Effectofparticle...
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stress is also very weak in the interspaces with leSS舢fimng, this cannot be tough enough to result in AI departing from the diamond particles.
Fig.3.Charactedstics of fracture surfaces for composite诵协100lma particles:(a)uneven distribution of AI;Co)interracial debonding.
diamond
particles in the cooling process【2】.T1lus,the gaps filled with large amounts of AI have a greater possibility to concentrate large residue tensile stress,which can make
Chu五et aL.Effect of particle size
on
the mlcrostructure and thermal conductivity of AI/diamond composites prepared…
647
A谢amond
composites.The mixed powders were packed
surface.More
seriously,
some diamond particles could even
scale of!f from the
matrix.
The significant difference in microstructures
between
salll-
pies l群.2#.and
spirit of the Hasselman-Johnson(H-J)model【12—141,according to the following equation:Fig.4-Thermal conductivity orC)and relative density OZD) of all specimens:1执2}}and3#represents samples们nI pal'tide sizes of40.70and100limb懈pectlvely;the dash line is the TC of AI matIix used in our experiments.
matrix.
Few bare diamond particles
Can be
seen
under this condition.
In contrast.in tIle case of
sample 3长cracks emerge
at
the
Al—diamond interface and most of diamond
can
be found that in botll,diamond par-
ticles ale closely embedded intO the AI matrix.Ductile dim-
pies
and the tear
ridges of theles
ale
bare
on
me fracture surfaces(Fig.2(c)),demonstrating weak
interfacial bonding where the Al matrix is not uniformly
close adhesion tO the entire diamond
The TC of sample3#is even lower than that of pure A1.as a
result of its high porosity and weak interfacial bonding.
Pores Call severely degrade the TC of the composite due to
used to investigate the microstructure of composites
by
scanning electron microscopy(SEM).Thermal
diffusivity
and specific heat WCTe measured by the laser flash method and
3并Can
be
explmned as
follows.
Fig.2.Microstruetures of fractures surfaces of the composites with various partide sizes'-(a)40Inn;Oh)70luna;(c)100pm.
。、暑矗口
At the beginning of the SPS,due to the hiigh fraction of diamond particles in the mixture powders,the applied uni—axial pressure can press diamond particles to be closely in contact with each other and locate Al powders in confined
along
the fracture surface.T协S phenomenon presents a
powerful
interfacial bonding between
the particles
and the
AI matrix,
which is
even
stronger thall the toughness of the AI
ttlat the distribution of砧powders into the diamonds’gaps cannot
be uniform.Pores can eventually form in some area with less A1(Fig.3(a)).In addition,due to the considerably large difference in coefficient of thermal expansion(CTE) between diamond(2.3×lif6)and AI(23.6×10-o),a large
gaps among the diamond particles.Diamond
gaps depend
on particle size,and larger particles have larger gaps.Under 廿lis condition.if the gaps ale very large it Call be considered
calorimetric techniques,respectively.Thermal conduc—
tivity was calculated
as the
product of density,thermal dif-
fusivity
and specific
heat.
3.Results and discussion
Kc=
r
2(Kp,足二一巧/ha一1)Vp+砗,K。+2群/ha+2(1)。(1一K口,x0+Kp/ha)re+Kp/K。+2Kp/ha+2 where‰,绵,and疋ale the TC of the matrix,reinforced particles
diamond
particles
debond from the AI matrix in the temperature declining process(Fig.3(b)).
Wmle
relatively small gaps call make AI evenly distribute among the diamonds’gaps,and the generated residue tensile
pie3#is only93%due to its high porosity.Besides this.it is
obviously seen that there is a sharp decrease in TC from325
W.m-I.K-1of sample2群to196W.m-1.K_1of sample3#.
RARE METALS,VoL28,No.反Dee2009
thermal stress can be generated at the interface of the corn-posites during the sintering process.Thermal stress exhibits as tensile stress on the
In addition.the TC of sample l#is lower than that of sample2群within nearly the same RD.Tllis is due to tIle fact Chat the composite with a smaller size of particles has more interraces than that with a larger size of particles.More in—teffaces bring higher ITR to decrease the TC of the compos-ite.and this result is in accordance with the
into
a
graphite die(10
nan3
in
diameter).The powder
COIII-
pacts were sintered at 5500C under a
fixed uniaxial pressure
of 30MPa in vacuum(1ess than 4Pa).田le sintering process is shown in Fig.1.Samples l筏2群,and 3群represent the as—sintered composites with particle sizes of 40.70and 100
岬l’respectively.
Time/s
Fig,1。SI S process of preparing Al/diamond composites.
2.2.Characterizafion
The bulk densi够of the composites was measured by Archimedes’principle.Fracture surface observation was
the fact that they can scatter the heat-carriers,such as elec—
trons from AI and phonons from diamond.Weak interfacial bonding leads to a large interface thermal resistance(ITR), which can greatly deteriorate the thermal conduction per-formance of the composites.
3.1.Microstructure
Fig.2shows the microstmcturas of fracture surfaces of the composites with various particle sizes.In t|le samples 1舟and
2#(Figs.2(a-b)),it
3.2.rI.1Iermal conductivity
Fig.4indicates the TC
and relative density(ItD)of all
specimens.It Call be seen that the RDs of samples1#and2#
are nearly the salne.which exceed97%.but the RD of saln—
Fig.3.Charactedstics of fracture surfaces for composite诵协100lma particles:(a)uneven distribution of AI;Co)interracial debonding.
diamond
particles in the cooling process【2】.T1lus,the gaps filled with large amounts of AI have a greater possibility to concentrate large residue tensile stress,which can make
Chu五et aL.Effect of particle size
on
the mlcrostructure and thermal conductivity of AI/diamond composites prepared…
647
A谢amond
composites.The mixed powders were packed
surface.More
seriously,
some diamond particles could even
scale of!f from the
matrix.
The significant difference in microstructures
between
salll-
pies l群.2#.and
spirit of the Hasselman-Johnson(H-J)model【12—141,according to the following equation:Fig.4-Thermal conductivity orC)and relative density OZD) of all specimens:1执2}}and3#represents samples们nI pal'tide sizes of40.70and100limb懈pectlvely;the dash line is the TC of AI matIix used in our experiments.
matrix.
Few bare diamond particles
Can be
seen
under this condition.
In contrast.in tIle case of
sample 3长cracks emerge
at
the
Al—diamond interface and most of diamond
can
be found that in botll,diamond par-
ticles ale closely embedded intO the AI matrix.Ductile dim-
pies
and the tear
ridges of theles
ale
bare
on
me fracture surfaces(Fig.2(c)),demonstrating weak
interfacial bonding where the Al matrix is not uniformly
close adhesion tO the entire diamond
The TC of sample3#is even lower than that of pure A1.as a
result of its high porosity and weak interfacial bonding.
Pores Call severely degrade the TC of the composite due to
used to investigate the microstructure of composites
by
scanning electron microscopy(SEM).Thermal
diffusivity
and specific heat WCTe measured by the laser flash method and
3并Can
be
explmned as
follows.
Fig.2.Microstruetures of fractures surfaces of the composites with various partide sizes'-(a)40Inn;Oh)70luna;(c)100pm.
。、暑矗口
At the beginning of the SPS,due to the hiigh fraction of diamond particles in the mixture powders,the applied uni—axial pressure can press diamond particles to be closely in contact with each other and locate Al powders in confined
along
the fracture surface.T协S phenomenon presents a
powerful
interfacial bonding between
the particles
and the
AI matrix,
which is
even
stronger thall the toughness of the AI
ttlat the distribution of砧powders into the diamonds’gaps cannot
be uniform.Pores can eventually form in some area with less A1(Fig.3(a)).In addition,due to the considerably large difference in coefficient of thermal expansion(CTE) between diamond(2.3×lif6)and AI(23.6×10-o),a large
gaps among the diamond particles.Diamond
gaps depend
on particle size,and larger particles have larger gaps.Under 廿lis condition.if the gaps ale very large it Call be considered
calorimetric techniques,respectively.Thermal conduc—
tivity was calculated
as the
product of density,thermal dif-
fusivity
and specific
heat.
3.Results and discussion
Kc=
r
2(Kp,足二一巧/ha一1)Vp+砗,K。+2群/ha+2(1)。(1一K口,x0+Kp/ha)re+Kp/K。+2Kp/ha+2 where‰,绵,and疋ale the TC of the matrix,reinforced particles
diamond
particles
debond from the AI matrix in the temperature declining process(Fig.3(b)).
Wmle
relatively small gaps call make AI evenly distribute among the diamonds’gaps,and the generated residue tensile
pie3#is only93%due to its high porosity.Besides this.it is
obviously seen that there is a sharp decrease in TC from325
W.m-I.K-1of sample2群to196W.m-1.K_1of sample3#.
RARE METALS,VoL28,No.反Dee2009
thermal stress can be generated at the interface of the corn-posites during the sintering process.Thermal stress exhibits as tensile stress on the
In addition.the TC of sample l#is lower than that of sample2群within nearly the same RD.Tllis is due to tIle fact Chat the composite with a smaller size of particles has more interraces than that with a larger size of particles.More in—teffaces bring higher ITR to decrease the TC of the compos-ite.and this result is in accordance with the
into
a
graphite die(10
nan3
in
diameter).The powder
COIII-
pacts were sintered at 5500C under a
fixed uniaxial pressure
of 30MPa in vacuum(1ess than 4Pa).田le sintering process is shown in Fig.1.Samples l筏2群,and 3群represent the as—sintered composites with particle sizes of 40.70and 100
岬l’respectively.
Time/s
Fig,1。SI S process of preparing Al/diamond composites.
2.2.Characterizafion
The bulk densi够of the composites was measured by Archimedes’principle.Fracture surface observation was
the fact that they can scatter the heat-carriers,such as elec—
trons from AI and phonons from diamond.Weak interfacial bonding leads to a large interface thermal resistance(ITR), which can greatly deteriorate the thermal conduction per-formance of the composites.
3.1.Microstructure
Fig.2shows the microstmcturas of fracture surfaces of the composites with various particle sizes.In t|le samples 1舟and
2#(Figs.2(a-b)),it
3.2.rI.1Iermal conductivity
Fig.4indicates the TC
and relative density(ItD)of all
specimens.It Call be seen that the RDs of samples1#and2#
are nearly the salne.which exceed97%.but the RD of saln—