The high-frequency induction sintering furnace is the core equipment for
preparing quartz fiber preform rods, responsible for sintering high-purity
quartz powder into dense and defect free rods (with sintering density ≥ 2.2g/cm
³ and bubble density ≤ 1/cm ³), directly determining the fiber transmission loss
and mechanical strength. If there is uneven sintering density (density deviation
exceeding 0.1g/cm ³) and more surface bubbles (exceeding 5/cm ³), it will lead
to an increase in fiber breakage rate and substandard transmission performance
during fiber drawing. Based on the extremely high purity (SiO ₂ ≥ 99.999%) and
microstructure requirements of quartz fiber preform, a three-step investigation
method is summarized to quickly restore sintering quality.
Step 1: Optimize the heating system and temperature field control - the core
power determines density uniformity
Improper configuration of induction coils, temperature distribution, and
heating rate are the main causes of uneven density.
Poor adaptation or aging of induction coil parameters: If the deviation of
coil turns exceeds 2 turns, it will lead to uneven distribution of heating
power, and the radial temperature difference of the prefabricated rod will
exceed 5 ℃; Oxidation or deformation of the coil surface, decrease in induction
efficiency, and insufficient local heating; The deviation between the coil and
the prefabricated rod exceeds 3mm, with high temperature near the coil end and
insufficient temperature at the far end. Accurately match the number of turns of
the coil according to the diameter of the prefabricated rod (such as 32 turns
for a diameter of 100mm), calibrate the coaxiality between the coil and the
prefabricated rod, and control the spacing between 2-2.5mm; replace the severely
oxidized coil, spray insulation and thermal conductivity coating on the surface
to improve heating stability.
Abnormal heating rate or temperature gradient: If the heating rate exceeds 5
℃/min, the moisture and gas inside the billet cannot be discharged in a timely
manner, which can easily form bubbles; A radial temperature gradient exceeding 3
℃/cm can lead to uneven sintering progress and significant density deviation
between the inside and outside. Adopting a "step heating" strategy: room
temperature -300 ℃ (2 ℃/min), 300-800 ℃ (3 ℃/min), 800-1200 ℃ (4 ℃/min), and
high temperature section insulation for 2 hours; Install an infrared thermometer
to monitor the surface temperature of the prefabricated rod in real time, adjust
the coil power to make the radial temperature difference ≤ 2 ℃.
Step 2: Calibrate atmosphere control and exhaust mechanism - precise control
of gas to reduce bubbles
Improper protection of atmosphere purity, flow rate, and exhaust path can
exacerbate bubble problems.
Insufficient purity or flow fluctuation of protective atmosphere: Ar gas with
purity below 99.999% contains O ₂ and H ₂ O, which will react with quartz powder
to produce gas; The flow rate fluctuates by more than 5L/min, the pressure
inside the furnace is unstable, and the exhaust is blocked. Select 99.9995%
high-purity Ar gas and install a gas purification device to further remove
impurities; Stabilize the flow rate at 20-25 L/min and maintain the furnace
pressure at 0.02-0.03 MPa through a mass flow controller to ensure smooth gas
flow.
Poor exhaust path or insufficient insulation: The billet is clamped too
tightly, and the exhaust channel is blocked; The insulation time in the
low-temperature range (300-600 ℃) is less than 1 hour, and the adsorbed water
and crystal water cannot be completely discharged. In the high-temperature
range, bubbles are formed by evaporation. Optimize the clamping method and
reserve a uniform exhaust gap; Extend the low-temperature insulation to 1.5-2
hours, and use vacuum assisted exhaust (evacuate to 10Pa at 300 ℃ and maintain
for 30 minutes) to thoroughly remove the gas inside the billet.
Step 3: Adapt to the characteristics of the billet and clean the furnace -
avoid defects at the source
The quality of the billet, contamination inside the furnace, and improper
cooling methods can affect the sintering stability.
Low density or high impurities in the green body: the green body density is
less than 1.6g/cm ³, with many internal pores and easy residual gas; The billet
contains over 5ppm of carbon and metal impurities, which can cause gas or
chemical reactions during sintering, resulting in the formation of bubbles.
Control the raw material preparation process to ensure a density of ≥ 1.7g/cm ³;
Select high-purity quartz powder raw materials, avoid pollution during the
processing, and wipe the surface of the blank with anhydrous ethanol before
entering the furnace.
Residual or rapid cooling in the furnace: residual dust and volatile
substances (with a thickness exceeding 0.1mm) on the inner wall of the furnace
cavity evaporate and merge into the prefabricated rod at high temperatures,
forming bubbles; The cooling rate exceeds 8 ℃/min, and the temperature
difference between the inside and outside of the prefabricated rod is large,
which is prone to stress cracks, and at the same time, bubbles cannot float up
and be discharged. After each sintering, blow the furnace chamber with
high-purity Ar gas and regularly clean the inner wall with a plasma cleaning
machine; Adopting the "gradient cooling" mode, natural cooling is carried out at
1200-800 ℃ (3 ℃/min), 800-500 ℃ (5 ℃/min), and below 500 ℃ to reduce
defects.
Daily maintenance should pay attention to: checking the purity, flow rate,
and coil status of Ar gas every day; Calibrate thermometers and mass flow
controllers weekly; Clean the furnace cavity and inspect the vacuum system every
month; Quarterly testing of coil inductance and heating uniformity. By
implementing the above measures, the sintered density of the preform can be
stabilized at around 2.25g/cm ³, with a bubble density of ≤ 0.5 bubbles/cm ³,
meeting the high-end requirements of optical fiber processing. If there are
still problems, it is recommended to contact the manufacturer to optimize the
heating coil design and atmosphere control program, and match it with the
exclusive sintering process for prefabricated rods.