Semiconductor Firing

[Shinigami]

Hello All,

 

Im an engineer at a semiconductor manufacturing plant that specializes in thermistor ceramics. Ive been experimenting on different formulas and sintering profiles. The parts are experiencing warpage problems. Upon my experiments the freshly pressed parts dipped in water then sintered come out straight. no warpage at all. The parts I dried come out warped. 

The mix consists of manganese, cupric oxide and nickel. Spraydried with binder. Then pressed 16 tons into 32mm discs 3mm thick.

Any ideas on why this is happening?  It seems backwards why the wet part is coming out perfect. In my pottery experience, it should of cracked or exploded.

Thank you!

[Hulk]

Hi Shinigami!

By "...then sintered..." do you mean pressureless sintering - high temperature only? If so, what does your firing profile look like (time and temperature)?

The materials, would you describe them as metallic powders (hence, not ceramic)?

Any road, in pottery, the water is trapped enough such that pressure may be generated as the water changes phase; perhaps your material allows moisture to escape as vapour without significant lag time (hence pressure).

What ingredients in the sprayed binder (are they soluble in water)?

The warped parts, were they subject to the same "...dipped in water..." step, but allowed to dry before the sintering? If so, any relationship between the drying process and subsequent warpage - which side "up", which side dried first, etc.

Interesting! Pictures?

[Shinigami]

Hello, Thank you for the reply!

Yes sintering through a 30 foot pusher style kiln. There are 3 zones of which is 2135F.  Speed of .5 inches a min.

Yes metallic powders, but its all very well oxidized before sintering. No raw shiny metals.

We use a cellulose binder which is water soluble. 

I suppose they could dry quickly inside of the pusher kiln. I tried to get them in there as quickly as possible to eliminate any drying before sintering. 

When they warp they get concaved upwards like a bowl. The parts are sitting  on alumina saggers of which are about 3/8 inch thick. 

Im suspecting too fast speed for sintering. If it get heated up on the top faster than the bottom (more mass on bottom due to the sagger) id imagine it would sinter unevenly right?

 

 

 

[Shinigami]

[neilestrick]

When they sit flat like that, the top and edges are going to heat faster and possibly heat more than the bottom and center, so that could definitely account for the warping. Slower firing or a perforated slab or some sort of stilt that allows for better heat distribution would help.

[Magnolia Mud Research]

On 5/12/2021 at 9:39 AM, Shinigami said:

Im an engineer at a semiconductor manufacturing plant that specializes in thermistor ceramics. Ive been experimenting on different formulas and sintering profiles. The parts are experiencing warpage problems. Upon my experiments the freshly pressed parts dipped in water then sintered come out straight. no warpage at all. The parts I dried come out warped. 

The mix consists of manganese, cupric oxide and nickel. Spraydried with binder. Then pressed 16 tons into 32mm discs 3mm thick.

Any ideas on why this is happening?  It seems backwards why the wet part is coming out perfect. In my pottery experience, it should of cracked or exploded.

 What I understand from your first post:  
the experiments are telling you that pieces dipped in water does NOT warp, non-dipped does warp.  This does not sound like the clay based drying vs warping:
therefore I would not force clay based techniques too much. 

From what you have posted, the mix does not contain any of the normal studio ceramic materials, especially clay.
  
The following are some questions I would ask if I were a member of a problem solving discussion.  
in random order (probably will need some in depth thinking and lab testing): 
0a. Is this a new reaction to the sintering testing in you company; 
0b. are the compositions significantly different from previous tested materials? 
0c. is the equipment the same as used previously without warping? What is different (other than the material composition) from previous testing that did not warp?    
1. What happens to the test pieces if no water is used? 
2. Why are the pieces dipped in water; could some other liquid be used?
3. Have you estimated the heat transfer rates from the kiln energy source to the test piece at various times in the kiln? Are these rates within what is normally used in making these test pieces?    
4. How long is necessary to burn out the cellulose binder, and at what oxygen levels are needed to maintain the burn; do these estimated times make sense with the time in vs temperature profiles in the kiln?  
5. What are the bonding between the particles dependent on the metal oxide layers, or are they depending on the binding of the metallic layers? 
6. Does this "problem" occur on only on these specific recipes or does it also occur on the "other" recipes? 
7. Are the orientation in the kiln the same as in the press; have you tried reversing the orientation to be different in the kiln from the pressing? 
8. have you "bent" the pieces opposite of the warping to see if that allows the pellets to finish flat? 

additional thoughts:

9. Have you make a test piece from the press and examined the distances among the solid particles of the pellets?  Uneven distances can lead to changes in shape of the pellet on sintering.  One approach is to embed the pellet in a plastic block, and slice the block into thin sheets and examine under a magnifying device.  same for the fired pellets.  compare and contrast the two sets of data. 
10. Warping is a result of uneven stresses and/or material strength; what would cause these conditions in the prepration and handling of the pellet and during time between the pellet is removed from the press and the time that the warping is discovered; what is actually happening taking minute by minute from press until warping is observed? 
11. What is the friction between the pellet and the surface its support in the kiln; is this enough force to cause warping?  
12. What is the material strength of the pellet at various times during the temperature profile from cold to cold; can you see the shape of the pellet at times during the firing; when does the warping become observable? 

 

(ik ben ook een ingenieur) LT