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A Thesis Presented to The Faculty of Alfred University
Revisiting the Role of Binder Burn-Out on Sintering of Alumina by
Stephen C. Hyde
In partial fulfillment of the requirements for
the Alfred University Honors Program
May 2021
Under the Supervision of:
Chair: Dr. William Carty Committee Members:
Dr. Steven Tidrow Dr. Tim Keenan
ii AKNOWLEDGEMENTS
There are several people who were crucial to my thesis work. First, I would like to thank Paul Johnson and Belvac for helping me dry press my powders. Fran Williams was also vital for creating the program to control gas flow in the tube and teaching me to use it.
Hyojin Lee has been invaluable for mentoring me for the last four years as I worked in the ceramic processing labs. Dr. William Carty’s passion and knowledge of ceramics is what motivated me to pursue this field and his guidance and advice through this thesis has been outstanding. My committee members have been important not only to my thesis but my time at Alfred. Dr. Tim Keenan’s teaching reassured me I had chosen the right field. Dr.
Steven Tidrow’s guidance as advisor to the Alfred chapter of Material Advantage has been quite useful. Dr. Carty’s grad students: Dan, Mackenzie, and Alicia, as well as Heidi Boettcher have been important to my academic and social life. Krishna Amin does an outstanding job at Alfred and has been crucial at many points in my Alfred career.
iii HONORS INTRODUCTION:
It has been amazing the last four years to get the honor to study ceramics at Alfred University. It is amazing to think that not that long ago I had no idea what ceramic engineering was. I have been able to learn so much these last few years and am excited to take what I have learned in Alfred out into the world. The ability to work in the labs both in classes as well as working for Dr. Carty was great experience that has prepared me to work in the field. Being in such a niche field means that there many opportunities to do important work.
Working on my thesis was quite insightful in several ways. Most importantly that procrastination is especially dangerous when combined with unforeseen setbacks. Often, I scheduled things so that I could get them done very close to a deadline, however when machines and instruments break, an item is backordered it becomes quite stressful. During these stressful times I would think back to several weeks before when I had put off working on my thesis, because I foolishly thought I had so much more time. I hope I will be able to remember this lesson later in life and improve my time management.
I chose this project for several reasons. First it delt with polymers. While ceramics are my main passion, I thought it would be a good opportunity to look at how other materials, specifically polymer binders, effect ceramics. A big part of this thesis was looking into how polymers break down and proposing ways they can break down that will affect ceramics. I also chose this project because it looks at something used in the ceramics field regularly and is not often thought about. It is often just assumed that binders will burnout, but my work creates evidence that binder burnout is more complex. I have been able to use lot of equipment in Alfred over the last four years, but I also chose this project
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because it would give me the opportunity to use different equipment, such as the tube furnace and a uniaxial die press.
To understand this paper, it is helpful to have a background in ceramics. Since not everyone has the pleasure to study ceramics a brief introduction has been prepared to teach and simplify topics that are mentioned later.
Ceramic Processing and Forming Background:
Ceramic engineering is a subset of material science that focus on inorganic non- metal materials. Traditional ceramics are made with clay components. Advanced ceramics are mostly oxides (alumina, MgO, and Zirconia) and carbides (silicon and titanium carbide.
Because ceramics have high melting points it is impractical to form them from melting so sintering is used. Ceramic powders are formed into the desired shape and then fired. The high surface area of powder allows the particles to join into grains and create a dense object at temperatures much lower than the melting point. Figure a shows what ideally happens during sintering. However, impurities and pores are inevitable as no material is perfect.
Figure a: Sintering Process
When a ceramic is formed in the desired shape and before it is sintered it is called green. This terminology comes from traditional ceramics having a green hue before they
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are fired. Polymer binders are often important when ceramics are green because they provide more strength to the delicate unfired pieces. During sintering binder will undergo pyrolysis, or thermal decomposition. Because organic binders are made up of chains of carbons, they breakdown by forming CO2 Or CO also known as carbon oxidation.
However, these chains of carbons have lots of hydrogens on them too, so at lower temperatures hydrogen stripping can occur where water vapor is formed. Binder is especially important when you form ceramics without liquid like in dry pressing.
In dry pressing ceramic powder is placed in a die between a top and lower punch.
The top punch of the press will lower and compress the powder, then the bottom punch will raise so that the part is being compressed from both sides. Figure b shows a more visual representation of this process. This is a fast and consistent way to make simple geometries and is often how ceramic tiles are made. This process is not only used for ceramics but is also used to make many medicinal pills.
Figure b: Uniaxial Die Pressing
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In dry pressing flowability of powder is important to allow the particles to rearrange during compression to create a higher density ceramic. To increase the flowability of powder ceramics are spray dried. Spray drying also allows binder to be incorporated into the dry powder. Figure c Shows how a spray dryer works: The ceramic suspension is sprayed into a chamber so droplets flow towards a flow of hot air. The inlet heat of the chamber is well above 100°C so the water in the suspension evaporates. The particles in the droplets will stick together and granules will form. The granules are round and have high flowability. The heavier granules will fall to the bottom of the chamber into collection. Smaller particles will follow air flow into a second cyclone that will collect smaller granules. The smallest particles that are considered dust will flow into the dust collector. The coarse granules have the highest yield and is the most useful product of this process.
Figure c: Spray Dryer Schematic
Project Scope:
The goal of this thesis was to look at binder burnout under different oxygen levels. Samples with and without binder were heat treated together to at several oxygen levels and then they were sintered all together. If oxygen had an impact on binder
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burnout, then there should be a trend in the density of samples with binder. Densities of the sintered samples were taken though immersion.
Results:
The densities of the samples with binder showed no trend from oxygen levels during heat treatment. This means the original hypothesis was null. However, all the densities for samples with binder were significantly different than samples without binder. This is evidence that binder burnout was never complete, and that binder burnout might not be as fully understood as previously thought.
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Table of Contents
LIST OF TABLES ... iv
LIST OF FIGURES ... iv
ABSTRACT ...v
INTRODUCTION ...1
EXPERIMENTAL PROCEDURE ...3
A. Sample Preparation ...3
B. Binder Removal ...4
C. Sintering ...4
D. Density Testing ...4
RESULTS AND DISSCUSSION ...4
E. Demonstration That Binder Alters Sintering ...4
F. TGA Data of A-16 S.G. Granules with PVA and PEG ...5
G. Densification Results ...6
H. Statistical Analysis ...6
CONCLUSIONS...7
FUTURE WORK ...7
REFERENCE ...9
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LIST OFTABLES
Table I. Shapiro-Wilk Test Results ...9 Table II. t-Test with Unequal Variance ...9
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LIST OF FIGURES
Figure 1. Chemical Structure of PVA (Kim1) ...1
Figure 2. Proposed Mechanism for Hydrogen Stripping (Keefe2) ...1
Figure 3. Ellingham diagram (Keefe2) ...2
Figure 4. TGA Data for Paraffin + Alumina (Keefe2) ... 3
Figure 5. Demonstration That Binder Addition Alters Sintering. ...6
Figure 6. TGA Data of A-16 S.G. Granules containing 2% PVA and 2% PEG ...7
Figure 7. Density of Sintered Samples with and Without Binder Vs. Oxygen Level During Heat Treatment ...8
xi ABSTRACT
Binders are commonly used to impart green strength for pressed ceramic parts prior to sintering. It is generally assumed that a simple heat treatment in air is sufficient to remove the binder prior to sintering. A simple experiment, however, in which a droplet of binder is introduced to a pressed compact without binder, demonstrates that the binder alters the sintering of the compact in the region of the droplet. It is proposed that this alteration in sintering behavior is due to residual carbon from incomplete binder removal.
To systematically evaluate binder removal, two sets of alumina compacts were prepared:
one contained binder (PVA and PEG) and one without binder. The samples were both subjected to binder removal heat treatments in which the ratio of oxygen to nitrogen in the gas was altered to produce seven atmospheric conditions with oxygen levels of 0, 10, 20 (dry air), 30, 50, 75 and 100%. After the binder removal stage, the resulting samples were sintered in air at 1550°C for one-hour, densities measured, and fracture surfaces of the microstructures evaluated. The results showed that oxygen level does not have a significant impact on binder removal. However, because the samples with binder always had a lower density than the samples without binder it can be concluded another factor is hindering binder removal.
1 INTRODUCTION
Polymer binders are often used in ceramics to increase the green strength of ceramics. Binders are especially useful for dry pressing. Polyvinyl alcohol (PVA) is one of the most common binders used. Figure 1 shows the chemical structure of PVA. PVA has a backbone of carbon with hydrogen and hydroxide groups on it.1 The three ways that PVA can break down is (1) chain scission (break-up into shorter organic molecules that can volatilize), (2) the direct oxidation of carbon and (3) hydrogen stripping. Oxidation of carbon results in CO or CO2 and Hydrogen stripping results in water vapor and the formation of unsaturated carbon bonds (double and triple bonded carbons) that are strongly bonded to each other. Figure 2 shows how hydrogen stripping occurs with PVA.
Figure 1. Chemical structure of PVA.1
Figure 2. Proposed mechanism for hydrogen striping.2
Keefe showed using Ellingham diagrams that temperatures below 525°C oxygen preferentially reacts with hydrogen to form water, stripping the hydrogen from the
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hydrocarbon molecule, rather than forming CO2.2Forming CO2 would be more favorable if CO were in the system, but it not abundant enough in the atmosphere to affect firing in air.The Ellingham diagram (Figure 3) plots the Gibb’s free energy (∆G) versus temperature showing the stability of a broad range of oxides. As ∆G becomes more negative, the oxide stability increases.
Figure 3. Ellingham diagram.2
Pyrolysis of organic binders happens at lower temperatures than typical ceramics sintering temperatures. PVA has been shown to undergo complete pyrolysis below 600°C while alumina is sintered at 1550°C.3 Work from Keefeshows that carbon yield in alumina containing paraffin (as a surrogate binder) could be increased by 20wt% if held at temperatures lower than 200°C.2 He proposed that the residue was carbon from incomplete binder burnout. In Figure 4 Keefe shows that oxygen level influenced
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residue. At a PO2of 21.5%, the heat treatment temperature effected the residue content, but at a PO2 of 5% the temperature had less of an effect. This can be seen as evidence that hydrogen stripping can affect binder burnout. Note that Keefe’s goal was to increase carbon yield, while binder removal for most systems is the opposite – no carbon residue is desirable.
Figure 4. TGA data for paraffin + alumina.2
4 EXPERIMENTAL PROCEDURE
Sample Preparation:
A 17 kg suspension with 20-volume percent calcined alumina (A- 16 S.G., Almatis, Leetsburg, PA) was prepared. This suspension was dispersed with PMAA at .25 𝑚𝑔
𝑚2 using Darvan C. The 0.25 𝑚𝑔
𝑚2 NH4-PMAA (Darvan C, R. T. Vanderbilt, Norcross, CT). This dispersant level does not fully disperse the alumina but provides for better milling and spray drying. The Al2O3 suspension was vibratory milled (Vibro-Energy Grinding Mill, SWECO, Florence, KY) for 100 hours to break up agglomeration. Whipkey showed that milling for 100 hours eliminates systematic agglomeration in alumina from the Bayer process.4 After milling a small sample of suspension was removed from the slurry to test the solids loading. The precise solid loading was needed to calculate the binder addition level. It was found that the solid loading decreased from 20-volume percent to 13.8-volume percent. The suspension was mixed thoroughly and then split into two equal batches. One was for the samples without binder and the other half with binder. The levels added for both PVA (Selvol 205, Sekisui, Dallas, PA) and polyethylene glycol (PEG 400, ChemWorld, Roswell GA) were 2% of dry weight in suspension. Both suspensions were spray dried in a laboratory spray drier (Bowen #1, Bowen Engineering Inc., North Branch, NJ). The inlet temperature was set to 250C° producing an outlet temperature of 110 ±5°C.
The samples were uniaxial pressed at 60 MPa in an industrial (80 Ton Uniaxial Press, Aeonic Press Co, Eason, PA). The disked formed had a diameter of 60mm and a height of 7mm. The disks were broken up to create smaller samples, roughly into eighths.
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Binder removal:
A tube furnace used previously for thesis work, that allows gas composition and heat treatment conditions to be controlled independently, was used for the binder removal.
The tube furnace was customized from a crucible furnace (Crucible Furnace, Lindberg/Blue M, Ashville, NC). The program to control the gas flow was made by Francis Williams using a LabView application (LabView v. 2014, National Instruments, Austin TX). Samples with and without binder were heat-treated together in the tube furnace for the binder removal heat treatment to ensure they received equivalent thermal history prior to sintering. The tube furnace was ramped up to 600°C at 10K/minute, held for 90 minutes and then cooled to room temperature. Gas flow was kept at a constant rate of 10 ml per second. Oxygen level was controlled by blending O2 and N2. Oxygen levels tested were:
0, 10, 20, 30, 75, and 100%. 8 samples were tested for each run, half with and without binder.
Sintering:
All samples and an additional set without a heat treatment were sintered together in a Deltech bottom loader furnace (Square Bottom Loading, Deltech, Denver, CO). Samples were heated 10K per minute to 1550°C with a 60-minute hold and then cooled to room temperature.
Analysis:
The density of the samples was taken with immersion testing. Following an ASTM standard test method modified for small sample size.
6 RESULTS AND DISCUSSION
A simple experiment was done to prove that binder alters the sintering of ceramics.
A drop of PVA (15 wt.%) was placed on the surface of a pressed A-16 S.G. powder compact initially prepared without any binder. The binder allowed to absorb into the surface and then excess liquid was removed. This sample was sintered with a specimen without binder and a specimen with binder. The comparison specimens (with and without binder) sintered as expected and exhibited smooth surfaces. But the specimen with the binder droplet sintered irregularly where the binder droplet was added illustrating a differential densification. This simple experiment illustrates that binder interferes with the sintering of alumina and it is proposed that this is caused by a carbon residue from incomplete binder removal. Figure 5 shows the process and the irregular sintering on the surface.
Figure 5. Demonstration that binder addition alters sintering.
7 Carbon Residue
Residue of carbon from binder burnout is feasible when compared to work from Tomas Lam. Lam added CaCl2 to alumina to introduce calcium as a dopant. It was assumed, much like carbon in binder, that chlorine would volatilize and leave the system. However even after 1700°C chlorine was present in the amorphous grain boundaries.5 Chlorine has a higher atomic number than carbon (17 and 5), so it feasible that carbon residue could remain after incomplete binder burnout. Carbon could remain as CO, CO2 or C-bonded to the oxide surface of alumina.
Binder Decomposition
Thermogravimetric analysis (TGA) was done on the spray dried granules that contained PVA and PEG. Figure 6 shows that 600°C is a sufficient temperature for PVA and PEG pyrolysis.
Figure 6. TGA data of A-16 S.G. granules containing 2% PVA and 2% PEG.
The densification results are shown in Figure7. It is evident that binder removal is independent of oxygen partial pressure. The data also indicates that there is a 0.97% reduction in density for samples that have binder compared to specimens that
99.4 99.5 99.6 99.7 99.8 99.9 100
0 200 400 600 800 1000
Perc ent R em aining
Temperaure (C°)
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did not contain binder. This suggests that there is a residue from binder removal in the system and it is proposed that this residue is small amounts of (unoxidized) carbon.
Figure 7. Density of sintered samples with and without binder Vs. oxygen level during heat treatment.
Statistical analysis was done to demonstrate that the decrease in density exhibited by the specimens containing binder was statistically significant. First a Shapiro-Wilk test was done to show that the density values were normally distributed so that the average and standard deviation could be used to execute a t-Test. Table I shows the P-values for both samples with and without binder. Since the P-value was above 0.05 the sets are normally distributed with a 95% confidence. Next t-Tests with unequal variance were performed. The reason that the unequal variance test was selected is because the samples with and without binder had different standard deviations. Table II shows the results for t-test the averages of sample sets and for all data. For 1-tail and 2-tail the P-
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values are below 0.01, so with 99% confidence the samples with and without binder are separate and distinct sets.
Table I. Shapiro-Wilk Test Results Binder No Binder
P-value 0.9737 0.2488
Table II. t-Test with Unequal Variance Average All data
P 1-tail 4.9x10-7 5.0x10-14 P 2-tail 9.8x10-7 1.0x10-13
Anomaly of 50% Oxygen Heat Treatment
The conditions for the 50% oxygen heat treatment varied from the rest. There was concern that the oxygen available for the test would be depleted during the run, so gas flow was not turned on until the sample had been heated to 190°C. The density at 50% oxygen is the highest of the samples with binder. The difference in test procedures for this run appears to have impacted binder burnout. There was no gas flow below 190°C and thus no added oxygen. It is proposed that limited oxygen below 200°C prevented hydrogen stripping and promoted chain scission. These results are different from the treatments with higher nitrogen levels because oxygen was only introduced in this run above 190°C. Once the gas flow was initiated and oxygen was introduced oxidation of carbon could occur. No gas flow below 190°C did not impact the density of samples
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without binder. To test to see if this hypothesis is correct additional work should be done in the future.
11 CONCLUSIONS
There was no trend for binder removal from oxygen level. Binder reduced the average density of dry pressed A-16 S.G samples by 0.97%. The hypothesis that higher oxygen levels do not promote full binder burnout was demonstrated to be true, however lower oxygen levels also did not have complete binder removal, with the residue proposed to be carbon.
12 FUTURE WORK
The conclusion that binder removal was incomplete in this work requires additional work.
The removal of organic is proposed to be a two-step process: first is preferential H- stripping and the second is oxidation of carbon. If binder undergoes H-stripping than oxidation of carbon will struggle to occur because of the triple bonds between leftover carbons. Two experiments are proposed to test this. First Hydrogen stripping should be avoided this can be done by eliminating oxygen below 200°C. This could further study the results found for 50% oxygen level. The same oxygen levels in this work (0, 10, 20, 30, 75, and 100%) should be used but with no gas flow below 200°C. If the hypothesis is correct this would prevent H2O from forming and triple carbon bonds from forming. A second experiment should be done to increase availability of oxygen for carbon oxidation by stagnating or pulsing gas flow. It is proposed that a constant flow of gas as done in this work hinders the ability of oxygen from reacting with binder. Flowing gas will fill the system with oxygen, then stopping gas flow will allow that oxygen to react with the ceramic system and react with binder. By repeating this process at temperatures favorable to carbon oxidation will allow for more efficient binder removal.
It is also important to verify that carbon is present in the samples after sintering.
Throughout this work it was assumed that carbon residue was the cause of lower density.
However no concrete proof was presented. Because carbon would be on the parts per million energy dispersive X-ray spectroscopy would not be able to detect residue.
Analyzing samples with transmission electron microscopy (TEM) or Fourier transform infrared spectroscopy would be required to detect carbon at that level. If residue from binder is present it would be in grain boundaries as C or CO.
13 REFRENCES
1. U. Kim, The Role of Polymer Compatibility in Ceramic Processing, Ph.D Thesis, Alfred University, Alfred, NY p.139, 2002.
2. K. Keefe, Carbon Retention: Pyrolysis of Phenolic Resin in SiC, M.S. Thesis, Alfred University, Alfred, NY, 27-29, 2015.
3. J. Reed, Principles of Ceramics Processing, 2nd Ed., John Wiley & Sons, New York, NY, p 591, 1995.
4. S. Whipkey, Investigating the Microstructure Evolution of Al2O3 With Glass Phase Chemistry inthe CaO-Al2O3-SiO2 System” Ph.D. Thesis, Alfred University, Alfred, NY, p103, 2020.
5. T. Lam, Glass Formation Boundary Approach to the Sintering of Alumina, Ph.D.
Thesis, Alfred University, Alfred, NY, p51, 2010.
