THE PENNSYLVANIA STATE UNIVERSITY SCHREYER HONORS COLLEGE

DEPARTMENT OF MECHANICAL ENGINEERING

3D Printing of Barium Titanate Using the Direct Ink Writing (DIW) Technique

KARIM BARSOM SPRING 2021

A thesis submitted in partial fulfillment of the requirements for a baccalaureate degree in Mechanical Engineering with honors in Mechanical Engineering

Reviewed and approved* by the following:

Zoubeida Ounaies Professor of Mechanical Engineering Thesis Co-supervisor

Amrita Basak Assistant Professor of Mechanical Engineering Thesis Co-supervisor

Bo Cheng Associate Professor of Mechanical Engineering Honors Adviser

* Electronic approvals are on file i

ABSTRACT

Barium titanate (BaTiO3) is a smart ceramic known for its piezoelectric and ferroelectric

properties. 3D printing, also known as additive manufacturing, of BaTiO3 has crucial

applications across the medical, robotics, and electronics industries. 3D printing allows engineers

to consider innovative designs with increasingly complex geometries which can lead to

innovations that are unimaginable today. The development of a process that produces ceramic

parts with 3D printing can also enhance the reliability and reduce the cost of manufacturing

small-scale electronic components. The Direct Ink Writing (DIW) process was selected for its

relatively low cost of entry, ease of use, and for its use of ceramic pastes and slurries similar to those used traditionally in tape casting and slip casting ceramic manufacturing methods. To execute the DIW process, I am using a modified off-the-shelf consumer 3D printer.

Understanding how the controllable parameters of the DIW process and hardware can

affect the quality, precision, speed, and capabilities when printing a BaTiO3 based slurry is the

first step towards optimizing the DIW process for prototyping and mass commercial

applications. Through preliminary slurry solid loading tests and a parameter impact assessment

looking at the flow rate, nozzle size, BaTiO3 solid loading in the slurry, print speed, and gap

height, I have shown how understanding the relative impact of each parameter can inform the

adjustments to each printing parameter for the desired outcome; may that be fastest print speed,

greatest resolution, or some combination of outcomes.

ii

TABLE OF CONTENTS

LIST OF FIGURES ...... iii

LIST OF TABLES ...... iv

ACKNOWLEDGEMENTS ...... v

Chapter 1 – Introduction ...... 1

1.1 - Background and Motivation ...... 1 1.1.1 – Barium Titanate ...... 1 1.1.2 – 3D printing of Ceramics ...... 3 1.1.3 – Traditional Ceramic Manufacturing ...... 7 1.1.4 – Direct Ink Writing of Ceramics ...... 13 1.2 Problem Statement ...... 19

Chapter 2 – Experimental Design ...... 21

2.1 - Barium Titanate Slurry Formulation ...... 21 2.1.1 – Procedure for Slurry Production ...... 22 2.1.1 – Manual Printing 3D Parts Using Prepared Slurries ...... 23 2.1.2 - Variation of the Solid Loading ...... 25 2.2 – DIW Printer Setup ...... 26 2.3 – Testing Procedure for Parameter Assessment ...... 30 2.3.1 – Design of Experiments Setup ...... 30 2.3.2 – 3D Model and Slicing ...... 31 2.3.3 – Setting Parameters ...... 32 2.4 – Data Collection and Analysis ...... 33

Chapter 3 – Results ...... 35

3.1 – Results of Slurry Processing ...... 35 3.2 – Printing Parameter Analysis Results ...... 42 3.3 – DIW of a 3D Part ...... 44

Chapter 4 – Conclusions ...... 48

4.1 – Summary of Significant Results ...... 48 4.2 – Conclusions...... 50 4.3 – Suggested Future Research ...... 52

Appendix A Full Test Matrix for Min-Max Test Plan ...... 53

Appendix B Binder Burnout and Sintering Ramp and Temperature Profiles ...... 55 iii

BIBLIOGRAPHY ...... 56

iv

LIST OF FIGURES

Figure 1.1. Crystalline structure of a BaTiO3 unit cell [3]...... 2

Figure 1.2. (a) Selective Laser Sintering (SLS) setup (b) Selective Laser Melting (SLM) setup [14] ...... 5

Figure 1.3. CIM component green body, brown body, and sintered part, respectively [9]. 9

Figure 1.4. A schematic of a standard tape casting machine [10] ...... 12

Figure 1.5. An example of a Direct Ink Writing setup [20]...... 14

Figure 1.6. Viscosity versus shear rate for a BaTiO3 slurry [29]...... 15

Figure 1.7. (a) Particle size distribution of the three types of powders, the SEM images of the corresponding particles with (b) D50=7.41 μm, (c) D50=3.00 μm, (d) D50=0.35 μm [29] ...... 16

Figure 1.8. Samples of clay fabricated by extrusion 3D printing. a) Side view of “twisted gear lamp”, b) top view of (a), c) pyramidal test object, d) 20 mm diameter cylindrical samples, e) 10 mm diameter cylindrical samples, and f) side view of “Ashtray” [25]...... 18

Figure 2.1. SEM images of Ferro barium titanate powder: a) 15000x magnification. b) 35000x magnification ...... 21

Figure 2.2. Processing of BaTiO3 slurry for extrusion ...... 22

Figure 2.3. Binder-burnout temperature profile ...... 24

Figure 2.4. Sintering temperature profile ...... 24

Figure 2.5. Model of the control and extrusion setup attached to the Ender 5 Pro 3D printer ...... 27

Figure 2.6. Hardened stainless steel 3D printer nozzle (dimensions in mm) ...... 28

Figure 2.7. a) 3D model of the shape that is used for print testing. b) showing the line that the 3D printer will make when executing the G-code sent to it ...... 32

Figure 2.8. Example of an image captured for each test case and used for data analysis 33

Figure 3.1. 75wt% parts printed by hand using a 5ml syringe with a 2mm diameter nozzle38

Figure 3.2. Close-up images of two of the printed parts shown in Figure 16...... 39 v

Figure 3.3. Rectangular 3D form printed using a 5ml syringe, 2mm diameter nozzle, and 70wt% slurry ...... 39

Figure 3.4. Dried parts prepared for binder burnout and sintering ...... 40

Figure 3.5. 75wt% (in blue boxes) and 70wt% parts (a) after binder burnout and (b) after sintering. (c) Crack formation in the 70wt% sample after the binder burnout step. 41

Figure 3.6. Parameter impact assessment plots. (a) Slurry wt% impact plot (b) Print speed impact plot (c) Nozzle size impact plot (d) Stepper motor step delay (flowrate) impact plot (e) Gap height impact plot ...... 42

Figure 3.7. DIW of a 2cm x 2cm x 2cm cube using printing parameters influenced by the parameter impact assessment results ...... 46

Figure 3.8. (a) Side profile of the printed cube (b) Top-down profile of the printed cube 47

vi

LIST OF TABLES

Table 1.1. Pros and cons of ceramic injection molding technology vs additive manufacturing [18]...... 10

Table 2.1. Initial recipe from Wei et al. [29] ...... 25

Table 2.2. Additional BaTiO3 slurry ceramic wt% variations tested for extrudability 26

Table 2.3. Parameters and levels for min-max testing ...... 29

Table 2.4. Reduced Min-Max test cases run for phase 1 of testing ...... 30

Table 3.1. Results of preliminary testing on the printability and visual observations on viscosity of four BaTiO3 slurries with different solid loadings ...... 36

Table 3.2. Measured density of the sintered parts shown in Figure 3.5 ...... 41

Table 3.3. Printing imperfections observed during the parameter impact assessment testing ...... 43

vii

ACKNOWLEDGEMENTS

I would like to thank my research advisors and thesis supervisors, Dr. Zoubeida Ounaies and Dr. Amrita Basak for their guidance and support throughout what has been a tumultuous time for research. I would also like to thank them both for allowing me to work on this project over the past three semester and for bringing me into their labs. I learned just about everything I know about how to conduct research and how to do so thoroughly through my time with them.

I would also like to thank Dr. Amira Meddeb in the Materials Research Institute for all

her guidance, instruction, and assistance with every step of the slurry production and processing

procedures. Her help was vital to my success making multiple batches of slurry for printing and

her expertise has made direct impacts on this thesis.

I would like to extend my gratitude to Professor Michael Lanagan for providing the

BaTiO3 ceramic powder for use in this project.

Finally, I am incredibly grateful for the existence of all the opportunities and privileges

I’ve had as a Penn State student thanks to the incredible hard work and sacrifice of my parents,

Mansour and Samia Barsom.

1

Chapter 1 – Introduction

1.1 - Background and Motivation

1.1.1 – Barium Titanate

Barium titanate (BaTiO3) is a piezoelectric and ferroelectric ceramic, first independently

discovered during the in 1945 by scientists in Japan, Russia, and the United States [1].

Piezoelectricity of a material is the creation of an electric charge as a result of mechanical stress

or stimulation. Ferroelectricity refers to the ability of a material to spontaneously polarize

without the influence of an external electric field, and the ability to have that polarization

reversed when a sufficiently strong applied electric field. This is somewhat analogous to

ferromagnetism, in which a material maintains a permanent magnetic field without the presence

of an external magnetic field. Throughout the decades following its discovery, more research

into the electrical properties of BaTiO3 led to its widespread use in a wide variety of application due to it being the first oxide material to demonstrate the ferroelectric phenomena [1].

BaTiO3 belongs to the perovskite crystal family, so-called after the mineral of the same

name, CaTiO3 [2]. The perovskite crystal family generally consists of compounds with the formula ABO3. The oxygen anion can often be replaced with other anions such as fluoride, chloride, nitride, hydride, and sulfide as long as these replacements still maintain the cubic unit cell that is associated with perovskite structure materials. Figure 1.1 shows the unit cell structure

of BaTiO3 in the cubic form [3]. The cubic structure of BaTiO3 is present at temperatures above

the material’s Curie point, approximately 130°C. The Curie point in ferroelectric ceramics is the 2 temperature at which the material undergoes a transition from the ferroelectric tetragonal phase

(<130°C) to the paraelectric cubic phase (>130°C) [4].

Figure 1.1. Crystalline structure of a BaTiO3 unit cell [3] The tetragonal structure is asymmetric and results from the displacement of the titanium atom

from its centrosymmetric position along the vertical C-axis of the unit cell. This displacement

introduces the permanent electric dipole within the material and the ability to hold this

polarization without the presence of an external electric field. BaTiO3 undergoes additional

transformations at continuously cooler temperatures, transitioning from a tetragonal unit cell

structure to an orthorhombic structure at 0°C, and orthorhombic to rhombohedral at -90°C. The

final phase is where the ferroelectric effect is again present [5].

BaTiO3 is a lead-free piezoceramic and shows a high piezoelectric constant measured at

between d33 ~ 620 pC/N to 700 pC/N. This is comparable to the piezoelectric constant for PZT, a

commonly used piezoelectric, of around 600 pC/N [6]. Applications of piezoceramics include

micro-actuators and transducers. The centrosymmetric cubic structure lends BaTiO3 a dielectric constant as high 7000 in some cases, while other ceramics such as titanium dioxide have dielectric constants between 20 and 70 [3]. Ferroelectricity is useful for a wide variety of

applications including multilayer ceramic capacitors, IR detectors, and in ferroelectric RAM [4]. 3 The piezoelectric effect in BaTiO3 is incredibly appealing in the face of growing environmental concerns over the toxicity of lead-based piezoelectric materials that have traditionally been used, such as PZT [7].

1.1.2 – 3D printing of Ceramics

3D printing has been used as a method to produce components using an incredibly wide range of materials with different mechanical, chemical, and electrical properties in all kinds of applications. Metals, plastics and polymers, cellular structures, ceramics, and many more materials have been shown to be successfully manufactured using 3D printing techniques that range across direct extrusion methods, powder-based polymerization, stereolithography, and laser sintering to name a few [8][9][10][11][12]. The rapid prototyping applications are quickly leading to rapid manufacturing of parts that meet or exceed the quality standards set by the traditional methods of manufacturing each parts with various materials [11]. 3D printing’s benefits come down to its speed, affordability, easy part customization, and possible geometric complexity relative to most traditional manufacturing methods. Some methods of 3D printing are also simpler to set up and run than a traditional method that would produce a similar component

[12].

The electrical properties of BaTiO3 and its relatively early discovery have made it a well- studied and often used ceramic throughout many industries. Traditionally, nearly all applications of ferroelectric oxides have involved planar geometries. Further development of the technology to implement BaTiO3 in electronics and sensors is beginning to revolve around placing the material in non-planar geometries, such as ceramic nanotubes. The drive for increased storage 4 density in FRAM (ferroelectric random-access memory) and DRAM (dynamic random-access

memory) components is requiring complex stacking geometries that cannot be achieved using

the wide-spread traditional plane-oriented manufacturing methods such as tape-casting, and

require a smaller feature size (greater resolution) than molding techniques can offer [13].

Through the development and implementation of 3D printing for ceramics, new applications

where the electrical and mechanical properties of ceramics could be introduced become possible.

3D printing methods for ceramics can be divided into two categories: direct and indirect.

Direct methods for 3D printing of ceramics include the sub-categories of powder, suspension, precursor, melting, and reaction -based methods. Indirect methods consist of producing sacrificial molds out of metal or plastic using another 3D printing processes and then casting a ceramic slurry into those molds [14]. Direct methods of 3D printing ceramics result in a printed part that is either immediately ready for use, or only needs to be sintered to full density before use. Parts manufactured with indirect methods have limited complexity when compared to direct methods, although the parts can be more complex than conventional molding techniques for ceramics. Due to the fewer number of processing steps, and therefore shorter overall production time, direct methods are preferable to indirect methods for manufacturing ceramics using 3D printing [14].

Within 3D printing for ceramics, there are a large variety of technologies that can be categorized into powder-based, bulk solid-based, and slurry-based methods for 3D printing ceramic parts [15]. Powder-based methods involve ceramic powders that are directly formed into a 3-dimensional part by a laser. Selective Laser Sintering (SLS) is a method that was initially intended to be used in the making of wax models for investment casting of metals. The process as it applies to ceramics consists of a build surface onto which a layer of powder is deposited, the 5 powder being heated, and then a high-power laser selectively sintering parts of the powder to

form a layer of the full part. After each layer, a new layer of powder is spread across the surface

and the process repeats until the form is complete. Figure 1.2 (a) shows a schematic of the SLS

process. A significant drawback of this method is that the sintering process is difficult to initiate

on a local level. Efforts to reduce the energy required to sinter the powder can be made by

coating the powder in a binder that melts when heated and introduces a medium for the powder

particles to bond through. SLS parts often still require a secondary sintering step to achieve full

density. Selective Laser Melting (SLM) is similar to SLS, but it uses a much higher-powered laser to melt the powder particles and produce a fully dense part without any post-processing.

Figure 1.2 (b) shows a schematic of the SLM process. However, parts produced using SLM are susceptible to cracking and distortion during printing due to the low thermal shock resistance of ceramics. The part experiences drastic cooling and heating rates during the printing process [14].

(a) (b)

Figure 1.2. (a) Selective Laser Sintering (SLS) setup (b) Selective Laser Melting (SLM) setup [14] The bulk solid-based ceramic 3D printing methods are Laminated Object Manufacturing

(LOM) and Fused Deposition Modelling (FDM). LOM begins with the creation of a sheet of

ceramic through a method such as tape casting. Those sheets are then cut into the shapes of each 6 cross-sectional layer of the desired part and adhered to each other in order. High density parts can be made from the laminated part by burning out the binder and sintering the part to its full density. The major difficulty with using LOM to produce ceramic parts is the limited resolution and complexity. The part features can only be as small as the thickness of the sheets used to

make it. FDM ceramic printing uses a thermoplastic polymer filament densely loaded with

ceramic particles. Up to 60 vol% of ceramic has been achieved in these filaments, with lower

loadings being typical [16]. With FDM ceramic printing, similar issues arise as those seen in

conventional plastic FDM printing. Surface roughness and dimensional accuracy are the primary

issues, with homogeneity of ceramic within the polymer binder being another issue [14].

Slurry-based ceramic 3D printing encompasses two main methods, with those being

polymerization based and deposition based. Slurries for polymerization 3D printing require a

photocurable suspension with nanometer sized ceramic particles suspended in an aqueous or

non-aqueous base. Stereolithography (SL), Digital Light Processing (DLP), and Twin Photon

Polymerization (TPP) are all ceramic 3D printing polymerization-based methods that use this

type of mixture as a feedstock. Surfactants need to be added to the base along with other

additives to aid with the dispersion of the nanoparticles. In SLS, a light source is projected onto

the surface of the feedstock to selectively cure sections [17]. DLP is an adaptation of SLS and

uses a mask in front of the light source to allow curing of a larger area at one time. TPP is

slightly different, and polymerization is achieved by the absorption of two photons of a near

infrared or green laser at a local point in the resin. This approach is more focused compared to

single photon absorption with SLS and DLP allows greater resolution than either SLS or DLP.

To obtain a dense ceramic component, the post-processes of binder burnout and sintering similar

to conventional ceramic processing must be used. All these photopolymerization processes, apart 7 from TPP, suffer from light scattering and producing unwanted polymerization of extraneous

locations due to the difference in the refractive index of the binder and the ceramic suspended

within it.

While each category of ceramic 3D printing has its own positives and negatives regarding

maximum printable resolution, compatible materials, post-processing effort, and other

limitations, deposition slurry-based technologies enable cheaper manufacturing when compared

to laser sintering or melting technologies while retaining a high degree of accuracy and

formability relative to ceramic FDM or LOM [15]. The expensive lasers and setups of

photopolymerization methods that pose problems for SLS, DLP, and TPP mean more affordable

methods may see adoption first. One such method is the Direct Ink Writing (DIW) method of

slurry-based ceramic 3D printing. Previous applications of ceramic-based slurries in the tape

casting and slip casting processes provide an existing foundation of knowledge for the

formulation of slurries in the DIW process. This aspect of slurry-based 3D printing for ceramics

lends the technology to commercialization and wide-spread use.

1.1.3 – Traditional Ceramic Manufacturing

Making components out of ceramics gives access to a whole host of desirable mechanical

properties such as high hardness, thermal properties such as heat resistance, and electrical

properties such as piezoelectricity and ferroelectricity when the proper ceramic is used. There are many manufacturing techniques that can be used to fabricate ceramic components. Moritz and

Maleksaeedi review the traditional methods of manufacturing ceramics and then compare the efficacy of a variety of 3D printing techniques to the traditional methods [18]. The main 8 categories of traditional ceramics manufacturing are dry shaping, (thermo-)plastic shaping, and

wet shaping.

Dry shaping methods consist of uniaxial pressing and isostatic pressing of a ceramic

powder. Small amounts of an organic additive may be used to assist with the particle-to-particle flow during the pressing process and reduce powder friction. Expensive dies must be manufactured and have a limited useful life before needing to be replaced. Dry forming allows increased geometric complexity through the use of “novel pressing equipment and pressing tools” [18].

After the initial molding in nearly every ceramic manufacturing technique, the part produced is referred to as a green body ceramic and is not yet ready for use. The green body ceramic is a formed ceramic part with the correct shape and relative dimensions, but it has not yet been sintered to full density and final size. Sintering describes the heating of the green body at a temperature sufficient to densifying the part and stimulate grain growth. At the temperatures that ceramics are sintered (typically >1000°C), the driving force behind sintering is the reduction of the interfacial free energy in the system [19]. On a global scale within the part, densification occurs as solid-gas interfaces are replaced by grain boundaries. Voids between the grains are reduced in this way and allow densification. Grain growth occurs due to larger grains having a larger radius of curvature than smaller grains, smaller surface to volume ratio and having a lower energy state relative to smaller grains [19]. The parameters for a sintering process include the particle size, temperature, time, pressure, and green density. A fully sintered part can have a theoretical density of the bulk material, but imperfections and leftover binder often decrease the realized density. Adjusting the parameters used in the sintering process can also affect the sintered part density [20]. 9 In the (thermo)plastic shaping methods, a ceramic powder is mixed with a plastic or

thermoplastic binder solution and typically pressed or extruded into the desired form. The

manufacturing methods under this category include extrusion and ceramic injection molding

(CIM). The process traditionally and most used for large-scale production of ceramic parts with a

high level of geometric complexity is CIM. CIM is best suited for medium to large series

production (<10,000 to 50,000 parts) [18]. Parts created using CIM are accurately reproducible

and require minimal post-processing if any. A progression of the processing that a CIM produced

part goes through is shown in Figure 1.3.

Figure 1.3. CIM component green body, brown body, and sintered part, respectively [9]. The typical CIM injection molding process begins by combining a ceramic powder and

binder into a feedstock, injection molding the feedstock into a mold, and finally binder burnout

and sintering the part. The binder burnout process is a lower temperature “baking” of the part intended to remove all of the organic binder mixed with the ceramic prior to the final densification during the sintering process [21]. The feedstock typically consists of ceramics with a particles size of 1-2 µm mixes with a thermoplastic binder system [21]. Ensuring that the

system is free of agglomerations and still has a proper ceramic to binder content to maintain

fluidity is important to a successful molding. CIM also allows the creation of multi-material

components utilizing multiple types of ceramics for parts with unique properties. However, a

large part of the cost of CIM is the initial tooling to produce the molds. Molds for CIM must be 10 made of hardened steel and the injection unit must be made from expensive wear-resistant materials due to the highly abrasive ceramic particles in the feedstock [21]. Table 1.1 summarizes a comparison between CIM and the current state of 3D printing of ceramics.

Table 1.1. Pros and cons of ceramic injection molding technology vs additive manufacturing [18].

Wet shaping methods of ceramic manufacturing include a wide range of distinct

processes including tape casting, slip casting, gel casting, direct coagulation casting,

electrophoresis, freeze casting, and pressure casting [18]. Of these processes, tape casting and 11 slip utilize a slurry mixture of ceramic powder most closely resembling that of the process

discussed later in this thesis, and therefore those two processes will be discussed in detail.

Tape casting is a process used to manufacture thin, flat sheets of ceramic by making a

thin layer of a raw powdered ceramic suspended in a liquid, drying the sheet, and then cutting the

dry material to the proper dimensions [22]. The final cut form is then sintered. This process is

low-cost relative to most other ceramic manufacturing methods, and is compatible with a large

range of ceramic/binder combinations [23]. The binder solution can be based on organic solvents

such as toluene and ethanol or simply water may be used. The binder used in tape casting is

typically polyvinyl butyral or polyvinyl alcohol. These binders are well known for their adhesive

and emulsifying properties which aid in the formation of a uniform mixture. The binder solution

can have various additives mixed in to achieve different effects and behaviors in the slurry, and a

dispersant and plasticizer are sometimes required to maintain homogeneity within the slurry.

Particle surface charges can be manipulated through the use of these additives. Dispersants are a type of surfactant used in a wide variety of industrial and household products and they function to create the necessary repulsion between suspended particles keeping them from agglomerating.

The dispersant coats individual particles of solid in the suspension and introduces a surface charge that repels other particles. Agglomerations result in a wide particle size distribution that may affect the uniformity of the slurry and the quality of the ceramic green body. Areas of low

solid concentration can then manifest as voids in the final dried and fired ceramic. The addition

of a dispersant adds stability to the suspension and ensures that the suspension can be stored for a

period of time with negligible degradation in print quality. Plasticizers are sometimes referred to

as “water-reducers” when used in a slurry or mixture and the addition of a plasticizer in the slurry aids in the flowability while reducing the need for additional water or solvent [24]. A 12 greater presence of water in a ceramic slurry can reduce the strength of the sintered ceramic

while increasing the shrinkage and cracking potential. A common dispersant is polyethylene

glycol alkyl ether and a range of plasticizers can be used including polyethylene glycol (liquid

form) and octyl phthalate [22]. A schematic of the standard tape casting equipment is shown in

Figure 1.4. The “slip” is the ceramic/binder slurry, and this is spread on a polymer film running through the machine. A hole in the bed of the machine, referred to as the “thickness-monitoring window” in Figure 1.4, is used to monitor and measure the thickness of the deposited film. The machine must either be long enough to allow the film to dry before being rolled and taken for further processing, or heating can be applied to the film as it travels to dry it to a point where it can be processed.

Figure 1.4. A schematic of a standard tape casting machine [10] The thickness of a tape cast ceramic sheet can be as thin as 0.01 mm to a few millimeters in

thickness, with most falling between 0.05 mm and 0.025 mm [25]. The drying time of the slurry

is an important consideration as a skin can form on the surface which prevents air pockets from

escaping through the surface [22]. 13 Slip casting is a process which uses a slurry very similar to that used in tape casting, but

the slurry is poured into a mold rather than being formed into a thin sheet. The slurry in the mold

is then left to set before the green body is removed and processed through the binder burnout and

sintering phases. This process allows complex geometries relative to only the flat sheets possible

with tape casting [18]. Slip casting has existed in some form or another since around 200 BCE,

with many common household items and dishware manufactured using this method today [26].

An additional modification that can be made to the slip casting process is to pressurize a porous

mold forcing the binder solution through the pores and leaving a layer of solid ceramic at the

surface of the mold. Any residual slurry is then poured out of the mold and the part can be

removed for drying and sintering [18].

1.1.4 – Direct Ink Writing of Ceramics

Direct Ink Writing (DIW) is a direct slurry-based 3D printing technique used to build 3D

structures layer-by-layer by depositing drops or streams of a semi-liquid paste [14][27–29]. This

method is also sometimes also referred to as robocasting, Freeze-form Extrusion Fabrication

(FEF), Ceramic Extrusion On Demand (CODE), paste extrusion, or extrusion free forming

[29][30][31][32]. The DIW process has promise as a multi-material deposition method, allowing the creation of innovative parts that integrate unique materials together to create a single part with varying properties [14][33]. Figure 1.5 shows one potential setup for the hardware that can be used in the DIW process. The primary elements are a storage area for the ink or suspension, a surface for the material to be deposited onto, a mechanism to move the nozzle in the x, y, and z directions relative to the surface being printed on, and a controllable extrusion system to release 14 the material. Additional components could include heating elements on the storage, extruder, nozzle, or print surface, and additional electronics to monitor the parameters of the printer.

Recent studies focused on DIW of BaTiO3, and other functional ceramics have used a wide variety of hardware setups to bring these elements together. Most studies acquire a prebuilt 3D printer originally intended for fused-deposition modeling (FDM) 3D printing and modify the extruder assembly to output a ceramic slurry rather than using a filament [29][32]. The use of a suspension as a feedstock means that DIW can be used to process and produce parts using a large range of materials [33]. While DIW began as an extrusion-based 3D printing process for other materials, using DIW for BaTiO3 allows the advantages of the process; namely the low cost, ease of use, and relatively high resolution, to be applied to this widely used material.

Figure 1.5. An example of a Direct Ink Writing setup [20]. Using the DIW method with ceramics requires the development of a slurry that possesses rheological and viscoelastic properties conducive to layered deposition [34]. A suitable slurry or paste for DIW has the ability to hold the weight of successive layers without losing dimensional 15 accuracy, while also flowing through a nozzle without clogging or requiring excessive force to

extrude. Figure 1.6 shows a plot of the desirable shear-thinning behavior expected from a slurry used in DIW by Wei et al. [29]. Shear-thinning behavior describes a material which decreases in apparent viscosity, as seen on the y-axis of the plot in Figure 1.6, as a shear force is applied to it, as seen along the x-axis in Figure 1.6. This rheological behavior is needed to allow the slurry to flow when under pressure in the reservoir and being extruded through the nozzle, but then become more viscous after deposition. Without shear-thinning, a slurry which flows through a nozzle appropriately may not hold its shape after deposition, while a slurry which does hold its form would not be possible to extrude.

Figure 1.6. Viscosity versus shear rate for a BaTiO3 slurry [29]. For the processing of the barium titanate slurries, there are some key requirements that must be

met in order to produce a slurry capable of being 3D printed. Properties such as solid loading,

viscosity, and particle size are all crucial to characterize to understand how they affect the

properties and printability of the slurry. Particle size and particle size distribution are two factors

which have been shown to have a significant impact on the rheology of a suspension and the

properties of the printed part [35][27]. First, the ceramic material used must be in a form that is 16 capable of being suspended in a binder solution. Micron-scale particles fit this description, and due to being readily available in the advantageous piezoelectric and ferroelectric phase, are the preferred particle size for a BaTiO3 powder for use in DIW [36]. Nanoscale ceramic particles can also be used but it has been shown that nanoscale barium titanate powders do not exhibit the same level piezoelectric and ferroelectric properties as powders of larger particle size after sintering [4]. One example of the characterization of a powder’s particle size and particle size distribution is shown in Figure 1.7 [30].

Figure 1.7. (a) Particle size distribution of the three types of powders, the SEM images of the corresponding particles with (b) D50=7.41 μm, (c) D50=3.00 μm, (d) D50=0.35 μm [29] The size of the particles is typically measured in terms of the diameter of an equivalently sized circle or sphere. The width of the particle size distribution chart indicates how uniform the 17 particle sizes are in the powder. Wide particle size distribution plots represent a less consistent

powder, while a narrow plot means that the majority of the powder particles have the same

width. Particles that are extremely non-uniform can cause damage to the printing hardware or negatively affect the print resolution [37]. However, there is evidence that a bimodal particle size distribution may be favorable in some cases, as it can enhance the piezoelectric properties of a

BaTiO3 ceramic after the part is sintered and has been shown by Renteria et al. to enhance the packing density of particles and reduce the shrinkage after sintering [27]. The effect that is seen

depends on whether the bimodal particle size distribution is dominated by a large or small

particle size. Large particle size dominant bimodal mixtures allow for better packing because the

smaller particles fill in the spaces between the larger particles. This type of bimodal particle size

distribution also enhances the piezoelectric property. A small particle size dominant bimodal

mixture will shrink less during sintering as the larger particles interspersed within the grain

matrix serve as a scaffolding for the rest of the grains to build around rather than condense into

[27]. Research on the impact that powder particle size has on the viscosity of a slurry largely

concludes that smaller powder particle sizes, less than 1 micron in diameter, give the slurry a

higher viscosity due to a stronger interparticle network than larger powder particle sizes [27,36].

Combinations of large and small BaTiO3 particles could therefore be combined to modify the

slurry viscosity without additional binders or solvents.

Although powder selection influences the suspension behavior, the binder and additives

used have a greater effect. Modifications to the slurry’s rheological properties can also be made

by adjusting the ratios of the binder, solvent, dispersant, and plasticizer [37]. These components

of a BaTiO3 slurry for DIW are used for the same purposes as in tape casting, as discussed in

section 1.1.3. The shear thinning behavior necessary in a DIW slurry primarily results from the 18 binder that is used [31]. Polyvinyl alcohol, polyvinylidene fluoride (PVDF), and polyethyleneimine (PEI) have seen the most use as polymer binders for DIW slurries, with the necessary solvent depending on the binder [32][33][35]. For polyvinyl alcohol, deionized water is an appropriate solvent while a binder like PVDF requires the use of dimethylformamide

(DMF) as a solvent. In some cases, deionized water alone can be used as the binder for the suspension, however additional steps must then be taken during the production of the slurry to ensure its printability [34]. Figure 1.8 shows a couple examples of DIW 3D printed parts simply made of a mixture of clay, a type of ceramic, and water. These parts show the viability of a simple slurry recipe; however, the resulting surfaces are rough when compared to a traditional manufacturing method for producing the same form, and the stability of the slurry is not ideal for extended use. While a simple mixture of ceramic powder and a solvent can be developed and used for BaTiO3 DIW, the printed parts would likely be limited to a low resolution and show a

large number of defects and imperfections [28].

Figure 1.8. Samples of clay fabricated by extrusion 3D printing. a) Side view of “twisted gear lamp”, b) top view of (a), c) pyramidal test object, d) 20 mm diameter cylindrical samples, e) 10 mm diameter cylindrical samples, and f) side view of “Ashtray” [25]. The properties of the slurry impact the printing parameters that must be used to achieve

an acceptable geometric tolerance [34]. High viscosity slurries are favorable for their ability to 19 maintain their shape after deposition but depositing a high viscosity slurry through a narrow

nozzle is difficult and clogging of the nozzle becomes a concern. Agglomerations within a high-

solid loading slurry can lead to poor flow in the stream of material through the nozzle and

eventually clogging. Using a larger nozzle can allow an unprintable slurry to be used at the

expense of print resolution. Larger nozzles also allow a higher flow rate of material which can be

advantageous by allowing the print speed to be increased [14]. More parts can be produced more

efficiently, but the problem of low resolution would still exist. Finding a method to balance and

optimize the flow rate, print speed, and resolution for a given BaTiO3 slurry is needed to produce

a three-dimensional part with acceptable tolerances and properties.

The also exists a need to understand what kind of imperfections can be present in a

BaTiO3 slurry printed using DIW. In the work of Hu et al., the issues with shape retention are

discussed for a kaolinite clay ceramic slurry [33]. A conical thin-walled shape was printed as a

benchmark, with the cone angle increasing from 0° up to 20° from vertical. As the angle of a

printed wall was increased, the ability of the printed slurry to hold the form decreased, likely

from the “slumping” of the printed lines and those lines falling to the side of the center-point of the line they are printed on. At and above 15° from vertical, the surface was not smooth, and the walls would tend to collapse. The parts were printed at two different layer heights, with the smaller layer height resulting in a steeper angle printable benchmark. At and above 25°, the smaller layer height prints also failed to hold their form.

1.2 Problem Statement

DIW of BaTiO3 is an ongoing research topic that has been studied extensively in recent

years [27][29][32][36]. The ultimate goal of investigating DIW for BaTiO3 is to bring the 20 manufacturing method out of the experimental phase and to the level of maturity required for

prototyping and commercial use. Optimizing DIW for commercial uses begins by defining the

parameters that affect print quality, how those parameters can be modified and adjusted, and

discovering which of these parameters has the greatest impact on the quality and precision

achievable by DIW. Using the DIW process for BaTiO3 is going to require greater knowledge on the potential imperfections that might appear specifically in a BaTiO3 slurry printed part. The objectives of this thesis are to 1) develop a series of BaTiO3 slurry recipes that can be used to

produce 3D parts which hold their form, 2) test the printing parameters available to determine

what their effects are and what imperfections they introduce when set incorrectly, and 3) use the

results of that testing to show how a printed part can be made. To do this, I will be using a

Design of Experiments (DOE) approach to investigate the parameter influence.

Chapter 1 of this paper begins by introducing BaTiO3 as a material, then the state of ceramic manufacturing for functional ceramics like BaTiO3 is explored, followed by a look into

the background and current state of 3D printing and DIW for BaTiO3. Chapter 2 begins by outlining how the BaTiO3 slurry is formulated and produced, followed by a description of the

testing setup and the parameters of interest in the setup. Then, a design of experiments is laid out that will provide the necessary results to understand the effect that each parameter has on a print.

The results of that testing are presented in Chapter 3 and used to perform a parameter impact

assessment for use in the model. Chapter 4 concludes this work by describing the findings and

takeaways from the study and pointing towards the areas which are deficient and require further

research.

21 Chapter 2 – Experimental Design

2.1 - Barium Titanate Slurry Formulation

The ceramic powder used here is a BaTiO3 powder supplied from Ferro Corp. with a particle size distribution between 0.8 µm and 2.1 µm (D50 = 1.3 µm). This ceramic has a density of 6.0 g/cc and a purity of at least 99.9% [38]. Figure 2.1 shows a set of SEM images captured of the ceramic used in this thesis. Figure 2.1(a) was taken at 15000x magnification and the scale in the bottom right is 5 µm. Figure 2.1(b) was taken at 35000x magnification and the scale shown is

2 µm.

Figure 2.1. SEM images of Ferro barium titanate powder: a) 15000x magnification. b) 35000x magnification The dispersant used in the slurries here is Lubrizol Solsperse™ 20000; an “active polymeric dispersant”. A plasticizer is an additive which increases the elasticity or decrease the viscosity of the binder solution. In this case, the plasticizer works to allow for a smoother flow of the slurry when extruding from a nozzle. The plasticizer used in this recipe is polyethylene glycol with a molecular weight of 400 g/mol (PEG-400). 22 2.1.1 – Procedure for Slurry Production

The process for creating each sample follows a procedure outlined in the work of Wei et

al. [29]. Figure 2.2 shows a flowchart as a reference for the processing procedure. First,

polyvinyl alcohol (PVA) is dissolved in deionized water to create the binder and solvent base

solution. The water is placed in a beaker and submerged in a silicon oil bath over a hotplate. A

thermocouple is placed in the oil close to the beaker of water to monitor the temperature and

adjust the regulate the hotplate temperature. The system is maintained at 90°C while PVA

granules are added to the water in small batches and stirred at 500 rpm. After approximately 2

hours of stirring, the PVA should be completely dissolved, and the solution will have increased

in viscosity. From there, a measured amount of the PVA solution is moved to another beaker and

the PEG-400 and Solsperse 20000 are added. The solution is stirred to homogeneity between the

additions. The whole solution is then poured into a mortar and the BaTiO3 powder is measured out. Finally, the BaTiO3 powder is added slowly and combined thoroughly with the binder

solution in the mortar with a pestle.

Figure 2.2. Processing of BaTiO3 slurry for extrusion

23 2.1.1 – Manual Printing 3D Parts Using Prepared Slurries

After making each slurry, a subjective test of the slurry’s printability was conducted to assess whether the slurry would be able to be printed using a syringe-based 3D printer setup. The test consisted of taking the prepared slurry, loading it into a syringe, and then extruding the slurry by hand onto a dish. The printability was assessed by evaluating how much effort it took to extrude the slurry through the 2mm nozzle of a 5ml syringe and a 100ml syringe, and then by looking at the extruded slurry and recording observations on the slurry’s viscosity, layering ability, and form holding ability. The two different sizes of syringes were selected due to their differing internal diameters of 12.07mm and 34.9mm for the 5ml and 100ml syringes, respectively. The larger the internal diameter, the less pressure is applied within the syringe for a given force applied on the plunger since the force is spread over a greater area. With a smaller and larger syringe tested, I could determine the relative ease with which a particular slurry could be extruded. When a syringe with the slurry is placed into an extruder that applied a set force, the internal pressure forcing the slurry out of the nozzle can be adjusted by changing the syringe into which the slurry is loaded and printed with. The four slurries tested in this way were those with a

BaTiO3 solid loading of 65wt%, 70wt%, 75wt%, and 80wt%.

Two samples of the hand-printed slurries were then left to dry over the course of a week

in a fume hood. Once dried, those samples were moved to an oven for binder burnout and

sintering. Plots of the rates and temperatures at which binder burnout and sintering were

completed can be seen in Figure 2.3 and Figure 2.4, respectively. The rates and temperatures

used for binder burnout and sintering were adopted from Wei et. al [29]. More detailed

information about the ramp rates and temperatures can be found in Appendix B. 24

Figure 2.3. Binder-burnout temperature profile

Figure 2.4. Sintering temperature profile 25 2.1.2 - Variation of the Solid Loading

One of the factors which impact print quality and printability is the amount of ceramic

solid in the slurry, or “solid loading” of the slurry [36]. While any amount of solid which produces a printable slurry may be used, it is advantageous to use the highest solid loading which achieves the task in order to minimize the opportunity for defects during the drying, binder burnout, and sintering process. When a greater amount of organic binder and water is present and must be removed during the binder burnout phase, defects have an increased opportunity to form in the sintered part [23]. As such, finding the highest amount of solid that may be added to the slurry as a percentage by weight of the total slurry mass is needed. To start, the recipe in Wei et al. was followed and a slurry was produced with a ~75 wt% of barium titanate ceramic powder

[29]. The exact amounts of each ingredient in this recipe are shown in Table 2.1.

Table 2.1. Initial recipe from Wei et al. [29]

Recipe 1 Solvent Binder Dispersant Plasticizer BTO Polyethylene Polyvinyl Alcohol Solsperse Glycol Material Deionized Water (PVA) 20000 (PEG-400) BTO Weight (g) 2 0.35 0.02 0.2 7.5 Weight% 19.861 3.476 0.199 1.986 74.479

Then a series of slurries at 65wt%, 70wt%, and 80wt% of BaTiO3 were produced to evaluate whether they could be extruded. The recipes for these slurries are shown in the tables shown in

Table 2.2. Proportions of BaTiO3 were determined based on a total produced mass of 100g for

simplicity, while the amount of PVA, PEG-400, Solsperse 20000, and H2O were set as an

equivalent proportion of the amount of BaTiO3 as in the 75 wt% recipe from Wei et al. 26 Table 2.2. Additional BaTiO3 slurry ceramic wt% variations tested for extrudability

65 wt % 70 wt % Material Wt (g) Wt% Material Wt (g) Wt% H2O 31.5185 31.52% H2O 23.7960 23.80% PVA 0.0252 0.03% PVA 4.1497 4.15% Sols. 0.3142 0.31% Sols. 0.1867 0.19% PEG 3.1417 3.14% PEG 1.8667 1.87% BTO 65.0000 65.00% BTO 70.0000 70.00% total weight 100.00 100.0000% total weight (g) 100.00 100.0000% b) a)

80 wt % Material Wt (g) Wt% H2O 15.0320 15.03% PVA 2.6214 2.62% Sols. 0.2133 0.21% PEG 2.1333 2.13% BTO 80.0000 80.00% total weight 100.00 100.0000% c)

2.2 – DIW Printer Setup

To evaluate the printability of the slurries produced, a DIW 3D printer setup had to be used. The fundamentals of a 3D printer used to print a ceramic slurry are the same as those of mainstream FDM (Fused Deposition Modeling) and therefore an off-the-shelf plastic FDM printer was used and modified. The printer used as the basis for the hardware setup is a Creality

Ender 5 Pro. This printer was selected for its square, rigid frame and for the way it is set up such that the print surface only moves in the up/down Z-direction, while the print head moves in the horizontal X and Y directions. If the print surface were to move in the X-Y direction, then the dimensions of the printed slurry may be affected by accelerations in those directions. The printer 27 only changes height of the Z position once every layer, which means that the deposited material is stationary for the duration of the layer it is printed on and only moves one time very slowly when the next layer is to be printed. This XYZ movement setup eliminates the need to compensate for any variability introduced due to accelerations in the X or Y directions.

The modifications to the printer focused on the extruder and the print head. The necessary features for ceramic slurry DIW are 1) a reservoir to contain the slurry, 2) a print head with interchangeable nozzles of varying sizes, 3) a method to move the slurry to the nozzle, and

4) movement in three-dimensional space. Figure 2.5 shows a model of the setup used in this work.

Figure 2.5. Model of the control and extrusion setup attached to the Ender 5 Pro 3D printer The reservoir can be as trivial as a plastic syringe that contains the slurry, which is what was used here. The syringes used in the setup are 20mL plastic syringes with a rubber plunger. The syringes are filled with the slurry immediately after the slurry is produced and then sealed until 28 ready to print. The nozzles used in this setup are the same as those used in an FDM printer and

varied from 0.2-2.0 mm in diameter. An example of this type of nozzle is shown in Figure 2.6.

Figure 2.6. Hardened stainless steel 3D printer nozzle (dimensions in mm) The hardened stainless-steel nozzles have male M6 threading on the back end and that was used

as the attachment point to the end of the syringe using a Luer-Lock to M6 female thread adapter.

The nozzles are then easily interchangeable to a wide variety of nozzle sizes for testing. The extrusion mechanism for pushing the plunger of the syringe consists of four main parts. A

NEMA 17 stepper motor with internal T8 threading is attached to the tail end of the plunger and the motor rides on a threaded rod. When the motor turns in one direction, the plunger is depressed, and the plunger can be retracted by reversing the motor. The motor is driven using a stepper motor driver and an Arduino that tells the driver the timing between to use between the steps of the motor. A longer delay between each step of the motor, measured in milliseconds, decreases the rate at which the plunger is depressed and therefore the flow rate of material through the nozzle. Finally, all the movement of the print head is accomplished using the exact same setup as the stock Ender 5 Pro shown in Figure 2.5. With this hardware setup, there are a range of parameters than can be optimized to produce a 3D printed ceramic part. The parameter 29 most important to realizing DIW technology in a small or medium scale production setup is the print speed. A faster print speed means more parts can be produced in less time. The downside of a faster print speed is that the print quality and dimensional accuracy may be reduced due to under-extrusion for a given print speed, a slurry that cannot be extruded at a fast enough rate to match the print speed, or other inconsistencies that may be introduced from going too fast. To optimize print speed, other parameters must be accounted for and quantified. The parameters tested here in addition to the movement speed of the print head are the gap height between the tip of the nozzle and the build surface, the flow rate of material through the nozzle, the size of the nozzle, and the solid loading of slurry used. Each parameter was given a range based on initial testing. For the print speed, a range between 10mm/s and 30mm/s was used. The gap height was set between 1mm and 4mm. The flow rate was set using the length of the delay between the steps of the extruder stepper motor and a delay between 1000ms and 4000ms was used. The nozzle sizes ranged between 0.5mm and 2mm. A range of slurry solid loadings between 65wt% and 75 wt% were used. A summary of these parameter levels can be seen in Table 2.3.

Table 2.3. Parameters and levels for min-max testing

Print Speed Gap Height (mm/s) (mm) 1 30 1 4 5 10 5 1

Slurry Flow Rate (wt%) (µs delay) 1 75 1 4000 5 65 5 1000

Nozzle Size (mm) 1 0.5 5 2

30 2.3 – Testing Procedure for Parameter Assessment

2.3.1 – Design of Experiments Setup

An experiment to test the parameters outlined in section 2.2 was designed by gathering the parameters and creating test matrices for two phases of testing. The first phase focused on the minimum and maximum values for each parameter and tested combinations of those levels for each parameter. A variation of Latin hypercube sampling was used to create the test matrix. First, all 2k test combinations were created, with k equaling the number of parameters being tested. In this case there are 5 parameters. Then the full matrix was divided into 4 equal parts and a random sample from each part was selected to create a half-size test matrix shown in Table 2.4.

Table 2.4. Reduced Min-Max test cases run for phase 1 of testing

Min-max test runs Run # Nozzle Width Slurry wt% Print Speed Gap Height Flow Rate 1 1 1 5 5 5 2 1 1 1 5 1 3 1 1 5 1 5 4 1 1 1 1 5 5 1 5 1 1 1 6 1 5 5 1 5 7 1 5 5 5 1 8 1 5 1 1 5 9 5 5 1 1 5 10 5 5 5 1 5 11 5 5 1 5 5 12 5 5 5 1 1 13 5 1 1 1 5 14 5 1 5 1 5 15 5 1 1 5 1 16 5 1 1 5 5 31

The full test matrix with the selections made for the reduced testing can be seen in

Appendix A. This methodology ensured that each parameter minimum and maximum level was

included in the testing without having to test all 2k test cases. For the second phase of testing, the

parameter influence assessment results from phase one are used to dial-in the correct parameters

that need to be used to create a 3D printed part with respectable dimensional accuracy while also

being printed with a relatively high print speed. The goal of the second phase is to demonstrate

how the influence of each parameter can be used as a guide as to which parameters needs

modification to correct inaccuracies or to fix visible flaws in the printed lines of the part. The

shape to be printed using this method is a 2cm x 2cm x 2cm cube.

2.3.2 – 3D Model and Slicing

The model used in the printability testing is a long thin rod. The length of the rod is set to

50mm, while the width and height of the rod are adjusted based on the nozzle being used. The width and height of the rod must be adjusted so that the software does not attempt to print a plane rather than a single line. A render of the model that is being used is shown in Figure 2.7(a).

A 3D model of the rod is run through the Prusa Slic3r slicing software. This software takes a 3D

.stl file and converts it into a G-code file. G-code is a numerical control programming language that the printer software reads to send instructions to the three movement axes of the printer. The line that the slicing software creates and then turns into G-code can be seen in Figure 2.7(b). A variety of parameters can be adjusted in the software, such as print speed, layer height, the inclusion of supports, infill, and many others, but the only parameters relevant to this testing are 32 the print speed and the nozzle diameter that the slicer is accounting for.

Figure 2.7. a) 3D model of the shape that is used for print testing. b) showing the line that the 3D printer will make when executing the G-code sent to it

2.3.3 – Setting Parameters

The procedure for testing starts by setting all the parameters according to the test case being run. Test cases were run in batches determined by the most recent slurry produced to print the slurry as close to its production as possible. Letting the slurry sit for an extended period would lead to evaporation of the water in the slurry and most likely slight agglomeration of the ceramic particles. The gap height is set manually by first bringing the tip of the nozzle down to the build plate and then moving the build plate down in the Z-axis through the printer’s software

controls. This is then set as the Z-axis offset for the printer to use on the first layer. The print

speed is set when the model is sliced, and the correct file for testing is loaded onto the printer.

The syringe is loaded with the correct weight percent slurry for the test case and the appropriate

nozzle is attached to the syringe. The Arduino code is then updated to reflect the desired step

delay for the flow rate needed in the test case. To start the print, all the axes are brought to their zero positions and the extruder motor is activated. Then the test can be prompted to begin using the 3D printer’s interface. 33 2.4 – Data Collection and Analysis

After each test case is run, images of the printed sample are taken from above as seen in

Figure 2.8. Each image contains a scale that can be referenced within software to measure the dimensions of the printed line and compare that to the theoretical or desired dimensions. This image shows primarily the width of the printed line as seen from above. The width of each line is measured at 4 points along the line in the open-source software ImageJ and averaged. If the line width average is greater than the predicted line width, then that test case is determined to have over-extruded. If under-extrusion is measured, then data is also collected regarding how much of the intended line was filled during the printing test. This can be seen in Figure 2.8 by the rectangle overlaying the printed line. In a case of under-extrusion, the image would be cropped to the size of this rectangle and a histogram of the pixel color values would be extracted using

ImageJ. A ratio of number of pixels with values corresponding to the white slurry versus the number of pixels with values corresponding to the background can be used to evaluate the percentage of the total expected print area that was printed.

Figure 2.8. Example of an image captured for each test case and used for data analysis Under-extrusion and over-extrusion occur when the flow rate, print speed, and nozzle size do not match up. In cases where the flow of material is too low, the printed line will appear either too thin or missing along the length of the print. When the flow rate of material is too 34 great, a line that is wider than expected or layers of material that seem to overlap each other in a

single location would be seen. There may also be other flaws in the printed line that, when combined with the line width data, can indicate which parameter caused the imperfection. These

will be explored further as they are discovered in the results. 35

Chapter 3 – Results

3.1 – Results of Slurry Processing

Table 3.1 shows the results of the preliminary testing performed to evaluate the initial printability and viscosity of slurries at 65wt%, 70wt%, 75wt%, and 80wt%. A representative image of each slurry is shown. Each slurry was produced and then tested by extruding through a

2mm nozzle of a 5ml- and a 100ml- syringe. The 65wt%, 70wt%, and 75wt% slurries could all be extruded through the 5ml syringe, while only the 65wt% and 70wt% slurries could be extruded using a 100ml syringe. The 80wt% slurry could not be extruded through either the 5ml or 100ml syringe. To describe the viscosity of each slurry qualitatively, an analogy to a typical household paste is made to give a better idea of how the slurry behaves. The 65wt% slurry

“slumps” more relative to the other slurries and shows more of a stringing effect. “Slumping”

here is defined as the printed lines spreading out to be wider than they are tall or filling in

between the rows of the preceding layer. The effect that this slurry shows can be described as

analogous to tacky glue. The 70wt% slurry holds its form better than the 65wt% as seen by the

slightly taller individual lines within the “printed” part in Table 3.1. There is less slumping

overall, and the form holds itself well enough for continued stacking of layers. The consistency

of the 70wt% slurry is analogous to toothpaste. The 75wt% slurry showed the best form holding

ability of all the slurries tested in this part of the study. Since the 75wt% slurry could not be

extruded using a 100ml syringe, all the tests were completed using a 5ml syringe. The lines

printed using the 75wt% slurry could support their own form and the weight of another layer

above them well enough to create single-line thick-walled parts as seen in the representative 36 image for the 75wt% slurry. The viscosity of this slurry is analogous to a silicone sealant.

Finally, the 80wt% slurry could not be extruded from either a 5ml or 100ml syringe. The viscosity of the slurry was more similar to a dry playdough than a paste, and the slurry would hold its form very well even if directly touched. Immense pressures would be required to print this slurry using a syringe setup, and multiple syringe plungers buckled when trying to apply that pressure by hand. The resulting extrusion is inconsistent and came out of the nozzle as “nuggets” rather than full lines.

Table 3.1. Results of preliminary testing on the printability and visual observations on viscosity of four BaTiO3 slurries with different solid loadings

BaTiO3 Slurry Printability Description Representative Image Viscosity content Composition Analogy (wt%) (ref. Chapter 2 (Subjective) for composition) 65 Table 2.2a Very runny in comparison to “Tacky glue” the 75 wt% slurry, but still holds its shape somewhat. Does not layer very well past 2- 3 layers. Printable using 100ml & 5ml syringe

70 Table 2.2b Stackable up to 2-3 layers “Toothpaste” without a problem. Supports slurry weight similar to 75wt%. Printable using 100ml and 5ml syringe. Somewhat difficult to extrude from 100ml syringe.

37 75 Table 2.1 Unprintable in 100ml “silicone syringe. Printable in 5ml sealant” syringe. Stable layers past 4-5 layers. Fairly easy to extrude from 5ml syringe. Shapes formed hold shape well

80 Table 2.2c Unprintable with 5ml syringe. Dry Playdough Plunger broke due to force required. Only “nuggets” of slurry were extruded before failure. Very dry relative to other slurries

The results of the preliminary slurry printability and viscosity testing pointed to the

75wt% slurry as being the best candidate for producing a series of printed parts which can hold their form and reach the binder burnout and sintering phase. Using a 5ml syringe, the 75wt% was hand-printed into the forms shown in Figure 3.1. The main shapes printed here are rectangular prisms and cubes, single-wall thick cylinders with a 1-layer thick base, solid cylinders, and larger rectangular shapes with holes in the center. These tests were conducted to demonstrate the printability of the 75wt% slurry and the form holding ability of the printed lines. After printing, these shapes were left to dry under a fume hood and two of the printed parts were taken for binder burnout and sintering. 38

Figure 3.1. 75wt% parts printed by hand using a 5ml syringe with a 2mm diameter nozzle Figure 3.2 shows two of the parts from Figure 3.1 in greater detail. Figure 3.2(a) shows how the lines of the 75wt% slurry do not fully fuse together when they are touching, and do not spread out. Rather, the lines maintain the form of the nozzle with which they were printed and stay in the place that they are deposited. The printed lines of the part in Figure 3.2(b) show a pockmarked outer surface caused by air bubbles that became trapped within the slurry after it was loaded into the syringe. These voids are likely present within the printed lines as well which may be an obstacle to achieving full densification if the part were to be sintered. 39 (a) (b)

Figure 3.2. Close-up images of two of the printed parts shown in Figure 16. Figure 3.3 shows a shape printed using the 70wt% slurry. This sample was created as a part to be compared to the 75wt% slurry printed parts during the binder burnout and sintering step. The sample in Figure 3.3 demonstrates inferior form holding compared to the 75wt% slurry parts and the lines have a greater tendency to spread out and fuse with neighboring lines.

Figure 3.3. Rectangular 3D form printed using a 5ml syringe, 2mm diameter nozzle, and 70wt% slurry In Figure 3.4, the parts selected for binder burnout and sintering are shown. These parts have been imaged after they have fully dried at room temperature for about 2 weeks, but before any heating has been applied to the parts. 40

75wt% BTO

70wt% BTO

Figure 3.4. Dried parts prepared for binder burnout and sintering Figure 3.5 shows the state of the parts after the binder burnout step in Figure 3.5(a) and

after sintering in Figure 3.5(b). After binder burnout the parts are almost entirely made up of

ceramic since all the water, binder material, and organic additives have been removed by

heating. During or after the binder burnout step, the 70wt% developed a crack along the bottom

of the part as seen in Figure 3.5(c). During sintering, this crack resulted in the part splitting into two separate pieces. 41 (a) (b)

(c)

Figure 3.5. 75wt% (in blue boxes) and 70wt% parts (a) after binder burnout and (b) after sintering. (c) Crack formation in the 70wt% sample after the binder burnout step. After sintering, the density for each part was measured and recorded in Table 3.2. Since they parts have irregular shapes, the Archimedes Principle was used to calculate the density.

There are two measured densities for the 70wt% part because both halves of the broken part were measured separately.

Table 3.2. Measured density of the sintered parts shown in Figure 3.5

75wt% BTO 70wt% BTO

S1 S2 S1 S2

Density (g/cc) 5.44 5.51 5.58 5.39

Relative density (%) 90.4 91.6 92.7 89.6

Theoretical density (g/cc): 6.02 42 3.2 – Printing Parameter Analysis Results

The results of the print parameter analysis were completed using the 3D printer hardware setup described in Chapter 2, rather than manual printing. The results are in two parts. First, a parameter impact assessment based on a comparison between the average measured width of a printed line and the expected width is presented through the plots in Figure 3.6(a – e).

(a) (b)

(c) (d)

(e)

Figure 3.6. Parameter impact assessment plots. (a) Slurry wt% impact plot (b) Print speed impact plot (c) Nozzle size impact plot (d) Stepper motor step delay (flowrate) impact plot (e) Gap height impact plot These plots are a line plotted between the lowest and highest level of each parameter’s overall average line width percent difference from the expected line width. The magnitude of the slope 43 of each line gives a relative indication of that parameter’s effect on the line width or accuracy of

a 3D printed line.

From the plots in Figure 3.6, we can see that the greatest impact on a printed line’s accuracy comes from nozzle size used and the solid loading level of the slurry. The flow rate also

has a significant impact, but not to the same extent as nozzle size and solid loading level. The

gap height and print speed have a minimal effect when compared to the effect of the nozzle size,

slurry solid loading, and flow rate.

The second part of the data analyzed in the parameter impact assessment deals with types

of imperfections seen in each test case. Every test case resulted in lines that were greater in width

than expected. However, the resulting line did not have the same shape in every run. The lines

exhibited additional imperfections that may help to indicate what the parameter that needs

adjusting is. Those imperfections are characterized in Table 3.3. Table 3.3 shows one example

test case of each of the printing imperfections seen during this testing.

Table 3.3. Printing imperfections observed during the parameter impact assessment testing Test Parameters Representative Image Imperfection Type and Features 2mm Nozzle Diameter Banding – Slurry 75wt% Slurry layering on itself as it 10mm/s Print Speed extrudes 1mm Gap Height 1000µs Stepper Motor Delay

44 2mm Nozzle Diameter Smearing – Nozzle 65wt% Slurry touches material as it is 30mm/s Print Speed extruded and pushes it 1mm Gap Height along the build path 1000µs Stepper Motor Delay

0.5mm Nozzle Diameter Inconsistent Extrusion 65wt% Slurry – Discontinuities along 10mm/s Print Speed the length of the line or 1mm Gap Height an incomplete line. 1000µs Stepper Motor Delay May result in under- extrusion as well

2mm Nozzle Diameter Tunneling – Nozzle is 75wt% Slurry touching the slurry just 10mm/s Print Speed enough to push material 1mm Gap Height to either side as it 1000µs Stepper Motor Delay progresses

0.5mm Nozzle Diameter Oscillation – The line 75wt% Slurry oscillates from side to 30mm/s Print Speed side in mid-air before 4mm Gap Height laying on the print 1000µs Stepper Motor Delay surface rather than extruding directly onto the surface in a straight line

3.3 – DIW of a 3D Part

To bring together all the data collected over the course of this study, it was decided to print a 2cm x 2cm x 2cm cube using the smallest diameter nozzle tested. The starting point for the printing parameters was chosen by looking for the case from the parameter assessment that gave the highest print accuracy. These parameters were determined to be a 0.5mm nozzle diameter, 75wt% slurry, 30mm/s print speed, 4mm gap height, and 1000µs stepper motor delay for the flow rate. The test case provided the most accurate line width, however that line would 45 oscillate from side to side rather than printing directly. The slurry for this test case was determined to be the most favorable for form holding in the preliminary printability testing, as described in Section 3.1. For that reason, the slurry solid loading parameter was held constant for the test. The nozzle diameter of 0.5mm was also held constant as the goal for the test. The first parameter that was changed was the flow rate, with the stepper motor delay set to 4000µs rather than 1000µs. This change did not result in too much of a difference to the oscillation problem, so the next parameter that was changed was the gap height. The gap height was reduced from 4mm to 1mm, and the oscillation went away. After the oscillation issue was solved, the next issue became that the extruded line was wider than the nozzle indicating that there was still a flow rate problem occurring. To decrease the flow rate, the stepper motor delay was increased in 4000µs increments until the printed line was measured to be the same width as the 0.5mm nozzle. This resulted in a stepper motor micro-step delay of 16000µs. The print was started and recorded, with the images in Figure 3.7 being taken during the print. Each layer of the printed cube is a series of 3 perimeters, with the center being filled with diagonal parallel lines offset by 90° from the preceding layer in a crisscrossing lattice pattern.

46

(a) (b)

Figure 3.7. DIW of a 2cm x 2cm x 2cm cube using printing parameters influenced by the parameter impact assessment results Figure 3.8 shows the final printed form. A little less than halfway through the print, the nozzle began to collide with the printed slurry, so the print was stopped in order to salvage the overall shape. Stopping the print means that the height dimension of the printed part cannot be compared to the expected height of 2cm. However, the width and length of the print can be measure from the overhead image shown in Figure 3.8(b). The average length and width measured along multiple points in those dimensions is shown in an overlay on Figure 3.8(b).

From those values, a percent deviation from the expected 2cm width and height can be calculated. The width average of 1.854cm is 7.3% less than expected and the length average of

1.904cm is 4.8% less than expected.

47

(a) (b)

Figure 3.8. (a) Side profile of the printed cube (b) Top-down profile of the printed cube

48 Chapter 4 – Conclusions

4.1 – Summary of Significant Results

In the slurry recipe tests, it was discovered that an 80wt% slurry could not be extruded from a nozzle when using a 5ml syringe or a 100ml syringe. This issue means that an 80wt% slurry made using the recipe described in Chapter 2 does not have the necessary rheological properties to be used in a DIW setup. The slurry with the highest solid loading demonstrated to be printable in this thesis is the 75wt% slurry, which is consistent with previous studies of

BaTiO3 DIW. Additionally, the 65wt% and 70wt% slurries were printable in the slurry recipe

testing but did not exhibit the same form-holding properties as the 75wt% slurry.

After binder burnout and sintering, the parts printed with the 70wt% and 75wt% BaTiO3 slurry showed very similar final relative densities of between 89.6% and 92.7% of the theoretical density of BaTiO3. The average density of the two 70wt% slurry parts was 91.2% and the

average density of the two 75wt% slurry parts was 91.0%.

The results of the parameter impact assessment indicate that the nozzle size and slurry

solid loading parameter levels should be the first parameters to be adjusted if a bad print is

produced. If those levels must be set as a constant for any reason, or if adjustments to those

parameters have already been made within a certain bound, then the next adjustment should be

made to the slurry flow rate. After that parameter is set, the gap height and print speed parameter

levels should be modified.

While every test case in the parameter impact assessment testing showed evidence of line

width inaccuracy and printing imperfections due to a flow rate that was too high, the different

imperfections visible in the printed lines can indicate which additional parameters would also

need to be adjusted to achieve a solid line with correct dimensions. Banding, smearing, 49 tunneling, oscillation, and inconsistent extrusion were all seen in the tests performed here. The

test cases exhibiting oscillation can be explained by looking at the gap height and print speed. If

the print speed were increased, the stream of slurry would most likely not have time to make the

sideways motions that resulted in the oscillation feature but would instead lay on the print bed

shortly after leaving the nozzle. A decrease in the gap height may also help in fixing the

oscillation, but if the flow rate of material is still too high for the print speed being used, then

banding or some other imperfection may appear. Tunneling is most likely caused by a combination of a fast flow rate and a small gap height. At small gap heights, the nozzle is more likely to be touching the printed slurry, especially if the flow rate is too high. Increasing the gap heigh can solve this issue, but risks introducing the oscillation feature. Inconsistent flow may have a variety of causes. One potential cause is that there is clogging occurring in the nozzle.

The solid particles in the slurry may agglomerate within the nozzle over time causing the printed

line to be inconsistent or stop entirely. Clearing the clogged nozzle can be a temporary fix, but

the issue is then likely to reoccur if all parameters are left the same. Inconsistencies in this

hardware setup were also present due to a poor mounting solution for the extruder assembly and

a stepper motor that may not have had adequate power to perform the task of extruding slurry, resulting in inconsistent extrusion. The test cases exhibiting banding as the imperfection are potentially the simplest to fix. Banding appears as periodic increases in the width of the printed line and is the result of a flow rate that is too high for the print speed being used. Increasing the print speed can be an easy solution. Changing the gap height either higher or lower could introduce oscillations or tunneling, respectively, so it is likely most effective to alter the print speed first. 50 The final part printed with this DIW setup was a 2cm x 2cm x 2cm cube. Using the

results of the parameter impact assessment testing, the printed cube had a width and length of

7.3% and 4.8% less than expected, respectively. The parameter impact assessment results were

used to gradually dial-in a level for each parameter which would result in a successful print,

starting with the parameters which would have the greatest impact. An improperly set flow rate

most likely contributed to the nozzle impacting the printed slurry about halfway through the print

and interrupting its completion.

4.2 – Conclusions

The inability to manually extrude the 80wt% slurry in the slurry processing portion of the test results is an obstacle that could be overcome with an extrusion setup that can apply sufficient pressure on the slurry to force it through the nozzle. Such a setup could make the 80wt% or even higher solid loading slurries viable. However, for the manual printing experiment and using the hardware setup described in section 2.2, this slurry is not viable.

After binder burnout and sintering of the 70wt% and 75wt% samples according to the temperature profiles in section 2.1.1, the calculated density values were similar enough to conclude that the change in BaTiO3 content in the slurries did not have the expected outcome of

increasing the density in the sintered part. What is known, however, is that in parts prepared

using a slurry with a lower solid content, there is a greater risk of imperfections and cracks due

to the greater amount of binder, solvent, and additives that must be removed from the material

during the binder burnout step. This effect was seen firsthand in the way that the part prepared

with the 70wt% slurry developed a crack as it was drying at room temperature leading to fracture 51 during binder burnout. It is therefore still appropriate to use a slurry with higher solid content when possible.

Another interpretation of the printing parameter assessment results can be that the levels for some of the parameters tested here did not extend into the ranges that would make an impact.

For example, changing the print speed from 10mm/s to 30mm/s may not have been a large enough increase to expose the impact that the print speed has on the print quality. Conversely, the difference between the minimum and maximum levels of the nozzle size and slurry solid loading parameters may have extended too far, exaggerating the impact that these parameters have on print quality. Without further testing, which of these conclusions is the true state of the system is not clear. The flow rate parameter as tested here almost certainly did not get tested at a broad enough range. To achieve a consistent printed line that did not show initial evidence of an improper flow rate or a printing imperfection, the delay between steps had to be increased to four times the original parameter’s set maximum level. However, even with that limitation, the part that was produced showed good dimensional accuracy in two dimensions, with the height being negatively impacted by the print getting stopped early. It became necessary to stop the print once the nozzle began to move slurry that had already been deposited. The reason that this occurred is suspected to be either a gap height that was set too low, or a flow rate greater than required that was not apparent until enough material had been layered to fill the available space between the printed slurry and the nozzle – also known as the gap height. With additional testing, the flow rate could have been decreased further and a full run of the model could have been completed.

Another solution could be to simply increase the gap height, allowing for a slightly greater flow rate than required, while allowing the print to finish without the nozzle touching the printed 52 slurry. The issue with this approach is that there is still too much material being extruded, and

the overall height of the part would not be accurate to the design.

4.3 – Suggested Future Research

One major step towards furthering the research completed in this thesis is to expand the

parameter levels and continue the printability testing. A test plan utilizing five or more levels for

each parameter would provide a more comprehensive picture of the test space and the impact

that each parameter has on a successful DIW produced 3D part. A broader range of tested

parameter levels would also provide a foundation to develop a comprehensive optimization

strategy when imperfections in the printed lines appear or dimensional inaccuracy is present.

Reevaluating the hardware setup as used here would also be an important step towards

proving the commercial viability of DIW for BaTiO3. The primary components of a DIW setup are all present, however the extruder setup was not stable enough to maintain constant pressure on the syringe or operate for an extended period without overheating. A more robust extruder setup is required to ensure consistency between test runs, which was unfortunately not seen here.

The variance between tests led to repeated testing of some test cases until two or more trials showed similar results. The ability of the stepper motor to handle a long period of continuous maximum torque operation became a major hurdle in this thesis, so reliability must be considered as well.

53

Appendix A

Full Test Matrix for Min-Max Test Plan

Table A 1. Full test matrix of min-max test cases. Runs marked with an asterisk in the far-right column are those selected for physical testing

Min-max test runs Run # Nozzle Width Slurry wt% Print Speed Gap Height Flow Rate 1 1 1 1 1 1 2 1 1 4 4 4 *

3 1 1 4 1 1 4 1 1 1 4 4 5 1 1 1 4 1 * 6 1 1 4 1 4 *

7 1 1 4 4 1 8 1 1 1 1 4 * 9 1 3 1 1 1 *

10 1 3 4 4 4 11 1 3 4 1 1 12 1 3 1 4 4 13 1 3 1 4 1 14 1 3 4 1 4 * 15 1 3 4 4 1 * 16 1 3 1 1 4 * 17 8 3 1 1 4 *

18 8 3 4 4 1 19 8 3 4 1 4 *

20 8 3 1 4 1 21 8 3 1 4 4 * 54 22 8 3 4 1 1 *

23 8 3 4 4 4 24 8 3 1 1 1 25 8 1 1 1 4 *

26 8 1 4 4 1 27 8 1 4 1 4 * 28 8 1 1 4 1 * 29 8 1 1 4 4 *

30 8 1 4 1 1 31 8 1 4 4 4 32 8 1 1 1 1

55 Appendix B

Binder Burnout and Sintering Ramp and Temperature Profiles

Table B 1. Binder burnout temperature and ramp profile

Segment Type Setpoint (°C) Rate (°C/min) Time (min) Cumulative Time (min) Ramp 500.000 2.500 194.800 194.800 Dwell 500.000 0.000 120.000 314.800 Ramp 600.000 2.500 40.000 354.800 Dwell 600.000 0.000 120.000 474.800 Ramp 400.000 2.500 80.000 554.800 Ramp 200.000 1.499 133.400 688.200 Ramp 20.000 1.000 180.000 868.200

Table B 2. Sintering temperature and ramp profile

Segment Type Setpoint (°C) Rate (°C/min) Time (min) Cumulative Time (min) Ramp 1300.000 2.500 505.200 505.200 Dwell 1300.000 0.000 300.000 805.200 Ramp 1200.000 7.463 13.400 818.600 Dwell 1200.000 0.000 300.000 1118.600 Ramp 1100.000 4.000 25.000 1143.600 Dwell 1100.000 0.000 300.000 1443.600 Ramp 1000.000 4.000 25.000 1468.600 Dwell 1000.000 0.000 300.000 1768.600 Ramp 750.000 4.000 62.500 1831.100 Ramp 400.000 2.500 140.000 1971.100 Ramp 200.000 1.499 133.400 2104.500 Ramp 20.000 1.000 180.000 2284.500

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