Sol-Gel Silica. Properties, Processing and Technology Transfer

170 Pages·1998·3.417 MB·English

by  Larry L. Hench (Auth.)

Preface
This book summarizes fifteen years of research in sol-gel processing of
silica leading to the commercial development of gel-silica optics and bioactive
gel-glasses for medical applications. Each of the seven stages of sol-gel
processing is described with emphasis on control of the ultrastructural
variables necessary to make reproducible materials. A unique feature of the
book is its emphasis on the practical outcome of the processing studies, i.e.,
commercially viable production of net shape, net surface, and monolithic
optical components. It is shown that reliable drying of gel-silica optics
requires control of gel texture, especially the distribution of pore sizes, which
is a function of the kinetics of hydrolysis and condensation reactions
occurring durinmgi xing and gelation, and also the kinetics and environment
of the castc omponent.
The environmental stability of porous gel monoliths is of special
concern in the book because it can limit the utility ofporous optical matrices,
Type VI gel-silica, and also severely restrict the ability to transform porous
gel into a fully dense silica optical component, Type V gel silica. The book
shows how theory and experiment are combined to solve the problem of
environmental stabilization; semi-empirical quantum mechanical calcula-
tions provide insight as to the mechanism of water attack of surface defects
within the interconnected pore network. Experiments are described which
determined the concentration of surface defects, the dimensions of the
hydrated layers within the network and the specific thermochemical treat-
ments required to stabilize the surface of the pores without collapsing them.
Vll ,i
viii Preface
The ability to make stable porous matrices with interconnected porosity
of controllable size makes it possible toi mpregnate the pores with optically
active second phases. The processing and properties these new generations
of optical composites are described in the book with numerous examples from
published papers and Ph.D. theses. The production of fully dense optical
silica components is also presented along with many advantages including:
improved physical properties and net shape, net surface processing of
microlenses, surface diffractive optics, aspheres, micro-optical arrays, and
laser written waveguides.
Addition of CaO and P205 to the silica precursor during mixing leads
to a new material, a biologically active gel-glass which enhances the repair
and regeneration ofbone. A chapter reviews the concept ofbioactive implant
materials and the important advances associated with sol-gel processing of
bioactive glasses.
A final chapter addresses one ofthe most difficult topics of all: how
to make the transfer of technology from the laboratory to a profitable
commercial enterprise. Principles of technology transfer are presented and
the separate pathways involved in moving from a research program to pilot
plant are discussed using examples from various chapters in the book. Thus,
this book presents an unique perspective of twentieth century research and
development in materials science. It starts with the concept of using
ultrastructure processing to produce a new generation of optical and medical
materials and shows step by step the sequence of experiments conducted to
reduce the concept to a commercial reality.
London, England Larry L. Hench
February, 1998
1
The Concept
This review is an odyssey that begins with a concept and ends fifteen
years later with a commercial technology for chemical processing ofnet shape
optics. The concept was bom in Montpellier, France,i n 1979 while visiting
Professor Jerzy Zarzycki's Laboratory of the Vitreous State. He and Jean
Phalliapou showed me a large (20 era) rod of pure vitreous silica made by
sintering a hydrothermallyprocessed aerogel. The glass was beautiful. Itwas
shocking to learn that it had been cast in its final shape and densified at
temperatures much lower than the liquidus of pure SiO .2
I discussed the Montpellier results with Dr. DonalUdl rich of the U.S.
Air Force Office of Scientific Research (AFOSR) and proposed that with
suffiucnideenrts tanding of soclh-egmeils ittr y, might be possible to mnaekte
shape ceramics and glasses without hydrothermal processing, a severe
restriction for eventual commercialization. Don approved a shift of empha-
sis, in my laboratory at the University of Florida, from glass surface
chemistry to what we called UltrastrPurcotcuerses ing.
The term and concept of ultrastructure processing was derived by
analforgoym the biological sciences. Intracellular organelles, revealed atthe
nanometer level by transmission electron microscopy, are called cellular
ultrastructures. llI Our ongoing studies of the interracial reactions between
bioactive glasses that formed a chemical bond wliitvhi ng tissues, discovered
in 1969 with funding from the U.S. Army Medical R&D command, were
being studied at a "cellular ultrastructure" level. ]7[-]2[ The mechanism of
bonding of glass to living bone involved a complex sequence o fion exchange
and inorganic polymerization reactions on the glass surface, all occurring at
2 Sol-Gel Silica
37~ body temperature (Fig. 1-1). The inorganic kinetics (Fig. 1-1, stages
1-5) matched the rate of cellular repaiprr ocesses (stages 6-10) which led to
incorporation of collagen fibers within the growing carbonate hydroxyapatite
mineral layers on the glass (stage 11), thereby providing a mechanically
strong interface.
Mechanical stress would break either bone or implant, but not the
interface. This behavior was unlike previous implant materials which all
exhibited interfacial failure due to formation ofa nonadherent fibrocuasp sule
at the implant-tissue interface. Newly developed surface spectroscopy
methods sensitive to the nanometer level were used to analyze the thickness
and composition of the reaction layers. ]7[-]5[ Transmission electron micros-
copy was needed to observe the cell-material interface. ]5[]4[ A critical
discovery was the measurement of rapid formation of a thick, 200 ,mt~
hydrated silica gel layer on the bioactive glasses (Fig. 1-2). The silica gel
formed by polycondensation of surface silanols (Fig. 1-1, stage 3), created
during ion exchange and network dissolution of the glass (stages 1 and 2).
Infrared reflection spectroscopy (IRRS) ofthe glass surfaces ]6[]2[ showed the
shift in Si-Ov-iSbir ational modes as the stage 1-3 reactions formed the silica
gel (Fig. 1-3). The IRRS spectra also showed evidence that the porous silica
gel provides sites for chemisorption ofa CaO, P205 rich layer (stage 4) and
subsequent rapid heterogeneous nucleation of hydroxy-carbonate apatite
(HCA) crystallites, i.e., bone mineral. Thus, during the 1970s and early
1980s the boundaries between molecubliaorc hemistry and molecular surface
chemistry were beginning to overlap. Consequently, it seemed logical that the
concept of molecular manipulation and heterogeneous surface chemistry be
extended to the processing of ceramics, glasses,a nd composites; however, to
do so required an application of chemical principles unprecedented in the
history of ceramics. In contrast, most technical and electronic ceramics are
primarily the products of applied physics with emphasis on the relation
between physicalb ehavior and microstructure.
In order to expand the role of chemistry in materials processing, the
first of a series of Ultrastructure Processing Conferences was organized
February 13-17, 1983 by Don Ulrich and myself. The objective of the
Conferences was ot cross-link the disciplines of inorganic and organic
chemistry, surface chemistry, polymers, ceramics and glass. ]8[ During
the first conference we proposed that "major advances in ceramics during
the next four decades will require an emphasis that relates chemical
processes to physical behavior via a route of ultrastructure control.
Organic chemistry, once anathema toc eramists, has become the potential
source of new ceramics. Figure 1-4 illustrates the major shift in thinking
The tpecnoC 3
Stage Reaction
Rapid exchange of Na+ or K+ with H+ or H30+ from solution
Si-O-Na § + § H -HO ~ Si-OH + )noitulos(*aN + OH
This stage is usually controlled by diffusion and exhibits a t ~'-
dependence ]36[-]06[.
Loss of soluble silica in form of Si(OH)4 to the solution resulting
from breaking of Si-O-Si bonds and formation of Si-OH (silanols)
at the glass solution interface.
Si-O-Si + H20 ~.- Si-OH + OH-Si
This stage is usually controlled by interfacial reaction and
exhibits a t ~.1 dependence. ~6[
Condensation and repolymerization of a SiO2-rich layer on the
surface depleted ni alkalis and alkaline earth cations.
O O O O
I I !
-O-Si-OH + HO-Si-O ~ Si-O-Si-O + H20
I I I I
O O O O
Migration of Ca §2 and -34OP groups to the surface through the
hcir_2OiS lafyoerrm ing a CaO-P205-rich film on top of the SiO -2
rich layer, followed by growth of the amorphous CaO-P2Os-rich
film by incorporation of soluble calcium and phosphates from
]171186[]76[.noitulos
Crystallization of the amorphous CaO-P205 film by incorpora-
tion of OH-, CO32, or F anions from solution to form a mixed
hydroxyl, carbonate, fluorapatite layer. ]17[
Adsorption of biological moieties, such as ,31-FGT BMP, ni the
porous silica geI-HCA layer.
Action of macrophages which deposit biological growth factors
in the inorganic gel layer.
Attachment of stem cells to the inorganic gel layer.
Differentiation of stem cells to form osteoblasts.
01 Generation of acellular organic matrix by the osteoblasts.
11 Mineralization of the organic matrix by nucleation and growth of
carbonate-hydroxyapatite crystals.
Figure 1-1. Reaction stages of a bioactive glass implant.
4 Silica Sol-Gel
required to realize this new perspective for ceramics and ceramic process-
ing. ''[8] The research projects pursued by our Florida team thereafter,
attempted to follow the concepts discussed in the 1983 Ultrastructure
Processing Conference, i.e., eht emphasis ni ultrastructureprocessing si no
limiting and controlling physical variability chemical of materials by the
production of structures, homogeneous uniquely extremely fine scale 01(
)mn compositional secondphases, controlledsurface ,stneidarg and unique
combinations of physical properties. ]8[
HCA Layer SiO2-rich Layer
100
--F
Z
uJ
O 80
Iz: SiO 2
U L.Q C
JJI
..J
O 60
Z=
JJI
:>
CaO
...i
uJ
tr
20
P205
2400 4800 7200 9600 12000
Depth (A)
Figure 1-2. Bilayer films formed no 45S5 ssalgoiB | h after 1 ni tar bone, ni vivo 1( = A
10 1" nm).
The Concept 5
SURFACE REACTION STAGES
SI,.j~ . 2 ,,..I-.- 3 ~l~5.jl 6
,'Vl-" ~ [-" i-"I, TM ~I -" f
960 ,_. u 690
-SI-O HCTERTS (2NBO) -O HCTERTS -
920 _ 650
- -
,~ 880
P-O ON=EB
I.-,
|
E <
m
Z
13:840 570 C
UL - I --~ . -
m J I - _ P-O = DNEB - 0~
-- m
).TSYRC(
Z 800 _ 530
UL r
> iS-)C-IS _HCTERTS
,<
,J
i SI-O-SI e END~ )ARTET(
760 490
720 --! k~ -t 450
680 410
0.0 1.0 2.0 3.0 4.0 5.0 6.0
log TIME (sec)
3
Figure 1-3. Time-dependent changes in IR vibrations of the surface of 45S5 Bioglass |
implant in a 37~ SBF. ysetruoc margaiD( of .G ).erroTaL
6 Sol-Gel Silica
A = ORGANIC CHEMISTRY
B = INORGANIC CHEMISTRY
C = SURFACE CHEMISTRY
D = SURFACE PHYSICS
ROMECHANICS
. s,cs I / /
'
~ ~,~. 40's - 70'st "~
Figure 1-4. Change ni eht roles of scisyhp dna yrtsimehc sa scimarec move sdrawot
erutcurtsartlu .gnissecorp
The MRS (Materials Research Society) Symposia on "Better Ceram-
ics Through Chemistry," organized by JeffBrinker, Sandia National Labs,
David Clark, University of Florida, and Don Ulrich, AFOSR, began in
1984 ]9[ and were held on alternate years to the series of Ultrastructure
Conferences ~1[ and still contribute to the growth of the field. Many
fruitful interdisciplinary and interinstitutional collaborations were fostered at
these Conferences and Symposia.
Our earliest research (1981-1984) was a collaboration between my
group at the University of Florida and Dr. Mike Prassas, Dr. Jean Phalliapou,
and Professor Zarzycki at the University of Montpellier, France. A series of
papers on the low temperature alkoxide-based processing of Na20-SiO 2
glasses culminated in a composition-temperature gel-glass processing dia-
gram for this silicate system (Fig. 1-5). ]32[-]51[ The gel-glass processing
diagram shows that full density alkali-silicate glasses can be prepared at
ambient pressures from sol-gel compositions with 3% Na20 to 40% Na20.
Densification temperatures range from 600~ fort he high silica glasses to
480~ for glasses with 85-60% silica. The transformation of a porous gel
The Concept 7
to a full density glass at temperatures as low as 480~ in an ambient
atmosphere, i.e., without hydrothermal conditions, provided impetus to our
University of Florida effort to make net shape optics. We discovered that
there was little problem in converting monolithic dried gel samples to a dense
silicate glass; however, the problem was maintaining monolithic objects
during drying. Thus, by 1984 the ultrastructure processing program at the
University of Florida became focused on control of sol-gel chemistry to
produce net shape monolithic components at ambient pressure. We decided
to emphasize sol-gel processing of pure silica glasses because of their
environmental stability and potential ufsoer in commerciaplr ecision optics,
as discussed in Ch. .8
m
A 0 1000 Pure Silica
o
ILl iii
:D I "
--,I 4000 ppm Na 20
< i (cid:12)9
I,U
.D
..t.... ~ Crystal
U.I
I-- ,
,eO i
500
0 10 20 30 40
Mol % Na 2 0
Figure 1-5. leG processing diagram for eht 2 Na20-SiO .metsys
2
Sol-Gel Kinetics
A study of various chemicals such as formamide or oxalica cid (called
DCCAs, Drying lortnoC )sevitiddAlacimehC added to the silicon alkoxide
derived sol led to the discovery that large (> 01 cm) gels could be cast and dried
routinely without [-]42[.gnikcarc ~3 Nearly one hundred percent yields of dried
gel silica monoliths were achieved. Figure 2-1 shows some of the dripeudr e
silica gel monoliths produced during 1984-86.
Figure 2-1. Examples of near net shape silica gels.
A key collaboration with Professor Jiri Jonas' lab at the Chemistry
Department of the University of Illinois occurred in 1984-1987. These
studies led to an understanding of the chemical variables that control the
evolution of a gel network from silicon alkoxide precursors. Gerard Orcel
(University of Florida), working with L. Artaki, W. Zerda and Professor