass surfactant was thus obtained and was stirred for 12 hours . 1 g of tergitol np9 ( which corresponds to a concentration of 1 % by mass tergitol in the liquid ) was added to the dispersion ( d ) prepared in the preceding step . the dispersion so obtained was placed in a bubbler constituted by a vertical pvc column ( height : 60 cm ; diameter : 5 cm ) and equipped with a sintered glass bottom having a pore diameter of 30 microns , through which nitrogen is injected at a constant flow rate of 0 . 2 ml / s . the dispersion was introduced through a pipe above the column , supplied by a peristaltic pump . after 15 minutes , a relatively stable foam ( m ) was obtained , which was recovered with the aid of a spatula and immediately placed in a beaker . the beaker was immediately placed at − 80 ° c . and was maintained at that temperature for 5 hours . at the end of this freezing , the resulting frozen foam was freeze - dried using a low - temperature freeze - dryer ( vessel under vacuum ). the water was thus removed by sublimation , yielding an aerogel . the aerogel obtained in the preceding step was subjected to heat treatment under the following conditions : temperature rise from ambient temperature ( 25 ° c .) to 400 ° c . in 2 hours , temperature drop from 400 ° c . to ambient temperature ( 25 ° c .) in 2 hours . porosity having a porous macrostructure with pores of dimensions of around 50 microns with , inside the walls of that macrostructure , pores of a size of from 2 to 50 nm .