[acs symposium series] inorganic and organometallic polymers volume 360 (macromolecules containing...

Download [ACS Symposium Series] Inorganic and Organometallic Polymers Volume 360 (Macromolecules Containing Silicon, Phosphorus, and Other Inorganic Elements) || Synthetic Routes to Oligosilazanes and Polysilazanes

Post on 27-Mar-2017

213 views

Category:

Documents

1 download

Embed Size (px)

TRANSCRIPT

  • Chapter 10

    Synthetic Routes to Oligosilazanes and Polysilazanes Polysilazane Precursors to Silicon Nitride

    Richard M. Laine 1, Yigal D. Blum, Doris Tse, and Robert Glaser 2

    Inorganic and Organometallic Chemistry Programs, SRI International, Menlo Park, CA 94025

    Organometallic polymer precursors offer the potential to manufacture shaped forms of advanced ceramic materials using low temperature processing. Polysilazanes, compounds containing Si-N bonds in the polymer backbone, can be used as precursors to silicon nitride containing ceramic materials. This chapter provides an overview of the general synthetic approaches to polysilazanes with particular emphasis on the synthesis of preceramic polysilazanes. The latter part of the chapter focusses on catalyzed dehy-drocoupling reactions as a route to polysilazanes.

    Non-oxide ceramics such as s i l i c o n carbide ( S i C ) , s i l i c o n n i t r i d e (S13N4), and boron n i t r i d e (BN) o f f e r a wide v a r i e t y of unique p h y s i c a l p r o p e r t i e s such as high hardness and high s t r u c t u r a l s t a b i l i t y under environmental extremes, as well as vari e d e l e c t r o n i c and o p t i c a l p r o p e r t i e s . These advantageous p r o p e r t i e s provide the d r i v i n g force f o r intense research e f f o r t s d i r e c t e d toward developing new p r a c t i c a l a p p l i c a t i o n s for these m a t e r i a l s . These e f f o r t s occur despite the considerable expense often associated with t h e i r i n i t i a l preparation and subsequent transformation into f i n i s h e d products.

    The expense in preparation derives from the need f o r high p u r i t y products. The cost in f a b r i c a t i n g three-dimensional shapes of SiC, ^ or BN a r i s e s from the need to s i n t e r powders of these materials at high temperatures (>1500C) and often under high pressure. Coatings of these materials are at present produced by chemical or

    Current address: Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195

    2On leave from the Department of Chemistry, Ben Gurion University, Beer Sheva, Israel

    0097-6156/88/0360-0124$06.00/0 1988 American Chemical Society

    Dow

    nloa

    ded

    by U

    CSF

    LIB

    CK

    M R

    SCS

    MG

    MT

    on

    Nov

    embe

    r 20

    , 201

    4 | h

    ttp://

    pubs

    .acs

    .org

    P

    ublic

    atio

    n D

    ate:

    Jan

    uary

    7, 1

    988

    | doi

    : 10.

    1021

    /bk-

    1988

    -036

    0.ch

    010

    In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

  • 10. LAINE ET AL. Synthetic Routes to Polysilazanes 125

    p h y s i c a l vapor deposition (CVD or PVD). In each instance, the process is equipment and energy int e n s i v e and, has numerous l i m i t a -t i o n s . I f inexpensive methods for the synthesis and f a b r i c a t i o n of non-oxide ceramics can be developed, then the number of u s e f u l a p p l i c a t i o n s could be enormous.

    One p o t e n t i a l s o l u t i o n to these problems, suggested some 20 years ago by C h a n t r e l l and Popper (1), involves the use of inorganic or organo-m e t a l l i c polymers as precursors to the desired ceramic m a t e r i a l . The concept (2) centers on the use of a t r a c t a b l e ( s o l u b l e , meltable or malleable) inorganic precursor polymer that can be shaped at low temperature (as one shapes organic polymers) i n t o a coating, a f i b e r or as a matrix (binder) for a ceramic powder. Once the f i n a l shape is obtained, the precursor polymer can be p y r o l y t i c a l l y transformed int o the desired ceramic m a t e r i a l . With c a r e f u l c o n t r o l of the py r o l y s i s c o n d i t i o n s , the f i n a l piece w i l l have the appropriate p h y s i c a l and/or e l e c t r o n i c p r o p e r t i e s .

    The commercialization of Nicalon (Nippon Carbon Co.), a SiC based ceramic f i b e r f a b r i c a t e d by p y r o l y s i s of f i b e r s of polycarbosilane (3), has sparked i n t e n s i v e e f f o r t s to synthesize polymer precursors of use in the f a b r i c a t i o n of Si^N^ and BN f i b e r s , coatings and binders. In t h i s regard, o l i g o - and pol y s i l a z a n e s have r e c e n t l y been shown to o f f e r considerable p o t e n t i a l as precursors to S13N4. The obje c t i v e of t h i s chapter is to provide an overview of the general methods for synthesizing o l i g o - and pol y s i l a z a n e s and to show how some of these routes have been used to synthesize o l i g o - and poly-s i l a z a n e preceramic polymers. In the l a t t e r part of the chapter, we w i l l emphasize our own work on t r a n s i t i o n metal catalyzed dehydro-coupling r e a c t i o n s .

    General Synthetic Approaches

    H i s t o r i c a l l y , o l i g o - and polysilazane s y n t h e t i c chemistry has passed through three stages of development with quite d i f f e r i n g goals. I n i t i a l e f f o r t s , beginning with the work of Stock and Somieski in 1921, were d i r e c t e d simply towards the preparation and c l a s s i f i c a t i o n of the general properties of polysilazanes (4,5). The commercial success of polysiloxanes or s i l i c o n e s in the f i f t i e s and s i x t i e s prompted studies on the synthesis of pol y s i l a z a n e s as p o t e n t i a l analogs (without much success). As mentioned above, current i n t e r e s t derives from t h e i r use as s i l i c o n n i t r i d e preceramic polymers.

    Reactions ( l ) - ( 5 ) i l l u s t r a t e known methods for forming s i l i c o n -n itrogen bonds of p o t e n t i a l use in the formation of o l i g o - and poly-s i l a z a n e s . The most common method of forming s i l a z a n e s is v i a ammo-n o l y s i s or aminolysis as shown in r e a c t i o n (1) (4,5):

    R 3 S i C l + 2R f 2NH > R 3 S i N R f 2 + R , 2NH 2 + C 1"" ( X)

    Si-N bonds can also be formed by a dehydrocoupling r e a c t i o n catalyzed by a l k a l i (6-11) or t r a n s i t i o n metals (12):

    Dow

    nloa

    ded

    by U

    CSF

    LIB

    CK

    M R

    SCS

    MG

    MT

    on

    Nov

    embe

    r 20

    , 201

    4 | h

    ttp://

    pubs

    .acs

    .org

    P

    ublic

    atio

    n D

    ate:

    Jan

    uary

    7, 1

    988

    | doi

    : 10.

    1021

    /bk-

    1988

    -036

    0.ch

    010

    In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

  • 126 INORGANIC AND ORGANOMETALLIC POLYMERS

    R 3SiH + R f 2NH c a t a l y s t > R 3 S i N R 2 + ^ ( 2 )

    In rare instances, one can also obtain displacement of oxygen by nitrogen as i l l u s t r a t e d by react i o n (3) (13). Reaction (3) proceeds despite the f a c t that the Si-0 bond is 25-30 kcal/mole stronger than the Si-N bond (14).

    R 3Si-0H + R'2NH > R 3SiNR f 2 + H 20 (3)

    Verbeek and Winter have used a deamination/condensation r e a c t i o n to form new Si-N bonds as exemplified by re a c t i o n (4) (15,16).

    2R 2Si(NHR f) 2 > (R fNH)R 2SiN(R ,)SiR 2(NHR l) + R'NH2 (4)

    It has been suggested that r e a c t i o n (4) passes through an R 2Si=NR f intermediate, although no su b s t a n t i a t i o n e x i s t s (17,18).

    Van Wazer (19) has described the use of a r e d i s t r i b u t i o n r e a c t i o n , i l l u s t r a t e d in (5), to form l i n e a r oligomers from c y c l i c species:

    2 R 2 S i C l 2 + i-[R 2SiNH] 3-l > ClR 2SiNHSiR 2Cl + C l - [ R 2 S i N H ] 2 - S i R 2 C l (5)

    With the exception of re a c t i o n (3), the u t i l i t y of a l l of the above reactions for the synthesis of o l i g o - and pol y s i l a z a n e s has been examined in varying d e t a i l . In the following paragraphs, we attempt to examine the pertinent studies in each area e s p e c i a l l y as i t re l a t e s to the synthesis of preceramic polymers.

    Polymerization by Ammonolysis and Aminolysis

    The s i m p l i c i t y of the ammonolysis/aminolysis of d i h a l o h a l o s i l a n e s , reactions (6) and (7), made them the o r i g i n a l method of choice f o r the synthesis of o l i g o - and po l y s i l a z a n e s , e s p e c i a l l y because of the analogy to the h y d r o l y t i c synthesis of po l y s i l o x a n e s . The ammono-l y s i s of d i h a l o s i l a n e s , r e a c t i o n (6), has been found to be extremely s e n s i t i v e to s t e r i c f a c t o r s (6).

    R 1 R S i C l 2 4- 3xNH3 > -[R 1RSiNH] x~ + 2xNH 4 +Cl" (6)

    For example, with R 1 - R - H, re a c t i o n (6) gives only o l i g o s i l a z a n e s ; with R 1 = CH 3 and R = H, both c y c l i c and o l i g o s i l a z a n e s are formed; and with R 1 > CH 3 and R = H then one obtains mostly c y c l o t r i - and c y c l o t e t r a s i l a z a n e s as the products.

    Dow

    nloa

    ded

    by U

    CSF

    LIB

    CK

    M R

    SCS

    MG

    MT

    on

    Nov

    embe

    r 20

    , 201

    4 | h

    ttp://

    pubs

    .acs

    .org

    P

    ublic

    atio

    n D

    ate:

    Jan

    uary

    7, 1

    988

    | doi

    : 10.

    1021

    /bk-

    1988

    -036

    0.ch

    010

    In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

  • 10. LAINE ET AL. Synthetic Routes to Polysilazanes 127

    Aminolysis of d i h a l o s i l a n e s , r e a c t i o n

Recommended

View more >