Acoustical ceramic panel and method

A rigid acoustic insulator panel for use as a sound insulator is disclosed. The panel is composed of a rigid matrix formed of randomly oriented, fused silica fibers having fiber diameters predominantly in the range between 0.5 and 2 .mu.m. The matrix has a three-dimensionally continuous network of open, intercommunicating voids, and a density of between about 2 and 6 lb/ft.sup.3. In one embodiment, the panel has greater flow resistance characteristics, progressing from a sound-absorbing side of the matrix to the opposite panel side. Also disclosed is a method of preparing the panel.

FIELD OF THE INVENTION 
The present invention relates to a lightweight, rigid, fibrous ceramic 
panel for acoustic sound insulation, and to a method of using and 
preparing the panel. 
BACKGROUND OF THE INVENTION 
Acoustical sound insulators are used in a variety of settings, such as 
vehicles, aircraft, and the like where it is desired to dampen noise from 
an external source. In general, such insulators should be lightweight, 
able to dampen sound over a wide sound-frequency spectrum, and relatively 
inexpensive in manufacture. 
With increased competitiveness in the aircraft industry, in particular, 
there is an interest in aircraft fuselage insulators which are lightweight 
and capable of serving as an effective sound barrier to jet and high-speed 
air noises. For use in aircraft, the acoustical insulator material should 
also be able be resist the uptake of moisture over time, and provide cabin 
protection against fires caused by aircraft impact. 
Current fuselage acoustic insulation used on civilian aircraft is 
fabricated from small diameter fiberglass strands held together in an 
organic matrix and berglass strands held together in an organic matrix and 
encased in a polymer film. The insulation is not water- or moisture-proof 
and tends to pick up significant amounts of water during use. The 
additional moisture pickup reduces the acoustic absorption performance and 
increases the aircraft's overall operational weight and cost. 
Current fiberglass insulations have relatively low porosities and a narrow 
range of pore sizes, and are typically used in mat thicknesses of 3-5 
inches. The material acts to dissipate sound, but does not form an 
effective sound barrier. To the extent that sound penetrates, but is not 
dissipated by the material, it is able to reach and pass through the 
interior panel of the fuselage into the passenger compartment. 
SUMMARY OF THE INVENTION 
The present invention includes, in one aspect, a rigid acoustic insulator 
panel for use as a sound insulator. The panel has a rigid matrix defining 
a sound-absorbing panel side and an opposite back side. The matrix (i) is 
formed of randomly oriented, fused silica fibers having fiber diameters 
predominantly in the range between 0.5 and 2 .mu.m, (ii) has a 
three-dimensionally continuous network of open, intercommunicating voids, 
and (iii) has a density of between about 2 and 6 lb/ft.sup.3. 
The matrix is preferably formed of fused silica and alumina fibers, where 
the alumina fibers make up 10-40 percent of the total fiber weight of the 
matrix. 
The fibers forming the matrix are preferably coated with a hydrophobic film 
effective to reduce water penetration into and retention in the matrix. 
The matrix has a preferred flow resistance between about 70-500K rayls/m. 
In one general embodiment, the matrix has a lower-to-higher flow resistance 
gradient, progressing in a direction from the sound-absorbing to the back 
side of the panel. Preferably the flow resistance measured at the 
sound-absorbing side panel is between about 20-100K rayls/m, and at least 
about 50% lower than that measured at the back side. 
The flow resistance gradient may be produced by a lower-to-higher density 
gradient across the panel, progressing in a direction from the 
sound-absorbing to the back side of the panel, or by a larger-to-smaller 
fiber diameter gradient, also progressing in a direction from the 
sound-absorbing to the back side of the panel. 
In another aspect, the invention includes a sound-absorbing panel for use 
as a sound barrier. The panel includes a rigid matrix of randomly 
oriented, fused silica fibers which form a three-dimensionally continuous 
network of open, intercommunicating voids, with a matrix density between 
about 2 and 6 lb/ft.sup.3. 
The matrix has a sound-absorbing sublayer (i) whose flow resistance is 
between about 20-100K rayls/m, and a backing sublayer whose fiber sizes 
are predominantly in the 0.5 to 2 .mu.m diameter size range, and (ii) 
whose flow resistance is at least 50% greater than that of the 
sound-absorbing sublayer. 
In one general embodiment, the sublayers form a continuous gradient of flow 
resistance between them, produced, for example, by a continuous density 
gradient between the two sides of the panel. 
In another general embodiment, the two sublayers form a discontinuous 
gradient of flow resistance between them, produced, for example, by a 
discontinuous density gradient or fiber-size gradient between them. 
In a related aspect, the invention includes a method for reducing the level 
of sound entering a compartment, such as a vehicle or aircraft 
compartment, from an external sound source. The method includes shielding 
the compartment with one or more panels of the type described above, where 
the sound-absorbing side of the panel is disposed to confront the external 
sound source. 
In still another aspect, the invention includes an improvement in a method 
for preparing a rigid, fused-silica matrix, by the steps of (a) forming a 
slurry composed of (i) silica fibers having selected fiber thicknesses and 
a selected fiber:liquid weight ratio, (ii) a thickening agent effective to 
give the slurry a selected viscosity, and (iii) boron nitride particles, 
in an amount between about 2-12 percent by weight of the total fiber 
weight, where the slurry contains silica fibers, a dispersing agent 
effective to enhance the dispersion of silica fibers in the slurry, (b) 
allowing the slurry to settle in a mold under conditions effective to 
produce a fiber block, and (c) drying the settled block to form a 
substantially dehydrated fiber block, and (d) heating the dehydrated block 
to a temperature of at least about 2200.degree. F. for a period sufficient 
to cause the silica fibers to form a fused-fiber matrix. 
The improvement includes selecting fiber sizes predominantly in the 0.5 to 
2 .mu.m size range for preparing the slurry, and allowing the slurry to 
settle under conditions effective to produce a density gradient in the 
fiber block in which the lower portion of the block has a density of at 
least about 50% greater than that of the upper portion of the block, and 
the average density of the block is between about 2 and 6 lb/ft.sup.3. 
Alternatively, the improvement includes selecting fiber sizes predominantly 
in the 0.5 to 2 .mu.m size range for preparing the slurry, allowing the 
slurry to at least partially settle, and adding to the at least partially 
settled slurry, a second slurry having fiber sizes predominantly greater 
than 2 .mu.m. 
These and other objects and features of the invention will become more 
fully apparent when the following detailed description of the invention is 
read in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
I. Acoustical Panel 
The acoustic panel of the invention is designed for use as a sound barrier, 
typically for sound-insulating a compartment for reducing the level of 
noise reaching the compartment from an external noise source. 
Because the panel is lightweight and able to reduce noise over a wide 
spectrum of sound frequencies, the panel is particularly suited for 
shielding the passenger compartments of high-speed vehicles and aircrafts. 
The use of the panel will be described below with respect to its use as a 
sound barrier for the passenger compartment of an aircraft, it being 
recognized that the invention is applicable to a variety of settings in 
which acoustical insulation is needed. 
A. Panel Configurations 
FIG. 1 shows a cross-sectional region of a passenger-area of an aircraft 
fuselage 20 containing an acoustic insulation panel 22 constructed in 
accordance with the invention. The fuselage conventionally includes an 
outer skin 24, a series of longitudinally extending stringers, such as 
stringers 26, and a series of circumferential frames, such as frames 28, 
encircling the fuselage. 
In accordance with an important feature of the invention, the panel is 
constructed of a rigid matrix which is formed of randomly oriented, fused 
silica fibers having fiber diameters predominantly in the range between 
0.5 and 2 .mu.m, and a three-dimensionally continuous network of open, 
intercommunicating voids, as detailed below with respect to FIGS. 3A-3D. 
The matrix has a density of between about 2 and 6 lb/ft.sup.3, where this 
density refers to the average bulk density of the matrix in fused form, 
i.e., considering the average density of the panel as a whole. 
The panel has a sound-absorbing side 30 which faces the fuselage skin, and 
an opposite back side 32 which is attached, e.g., by adhesive attachment, 
to an interior wall 34 of the aircraft passenger compartment. The panel 
and attached wall are attached to the fuselage frames by direct attachment 
of the panel's sound-absorbing side to the frames, as shown. The rigid 
acoustical panel thus serves both as an acoustical barrier between the 
fuselage skin and passenger compartment, and as a structural member for 
attaching the interior wall to the fuselage. 
FIG. 2A shows a configuration for mounting an acoustical panel, here 
indicated at 36, to an aircraft fuselage 38 in a non-passenger area of the 
aircraft. The figure shows a fuselage skin 40, a stringer 42, and frames, 
such as frame 44 of the fuselage. 
The panel has a sound-absorbing side 46 facing the outer skin of the 
aircraft, and a back side 48 which here serves as the interior wall of the 
non-passenger region of the aircraft compartment. As above, the panel is 
attached directly to associated frames of the aircraft fuselage. In other 
words, the configuration is identical to that in FIG. 1, except that the 
panel in the non-passenger compartment serves both as an acoustical 
insulator and as the interior wall surface of the compartment. 
FIG. 2B shows another configuration for mounting acoustical panels, such as 
panels 50, 52, 54 in a non-passenger compartment region of an aircraft 
fuselage 54. As above, the fuselage structure includes an outer skin, 
stringers, such as stringer 58, and a series of frames, such as frame 60. 
The panel configuration in this figure includes a series of longitudinally 
spaced interior panels, such as panel 50, attached directly to associated 
stringers, such as stringer 56, by adhesive or mechanical attachment. Each 
set of frames, such as the set including frame 58, is covered by a shorter 
frame panel, such as panel 54 attached directly to those frames. Side 
panels, such as panel 52, are used to fill the space between the interior 
and frame panels, and are attached, e.g., adhesively, to the overlapping 
edge portions of the interior and frame panels, as shown. Alternatively, 
the U-shaped members formed by the frame and adjacent side panels may be 
fabricated as a single piece and adhesively or mechanically attached to 
the associated frames. 
As in FIG. 2A, the panels serve both as acoustical insulators and 
structural members forming the interior wall surfaces of the non-passenger 
region of the compartment. The sides of the panels opposite the 
wall-surfaces of the panels are the sound-absorbing sides of the panels. 
It will be appreciated that the above configurations are representative of 
many different fastening and insulation configurations that may be 
suitable for sound insulating a chamber, such as an aircraft fuselage, a 
vehicle passenger compartment, or the like. 
B. Panel Microstructure 
FIGS. 3A-3D are scanning electron microscopy (SEM) photomicrographs of a 
fused-fiber matrix 60 making up the acoustical panel of the invention. The 
matrix is composed typically of 60-90% by weight silica fibers and 10-40% 
by weight alumina or alumina/silica (mullite). In the embodiment shown the 
matrix is composed of 80 percent of fiber weight of silica fibers and 20 
percent by fiber weight of alumina fibers. A matrix of this type will be 
referred to herein as a fused-silica matrix, it being recognized that the 
matrix is composed of silica fibers or a composite of silica and alumina 
fibers fused with one another, typically above 2,000.degree. F. 
The figures are electron micrographs of the matrix taken at 200.times. 
(3A), 1,000.times. (3B), 2,000.times. (3C), and 7,000.times. (3D) 
magnification. The portion of the matrix in FIG. 3A shows a "nest" of 
fused silica and alumina fibers, such as fibers 62, 64, respectively, 
ranging in size from about 200 .mu.m to 10 mm in length. The higher 
magnification SEM micrograph seen in FIG. 3B shows how the fibers are 
fused at their points of intersection to form a rigid fiber structure 
having 3-dimensionally continuous network of interconnecting voids or 
pores, such as pores 66, which tend to have "long" (uninterrupted) 
dimensions between about 10-100 .mu.m, and short "width dimensions between 
about 0.1 to 5 .mu.m. That is, the fused fibers are substantially randomly 
oriented, forming in all directions, interconnecting pores defined by 
groups of fused fibers, where the pores can range in size between about 
0.1 to 100 .mu.m depending on pore orientation and distance between 
adjacent fibers. 
The 2,000.times. magnification micrograph (FIG. 3C) clearly shows both 
silica fibers, which are smooth surfaced, and alumina fibers, which have a 
textured or mottled surface. The silica fibers in the matrix, which 
constitute the predominant fiber species, preferably 60-90 weight percent, 
have diameters in the 0.5 to 2 .mu.m size range. The alumina fibers, which 
preferably constitute between 10 and 40 weight percent of the matrix, may 
have sizes in the same range, or as shown here, larger fiber diameter 
sizes, e.g., 2.5-3.5 .mu.m. 
The mottled regions on the alumina fibers presumably represents grain 
growth that occurs during the high-temperature sintering step used in 
forming the matrix. Clearly visible in FIG. 3C are fusion junctions 
between two silica fibers, such as junction 68; fusion junctions between 
silica and alumina fibers, such as junction 70 between silica and alumina 
fibers; and fusion junctions, such as junction 72 between two alumina 
fibers. 
The junction region at the lower center in FIG. 3C is shown at 7,000.times. 
magnification in FIG. 4D. The micrograph shows more clearly the textured 
grain-growth regions of the alumina fibers, and both silica/alumina and 
alumina/alumina fiber junctions. 
C. Panel Properties 
The acoustic panel of the invention is designed to provide (i) an effective 
sound barrier over a wide range of lower frequencies, (ii) surface pore 
sizes which allows absorption of sound over a wide range of higher 
frequencies, and (iii) the ability to reflect non-absorbed sound and 
dissipate absorbed sound. 
The property of the panel as a sound barrier, particularly at lower sound 
frequencies, is related to (i) the relatively high flow resistance of the 
panel material, and (ii) to its material strength. 
In one general embodiment of the invention, the panel has a relatively high 
flow resistance at both panel sides and a relatively uniform flow between 
panel sides. The flow resistance at both panel sides is preferably between 
about 70-500K rayls/m. In this embodiment, the sound-absorbing side of the 
panel acts as a barrier to sound, particularly in lower-frequency ranges, 
e.g., at frequencies below about 1,000 Hz. 
FIG. 4 shows a device 74 for use in measuring the flow resistance of a 
sample, here indicated at 76. The device includes a sample chamber 78 for 
holding the sample between a pair of screens 80. 82. An air supply or 
vacuum source 84 pumps air into or evacuates air from, respectively, a 
lower region 86 of the chamber. The rate of air flow between chamber 
region 86 and source 84 is measured by a flowmeter 88. Chamber region 86 
is also in fluid communication with a differential pressure measuring 
device 90 which measures the pressure differential across the sample. 
In operation, source 84 is adjusted to a desired pressure or vacuum level. 
The resistivity of the sample in the sample chamber is then measured from 
the pressure differential across the sample and the rate of flow through 
the device, with high pressure differential measurements and low flow 
rates being associated with high resistivity, and low pressure 
differential and high flow rates being associated with low resistivity. 
FIG. 5 shows the relationship between flow resistivity and mean pore size 
in a panel constructed in accordance with the invention. The panel 
matrices examined were formed to have varying bulk densities and/or fiber 
diameters, as discussed in Section III below. Mean pore size of each 
matrix was determined by percent intrusion of mercury into a matrix, as a 
function of mercury intrusion pressure, measured using a Micromeretics 
PoreSizer 9320 mercury porosimeter. Sample sizes with dimensions of 0.5625 
inch diameter by 0.4 inch height were cored from a fused matrix formed in 
accordance with the invention. The intrusion pressure was varied from 0.15 
to 30 psixA (area=1 in.sup.2), over 85 points of increasing pressure. From 
this data, a instrument program calculated the incremental volume (ml/g) 
intruding into the sample. An internal program is used to calculate a pore 
diameter in microns for a given pressure level. From this, the mean pore 
diameter for the sample is determined. 
As seen, flow resistance increases logarithmically with decreasing mean 
pore size over a mean pore size range of about 20-150 .mu.m, with the 
desired flow resistance in the range between 70-500K rayls/m corresponding 
to mean pore sizes in the range of about 80-90 .mu.m or less. 
FIG. 6 shows the distribution of pore sizes in a panel constructed in 
accordance with the invention, and in particular, a panel having a 
lower-to-higher density gradient progressing from the panel's 
sound-absorbing to its back side. As described above, the pore size 
distribution is determined from the extent of Hg intrusion into a 
defined-area surface of the panel, at each of a number intrusion pressure 
from about 0.15 to 30 psiA. 
For the sound-absorbing side of the panel, pore sizes ranged from about 0.1 
to 850 .mu.m, with a mean pore size of 65.8 .mu.m. With reference to FIG. 
5, this mean pore size corresponds to a flow resistivity of about 180K 
rayls/m. For the back panel side, pore sizes ranged from about 0.1 to 100 
.mu.m, with a mean pore size of about 51.2 .mu.m, corresponding to a flow 
resistivity of about 334K rayls/m. These measurements illustrate how a 
flow resistivity gradient in a panel constructed in accordance with the 
invention can be demonstrated. 
In addition to high flow resistivity, the barrier properties of the panel 
also rely on high strength (stiffness). Without material stiffness, or 
alternatively, material mass, sound pressures that build up by the high 
resistance on the incoming side of the panel will merely cause the 
material to move as a unit and transmit this motion into acoustical 
pressure on the other side of the panel. Stiffness is more desirable than 
mass, since lighter weight is desirable, particularly for vehicle/aircraft 
use. The stiffness properties of the material are discussed below. 
According to another feature of the panel matrix, the wide range of pore 
sizes is effective to absorb sound over a broad range of higher 
frequencies, e.g., above about 1,000 Hz. As already noted, the range of 
pore sizes in the panel of the invention is between about 0.1 to 100 
.mu.m. 
Once absorbed, sound waves of a particular frequency are deflected and 
dissipated by the randomly oriented, fused silica fibers. In particular, 
the relatively high internal flow resistivity of the material, combined 
with high material strength, acts to dampen sound waves by localized 
vibrations within the matrix. 
To be effective in dissipating absorbed sound, the material must also have 
a thickness of at least about one-quarter wavelength. This is to insure 
that some portion of the wave having high particle velocity is within the 
dissipative medium. A preferred panel thickness is at least about 1/2 
inch, preferably 1/2 to 2 inches. 
As indicated above, the ability of the panel material to reflect 
non-absorbed sound, and to dissipate absorbed sound depends on 
panel-matrix stiffness, due to the fused-fiber construction of the 
material. One measure of material stiffness is compression modulus, which 
provides a measure of the material resistance to deformation under a 
compressive force, measured according to standard methods. The compression 
modulus of the panel matrix is preferably between 100 and 2,500 psi. 
With reference again to FIGS. 1-3, it can be appreciated how the panel of 
the invention acts to insulate an aircraft against outside noise, e.g., 
engine noise. In the embodiments shown in FIGS. 1 and 2, sound impinging 
on the insulating panel from the outside is partially reflected, 
particularly at lower frequencies, and partially absorbed and dissipated, 
particularly at higher frequencies. Some of the reflected sound will pass 
through the fuselage skin, and some will be back reflected at higher 
frequencies, leading to greater sound absorption. 
Because sound that is absorbed tends to be dissipated within the panel, due 
both to the high flow resistivity of the panel and to its stiffness, the 
panel provides an effective insulator against outside sound over a broad 
range of sound frequencies, such as are characteristic of jet engine and 
high-speed air noises. 
In addition to the ability of the panel material to act as a sound 
insulator, by reflecting non-absorbed sound and dissipating absorbed 
sound, the panel also has useful properties, particularly in the context 
of aircraft sound insulation, of (i) rigid construction, (ii) low density, 
(iii) ability to resist uptake of moisture, and (iv) ability to provide 
good heat insulation against fire. 
The rigid construction of the panel allows its use as a structural wall 
member, as indicated in the FIG. 2 and FIG. 3 configurations. 
The ability of the panel to resist moisture uptake is achieved by coating 
the fibers making up the matrix with a hydrophobic surface coating, such 
as a surface coating of an alkyltrialkoxyysilane, such as methyl 
trimethoxysilane, polyethylene, polystyrene, or polytetrafluoride. Methods 
for coating the fibers of a matrix with a hydrophobic polymer are 
considered in Section IV below. 
II. Panel with Flow-Resistance Gradient 
In a second general embodiment of the invention, the panel matrix has a 
lower-to-higher flow resistance gradient, progressing in a direction from 
the sound-absorbing panel side to the back panel side. As will be 
described, the gradient results from a lower-to-higher density gradient, 
progressing in a direction from the sound-absorbing to the back side of 
the panel, and/or to a larger-to-smaller fiber size gradient, progressing 
in the same direction. 
More generally, the panel of the invention may include a rigid matrix of 
the type described above, having (i) a sound-absorbing sublayer whose flow 
resistance is between about 20-100K rayls/m, and (ii) a backing sublayer 
whose fiber sizes are predominantly in the 0.5 to 2 .mu.m diameter size 
range, and whose flow resistance is at least 50% greater than that of the 
sound-absorbing sublayer. The two sublayers may form a continuous 
flow-resistance gradient between opposite panel sides, as described in 
FIGS. 7 and 8 below, or may be joined at a relatively steep gradient 
region, as described in FIGS. 9 and 10 below. 
FIG. 7 shows a side view of a panel 92 having having a continuous 
flow-resistance density gradient between its sound-absorbing and opposite 
sides 94, 96, respectively. In this embodiment, the flow-resistance 
gradient in the panel is due to a lower-to-higher density gradient on 
progressing from the sound-absorbing to the opposite panel side. The 
fibers forming the matrix are preferably in the range 0.5 to 2 .mu.m, 
although larger fiber diameters, e.g., in the range 1-5 .mu.m may be 
employed. 
Specifically, the fiber sizes should be such as to produce a flow 
resistance, at the back side of the panel opposite the sound-absorbing 
side, of between about 70-500K rayls/m, such that the panel can act an 
effective barrier to sound penetration. The flow resistivity at the 
sound-absorbing side of the panel is preferably between about 20-100K 
rayls. 
As discussed above with respect to FIG. 6, higher flow resistivity is 
achieved in the panel of the invention by reducing mean pore size. Mean 
pore size, in turn, can be reduced by reducing the average fiber diameter 
size or increasing the matrix density. Therefore, if larger diameter 
fibers are used, a greater matrix density will be required at the back 
side of the panel, to achieve the desired high flow resistivity. 
FIGS. 8A and 8B illustrate the different fiber densities in front and back 
regions 98, 100, respectively, of the panel, i.e., regions of the 
sound-absorbing sublayer and backing sublayer, respectively. As seen, the 
fibers forming each sublayer, such as fibers 102 forming sublayer 98 and 
fibers 104 forming sublayer 100 have substantially the same fiber 
diameters, but are more closely packed in the backing sublayer, producing 
a lower mean pore size and thus a higher flow resistivity than in the 
panel's sound-absorbing sublayer. 
In the embodiment shown, in which the flow-resistivity gradient is due to a 
bulk phase density gradient, the density of the panel's backing sublayer 
is preferably at least about 0.5 lb/ft.sup.3 greater than that of the 
panel's sound-absorbing sublayer, where each sublayer is considered to be 
a finite-width slice of the panel taken at either panel side. In the 
density range particularly between 2-3 lb/ft.sup.3, this density 
difference across the panel sides can produce a difference in flow 
resistivity between the two sublayers of twofold or more, as can be 
appreciated from FIG. 6B. Methods for forming a panel having a continuous 
density gradient of this type will be described below in Section V. 
Alternatively, or in addition, the continuous flow-resistivity gradient in 
the panel may be formed by side-to-side variations in fiber diameter 
sizes, as illustrated for the discontinuous gradient panel now to be 
described. 
FIG. 9 shows a side view of a panel 104 having having a discontinuous 
flow-resistance density gradient between its sound-absorbing and backing 
sublayers 104, 106, respectively. The discontinuity, indicated at 106, 
defines the boundary between the upper sound-absorbing sublayer, indicated 
at 108, and the lower backing sublayer, indicated at 110. 
In this embodiment, the flow-resistance gradient in the panel is due to a 
larger to smaller fiber diameter gradient on progressing from the 
sound-absorbing to the back side, i.e., between the sound-absorbing and 
backing sublayers. In particular, and as illustrated in FIGS. 10A and 10B, 
the fibers, such as fibers 112, forming the sound-absorbing sublayer are 
in a size range preferably between about 2-9 .mu.m, more preferably 3-6 
.mu.m, and the fibers, such as fibers 114 forming the backing sublayer, 
are preferably in the size range 0.5-2 .mu.m. 
The properties of the gradient panel, as it functions as a sound insulator, 
are similar to the uniform-matrix panel described in Section I. However, 
the gradient panel differs in an important respect. Because of the lower 
flow resistivity of the sound-absorbing face, e.g., less than 100K 
rayls/m, the panel absorbs more sound, particularly at lower sound 
frequencies. However, because of the high flow resistivity of the backing 
sublayer, as well as the stiffness of the panel, absorbed sound is still 
effectively dissipated as it moves through the panel. In either the 
continuous or discontinuous gradient embodiments, the backing sublayer, 
which serves as a barrier to sound penetration, particularly for lower 
frequency sound, may be a relatively thin portion of the total panel 
width, for example, 1/8-1/4 inch out of a total to 1-2 inch panel. The 
sound-absorbing sublayer, which functions to absorb and dissipate absorbed 
sound, is preferably at least about 1/2 inch, preferably 1/2 to 2 inches, 
as discussed above for the uniform panel described in Section I. 
III. Sound-Insulation Method 
In another aspect, the invention includes a method for reducing the level 
of sound entering a compartment, such as a vehicle or aircraft 
compartment, from an external sound source. The method includes shielding 
the compartment with a one or more panels of the type described in Section 
I or II, where the sound-absorbing panel side is placed to confront the 
external sound source. 
In one embodiment, illustrated in Section II, the panel matrix has a 
lower-to-higher flow resistance gradient, progressing in a direction from 
the sound-absorbing to the back side of the matrix. 
In this embodiment, the matrix may have a lower-to-higher density, 
progressing in a direction from the sound-absorbing to the back side of 
the lattice, and/or a larger-to-smaller fiber diameter on progressing in 
the same direction. 
As discussed above, the method is effective to reflect impinging sound, 
particularly at lower frequencies, and to absorb and dissipate sound over 
a broad range of higher frequencies, providing effective sound insulation 
over a wide sound frequency spectrum. 
IV. Method of Panel Preparation 
This section describes the preparation of the acoustical insulator panel of 
the invention, and in particular, one having a substantially uniform 
flow-resistivity between its sound-absorbing and opposite sides. 
The basic preparation method involves the steps of (i) forming a fiber 
slurry having desired viscosity and fiber dispersion characteristics, (ii) 
allowing the slurry to settle under conditions that produce a selected 
fiber density and orientation, (iii) drying the resulting fiber block, and 
(iv) sintering the block to form the desired fused-fiber matrix. 
A. Fiber Treatment 
The silica (SiO.sub.2) and/or alumina (Al.sub.2 O.sub.3) fibers used in 
preparing the matrix are available from a number of commercial sources, in 
selected diameters (fiber thicknesses) between about 0.5 and 2 .mu.m, or 
larger fiber sizes, e.g., 2-8 .mu.m where a panel with a fiber-size 
gradient is produced, as described below. A preferred silica fiber is a 
high purity, amorphous silica fiber (99.68% pure), such as fabricated by 
Manville Corporation (Denver, Colo.) and sold under the fiber designation 
of "Q-fiber". High purity alumina fibers (average 2.5 to 3.5 .mu.m) may be 
procured, for example, from ICI Americas, Inc. (Wilmington, Del.). 
In a preferred heat treatment, the silica fibers are compressed into 
panels, e.g., using a Torit Exhaust System and compaction unit. The 
compressed panels are passed through a furnace, e.g., a Harper Fuzzbelt 
furnace or equivalent, above 2100.degree. F. for a minimum of 60 minutes, 
corresponding to a speed setting of about 5.4 inches/minute. The heat 
treatment is used to close up surface imperfections on the fiber surfaces, 
making the matrix more stable to thermal changes on sintering. The heat 
treatment also improves fiber chopping properties, reducing fabrication 
time. The method is illustrated in Example 1, Part A. 
B. Preparing a Fiber Slurry 
Silica, and optionally including alumina and/or mullite fibers, from above 
are blended to form a fiber slurry that is used in forming a "green-state" 
block that can be sintered to form the desired matrix. 
The slurry is formed to contain, in an aqueous medium, silica, or silica 
and alumina fibers of the type described above, at a fiber:liquid weight 
ratio of between about 1:20 to 1:200, where the liquid weight refers to 
the liquid weight of the final slurry preparation. For producing a panel 
with a uniform density gradient, a relatively low fiber:liquid ratio, 
e.g., 1:20-1:50 is preferred. 
The slurry preferably includes thickening agents effective to give the 
slurry a viscosity between about 500 and 10,000 centipoise, as measured by 
standard methods (Example 1). The viscosity agent may be any of a number 
of well-known hydrophilic polymers, such as polyvinylalcohol, 
polyvinylacetate, polyvinylpyrrolidone, polyurethane, polyacrylamide, food 
thickeners, such as gum arabic, acacia, and guar gum, and methacrylate 
type polymers. The polymers preferably have molecular weights greater than 
about 25-50 Kdaltons, and are effective to increase solution viscosity 
significantly at concentrations typically between about 2-50 weight 
percent (based on total fiber weight) solution. For producing a panel with 
a relatively uniform matrix density, a relatively high slurry viscosity is 
preferred. 
One preferred thickening agent is Acrylic Acid Polymer, e.g., the polymer 
sold under the tradename Acrysol ASE-108 and available from Rohm and Haas 
Company (Philadelphia, Pa.). An acrylate solution used in the method is 
detailed in Example IB. 
The slurry is also preferably formed to contain a source of boron that 
functions, during sintering, to form a boron/silica or boron/alumina 
surface eutectic that acts to lower the melting temperature of the fibers, 
at their surfaces, to promote fiber/fiber fusion at the fiber 
intersections. In a preferred embodiment, the boron is supplied in the 
slurry as boron nitride particles 15 to 60 .mu.m in size particles. Such 
particles can be obtained from Carborundum (Amherst, N.Y.). The amount of 
boron nitride is preferably present in the slurry in an amount 
constituting between about 2-12 weight percent of the total fiber weight. 
The adhesive property of the thickening agent described above is useful in 
adhering particles of boron nitride to the fibers in the slurry, to 
produce a relatively uniform dispersion of particles in the slurry, and to 
prevent the particles from settling out of the slurry during the molding 
process described below. 
Scanning electron micrographs of a green-state block shows an even 
distribution of boron nitride particles within the fiber matrix. The even 
distribution of particles throughout the block is advantageous in 
achieving effective and relatively uniform boron concentrations throughout 
the matrix during sintering, as described below. 
Fragments of the silica fiber are mixed in a desired weight ratio with 
alumina fibers, e.g., 10-40 weight percent alumina fibers, and the fibers 
are dispersed in an aqueous solution containing the dispersing agents. At 
this point, the fibers are uniformly dispersed in the liquid medium using 
a low-shear mixer. The boron nitride and acrylate suspension is mixed into 
the slurry, then a Methocel.TM. gel stock solution and reagent grade 
ammonium hydroxide are added as thickening agents to bring the viscosity 
of the slurry to a desired value between 500-10,000 centipoise. Generally 
the slurry is not chopped, since a greater degree of chopping produces 
shorter fibers leading to tighter packing and a less open matrix. 
Similarly, longer fibers lead to more open matrix structure and lower bulk 
densities. 
The fiber mixing is preferably carried out under condition to produce 
average fiber sizes of a selected size in the 3-20 mm fiber-length range. 
After dispersing the fibers uniformly in the liquid medium, the acrylate 
acid polymer solution and boron nitride suspension is added, then 
dispersed into the fiber slurry medium using a low shear mixer. The method 
is illustrated in Example 2A. 
C. Forming a Dried Fiber Block 
The method of forming a green-state block, i.e., a dried, rigid matrix of 
unfused fibers, from the above fiber slurry, is illustrated in FIGS. 
11A-11D. 
In the first step, illustrated in FIG. 11A, a slurry 120 is added to a mold 
122 equipped with a lower screen 124 sized to retain slurry fibers. For 
fiber sizes (lengths) in the range 1-15 mm, the screen has a mesh size 
between about 8 to 20 squares/inch. The mold has a lower collection trough 
126 equipped with a vacuum drain port 128. 
A vacuum of between 4 and 28 inches of mercury is applied to the port. In 
forming a uniform-density block, it is desirable to employ a compression 
plate (not shown) placed over the slurry. The compression plate acts to 
compress the slurry from above, to achieve a relatively uniform fiber 
packing as the slurry is dewatered. This is in contrast to the method 
described in the section below for constructing a green-state block with a 
pronounced top-to-bottom density gradient. In this method, it is desirable 
to promote fiber packing preferentially at the bottom of the mold, by 
applying only vacuum (without a packing plate). 
The vacuum is applied over a vacuum forming time, defined as the time 
required to reduce the slurry to the desired block height, and enough 
water is removed from the block so that standing liquid is removed from 
the top of the block, and the vacuum starts to pull air. A total vacuum 
forming time between about 5 and 300 seconds is sufficient to evacuate the 
water to form the desired block height, as illustrated in FIG. 11B. 
The complete vacuum dewatering process continues for 5 to 15 minutes after 
the vacuum forming time, until approximately 50% of the water is removed 
and/or little water is being drawn from the formed matrix, as illustrated 
in FIG. 11C. 
Finally, the dewatered panel is removed from the mold (FIG. 11D), placed 
onto a handling fixture, such as a metal plate, to prevent block damage 
during handling. The wet block is dried in an oven, typically at a 
temperature between 150.degree.-500.degree. F. 
In the dried matrix, the viscosity agent acts to bond the fibers at their 
intersections, forming a rigid, non-fused panel. The target density of the 
matrix after drying is between about 1.8 to 5.5 pounds/ft.sup.3. Details 
of the molding and drying steps, as applied to producing an exemplary 
silica/alumina fiber block, are given in Example 2, Parts A and B. 
The green-state block may be formed to include sacrificial filler(s) that 
will be vaporized during sintering, leaving a random dispersion of desired 
voids in the final fused matrix panel. The fillers are preferably formed 
of polymer or graphite. In the embodiment of the invention in which the 
panel matrix has a uniform flow resistivity throughout, the sacrificial 
filler is uniformly dispersed throughout the slurry used in forming the 
green-state block. 
D. Fused Fiber Matrix 
In the final step of matrix formation, the green-state block from above is 
sintered under conditions effective to produce surface melting and 
fiber/fiber fusion at the fiber crossings. The sintering is carried out 
typically by placing the green-state block on a prewarmed kiln car. The 
matrix is then heated to progressively higher temperature, typically 
reaching at least 2,000.degree. F., and preferably between about 
2,200-2,400.degree. F., until a desired fusion and density are achieved, 
the target density being between 2 to 5 pounds/ft.sup.3. One exemplary 
heating schedule for a silica/alumina matrix is given in Example 2C. 
In a preferred method, discussed above, the matrix is formed with 
high-purity silica and alumina fibers that contain little or no 
contaminating boron. In order to achieve fiber softening and fusion above 
2,000.degree. F., it is necessary to introduce boron into the matrix 
during the sintering process, to form a silica/boron or alumina/boron 
eutectic mixture at the fiber surface. Boron is preferably introduced, as 
detailed above, by including boron nitride particles in the green-state 
block, where the particles are evenly distributed through the block. 
During sintering, the boron nitride particles are converted to gaseous 
N.sub.2 and boron, with the released boron diffusing into the surface of 
the heated fibers to produce the desired surface eutectic, and fiber 
fusion. The distribution of boron nitride particles within the heated 
panel ensures a relatively uniform concentration of boron throughout the 
matrix, and thus uniform fusion properties throughout. 
Also during fusion, the viscosity agent and dispersant agents used in 
preparing the green-state block are combusted and driven from the block, 
leaving only the fiber components. 
Where the green-state panel has been constructed to include a high content 
(greater than 25 percent) of sacrificial element, an intermediate 
temperature treatment is required to effectively ensure all the 
sacrificial fibers are vaporized during sintering. In sacrificial element 
concentrations of less than 25 percent, the high temperature sintering is 
also effective to vaporize this element, leaving desired voids in the 
matrix, such as voids randomly distributed throughout the panels upper 
surface that is subjected to the sound waves (Section V below). 
Example 5 illustrates the preparation of a fused-silica matrix containing 
30% sacrificial fibers. It will be appreciated that the presence of 
sacrificial fibers, by effectively expanding the void space in the matrix, 
can be used to reduce matrix density in a systematic way. 
After formation of the fused-fiber matrix in flat, curved or complex shape, 
the matrix panel may be machined to produce the desired finished contours 
and configuration. 
E. Waterproofing 
The matrix is waterproofed to prevent moisture or water absorption into the 
panel. A chemical vapor infiltration process, as detailed in Example 4, is 
used to vaporize the methyltrimethoxysilane solution, which in the 
presence of a dilute acetic acid solution (catalyst) hydrolyzes the silane 
to react with active sites on the fibers and causes a self-polymerization 
to occur. The mono layer coating changes the surface tension of the 
individual fibers to make them hydrophobic which prevents any water 
molecules from wetting the fiber surface or absorbing into the bulk fused 
fiber matrix. Successful application of the waterproofing agents prevents 
moisture or water absorption into the matrix up to approximately 
1050.degree. F. 
Waterproofing agents that possess film-forming characteristics over the 
fiber matrix are preferred, e.g., methyltrimethoxysilane (MTMS), 
hexa-methyl-disilazane (HMDS), dimethylethoxyl (DMES), disilazane are 
examples of silane compounds applicable for waterproofing the rigid 
fibrous matrix. Other film-forming chemicals such as the commercially 
available product Scotchguard.TM., which are externally applied, provide 
limited moisture and water absorption protection (less than 100 percent 
effective). The preferred waterproofing agent for this application is a 
methyltrimethoxysilane, commercially manufactured by Dow Corning under the 
product name DC-Z6070. 
V. Forming a Panel with a Flow-Resistivity Gradient 
The invention also provides improvements in the above panel-forming method, 
for forming panels having a flow-resistivity gradient between its 
sound-absorbing and back side. 
A. Matrix with a Density Gradient 
As discussed in Section III, with reference to FIGS. 7 and 8, the 
flow-resistivity gradient may be produced by a matrix density gradient 
between front and back panel sides. Preferably the matrix density at the 
back of the matrix, or in the backing sublayer, is at least about 0.5 
lb/ft.sup.3 higher than that of the panel's sound absorbing side or 
sublayer. 
A panel of this type can be produced, in accordance with the invention, by 
a modification of the panel-forming method described with reference to 
FIGS. 12A-12D. The modification is designed to produce greater initial 
packing of the slurry, in the lower region of the mold, and consequently 
less packing at the upper region of the mold. 
In one embodiment, this slurry packing feature is achieved by reducing the 
fiber:water ratio of the slurry, typically to a range of about 1:80 to 
1:400. The more dilute slurry tends to become more highly compacted in its 
lower region, with vacuum removal of water in the mold, because a greater 
amount of water is being pulled through the compacting slurry. This 
greater packing at the lower portion of the mold, in turn, reduces the 
rate of water removal from the slurry, producing progressively looser 
packing as more of the slurry becomes dewatered. 
At the same time, the viscosity of the slurry is preferably made relatively 
low, preferably in the range between about 500 and 1,000 centipoise. The 
lower viscosity assures that the fibers in the slurry will settle readily 
under gravity during initial dewatering, to form a relatively high fiber 
density at the bottom of the mold. 
In an alternative embodiment, a fiber density gradient in the settling 
slurry is established by compacting the slurry under a relatively low 
vacuum. The lower vacuum causes a slower rate of water removal from the 
slurry, allowing more fiber settling under the influence of gravity, and 
therefore greater fiber compaction at the initial stages of water removal 
from the slurry. As above, initial fiber compaction leads to a reduced 
rate of water removal, producing progressively less packing in the 
remaining slurry. 
As noted above, typical vacuum pressures applied to the mold during slurry 
compaction are between about 16-28 inches of Hg, typically about 20-26 
inches of Hg. In forming a block with a fiber density gradient, the vacuum 
is reduced typically to between about 7-14 inches of Hg. As indicated 
above, slurry viscosity and fiber:water ratio, in addition to vacuum, will 
determine the rate of settling of the fibers, and thus the gradient 
produced in the block. 
In still another approach, the matrix density gradient is formed by 
introducing sacrificial fibers or particles into the upper portion of the 
slurry, after a substantial portion of the slurry has already settled. The 
sacrificial material is added to create a upper sublayer in a green-state 
block (i) containing preferably between about 20-40 by weight sacrificial 
material, (ii) a total thickness of at least about 1/2 inch, and (iii) a 
continuous-gradient interface with the lower portion of the block. 
In this embodiment, the green-state block itself may be formed to have a 
relatively uniform density throughout, since the reduced fiber density is 
created during sintering, when the sacrificial material in the upper 
(sound-absorbing) sublayer of the mold is vaporized. 
More generally, this embodiment of the invention is an improvement in a 
method of preparing a rigid, fused-silica matrix, by the steps of (a) 
forming a slurry composed of (i) silica fibers having selected fiber 
thicknesses and a selected fiber:liquid weight ratio, (ii) thickening 
agents effective to give the slurry a selected viscosity, and (iii) boron 
nitride particles, in an amount between about 2 and 12 percent by weight 
of the total fiber weight, where the slurry contains silica fibers, a 
dispersing agent effective to enhance the dispersion of silica fibers in 
the slurry, (b) allowing the slurry to settle in a mold under conditions 
effective to produce a fiber block, and (c) drying the settled block to 
form a substantially dehydrated fiber block, and (d) heating the 
dehydrated block to a temperature of at least about 2200.degree. F. for a 
period sufficient to cause the silica fibers to form a fused-fiber matrix. 
The improvement includes selecting fiber sizes predominantly in the 0.5 to 
2 .mu.m size range for preparing the slurry, and allowing the slurry to 
settle under conditions effective to produce a density gradient in the 
fiber block in which the lower portion of the block has a density of at 
least about 0.5 lb/ft.sup.3 greater than that of the upper portion of the 
block, and the average density of the block is between about 2 and 6 
lb/ft.sup.3. 
B. Matrix with a Fiber-Size Gradient 
As discussed in Section III, with reference to FIGS. 9 and 10, the 
flow-resistivity gradient may be produced by a fiber-size density gradient 
between front and back panel side, with smaller fiber sizes on progressing 
from the sound-absorbing to the back side of the matrix panel. 
A panel of this type can be also be produced, in accordance with the 
invention, by a modification of the panel-forming method described with 
reference to FIGS. 12A-12D. The modification is designed to produce a 
green-state block with fiber diameters preferably in the size range 
between 0.5 and 2 .mu.m in the lower block sublayer, and fiber diameters 
preferably above about 2 .mu.m, typically 3-8 .mu.m, in an upper block 
sublayer, with a smooth or continuous fiber-size gradient between the two 
sublayers. 
In preparing a panel of this type, a slurry with the smaller-size fibers is 
introduced into a mold, and partially compacted under vacuum, as above. At 
this stage, a second slurry containing the larger-diameter fibers is 
added, preferably with some stirring of the interface to produce localized 
mixing of the smaller and larger fibers. The two slurries are then 
compacted, and dewatered, as above, to form the desired green-state block 
for sintering. 
More generally, this embodiment of the invention is an improvement in a 
method of preparing a rigid, fused-silica matrix, by the steps of (a) 
forming a slurry composed of (i) silica fibers having selected fiber 
thicknesses and a selected fiber:liquid weight ratio, (ii) thickening 
agents effective to give the slurry a selected viscosity, and (iii) boron 
nitride particles, in an amount between about 2-12 percent by weight of 
the total fiber weight, where the slurry contains silica fibers, a 
dispersing agent effective to enhance the dispersion of silica fibers in 
the slurry, (b) allowing the slurry to settle in a mold under conditions 
effective to produce a fiber block, and (c) drying the settled block to 
form a substantially dehydrated fiber block, and (d) heating the 
dehydrated block to a temperature of at least about 2200.degree. F. for a 
period sufficient to cause the silica fibers to form a fused-fiber matrix. 
The improvement includes selecting fiber sizes predominantly in the 0.5 to 
2 .mu.m size range for preparing the slurry, allowing the slurry to settle 
partially, then adding a second slurry composed of larger-diameter fibers, 
and compacting and dewatering the slurry mixture to form a fiber block for 
sintering. 
The following examples are intended to illustrate methods for forming and 
testing an acoustical panel formed in accordance with the invention, but 
are in no way intended to limit the scope of the invention. 
EXAMPLE 1 
Forming a Fiber Slurry 
A. Fiber Pretreatment 
The silica fibers were heat treated as described above. The bulk fiber is 
compressed into panels, e.g., using a Torit Exhaust System and compaction 
unit. The compressed panels are passed through a furnace, e.g., a Harper 
Fuzzbelt furnace or equivalent at 2150.degree. F. for a minimum of 60 
minutes, corresponding to a speed setting of about 5.4 inches/minute. The 
heat treatment is used to close up surface imperfections on the fiber 
surface, making the matrix more stable to thermal changes during fusion. 
B. Preparation of Stock Acrylate Solution 
The acrylate stock was prepared for dispersing the boron nitride powder 
into the fiber slurry. 18 parts by weight of acrylic acid polymer (Acrysol 
ASE-108 from Rohm Haas) was dissolved in 80.2 parts by weight deionized 
water (1 megohm) using a spatula. Ammonium hydroxide (reagent grade 28-30% 
W) at 1.80 parts by weight was added to the mixture during the stirring to 
help dissolve the acrysol. Mixing was continued until almost all the 
milkiness color was gone. Preparation of the stock solution is performed 
at room temperature of 68.degree..+-.2.degree. F. 
Upon completion of mixing, the solution's viscosity was measured after a 24 
hour waiting period. Using a Brookfield Synchro-Lectric Viscometer (Model 
LVT) with a number 3 spindle installed in the instrument, an appropriate 
sample size was adjusted for a temperature of 75.degree..+-.5.degree. F. 
The viscosity expressed in centipoise was measured a four spindle speeds 
(0.3, 0.6, 3 and 30 rpm) in ascending order. The solution must have the 
minimum viscosity reading defined in the table below. 
______________________________________ 
Minimum 
Spindle Speed Viscosity 
(rpm) (Centipoise) 
______________________________________ 
0.3 32,000 
0.6 24,000 
3 12,000 
30 4,000 
______________________________________ 
C. Preparation of Gel Stock 
A gel stock was prepared for use as a thickening agent in the fiber slurry. 
A 2 parts by weight methyl cellulose (Methocel A4M commercial grade powder 
from Dow Chemical Co.) was dissolved in 98 parts by weight, of hot 
deionized water (1 megohm) and vigorously stirred to produce a homogeneous 
solution. The methyl cellulose solution was slowly gelled by placing the 
mixture container in an ice bath with a maximum temperature of 45.degree. 
F., for a minimum time of 40 minutes. Upon completion of gelling, the 
solution's viscosity was measured using a Brookfield Synchro-Lectric 
Viscometer (Model LVT) with a number 1 spindle installed in the 
instrument. 
Prior to testing the appropriate sample size was adjusted for temperature 
to 68.degree..+-.2.degree. F. while stirring slowly to avoid air 
entrapment. Viscosity measurements were recorded at one spindle speed (0.6 
rpm) and expressed in centipoise. The solution should have a minimum 
viscosity of about 4000 centipoise. 
D. Preparing a Fiber Suspension 
A suspension of boron nitride and acrylate stock solution from Part B above 
was prepared by thoroughly mixing the constituents together. The weight 
percentages of the boron nitride was measured from between 2 and 12 
percent of the total fiber weight. 
The acrylate stock solution from Part B was added from between 5-30 percent 
of the total fiber weight. The stock solution is used to attach the boron 
nitride powder to the fibers, increase the slurry viscosity, and provide 
the dehydrated green-state block with low-temperature strength for 
handling. 
E. Mixing the Fibers 
The silica/alumina fiber compositions were placed into a partially filled 
mixing container filled with deionized water and a wetting agent, such as 
Darvon 821A, was added at a concentration of 0.2 to 5 percent by liquid 
weight to enhance fiber dispersion. The remaining DI water was added until 
the desired fiber:water ratio was achieved. The slurry was mixed using a 
variable low-shear double impeller blade to disperse, but not chop, the 
fibers and allowed to age for an appropriate time (typically 1-24 hours; 
aging greater than 24 hours may be required for fiber diameter sizes from 
0.5-1 microns). The boron nitride and acrylate stock solution suspension 
was added and blended into the slurry. A gel stock solution prepared in 
Section C was added in a concentration of 2-30 percent by weight of the 
total fibers. A reagent-grade ammonium-hydroxide (25%) at a volume of 0.1 
to 1 ml per pound of fibers was added to stabilize the slurry viscosity, 
and the slurry was transferred to the vacuum forming mold. 
EXAMPLE 2 
Preparation of Fused-Fiber Matrix 
A. Forming the Fiber Slurry 
The vacuum forming system used to form the matrix is equipped with a 
variable vacuum drain control to 28 inches of mercury. 
The fiber slurry was transferred into the forming tank equipped with a 
paddle mixer. The mixer is used to stir the slurry to keep the fibers from 
settling between block forming. The vacuum forming mold is placed into the 
slurry with the screen side up. Once the mold is immersed in the slurry 
vacuum is applied to the mold so that he fibers are drawn into the mold 
and compacted. When the desired fiber height is achieved the forming mold 
is raised out of the forming tank. The vacuum forming time ranges from 5 
sec to 300 sec, and is timed when the vacuum is first applied to when the 
standing water is drained from the top of the block. 
The vacuum is continued (dewatering step) until about 50 percent of the 
remaining water is removed from the block, or little water can be pulled 
from the block. The dewatering period typically ranges from 5-30 minutes. 
B. Drying the As-Cast Matrix 
The as-cast matrix was placed on an Armalon lined handling fixture mounted 
on a baker's cart, and dried in an electrically heated drying oven set 
between 150.degree. F. to 500.degree. F. for a minimum of 16 hours. The 
target density of the matrix after drying is between 1.8 to 5.5 pcf. 
C. Fusion of the Matrix 
The dried matrix was sintered above 2200.degree. F. using a bottom loading 
Harper Elevator Kiln or equivalent; equipped with a programmable 
controller, to achieve fired densities between 2.0 to 5.0 pcf. Kiln cars 
were pre-warmed to increase temperature uniformity in the kiln and around 
the materials being fired. The firing schedule includes the following ramp 
rates, temperature settings, and estimated soak times. 
______________________________________ 
Ramp Temp Soak Time 
______________________________________ 
start 1800.degree. F. 
12 minutes 
2.degree. F./min 
1900.degree. F. 
6 minutes 
1.degree. F./min 
2100.degree. F. 
6 minutes 
2.degree. F./min 
2200.degree. F. 
as required to 
achieve target den- 
sity 
______________________________________ 
The kiln was then cooled to 1800.degree. F. prior to kiln car removal. The 
panel is cooled to below 200.degree. F. and the fused matrix is removed 
from the car. 
EXAMPLE 3 
Panel Containing 78% Silica Fiber and 22% Alumina Fiber 
91.1 pounds of high purity (99.68+%) heat treated silica fibers (Schuller, 
code 108 "Q" fibers, 1.2 .mu.m to 1.8 .mu.m in diameter) and 25.7 pounds 
of alumina fibers (2.5 to 3.5 microns in diameter, ICI America) were 
dispersed in 686 gallons of deionized water (approximately 5722 pounds) 
and mixed for 240 minutes using a low-shear double propeller mixer at 500 
rpm. 
A dispersion mixture of 3.3 pounds boron nitride powder (325 mesh, Type 
SHP, Carborundum) and 11.7 pounds of a stock acrylate solution was added 
to the mixing tank. 30 pounds of a 2 percent methocel solution (Rohm and 
Haas) was added and mixed into the slurry for 10 minutes. Next, the fiber 
slurry mixture was dumped into the forming tank and 17.5 milliliters of 
reagent grade ammonium hydroxide was mixed into the forming tank for 25 
minutes. The vacuum forming mold was submerged into the forming tank 
screen side up, allowing the slurry to fill the mold. The slurry was 
compressed using 17 inches of mercury for 5 seconds to fabricate a 2 inch 
thick panel. The mold was raised out of the tank with the vacuum pressure 
on to remove excess water from the panel. When very little water could be 
withdrawn from the panel (.about.50 percent of water removed), the vacuum 
was turned off. 
The panel, 27".times.27".times..about.2" thick in size, was removed from 
the mold and dried for a minimum of 48 hours at 350.degree. F. The dry 
density of the panel was 3.91 pcf (0.06 g/cc). The block was fired at a 
ramp rate of 2.degree. F./minute to 2350.degree. F. for 30 minutes. The 
fired density of the block was 4.50 pcf (0.08 g/cc). 
The median pore size for the front surface of the panel was 65.8 microns as 
measured by the mercury porosimetry method having an air flow resistivity 
of 179,713 mks rayls per meter per ASTM C522-87. The back surface median 
pore size was measured at 51.2 microns and the air flow resistivity 
measured at 334,332 mks rayls per meter. 
EXAMPLE 4 
Waterproofing a Panel 
Finished panels are waterproofed using a chemical vapor infiltration (CVI) 
process to apply the methyltrimethoxysilane solution. The 
methytrimethoxysilane vapors deposit a thin film coating over each fiber 
that changes its surface tension; making the fibers hydrophobic. The 
resulting process causes the water droplets to bead on the panel surface 
rather than be absorbed into the high porosity open cell structure. 
The panels are placed inside a temperature controlled vacuum oven having 
.+-.15.degree. F. control capability. The oven is closed and evacuated to 
remove its air content. The chamber is heated to 350.degree..+-.10.degree. 
F. Once evacuated to greater than 29 inches of Mercury, the oven is purged 
with nitrogen gas. The oven is re-evacuated to more than 29 inches of 
Mercury. The exterior reservoirs are evacuated of air and purged with 
nitrogen gas. Dilute acetic acid solution (45 ml) is added to one 
reservoir and 225.+-.5 ml of silane in the other. The dilute acetic acid 
solution is prepared by carefully mixing 50 parts by volume of glacial 
acetic acid to 100 part by volume deionized water in a clean plastic or 
glass container. The solution is slowly mixed and stirred. 
After adding the acetic acid solution and silane to their respective 
reservoirs; the caps are closed and the acetic acid solution is heated to 
350.degree..+-.10.degree. F. and the silane heated to 
375.degree..+-.10.degree. F., respectively. Once the acetic acid is 
vaporized and the pressure inside the reservoir reads 20 psi, minimum, the 
vapors are released into the vacuum chamber (previously evacuated to 
greater than 29 inches of mercury and held steady at 
350.degree..+-.10.degree. F.). The valve is kept open until the vacuum 
pressure in the oven has stabilized for 15 seconds. After a 5 minute timer 
is set and goes off; and the silane reservoir pressure reads greater than 
20 psi, minimum; the silane vapors are released into the vacuum chamber. A 
timer is set for 60 minutes. When the pressure stabilizes in the vacuum 
chamber, the silane injector valve is closed. After the 60 minute timer 
goes off, nitrogen is purged through the system then evacuated. The 
nitrogen purge and evacuation is repeated 4 more times to ensure all 
silane vapors (extremely hazardous) have been removed. After the last 
evacuation, air is slowly bled into the chamber until the vacuum gauge 
reads zero. The valves are closed and the chamber door opened and the 
panels removed. The waterproofing process can be repeated a maximum of one 
more time as necessary to ensure water resistance of the ceramic panels. 
While the invention has been described with reference to specific methods 
and embodiments, it will be appreciated that various modifications and 
changes may be made without departing from the invention.