1. Characteristics of HPHT Hydraulic Cubic Press and Overview of the Multi-Seed Method

HPHT Hydraulic Cubic Press Apparatus: This is one of the most popular and widely used high-pressure devices today. It is characterized by fast pressure increase and decrease speeds, lower cost, relatively high accuracy in pressure and temperature control, simple operation, and fast pressure transmission. Its maximum achievable cavity pressure is 5.5–6.0 GPa, which fully meets the requirements for conventional catalyst-based synthesis of high-quality diamond crystals and laboratory scientific research. The specific model used in this experiment is the SPD 6×1200 type.

Multi-Seed Method: Unlike the traditional method where only one seed crystal is placed in a single cavity, the multi-seed method involves placing 3–5 diamond seed crystals within a single synthesis cavity. This enables the simultaneous growth of crystals, ensuring that the crystals do not come into contact with each other after growing, while fully utilizing the space within the reaction cavity. This method can synthesize multiple crystals with consistent crystal habits and quality.

2. Principle of High-Temperature High-Pressure Temperature Gradient Method:

The multi-seed method for synthesizing gem-quality diamond single crystals is based on the high-temperature high-pressure temperature gradient method. Its principle is as follows:
Driving Force: Temperature gradient serves as the driving force for crystal growth.
Material Transport: This method facilitates the dissolution of carbon and its re-precipitation as diamond on the seed crystals.
Cavity Setup: The carbon source is placed in the high-temperature region of the reaction cavity, diamond seed crystals are placed in the low-temperature region, and the catalyst metal is positioned between the two.
Diffusion Mechanism: Due to temperature differences within the crystal growth cavity, there is a variation in carbon concentration within the solvent. Driven by the temperature gradient, carbon is transported downward through the molten catalyst, diffusing from the high-concentration area at the high-temperature region to the low-concentration area at the low-temperature region, where it precipitates onto the seed crystals, enabling crystal growth. Gravity is also an important factor influencing crystal growth.

3. Experimental Material Preparation
Catalyst: The experiment utilized a self-developed FeNi catalyst, where the Fe content ranged from 60 wt% to 70 wt%, and the Ni content from 30 wt% to 40 wt%.
Carbon Source: Flake graphite powder with a particle size of 200 mesh was used as the carbon source.
Diamond Seed Crystals: High-quality 0.6 mm × 0.6 mm Ib type diamond crystals with {100} facets were selected as the initial growth surfaces.

4. Synthesis Parameters and Cavity Assembly
Growth Pressure: The experimental pressure range was 5.4–5.7 GPa.
Pressure Calibration: The synthesis pressure was calibrated based on a curve relating oil pressure to internal cavity pressure, established using the high-pressure phase transition points of Bismuth (Bi), Barium (Ba), and Thallium (Tl).
Pressure Adjustment for Multi-Seed Method: With multiple diamond seed crystals distributed at different positions within the cavity, the stability range for diamond synthesis within the cavity becomes more demanding. To ensure that fluctuations in the crystal growth environment remain within the diamond's stable region, it is necessary to appropriately increase the synthesis pressure. This effectively widens the diamond synthesis window, compensating for the increased requirements introduced by multiple seeds.
Growth Temperature: The experimental temperature range was 1280–1400 °C.
Temperature Calibration: The synthesis temperature was calibrated based on a curve relating input power to temperature, measured by a Pt6%Rh—Pt30%Rh thermocouple. The thermocouple's measurement position was at the exact center of the cavity.
Temperature Gradient: This is a critical factor for ensuring crystal quality. An excessively large temperature gradient can lead to a significant concentration gradient, causing carbon to diffuse too quickly in the melt. If the seed crystals cannot absorb it rapidly enough, spontaneous nucleation near the seeds or precipitation of recrystallized graphite may occur. It can also easily lead to the formation of metal inclusions, affecting crystal quality. Conversely, an excessively small temperature gradient can result in the seed crystals failing to grow and instead graphitizing.
Growth Time: The experimental growth duration was 20–30 hours.
Cavity Assembly and Temperature Gradient Adjustment:
The design concept of the multi-seed method is to increase the area for receiving diffused carbon, thereby increasing the carbon absorption capacity, which requires sufficient carbon.
The cavity assembly design must consider creating a sufficiently large carbon concentration gradient through a greater temperature gradient.
The temperature gradient within the crystal growth cavity can be adjusted by modifying the synthesis cavity assembly (e.g., adjusting the height of the seed bed).
In a HPHT Hydraulic Cubic Press, the top and bottom anvils are typically energized to heat the synthesis cavity. The two pairs of lateral anvils are insulated relative to the press cylinder, allowing for the placement of two sets of thermocouples to measure temperature differences at various positions.
Diamond Stable Region: Diamond growth must occur within the diamond's stable region, which is defined by the V-shaped area bounded by the equilibrium line between diamond and graphite and the eutectic line between the solvent and diamond.

5. Specific Implementation of the Multi-Seed Method

In the China-type cubic high-pressure apparatus, through reasonable adjustment of the cavity assembly for high-temperature high-pressure synthesis, experiments were conducted to grow gem-quality diamond single crystals with 3–5 seed crystals within the diamond synthesis temperature range.

In the experiments, 3, 4, and 5 diamond seed crystals were used, all with {100} facets as their initial growth surfaces.
6. Post-Synthesis Treatment and Sample Characterization

Sample Retrieval: After growth, the pressure was released, and the samples were retrieved.

Acid Treatment: The retrieved samples underwent acid treatment for the purification of diamond single crystals.

Characterization Analysis:

An optical microscope was used to observe the morphology, color, and distribution of internal inclusions in the diamond samples.

Vernier calipers were used to measure the crystal dimensions.

A precision electronic balance was used to measure the crystal mass and calculate the crystal growth rate.
7. Multi-Seed Method Synthesis Results and Advantages

Consistent Crystal Quality and Morphology: Through the multi-seed method, multiple (3–5) high-quality Ib type gem-quality single crystals diamond were successfully synthesized in a single experiment. These crystals exhibited consistent crystal habits and quality. The crystals displayed a light yellow color.

Crystal Size:

When using 3 seed crystals, the average crystal size was between 3.5–4.0 mm.

When using 4 seed crystals, the average crystal size was between 3.1–3.6 mm.

When using 5 seed crystals, the average crystal size was between 2.5–3.0 mm.

(Note: Crystals with a particle size greater than 1 mm are classified as gem-quality diamonds.)

Significantly Improved Growth Rate:

To grow high-quality Ib single crystalsdiamond , the growth rate of a single crystal generally needs to be controlled at around 2.5 mg/h.

The growth rate of individual crystals grown by the multi-seed method was around 2.0–2.5 mg/h.

However, the total growth rate of all crystals combined (taking four seeds as an example) could reach 6.45–7.1 mg/h. This indicates a significant increase in the overall growth rate of single crystals diamond using the multi-seed method.

Efficient Utilization of Cavity Space: The multi-seed method effectively utilizes the space within the high-temperature high-pressure cavity, addressing the issue of low utilization efficiency of high-temperature high-pressure resources in larger presses.

Potential for Commercial Production: The multi-seed method provides an important basis for the batch synthesis and industrialization of single crystals gem-quality diamond . It represents one of the significant directions for their commercial growth.
In summary, by reasonably adjusting the cavity assembly of the HPHT Hydraulic Cubic Press and precisely controlling the synthesis pressure, temperature, and temperature gradient, the multi-seed method can efficiently and in batches synthesize high-quality, consistently shaped, and high-quality single crystalsgem-grade diamond , providing important theoretical and experimental guidance for large-scale industrial production.