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Development of Copper-Alumina Laminated Compact by Microwave

time:2008-02-27 11:27:14  View:        

Microwaves (MW) have the distinctive feature of generating heat in metal powders and   ceramics.   Therefore,   the   development   of   composite   copper-alumina   laminated compacts  consisting  of  a  metal  powder  and  ceramic  powder  by  MW  sintering  was investigated.   The   general   sintering   temperature   of   alumina   is   in   the   vicinity   of

1500-1600°C. This greatly exceeds the melting point of copper (1080°C), which means that Cu melts in the conventional sintering process. For this reason, application of MW sintering,  which  is  considered  particularly  effective  for  low  temperature  sintering  of alumina, was examined.

In order to sinter alumina at a temperature below the melting point of Cu, Bi2O3  was

added to the alumina. Bi2O3  has higher permittivity (İ) than alumina and melts at around

850°C. When Bi2O3-added alumina was sintered in a MW furnace, it was found that a relative density exceeding 95% can be obtained at 1030°C. In sintering under the same temperature conditions with an ordinary electric heating furnace, alumina with the same chemical composition showed a relative density of 90%. Thus, these results demonstrated MW sintering is effective for achieving higher densification.

On the other hand, laminates of Cu and alumina tend to crack due to the difference in the thermal expansion coefficients of the materials during sintering. To solve this problem, mixed  layers  were  provided  between  the  Cu  and  alumina  in  order  to  alleviate  the difference  in  the  thermal  expansion  coefficients  of  the respective  materials  being used, and  the  sintering  temperature,  heating  rate,  and  atmosphere  in  the  MW  furnace  were optimized.  As  a  result,  it  was

Until   recently,   microwave   (MW)   heating   was considered a type of spontaneous heat generation caused by  absorption  of  electromagnetic  waves  by  a  dielectric, and  thus  was  thought  to  be  unsuitable  for  sintering  or heating  of  metals.  However,  since  the  publication  of  a report on the possibility of sintering metal powders using MW  in  the  United  States  in  1999,1)   much  research  has been carried out on this subject.

The principle of heating in green compacts of metal powders by MW has not yet been clarified, and detailed data remain extremely rare. In recent years, however, it has  been  discovered  that  hot  spots  with  

from  the  submicron  level  to  several  microns  occur  for

some  reason  in  the  interior  of  compacts  under  MW irradiation,  and  these  hot  spots  migrate  randomly.2)   In other  words,  from  the  microscopic  viewpoint,  sintering of  compacts  by  MW  is  considered  to  be  the  result  of bonding    between    particles    due    to    repeated    local occurrence  and  extinction  of  temperature  regions  (“hot spots)  with  a  temperature  exceeding  the  melting  point of    the    low    temperature    bulk,    and    accompanying migration of these hot spots in the material.

Focusing on the distinctive feature that MW acts on metal powders and ceramic material systems, generating

a   single   compaction   and   sintering   process.3)     The

properties required in these laminated compacts include electrical  conductivity  to  transmit  the  generated  power without  loss, an  insulating property  for  formation  of  an electric   circuit,   and   relaxation   of   the   thermal   stress generated in heat conduction elements. In order to satisfy these  property  requirements,  the  laminated  compact  is composed  of  copper,  which  is  an  excellent  conductor, and alumina, which is an insulator. However, differences in   the   physical   properties   of   Cu   and   alumina,   for example, the deformation capacities of the powders and their  behavior  during  compaction,  melting  points,  and expansion-contraction behavior during sintering, make it extremely  difficult  to  obtain  laminated  compacts  by  a single   compaction   and   sintering   process.   Therefore, various production  methods  such  as discharge  activated pressure sintering had been studied from an early date.

the multi-mode    type, which produces diffuse reflection

by  generating  electromagnetic  waves  in  a  box-shaped container  like  that  used  in  electronic  ranges,  and  the other, the single-mode type, in which the electromagnetic waves are adjusted to the amplitude of the electric field. The  MW  furnace  used  here  was  a  single-mode  type, which allows easy study of the effect of output. Figure 1 shows   a   schematic   diagram   of   a   single-mode   MW furnace.  The  generated  electromagnetic  waves  can  be adjusted  so   that  the  electric  field  has  its   maximum amplitude  in  the  vicinity  of  the  object  being  sintered. With the single-mode type, high energy can be applied at low power, but this has the corresponding drawback that local  thermal  runaway  can  easily  occur  in

2.1  Study  of  low  temperature  sintering  method  for

alumina

Alumina is normally sintered at a temperature in the vicinity  of  1500-1600°C.  However,  the  present  study investigated  sintering  at  below  1080°C,  as  this  is  the melting  point  of  Cu.  The  alumina  powder  used  in  this study was TM-DAR (manufactured by Taimei Chemicals Co.,  Ltd.),  which  is  sold  commercially  as  an  alumina powder for low temperature sintering. Table 1 shows the chemical composition and particle size distribution of the alumina powder.

 

Table 1 Properties of alumina powder.

Crystal form          Purity (%)           Mean particle size (µm)

Į                    99.99                             0.18

 

 

From  the  viewpoint  of  sintering  performance,  the fluidity of this alumina powder is extremely low due to its  small  mean  particle  size  of  0.18Ǵm.  Therefore,  the alumina  powder  used  in  this  study  was  granulated  to  a mean particle size of 25Ǵm by a spray dry technique. As

a   binder,   0.3mass%   of   PVA  was   added   considering

compaction, shape retention, and debinding properties.

The  granulated  powders  were  pressed  to

Alumina is characterized by poor absorption of MW at    low   temperatures    (around    room         temperature). Therefore,  heating  in  this  low  temperature  region  was supported  by  an  SiC  heating  support  ring.  The  area around the sample was covered with a heat insulator to reduce heat radiation. Figure 2 shows the sample setting method.

 

Heat insulator

SiC heating support ring

 

 

 

 

 

 

Sample (Ǿ11.3)

 

 

 

 

 

Fig. 2   Sample setting method (single-mode MW sintering furnace).

  

 
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