1. Bibliographic Information

1.1. Title

The Effect of $\mathrm{TiO_2}$ Addition on Low-temperature Sintering Behaviors in a $\mathrm{SnO_2}$-CoO-CuO System

1.2. Authors

• Jae-Sang Lee
• Kyung-Sik Oh
• Yeong-Kyeun Paek (Corresponding Author, Andong National University, South Korea)

1.3. Journal/Conference

Publication Venue: This paper is formatted as a research article, likely published in a materials science or ceramics journal (e.g., Journal of the Korean Ceramic Society, based on the reference style and authors' history). Status: Published.

1.4. Publication Year

2024 (Published online: April 30, 2024)

1.5. Abstract

This study addresses the challenge of densifying pure Tin Dioxide ($\mathrm{SnO_2}$), which typically requires high sintering temperatures (approx. $1300^\circ\mathrm{C}$) to achieve a usable porous microstructure (porosity $> 40\%$). The researchers propose a low-temperature sintering approach ($950^\circ\mathrm{C}$) by introducing Titanium Dioxide ($\mathrm{TiO_2}$) into a $\mathrm{SnO_2}$-CoO-CuO system. The study synthesized samples with the composition $\mathrm{Sn}_{1-x}\mathrm{Ti}_{x}\mathrm{O}_{2}$-$\mathrm{CoO}(0.3\mathrm{wt}\%)$-$\mathrm{CuO}(2\mathrm{wt}\%)$ (where $x \le 0.04$). The results demonstrated that the addition of $\mathrm{TiO_2}$ improved densification even at lower temperatures, with grain-boundary diffusion identified as the dominant mass transport mechanism.

1.6. Original Source Link

/files/papers/69424d3c30b8b1927efc8a3f/paper.pdf (Official Publication)

2. Executive Summary

2.1. Background & Motivation

The Core Problem: $\mathrm{SnO_2}$ (Tin Dioxide) is a critical material for electronic devices like gas sensors. For these applications, a porous microstructure is essential to allow gas to penetrate the material. However, pure $\mathrm{SnO_2}$ is notoriously difficult to sinter (densify) due to its dominant non-densifying diffusion mechanisms (evaporation-condensation). Current Challenges: To achieve the necessary structural integrity and porosity ($>40\%$) with pure $\mathrm{SnO_2}$, extremely high sintering temperatures ($~1300^\circ\mathrm{C}$) are required. This high-temperature processing is energy-intensive and costly. The Innovation: The paper aims to significantly lower this required temperature to $950^\circ\mathrm{C}$ by using a multi-component additive system. It builds upon known sintering aids (Cobalt Oxide and Copper Oxide) and introduces Titanium Dioxide ($\mathrm{TiO_2}$) to create a synergistic effect that promotes densification at lower temperatures without sacrificing the desired porous characteristics.

2.2. Main Contributions / Findings

1. Low-Temperature Sintering Success: Successfully sintered $\mathrm{SnO_2}$ ceramics at $950^\circ\mathrm{C}$ by adding small amounts of $\mathrm{CoO}$, $\mathrm{CuO}$, and $\mathrm{TiO_2}$.
2. Microstructure Control: Achieved a desirable porous microstructure with approximately 40% porosity, which is ideal for gas sensor applications.
3. Mechanism Identification: Confirmed that grain-boundary diffusion is the dominant mass transport mechanism at this low temperature, activated by the additives.
4. Solid Solution Formation: X-ray diffraction analysis revealed that $\mathrm{TiO_2}$ forms a solid solution with $\mathrm{SnO_2}$ (rutile structure), modifying the lattice parameters without creating secondary phases.
5. Dielectric Properties: Analyzed the dielectric constant, finding it is enhanced at low frequencies due to space charge and rotation direction polarization mechanisms linked to oxygen vacancies.

3. Prerequisite Knowledge & Related Work

3.1. Foundational Concepts

To understand this paper, a novice needs to grasp several materials science concepts:

• Sintering: A thermal process where a powder compact is heated below its melting point to bond particles together. The goal is often densification (removing voids/pores) and strengthening.
• Densification vs. Porosity:
  • Densification: The process of particles moving closer, shrinking the material and reducing holes.
  • Porosity: The volume of void space in the material. For gas sensors, high porosity is good (more surface area for gas detection), but some densification is needed for mechanical strength. It's a trade-off.
• Mass Transport Mechanisms: How atoms move during sintering to cause bonding:
  • Non-densifying Mechanisms: Surface diffusion and evaporation-condensation. These coarsen particles but do not shrink the material (common in pure $\mathrm{SnO_2}$).
  • Densifying Mechanisms: Grain-boundary diffusion (atoms move along the boundaries between grains) and Volume (Lattice) diffusion (atoms move through the crystal lattice). These cause shrinkage and densification.
• Solid Solution: A mixture where one solid dissolves into another. Here, $\mathrm{Ti}^{4+}$ ions replace $\mathrm{Sn}^{4+}$ ions in the crystal lattice structure, forming a single homogeneous phase.
• Rutile Structure: The tetragonal crystal structure commonly found in $\mathrm{TiO_2}$ and $\mathrm{SnO_2}$. Because they share this structure, they mix easily.
• Dielectric Constant ($\epsilon_r$): A measure of a material's ability to store electrical energy in an electric field. In this context, it helps characterize the defect structure (like oxygen vacancies) of the material.

3.2. Previous Works

The authors cite specific prior research that established the foundation for this study:

• Pure $\mathrm{SnO_2}$ Limitations: Kimura et al. [4] and Bueno et al. [2] established that pure $\mathrm{SnO_2}$ is hard to densify even at $1400^\circ\mathrm{C}$ due to non-densifying mechanisms.
• CoO Doping: Varela et al. [7] showed that Cobalt Oxide ($\mathrm{CoO}$) allows $\mathrm{SnO_2}$ to reach 99% density at $1400^\circ\mathrm{C}$ by creating oxygen vacancies (defects) that activate grain boundary diffusion.
• CuO Doping: Lalande et al. [6] demonstrated that Copper Oxide ($\mathrm{CuO}$) initiates shrinkage at very low temperatures ($1000^\circ\mathrm{C}$), acting as an "activator" for sintering.
• TiO_2 Doping: Bueno et al. [2] showed that $\mathrm{TiO_2}$ forms a solid solution with $\mathrm{SnO_2}$ and also utilizes grain boundary diffusion.

3.3. Differentiation Analysis

• Technological Evolution: Previous works focused on individual dopants ($\mathrm{CoO}$ OR $\mathrm{CuO}$) or high-temperature sintering ($1300^\circ\mathrm{C}-1400^\circ\mathrm{C}$).
• This Paper's Innovation: This study combines three components ($\mathrm{CoO} + \mathrm{CuO} + \mathrm{TiO_2}$) to exploit a synergistic effect. Specifically, it targets a much lower temperature ($950^\circ\mathrm{C}$) to achieve a specific balance: enough density for strength but enough porosity ($>40\%$) for sensors. Most prior high-density studies aimed to eliminate porosity; this study intentionally preserves it.

4. Methodology

4.1. Principles

The core principle is Activated Sintering. By introducing specific impurities (dopants) into the $\mathrm{SnO_2}$ matrix, the researchers manipulate the defect chemistry (creating vacancies) and atomic mobility. This lowers the activation energy required for atoms to diffuse across grain boundaries, allowing the material to bond and densify at $950^\circ\mathrm{C}$ instead of $1300^\circ\mathrm{C}$.

4.2. Core Methodology In-depth (Layer by Layer)

Step 1: Raw Material Preparation

The process begins with high-purity powders:

• Matrix: $\mathrm{SnO_2}$ (99.0% purity).
• Additives: $\mathrm{CoO}$ (<325 mesh), $\mathrm{CuO}$ (99.9%, $2\mu\mathrm{m}$), and rutile $\mathrm{TiO_2}$ (99.9%, $2\mu\mathrm{m}$).

Step 2: Powder Synthesis (Ball Milling)

To ensure homogenous mixing, the powders were processed as follows:

• Composition: The base mixture was $\mathrm{SnO_2}$ with $0.3\mathrm{wt}\% \mathrm{CoO}$ and $1\mathrm{wt}\% \mathrm{CuO}$. $\mathrm{TiO_2}$ was added in varying amounts ($x = 0, 0.01, 0.02, 0.04$ moles) based on the formula $\mathrm{Sn}_{1-x}\mathrm{Ti}_{x}\mathrm{O}_{2}$.
• Milling: Mixed for 24 hours in a polyethylene bottle using Zirconia balls (grinding media) and ethyl alcohol (solvent). This "wet-milling" breaks down agglomerates and mixes the powders intimately.
• Drying & Sieving: The slurry was dried at $90^\circ\mathrm{C}$ for 24 hours. The dried powder was crushed and passed through a 500 mesh sieve to ensure fine, uniform particle size.

Step 3: Forming (Pressing)

The powder was shaped into solid pellets:

1. Uniaxial Pressing: $\sim 2\mathrm{g}$ of powder was pressed into a mold at 0.5 MPa.
2. Cold Isostatic Pressing (CIP): The pellets were subjected to high pressure (100 MPa) from all directions for 3 minutes. This ensures uniform density in the "green" (unsintered) body.
   • Dimensions: Diameter $\sim 13\mathrm{mm}$, Thickness $\sim 4\mathrm{mm}$.

Step 4: Sintering (Heat Treatment)

The green pellets were fired in a furnace:

• Temperature: $950^\circ\mathrm{C}$.
• Duration: 8 hours.
• Atmosphere: Air.
• Ramp Rate: Heating and cooling at $5^\circ\mathrm{C}/\mathrm{min}$.

Step 5: Characterization & Dielectric Measurement

The sintered samples were analyzed using several techniques. A key quantitative measurement was the Dielectric Constant.

Dielectric Constant Measurement: To measure this, silver ($\mathrm{Ag}$) electrodes were pasted on the polished surfaces of the pellet and baked at $400^\circ\mathrm{C}$. Capacitance ($C$) was measured using a source meter.

The relative dielectric constant ($\epsilon_r$) was calculated using the following standard formula:

$$\epsilon_{r} = \frac{C d}{\epsilon_{o} A}$$

Symbol Explanation:

• $\epsilon_{r}$: Relative Dielectric Constant of the sample (dimensionless). This is the value being determined.

• $C$: Capacitance, the measured value from the LCR/source meter (Farads, F).

• $d$: Thickness of the sample pellet (meters, m).

• $\epsilon_{o}$: Vacuum Permittivity (or dielectric constant in vacuum), a physical constant approx. $8.854 \times 10^{-12} \mathrm{F/m}$.

• $A$: Area of the electrode on the sample surface (square meters, m²).

  This formula relates the geometry of the capacitor (Area $A$, distance $d$) and the material property ($\epsilon_r$) to the measured electrical capacity ($C$).

5. Experimental Setup

5.1. Datasets / Samples

The "dataset" in this physical science context consists of the physical samples synthesized with varying Titanium concentrations.

• Sample Series: $\mathrm{Sn}_{1-x}\mathrm{Ti}_{x}\mathrm{O}_{2} - \mathrm{CoO}(0.3\mathrm{wt}\%) - \mathrm{CuO}(1\mathrm{wt}\%)$
  • Sample 1: $x = 0$ (Baseline, no $\mathrm{TiO_2}$)
  • Sample 2: $x = 0.01$
  • Sample 3: $x = 0.02$
  • Sample 4: $x = 0.04$
• Why these? This range allows the authors to observe the trend of how increasing $\mathrm{TiO_2}$ affects density and porosity incrementally.

5.2. Evaluation Metrics

1. Sintered Density ($\rho$):
   • Definition: The mass per unit volume of the sintered ceramic. Higher density implies more shrinkage and fewer pores.
   • Method: Measured via the Archimedes method (using water displacement).
2. Porosity (%):
   • Definition: The percentage of the total volume that is void space.
   • Relevance: Crucial for gas sensors; target is $>40\%$.
   • Calculation: Derived from the theoretical density and measured density.
3. BET Surface Area ($S_{BET}$):
   • Definition: The total surface area of the material per unit of mass ($m^2/g$). Measured by Nitrogen adsorption. Higher surface area usually correlates with higher porosity and smaller grain size.
4. Grain Size:
   • Definition: The average diameter of the individual crystals (grains) within the ceramic. Determined from SEM images.
5. Dielectric Constant ($\epsilon_r$):
   • Definition: As defined in section 4.2. It indicates the material's polarization behavior and defect density.

5.3. Baselines

• Baseline: The sample with $x=0$ (only doped with $\mathrm{CoO}$ and $\mathrm{CuO}$, no $\mathrm{TiO_2}$).
• Comparison: This allows the isolation of the specific effect of $\mathrm{TiO_2}$ addition.

6. Results & Analysis

6.1. Phase Analysis (XRD)

The X-ray Diffraction (XRD) results confirmed the chemical structure of the sintered samples.

The following figure (Figure 1 from the original paper) shows the X-ray diffraction patterns:

该图像是图表，展示了 ext{Sn}_{1-x} ext{Ti}_{x} ext{O}_{2} - ext{CoO}(0.3 ext{wt ext{%}}) - ext{CuO}(2 ext{wt ext{%}}) 的 X 射线衍射图谱，包括不同 $x$ 值的样本： (a) (i) $x=0$，(ii) $x=0.01$，(iii) $x=0.02$，(iv) $x=0.04$；(b) 是 $33^{ ext{o}}$ 到 $40^{ ext{o}}$ 之间的详细视图，样本在 $950^{ ext{o}} ext{C}$ 下烧结 8 小时。

• Analysis:
  • The patterns show peaks corresponding to $\mathrm{SnO_2}$ (rutile) and $\mathrm{CuO}$.
  • Crucially, no separate $\mathrm{TiO_2}$ peaks were found. This proves that $\mathrm{TiO_2}$ fully dissolved into the $\mathrm{SnO_2}$ lattice, forming a solid solution $(\mathrm{Sn}, \mathrm{Ti})\mathrm{O}_2$.
  • Shift: In Fig 1(b), the peaks shift slightly to the left (lower angle). This indicates a change in lattice parameters (cell size) due to the incorporation of Ti atoms, which further confirms the solid solution formation.

6.2. Densification Results

The study aimed to see if $\mathrm{TiO_2}$ improves density at low temperatures ($950^\circ\mathrm{C}$).

The following figure (Figure 2 from the original paper) shows the sintered densities:

该图像是一个图表，展示了不同 ext{TiO}_2 含量下，经过 $950^{ ext{°C}}$ 烧结 8 小时的 ext{Sn}_{1-x} ext{Ti}_{x} ext{O}_2- ext{CoO}(0.3 ext{wt ext{%}})- ext{CuO}(2 ext{wt ext{%}}) 样品的烧结密度。随着 ext{TiO}_2 含量的增加，样品的烧结密度逐渐提升。

• Core Finding: The graph shows a clear trend: as the $\mathrm{TiO_2}$ content ($x$) increases from 0 to 0.04, the sintered density increases.
• Interpretation: Even though the temperature is low ($950^\circ\mathrm{C}$), the combination of additives activates the grain boundary diffusion mechanism. Pure $\mathrm{SnO_2}$ would not densify here. The additives make the atoms mobile enough to close some pores and increase density.

6.3. Microstructure and Porosity

While density increased, the goal was to maintain porosity.

BET Surface Area: The following figure (Figure 3 from the original paper) shows the BET surface areas:

该图像是一个图表，展示了不同浓度 $TiO_2$ 添加量对样品 BET 表面积的影响，横轴为 $TiO_2$ 含量 (x)，纵轴为 BET 表面积 (m²/g)。从图中可以看出，当 $TiO_2$ 含量增大时，BET 表面积先增后减。

• Observation: The surface area generally decreases as $\mathrm{TiO_2}$ content increases. This is consistent with the density data: as the material becomes denser, there is less internal surface area available (pores are closing).

Pore Structure: The following figure (Figure 4 from the original paper) shows the Nitrogen adsorption isotherms:

该图像是图表，展示了不同TiO2含量（$x=0, 0.01, 0.02, 0.04$）的$\mathrm{Sn_{1-x}Ti_xO_2-CoO(0.3wt\%) CuO(2wt\%)}$样品在$950^{\circ}\mathrm{C}$下烧结8小时后的氮气吸附等温线。图中横坐标为相对压力（P/Po），纵坐标为吸附量（cm³/g STP）。不同颜色的线条代表了不同的TiO2掺杂浓度。

• Analysis: The abrupt increase at high pressure indicates the presence of a porous structure in all samples.

The following figure (Figure 5 from the original paper) shows the pore size distributions:

* Analysis: The pore size distribution remains relatively constant across samples. This suggests that while the amount of porosity changes slightly (density increases), the nature of the pores (their size) is stable.

Visual Confirmation (SEM): The following figure (Figure 6 from the original paper) shows SEM images of the microstructure:

该图像是图6，显示了经 950^{ullet}C 烧结8小时的 ext{Sn}_{1-x} ext{Ti}_x ext{O}_2 ext{CoO}(0.3 ext{wt ext{%}})- ext{CuO}(1 ext{wt ext{%}}) 样品的扫描电子显微镜(SEM)图像。图中(a)为 $x=0$ 的样品，(b)为 $x=0.04$ 的样品，展示了它们的断裂表面微观结构。

• Visual Analysis:
  • Image (a) ($x=0$) and Image (b) ($x=0.04$) both clearly show a porous network (dark voids between grains).
  • Grain Size: The text notes that grain size decreased from 172 nm ($x=0$) to 132 nm ($x=0.04$) with $\mathrm{TiO_2}$ addition. Smaller grains are often desirable for mechanical strength and sensor sensitivity.

6.4. Dielectric Properties

The following figure (Figure 7 from the original paper) shows the frequency-dependent dielectric constants:

该图像是图表，展示了不同频率下 ext{Sn}_{1-x} ext{Ti}_x ext{O}_2- ext{CoO} (0.3 ext{ wt ext{ extpercent}}) 和 $ext{CuO} (2 ext{ wt ext{ extpercent}})$ 样品的介电常数，x的取值分别为0、0.01、0.02和0.04，且在950°C下烧结8小时。

• Low Frequency (< 1 kHz): The dielectric constant is very high. This is attributed to Rotation Direction Polarization (RDP) and Space Charge Polarization (SCP).
  • Mechanism: Oxygen vacancies (created by CoO doping) form dipoles. At low frequencies, these dipoles have time to align with the electric field, increasing the dielectric constant.
• High Frequency (> 1 kHz): The dielectric constant drops to the static value ($\sim 24$). The dipoles cannot keep up with the fast-switching field (relaxation).
• Significance: This confirms the presence of oxygen vacancies and defects at the grain boundaries, which supports the hypothesis that grain boundary diffusion is the active sintering mechanism.

7. Conclusion & Reflections

7.1. Conclusion Summary

This study successfully demonstrated that adding $\mathrm{TiO_2}$ to a $\mathrm{SnO_2}$-CoO-CuO system enables effective low-temperature sintering at $950^\circ\mathrm{C}$.

• Synergy: The combination of dopants activates grain-boundary diffusion, a mechanism that usually requires much higher temperatures.
• Outcome: The process yielded a ceramic with improved density (compared to pure $\mathrm{SnO_2}$) while preserving a porous microstructure (~40% porosity) essential for gas sensors.
• Control: $\mathrm{TiO_2}$ content can be used to fine-tune the density, grain size, and mechanical characteristics.

7.2. Limitations & Future Work

• Limitations:
  • The study only investigated $\mathrm{TiO_2}$ content up to $x=0.04$. The effects of higher concentrations are unknown.
  • Mechanical strength was discussed theoretically (relating to porosity and grain size) but not explicitly measured (e.g., via hardness or fracture tests).
• Future Directions: While not explicitly listed as "Future Work" in a separate section, the results suggest that this material system is ready for testing in actual gas sensor devices to verify if the electrical sensitivity matches the structural suitability.

7.3. Personal Insights & Critique

• Relevance: This is a classic "materials engineering" paper where the goal is to optimize processing parameters (temperature) to save energy. Lowering sintering temp from $1300^\circ\mathrm{C}$ to $950^\circ\mathrm{C}$ is a massive energy saving for industrial manufacturing.
• Methodological Soundness: The use of multiple characterization techniques (XRD, SEM, BET, Dielectric) provides a robust picture of the material. The link between the defect chemistry (oxygen vacancies) inferred from dielectric measurements and the sintering mechanism (grain boundary diffusion) is a strong theoretical connection.
• Critique: The paper would be stronger with direct mechanical testing data. They claim "improved densification... can be useful for controlling strength," but without stress-strain or hardness data, this remains a qualitative inference based on grain size reduction. However, for a fundamental sintering study, the microstructural analysis is sufficient.