Educational Video on Belt Furnaces by THT/Hengli

If you want to see how belt furnaces work and why they're used check out this educational video on the technology

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What is a Dye Sensitized Solar Cell (DSSC)

The electrochemical dye solar cell was invented in 1988 by Professor Graetzel of Lausanne Polytechnique, Switzerland. The “Graetzel” dye cell is based on a layer of nano-sized titanium dioxide particles impregnated with dye. Dye cells have been a subject of intense academic interest as more than 50 university research teams around the world have worked to enhance their lifetime, size and efficiency.

Dye cells generate electricity from solar energy using nano-sized titanium dioxide particles impregnated with dye, rather than silicon or similar semiconductors. Using DSSCs will result in unprecedented low costs, both manufacturing line capital cost and module cost per peak watt (ppw). Other solar cell technologies (silicon, thin film) rely on complex vacuum deposition techniques for cell active layer preparation. Not only is the production line for these systems large, complex and very expensive, but the raw materials (such as silicon) are costly and undersupplied. By comparison, dye requires simple equipment (screen printing, air ovens) and benign materials like Titania powder available at low cost.

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Furnace Process Applications and Temperature Profiles

Efficient design and customization capabilities have enabled our furnaces to be applied in a wide range of processes such as firing, brazing, annealing, sintering, hardening, glass to metal seal, reflow soldering, epoxy curing, hermetic sealing, LTCC (Low Temperature Co-fired Ceramics ), etc.

Thick Film
After a paste is screened onto a substrate and it settles for 5-15 minutes at room temperature, it undergoes oven drying at 100-150°C for 10-15 minutes to remove solvents. Firing is then completed in conveyor belt furnaces at temperatures between 500-1000°C.

Typical Temperature Profile:

Recommendations:
HSG Series Thick Film Drying Furnace
HSK Series Thick Film Firing Furnace

Thin Film Solar Cells
Take CIGS solar cells for example, thin film active layers are commonly formed using sputter deposition. After sputtering, the thin film needs to be annealed at 400-500°C to achieve optimum results. It is also possible to inject additional chemicals during the annealing process.

Typical Temperature Profile:

Recommendations:
HSG Series Photovoltaic Drying Furnace
HSH Series Photovoltaic Fast Response Furnace


Crystalline Silicon Solar Cell
Electrical contacts are usually formed by screen printing. The firing is done in conveyor belt furnaces at a temperature of about 700°C for a few minutes. Upon firing, the organic solvents evaporate and the metal powder becomes a conducting path for the electrical current.

Typical Temperature Profile:

Recommendations:
HSG Series photovoltaic Drying Furnace
HSH Series Photovoltaic Fast Response Furnace

Dye Sensitized Solar Cells (DSSC)
TiO2 nanoparticles have been used extensively to increase the interfacial surface area in Dye Sensitized Solar Cells. Nanoparticle films are generally made by screen printing a paste of titania nanocrystals and then sintering the particles together at 450-500°C.

Typical Drying and Firing Temperature Profiles:

Recommendations:
HSG Series Photovoltaic Drying Furnace
HSK Series Photovoltaic Fast Response Furnace
HSH Series Photovoltaic Fast Response Furnace

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Furnace for Firing Operation

The HSH series furnace is a specially designed infrared furnace that caters to the needs of the photovoltaic metallization firing requirements. The heating in this furnace is achieved with the help of short wave infrared lamp heaters. The fast response of the IR lamps allow for quick heating. The furnace is rated at 1000 oC and can operate very well in the 750-800 oC range required for sintering of front contact metallization. The belt width comes in various standard sizes, including, 250mm, 300mm and 380mm to match with the requirements of the wafer size. Cooling can be achieved through forced air, as well as, water per requirement. The presence of a muffle helps to control the atmosphere within the furnace, as well as, prevent the external atmosphere from entering. In short, the muffle design helps maintain a cleaner furnace atmosphere. As a standard feature, this furnace is equipped with a steel brush and helps in the cleaning process of the conveyor belt. Ultrasonic belt cleaning is available as an extra option as well.

A microprocessor based PID controller controls the furnace. Type K thermo-couples are used for determining the zone temperatures. Controls are located on the right hand side and can be viewed from the entrance of the furnace. The central processing unit (CPU) is mounted under the exit table. The furnace is controlled by a microprocessor based controller system and the CPU is loaded with a Windows operating system that allows for easy computing. The computer system comes with a pre-installed program for controlling the Confurnace parameters including the belt speed and the zone temperatures. Temperature profiles can be stored and retrieved for future purposes. Thermocouple ports are located at the entrance table for connecting the profiling thermocouple directly to the microprocessor. This feature allows the monitoring and recording of the actual temperature experienced by the part. Software is also provided with the computer to capture, display, printout and store the furnace profile. The furnace is equipped with a redundant overheat safety protection system which incorporates an additional type “K” thermocouple in the center of each controlled zone and the multi-loop alarm.

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Torrey Hills Technologies' photostream

Torrey Hills Technologies' photostream on Flickr.

If you're interested in seeing photos of our furnaces check out our flickr photostream! Tons of belt furnace pictures from the inside out!

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The HSK Series Furnace for DSSC Application

The HSK series furnace is an energy efficient precision thermal processing system specifically designed, and most often used, for DSSC applications. It has six channel temperature profiling units for independent temperature profiling with an LCD data display and check, analysis software, sampling unit, 3 T.C., and an RS232 CPU interface. The HSK Series is designed to support continuous on/off heating and cooling cycles resulting from alternating production periods and inactive operation. The heating length of the HSK Series is 3220mm (127”) and includes seven independently controlled heat zones. Process materials are transported through the furnace on a belt that is 350mm (14”) in length with 50mm (2”) of product clearance. The speed of the belt ranges from 40-200mm (2”-8”) per minute and is administered using a digitally displayed variable frequency motor controller. The belt speed is also programmable in IPM with readout right on the PC. The belt material on the HSK Series furnace is Nichrome V mesh (Balanced Spiral) and operates from 480V, 3 phase, 5 wire, 60Hz with a maximum load connection of 42kVA.

The performance of the HSK Series furnace is unparalleled as it can protect itself from over heating, over loading, and low gas pressure. It has an ultra-clean low-mass refractory heating chamber that can increase heat from ambient temperature to 1,050°C in approximately 40 minutes. The temperature of the furnace is controlled by a microprocessor that typically operates from 200-900°C. Each zone is managed using a high performance, single ASIC full auto-tuning PID and a single loop intelligent temperature controller. The HSK Series atmosphere distribution and management system can terminate thermal shock and process contamination, as well as, extract burn-off effluents across the entire width of the chamber for yield improvement. The HSK Series is assembled with entrance/exit curtains and an air powered Venturi exhauster (200mm /8” in diameter) to keep the firing chamber clean while, at the same time, improving temperature stability for drying and firing. The exhaust flow can also be easily adjusted using the flow meter. The HSK Series is readied with a redundant overheating safety protection system that incorporates a type “K” thermocouple (located in the center of each heated zone) and a multi-loop alarm. It ensures consistent ‘firing’ results because of its exceptionally reliable temperature uniformity control. The HSK Series furnace has a removable condensate collection trap and provides emergency off buttons located at each end of the furnace (connected to a 24v emergency off circuit). To see a complete list of the HSK Series specifications please see the chart below.

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Equipment Maintenance and Belt Cleaning Specifics

- Keep the belt speed under 150 mm/min while cleaning
- Use distilled water rather than tap water as the solvent. Tap water can have impurities, but distilled water that has been degassed allows for a more even distribution of cavitations
- Always monitor the quality of the solvent
- After use, clean out the rinse tanks
- After use, change out the filters

For more on Ultrasonic Belt Cleaning check out the "Technical Data" page at www.beltfurnaces.com

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Steel Brush verses Ultrasonic Belt Cleaner

Using a steel brush as a tool for cleaning the belt can often be time-consuming, imprecise, and inconsistent. A steel brush uses steel wire bristles to clean the surface area around the belt, however, due to its abrasive nature and inability to reach abstruse regions of the belt, it does not obtain the cleaning capabilities that an ultrasonic belt cleaner can easily handle.

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Advantages of Ultrasonic Belt Cleaning

Simply put, the advantages of using ultrasonic belt cleaning technology have to do with precision, speed, and consistency. Ultrasonic energy infiltrates obscure regions of the belt, which means that all areas of the belt will be categorically cleansed. Ultrasonic cleaning also works faster than any other conventional cleaning procedure in the elimination of contaminants. Due to the efficiency of ultrasonic antisepsis, the labor saving advantages designate ultrasonic technology as the preeminent economical design in cleaning furnace belts. And, unlike manual cleaning (like using a steel brush) ultrasonic technology offers incomparable cleaning consistency throughout the entire belt.

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Principles of Ultrasonic Belt Cleaning

In an ultrasonic belt cleaner, the solvent vibrates at ultrasonic frequencies. These ultrasonic waves cause extremely rapid pressure decreases and increases in the fluid. The sudden decrease develops instants where the water can no longer exist in a fluid state and, as a result, gas bubbles form. In other words, water can only exist in a liquid state above a minimum pressure (the threshold of cavitation). When the pressure decreases below the critical pressure, the water becomes a gas and evaporates. Then, at the ensuing increase in pressure, the gas bubbles will collapse again. This process of creating minute bubbles in the liquid is known as cavitation and it is responsible for the scrubbing effect that propagates ultrasonic cleaning.

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What is an Ultrasonic Belt Cleaner

An ultrasonic belt cleaner is an appliance that uses ultrasound technology and a cleaning solvent to thoroughly scour the furnace belt. The intention is to remove all traces of contamination deposited onto the belt. Sound waves are submitted into a cleaning liquid by means of transducers that are positioned within the cleaning tank. A transducer is an instrument that transforms one form of energy into another. It produces ultrasonic waves in the fluid by altering stature in harmony with an electrical signal vibrating at ultrasonic frequencies. The sound travels throughout the tank creating waves of compression and expansion in the liquid, leaving behind an immense amount of microscopic bubbles that implode before dislodging impurities from the furnace belt. Ultrasonic technology is significantly valuable in furnace belt sanitation as it can deliver ultra-precise and efficient cleaning capabilities.

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The Process of Cavitation in Ultrasonic Belt Cleaning

Cavitation (essentially a microscopic brush) is the process in ultrasonic cleaning creating rapid formations and violent collapses of gas bubbles or cavities in a cleaning liquid (solvent). Cavitation is responsible for the scrubbing effect, which produces ultrasonic cleaning. It is basically a method of using a transducer in a cleaning solution to create bubbles that literally implode around the parts of the belt that needs to be cleaned. These bubble implosions create a scrubbing action that causes contaminants to dislodge from the belt. The agitation by countless small and imploding bubbles create a highly effective scrubbing action of both exposed and hidden areas of the belt immersed in the cleaning solution. As the frequency increases, the number of these bubbles also increases, while, at the same time, the energy released by each bubble decreases. This means that higher frequencies are ideal for small particle removal without damaging the belt. Cavitation will occur throughout the cleaning solvent if the energy intensity is sufficient. It will also accelerate chemical reactions and the rate at which surface films are dissolved.

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Cavitation: a process that ultrasonic cleaning depends on

Ultrasonic cleaning depends upon this process of cavitation, the rapid formation and violent collapse of minute bubbles or cavities in a cleaning liquid. This agitation by countless small and intense imploding bubbles creates a highly effective scrubbing of both exposes and hidden surfaces of parts immersed in the cleaning solution. As tyhe frequency increases, the number of these cavities also increases but the energy released by each cavity decreases making higher frequencies ideal for small particle removal without damage of the items being clean.

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THT installation for IBM Research

Ultrasonic Belt Cleaner

An ultrasonic belt cleaner, or a sonicator, is a cleaning device that uses ultrasound (usually from 20–400 kHz) and an appropriate cleaning solvent (sometimes ordinary tap water) to clean delicate items. However, ultrasonic cleaners are often used to clean industrial parts and electronic equipment such as a belt furnace! They are even more effective than using a steelbrush. Stay tuned over the next few days as we go more in depth on the details, uses, and pros and cons of using an ultrasonic belt cleaner.

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Temperature Profile and The Temperature Upper Limit

The temperature profile comparisons clearly indicate that higher sintering temperatures result in larger TiO2 nanoparticles. Which, in turn, result in an overall improvement on the efficiency of the solar cell. The reason being that the larger particles allow for prominent dye absorption, producing higher electron generation. The temperature upper limit is around 600 oC. As you can see, the efficiency drops suddenly due to the instability of the TiO2 nanoparticles. In conclusion, a sintering temperature between 400 oC and 500 oC will result in a highly efficient DSSC.

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Influence of Sintering Temperature on DSSC Performance

Extensive research has been accumulated on the effects of various temperatures and the efficiency of the DSSC. The current ideal firing temperature is preferred around 450-500 oC, as shown below. High sintering temperatures at 450 oC result in a more desirable contact between the nanoparticles and a stronger adhesion to the substrate than those sintered at lower temperatures. It is important to keep in mind, however, that the DSSC will become unstable at very high temperatures because they have an upper limit of 600 oC to 650 oC.

Process of Sintering in a Belt Furnace

First, the titanium dioxide layer is prepared by sintering TiO2 nanoparticles at temperature range from 300 oC to 500 oC. The sintering process takes place on the transparent conductive oxide (TCO) glass plate, which is put into furnace heated uniformly for about twenty minutes. This process gets rid of the ambient moisture within the Titania layer, which is needed to ensure electrical contact between the titanium dioxide nanoparticles, and a good adhesion to the TCO (transparent conductive oxide) glass plate. Sintering of the layer can be done at 150 oC, however, it results in less performance than those sintered at 450 oC as mentioned in the research paper written by P.M. Sommeling in ECN Solar Energy. Then the layer is soaked in the dye solution to let the dyes get absorbed into the TiO2 surfaces. At last, the layer is put into a drying furnace. The titania is baked at 100 oC, and allowed for cooling.

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